key: cord-009836-7o6htufh
authors: Borrow, Persephone; Shaw, George M.
title: Cytotoxic T‐lymphocyte escape viral variants: how important are they in viral evasion of immune clearance in vivo?
date: 2006-04-28
journal: Immunol Rev
DOI: 10.1111/j.1600-065x.1998.tb01206.x
sha: 
doc_id: 9836
cord_uid: 7o6htufh

Summary: Although viral variants which are not recognized by epitope‐specific cytotoxic T lymphocytes (CTL) have been shown lo arise during a number of persistent virus infections, in many cases their significance remains controversial: it has been argued that the immune response is sufficiently plastic to contain their replication. In this review, we describe the mechanisms by which amino acid changes in viral proteins may affect epitope recognition by virus‐specific CTL, and discuss the viral and immunological basis for the emergence of viral variants bearing such amino acid changes during infection. We then consider the impact that viral variation may have on the host CTL response and its ability to contain virus replication. We argue that the emergence of a viral variant demonstrates that it must have an in vivo replicative advantage, and that as such, the variant must tip the balance between virus replication and immune control somewhat in favor of the virus. Further, we suggest that although the immune response can evolve to recognize new viral epitopes, the CTL generated following such evolution frequently have a reduced ability to contain virus replication. We conclude that this escape mechanism likely does make a significant contribution to persistence/pathogenesis during a number of different virus infections.

Following virus infection, a series of complex interactions occur between the virus and the host immune system. The host aims to ehminate the infection and minimize associated pathological consequences, whilst the virus tries to avoid clearance by the host immune response so that it can persist and be disseminated to other hosts over a long time period. Viruses have evolved a variety of different strategies for avoiding clearance by the host immune response; a single virus often employs multiple strategies simultaneously to increase its chances of persisting in the face of an adaptive host defense system with an array of different effector mechanisms (1). As cell-mediated immime responses, in particular the virus-specific CD8+ cytotoxic T-lymphocyte {CTL) response, frequently play a key role in the ehmination of established virus infections, many viral evasion strategies are targeted to this arm of the immune response.

Viral immune evasion strategies may broadly be divided into two categories: those that enable the virus to avoid detec-jrrow & Shaw • In vivo importance of CTL escape viruses tion by the host immune response, and those that impair the functioning of the host immune system. Viral impairment of the functioning of the host immune system may be non-antigen-specific, as exemplified by the production of homologs of host cytokines or rheir receptors that mimic (in the case of immune downregulatory) or block (in the case of immunestimulatory or antiviral effector) the actions of host cytokines (2), or hy infection of cells of the immune system, causing either impairment of their functions or resulting in their destruction; the latter may occur either directly or hy the cells being rendered targets for immune lysis (3). However, as too severe a generalized impairment of host immune functions can lead to the death of the host, thus reducing the opportunity for virus spread, a preferable strategy is impairment of the virusspecific immune response. Examples of how this may be achieved are by viruses infecting and targeting the destruction of antigen-specific B cells (4) or inducing exhaustion of highaffmity clones of virus-specific T cells (5), Strategies viruses may employ to avoid detection hy the host immune response include i) the establishment of latent infections during v\'hich viral protein production is minimized, a strategy commonly employed hy herpesviruses and perfected by herpes simplex virus (6); ii) infection of immune-privileged sites (to which access of the immune system is restricted and where levels of major histocompatibility complex (MHC) antigen expression are iow and immune downregulatory mediators may be present), the best example being the hrain where a surprisingly large number of different viruses persist (7); iii) virus-induced downregulation of levels of MHC antigens or adhesion molecules on the surface of infected cells so :hat they do not trigger host T-cell recognition (8); and iv) evolution of antigenic variants whose recognition by the specific immune response is impaired, Antigenic variation was initially documented as a means for viral escape from antibody neutrahzation (9), but more recently has also heen shown to confer viral escape from CTL control (10).

Although the above immune evasion mechanisms all have [he potential to promote viral persistence, the contribution that some of these mechanisms in fact make to persistence in vivo, particularly during human virus infections, is not completely dear. For example, although viral variants which are not recognized by epitope-specific CTL bave been found during a number of persistent virus infections (11), in many cases their significance remains controversial: it has heen argued that the immune response is sufficiently plastic to contain their rephcation (12), In this review, the mechanisms hy which amino acid changes in viral proteins may affect epitope recognition by virus-specific CTL are described, and the viral and immunoiog-ical basis for the emergence of viral variants bearing such amino acid changes during infection is discussed. The impact that viral variation may have on the antiviral CTL response during the course of virus infections is then considered, and the contrihution that this escape mechanism may make to persistence/pathogenesis in different virus infections is discussed.

Mechanisms by which amino acid changes may affect epitope recognition by virus-specific CTL For a viral epitope to be recognized by CD8-^ T cells, it must be processed and presented on the cell surface in association with MHC class I molecules. Amino acid changes may thus affect epitope recognition by specific CD8+ T cells hy altering any of the steps involved in this pathway, from the initial generation of the epitopic peptide to its formation of a stable complex with MHC, They may of course also affect the interaction of the peptide-MHC complex with the T-cell receptor (TCR), either ablating TCR binding altogether, or altering the signahng which occurs via the TCR, As described below, examples have been found of amino acid changes in viral proteins that act via each of these mechanisms.

Alteration ofantigen processing/peptide transport The efficiency of processing and presentation of CTL epitopes is determined not only by the sequence of the epitope itself, but also by the residues which surround it in the protein. Amino acid mutations in flanking sequences may thus affect the recognition of viral CTL epitopes, as was originally demonstrated in experiments using cytomegalovirus and influenza virus-specific CTL (13, 14), The precise mechanism by which antigen processing was affected was not defmed in these studies or a subsequent paper describing mutations in the Nef protein of human immunodeficiency virus type 1 (HIV-1) which also affected presentation of a nearby CTL epitope (15), However, in a more recent report, alteration of proteasome-mediated degradation due to a single amino acid difference within a CTL epitope was shown to form the basis of lack of presentation of this epitope by infected cells (16). In mice infected with AKV/MCF type murine leukemia virus, an epitope in the pi 5E transmembrane viral envelope protein constitutes the immunodominant sequence recognized by virus-specific CTL, These CTL do not recognize cells infected with Friend/Moloney/Rauscher type murine leukemia virus, where the epitope sequence differs by a single residue (17); this is because the amino acid difference causes epitope destruction by specific proteasomal cleavage. Mice infected with the latter virus thus do not mount a CTL response to this epitope, Amino acid changes that alter the processing of CTL epitopes can therefore have a dramatic effect on the in vivo antiviral CTL response.

It is unclear how commonly such mutations are selected for during persistent virus infections. That relatively few examples of amino acid changes in viral proteins which affect CTL recognition via this mechanism have been described could be a reflection of the fact that such mutations are not readily picked up by the methods most frequently used to analyze antiviral CTL responses. Alternatively, the requirements for antigen processing/peptide transport may be so much less stringent than those for peptide interactions with MHC and the T-cel! receptor that amino acid changes which affect the former arise less often.

Amino acid mutations within epitopic peptides may ablate peptide binding to MHC altogether, or reduce the affmity of peptide-MHC interaction so that the peptide-MHC complex has an extremely short half-hfe and is unlikely to trigger T-cell activation. Viral mutations which alter peptide interactions with MHC class I molecules in each of these ways have been described; examples in HIV were recently reviewed (19). As mutations which prevent the stable association of peptides with MHC confer escape from recognition by all epitope-specific T cells regardless of their TCR usage, this is an efficient mechanism for viruses to avoid CTL recognition. It is of interest that the human leukocyte antigen (HLA)-A 11 epitope loss from Epstein-Barr virus (EBV) isolates derived from populations where the frequency of HLA-Al 1 is extremely high is conferred via mutations in anchor residues of the epitope that are important for binding to HLA-Al 1(19, 20). EBV is a genetically stable DNA virus which generates mutants at a relatively low frequency; mutants will thus only come to prevail in the population if they confer advantages for the spread of virus to new hosts, in which very different TCRs may he used to recognize the same peptide-MHC complex. Mutations that ablate binding of the immunodominant HLA-A1 ] -restricted EBV CTL epitope to MHC will allow escape from recognition from epitope-specific T cells in any host.

Alteration of peptide interaction with the TCR Amino acid changes in a peptide which do not prevent it from being presented in association with MHC may result in alterations to the surface recognized by the TCR. This surface may be altered by changes in the TCR contact residues, or by changes in other residues in the epitope that cause the peptide to bind to MHC in a distorted conformation (21). Epitopes bearing mutations that cause alterations in the TCR contact sur-face have been termed altered peptide ligands (APL) (22). Epitope-specific T cells bearing distinct TCRs may be affected by APL in different ways. The altered peptide-MHC complex may fail to interact with a particular TCR altogether; or it may be recognized, but the T cell may receive a reduced or even a different type of signal when it recognizes an APL-bearing cell (23-26), The responding T cell may thus be only partly activated (e.g. to proliferate but not induce cell lysis (27)), or even anergized (28). Presentation of certain APL to T cells can therefore inhibit the response to the index peptide not just by competing with it for binding to MHC, but by negatively signaling the responding T-cell population: this phenomenon is known as T-cell antagonism. The abihty of APL to act as CTL antagonists can be tested by measuring the capacity of cells presenting the APL to inhibit lysis of labeled target cells presenting the index peptide (29), Using this technique, mutations which confer antagonistic properties on epitopic sequences have been shown to exist in the persisting virus population in individuals chronically infected with both hepatitis B virus (HBV) (30) and HIV-1 (31),

Although viral mutations which result in the generation of APL may not provide as efficient a means of simple escape from immune recognition as mutations which abrogate epitope presentation altogether (because the APL may still be immunogenic to a proportion of host T cells which could be expanded following emergence of the mutant), they could potentially have powerful effects on the overall control of virus replication via other mechanisms. If they are able to act as T-cell antagonists, they may not only inhibit the lysis of cells on which the mutant epitope is presented, but also that of cells in which nonmutant virus is replicating, hence conferring protection from immune control on the entire viral quasispecies. Further, as some APL are able to stimulate and sustain the growth of CTL despite the fact that they do not induce CTL lysis (27), they could drive an ineffectual CTL response and hence modulate the CTL repertoire in a detrimental fashion, reducing the overall efficiency of CTL control of virus replication.

Genetic variation is a strategy viruses exploit to promote their survival not just in the face of the host immune response, but under any environmental conditions they may encounter. They can achieve variation by a number of different mechanisms, including mutation, homologous and non-homologous recombination and (for viruses with a segmented genome) reassortment: different virns families utihze these to different extents. RNA viruses (including retroviruses) and DNA viruses such as Immtinologicd Reviews 164/1998 bepadnaviruses, whose genome replication involves an RNA intermediate, have extremely high mutation rates. This is due in large part to the absence or very low efficiency of proof-reading-repair activities associated with RNA replicases and transcriptases (32), and the lack of post-rephcative error correction mechanisms such as those diat normally operate during rephcation of cellular DNA. Misinsertion errors during RNA rephcation and reverse transcription have been estimated to be in the range of 10""^ to 10"^ substitutions per nucleotide per round of copying; for a 10 kb genome, this w(mld result in each progeny RNA strand including an average of 0.1 to 10 mutations (33). RNA viruses thus exist not as homogeneous populations, but as complex, dynamic mixtures of heterogeneous sequences termed quasispecies (34, 35). If a virus is replicating under a constant set of environmental conditions to which it is optimally adapted, although the precise composition of the quasispecies will continually be fluctuating, the average sequence of the viral population will remain unchanged. However if conditions alter, mutations which confer an increase in fitness (the relative ability of the virus to produce infectious progeny) will be selected for and come to predominate in the viral population.

The quasispecies nature of RNA viruses, conpled with the short replication times and high viral yields they frequently exhibit, favors rapid adaptation to environmental changes. Dramatic examples of viral evolution in response to environmental pressure have been provided by the emergence of drug-resistant viral variants in HIV-1-infected patients treated with antiretroviral agents (e.g. (36-38)), HIV is a retrovirus with an in vivo mutation rate of approximately 3x10"^ nucleotides per rephcation cycle (39). Human infection with this virns is characterized by high levels of persisting virus, much of which is actively replicating and turning over at remarkable rates (40-42). As many as 10"^ virions may be produced per day (42), allowing great potential for variation (43), although not all of these will be infectious, and the effective population size may be much smaller (44). W^here a single nucleotide change is sufficient to confer a high level of resistance to antiretroviral treatment, resistance may be selected for very rapidly Eor example, a single nucleotide change can reduce HIV's susceptibihty to the non-nucleoside reverse transcriptase inhibitor nevirapine by 100-1,000-fold. Viral variants bearing this mutation have been shown to pre-exist in the plasma HIV RNA in untreated patients (45), and in some patients high level resistance to this drug may be acquired within weeks of starting therapy (3 7), In cases where multiple mutations are required to achieve drug resistance, either because a series of mutations needs to occur before a high level of resistance to one antiretrovirai agent is achieved (38, 46) or because the patient is being treated simultaneously with a combination of several antiretroviral agents (47, 48), it takes longer for resistance to evolve. Indeed, in some patients, particularly where virus replication has been reduced to very low levels, it may not do so (49, 50). However, these observations illustrate the tremendous potential for RNA viruses to overcome controlling forces during the course of an infection via selection for resistance-conferring mutations.

Under what conditions are CTL escape viral variants selected in vivol

The above outline of how mutations are selected for in a virus population allows a number of predictions to be made about the in vivo conditions under which CTL escape virns variants are likely to emerge. Firstly, as a mutation will only be selected for if it confers a fitness advantage on the virus hearing it, CTL escape virus variants will only grow out in infections where CTL pressure exerts significant control over virus replication. It is thus perhaps not surprising that the first report of the in vivo emergence of a CTL escape virus variant came from a study performed using lymphocytic choriomeningitis virus (LCMV) (10), a murine virus: during infection with LCMV the CD8+ CTL response is well known to be the critical determinant of control of virus rephcation (reviewed in (51)), Conversely, demonstration that viral variants bearing mutations which confer escape from CD8"^ T cells come to dominate the viral quasispecies during a particular virus infection can be used to provide evidence for the importance of this arm of the immune response in controlling the infection, which in human virus infections may otherwise be difficult to demonstrate convincingly (52) (53) (54) . Secondly, as the stronger the control over virus rephcation exerted by the CTL response directed against a particular epitope, the greater the selective pressure it will exert for escape mutations in this epitope to emerge in the viral population, the frequency of CTL directed against a particular epitope and the avidity of their target cell interaction will clearly influence the selection of escape mutations in this epitope. The original demonstration of CTL escape virus selection in the LCMV system was actually made in TCR transgenic mice where CD8+ T cells specific for the epitope in which escape mutations were observed made up 75-90% of the peripheral T-cell population prior to infection (10), This was an artificial system; however, due to advances in methods for quantitating epitope-specific T cells, it has recently become apparent that epitope-specific CD8"^ T cells can reach extremely high frequencies in natural virus infections in both mice and humans, particularly during the primary immune response (55) (56) (57) . Primary infection is thus clearly a setting under which escape-conferring mutations may potentially emerge very rapidly Although lower frequencies of epitope-specific CTL will also have the capacity to drive the selection of escape viral variants, this selection will occur more gradually over the course of a greater number of rounds of viral rephcation.

The greater the selective advantage conferred hy a particular viral mutation, the faster this mutation will be selected for in the viral quasispecies. Thus, not only the strength of the CTL response to a particular epitope hut also the immunodominance of this response will have a great impact on the likelihood that escape mutations will be selected for within the epitope. Escape mutations in subdominant or even co-dominant CTL epitopes may confer such a shght selective advantage on the viral variants bearing them (whose replication will still he controlled by the more dominant or co-dominant CTL responses in the host (58, 59) ) that they may never emerge in vivo. Virus rephcation must clearly be ongoing for viral variants to be generated and selected. Particularly where the selective advantage conferred by a certain amino acid mutation is not very great, many cycles of virus replication may need to occur before viruses bearing it become predominant in the viral quasispecies. How dominant the CTL response to a certain viral epitope is in the overall control of virus replication is thus also an important determinant of the emergence of escape mutations in this epitopic sequence from this perspective. If other arms of the immune response or CD8+ T-cell responses to other epitopes in the virus which are not affected by the mutation concerned are able to reduce virus replication to very low levels or mediate viral clearance, the escape mutation will not have time to hecome fixed in the viral population during the course of infection. CTL escape virus variants will thus be most likely to emerge in the face of a host CTL response which is highly dominated by T cells of a single epitope specificity; even a very strong response to a particular epitope is unhkely to have the chance to select for escape viral variants in the context of strong responses to other epitopes. This is illustrated by observations made in the murine LCMV infection model. The CTL response moanted to LCMV by infected C57BL/6 mice has a broad epitope specificity: in the viral glycoprotein (GP) and nucleoprotein (NE) alone, at least five different CTL epitopes have heen identified (60, 61). Although the response to some of these epitopes is extremely strong (T cells recognizing the epitope at NE 396-404 constitute up to 40% of the CD8-^ T-cell population, and those recognizing the epitope at GE 33-41 up to 29% at the peak of the antiviral immune response (56)), the infection is rapidly cleared, and CTL escape viral variants do not emerge. If mice are immunized with one of these epitopes in a carrier protein prior to infection, the CTL response mounted upon LCMV infection is then biased in favor of this epitope. Viral clearance still occurs; however, if virus is isolated from the mice just prior to clearance and grown up in vitro, it is found to contain escape mutations in the epitope to which the CTL response was biased (62) , Escape viral variants thus arise in the context of this more immunodominant CTL response, but they do not grow out in vivo as the infection is cleared too rapidly The LCMV TCR transgenic mice discussed above represent a situation of epitope immunodominance at the extreme of the spectrum: their T-cell repertoire is so dominated by transgene-expressing cells recognizing the LCMV GE 33-41 epitope that CTL responses cannot be effectively mounted to other viral epitopes. Thus, when they are infected with LCMV and viral mutants arise which cannot be recognized by the GE 33-4]-specific CTL, their replication is not contained by CTL of other specificities and they emerge in the context of viral persistence (10).

The ahove predictions that CTL escape viral variants are most likely to. emerge in the context of a strong host CTL response which is highly focused on a single viral epitope are supported by the observation that one of the clearest examples of the emergence of CTL escape virus variants during a human virus infection occurred under just these conditions. Both we and others have shown that strong CD8+ CTL responses are mounted very early following infection with HIV-1, prior to seroconversion, and have hypothesized that they play an important role in containing viral replication (63, 64), In one patient we studied (53), the early CTL response (16-20 days following the onset of symptoms indicative of acute HIV-1 infection, which occurred 20 days after the homosexual encounter during which he initially contracted the virus) appeared to he strongly directed against a single viral protein, Gpl60. Epitope mapping performed using the Gpl 60 sequence of the patient's autologous early HIV-1 population indicated that this response was in fact extremely focused on a single epitope encompassing Gpl60 amino acids 30-38(9), recognized in association with HLA-B44, The frequency of epitope-specific CTL was extremely high: at the earhest timepoint available for study, which may have been shghtly after the peak of the primary immune response, 1 in 1 7 peripheral blood mononuclear cells (EBMCs) were found to score as virus-specific CTL precursors by limiting dilution analysis, a technique which has recently been shown to greatly underestimate the total numher of epitope-specific T cells (55, 56) , As shown in Fig, 1 , viral variants bearing mutations in the epitopic sequence which conferred escape from recognition by epitope-specific CTL rapidly appeared in this patienc, and then increased in frequency until 164/1998 they had cotnpieteiy repiaced the transmitted virai strain. Interestingly, the variants which came to predominate in the virai quasispecies aii possessed changes at Gpi 60 amino acid 3 i, the position which constituted the major anchor residue for HLA-B44. As discussed above, mutations which abiate peptide binding to MHC confer escape from recognition by aii epitope-specific T ceiis, and indeed, not oniy individual CTL ciones but aiso bulk CTL derived from this patient were unabie to recognize peptides corresponding to the mutant virus sequences in in vitto assays (Fig. 1) .

Thus in both human and murine systetns, there is evidence that mutations which confer resistance to controi by epitopespecific CTL are most iiiieiy to be seiected for in highiy immunodominant epitopes. Under conditions of epitope immuno-dominance, a singie mutation which provides escape from recognition by CTL of one specificity wiii confer a greater repiicative advantage on the mutant virus, thus the seiection for it wiii be stronger; pius there wiii be a greater chance that virus replication wiii be abie to continue for iong enough for the mutation to compieteiy repiace the index residue in the virai quasispecies.

Evolution of the CTL profile in response to the emergence of escape viral variants

The virai quasispecies can ciearly evoive in response to host immune pressure: as the host immune response is aiso adaptive, can it in turn evoive in response to the emergence of escape viral variants? The specific immune response is nltimately driven by antigen; the emergence of a CTL escape mutation in a virus population during the conrse of an infection could potentially affect the antigen to wbicb tbe immune system is exposed, and hence the CTL profile, in several ways. Firstly, unless the mutation selectively affects CTL stimulation so that only effector functions are diminished, the level of CTL specific for the index epitope will decline as the level ofantigen available to stimulate them decreases. Secondly, if the mutant sequence is presented to and can stimnlate novel populations of host CTL, these will increase in frequency. Thirdly if the mutation reduces the efficiency with which the epitope is presented, this may allow the level of presentation of other viral epitopes to increase, and hence CTL of different specificities to increase in frequency, or novel populations of host CTL to be stimulated. Finally, as tbe mutant virus must bave a rephcative advantage over tbe original virus in order to be selected, the antigen load will increase, which will drive the overall expansion of virusspecific CTL. As discussed earlier, CTL escape mutations are most likely to be selected for in immunodominant CTL epitopes. The end result of the above effects will be that the breadth of tbe CTL response will increase, with the previously dominant T-cell clones declining in frequency, and subdominant or novel responses being stimulated. Sucb broadening of CTL specificity in response to tbe emergence of viral variants able to escape recognition by CTL directed against a highly immunodominant epitope was apparent in the HIV patient described in the previous section (53) . As illustrated in Fig. 2 , whereas PBMC derived from tbe patient very early during tbe infection mediated detectable lysis after in vitro restimulation of only target cells expressing the immunodominant Gpl60 30-38(9) epitope, as viral variants bearing mutations in this epitope emerged, responses to epitopes elsewhere in Gpl60 and in other viral proteins became apparent. A decline in the frequency ofCTL directed against the Gpl 60 30-38(9) epitope occurred simultaneously (53) .

This patient clearly bad the capacity to make CTL responses to a broad range of epitopes in HIV; why then was his initial response so highly foctised on a single epitope? How epitope immunodominance is dictated dnring an antiviral immune response is not clear: both the level of presentation of different viral epitopes and the available T-cell repertoire impact on this. The former can be affected by epitope processing (65), peptide transport into tbe endoplasmic reticulum (66, 67) , tbe binding affinities of peptides to class I molecules (68) and the stabihty of peptide-MHC complexes on the cell surface (69), If CTL epitopes overlap, different MHC molecules may compete for their presentation (70) . The T-cell repertoire is initially deter-mined by positive and negative selection in tbe tbymus, and subsequently reshaped in tbe periphery during the course of successive immune responses. Tbe influence that tbe previous infection history may have on the subsequent immune response to a virus infection bas been illustrated in mnrine arenavirus infections (71) . In the HIV patient discussed above, both properties of the epitopic sequence and the T-cell repertoire at the time of infection may bave contributed to the extreme immunodominance of the early CTL response, Tbe Gpl 60 30-38(9) peptide may have had advantages over other potential epitopic sequences in its processing and transport: there appear to be differences in the processing of transmembrane and cytoplasmic proteins for presentation with class I whicb may give tbe former a presentation advantage under certain circumstances (72); further, it is of interest that during the natural processing of Gpl60, signal peptide cleavage occurs between amino acids 29 and 30, thus generating the same N terminns as the 30-38(9) epitope. In addition, this epitope is predicted to have a very high binding affmity to HLA-B44. T-cell repertoire effects may also bave played a role: it is possible tbat prior unrelated infections in this patient left bim with a population of memory T cells that cross-recognized this epitope, and that these cells, being present at higher frequency and more readily activated than naive T cells, dominated the initial HIV-1-speciflc immune response. This cannot be demonstrated conclusively as no samples are available from tbis patient prior to infection; bowever, the fact that epitope-specific CTL were contained in an ohgoclonal population of Vp 19+ T cells which underwent a massive expansion very early after infection in this patient (73) would be consistent with this.

Another study in the HIV system wbich emphasized the effect that escape mutations in CTL epitopes may bave on CTL specificity focused on the CTL responses of two HLA-identical hemophiliac brothers who were exposed to the same batch of contaminated Factor VIII and became seropositive within 10 weeks of one another (74) . Their CTL responses were very dissimilar: one made strong responses to two HLA-A2 and HLA-A3-restricted epitopes in Gag, whilst the other instead responded to two HLA-B 7-restricted CTL epitopes. Mutations in botb immunodominant epitopes of the Gag responder were seen in provirai sequences from the non-responder; it was thus very hkely that be had initially made a CTL response to these epitopes, and tbat following the emergence of escape mutant viruses, tbis response had been downregulated and the B7restricted responses stimulated.

In this individual, the escape mutant viruses did not revert to the index sequence when the CTL response against them diminished, implying tbat the mutations they bore were not detrimental to viral replication. In other cases, however, CTL escape mutations may reduce viral fitness (75) . If they impair virus replication too severely, tbey will not be selected for in the first place; this may contribute to the fact that in some studies escape variants have not been seen to arise in particular CTL epitopes despite the fact tbat CTL pressure is exerted on them over long periods of time (76. 77). If they have a smaller impact on viral fitness, they will emerge when epitope-specific CTL are exerting a strong pressure on viral replication, as overall they will confer an advantage on the virus, but as the frequency of epitope-specific CTL declines, they will tend to be replaced in the viral quasispecies by the index sequence. This in fact occurred in the HIV patient we described, where a strong selection occurred in tbe viral quasispecies for viral variants bearing mutations at the residue w-bich constituted tbe anchor motif for the HLA-B44-restricted CTL epitope to which a dominant response was made in tbe early phase of the infection. Later in tbe infection, wben the frequency of epitope-specific CTL bad declined significandy, clones bearing the index sequence again began to emerge in tbe viral quasispecies (X. Wei, P Borrow, . Epitope immunodominance and mutant virus frequency tbus drive one another, which can lead to complex fluctuations in the makeup of tbe viral quasispecies and CTL specificity over the course of a persistent infection, Sucb fluctuations have been observed in patients cbronically infected with HIV-1, and mathematical models developed to describe them (78-80), These models predict tbat antigenic variation in immunodominant epitopes can shift responses to weaker epitopes and tbereby reduce immunological control of the virus. That this certainly can occur bas been illustrated in experiments performed in the LCMV model system, CTL clones directed against the three most dominant epitopes recognized during the antiviral CTL response in C57BL/6 mice were used to select in vitro for LCMV variants bearing escape-conferring mutations in one, two or all three of these epitopes (81) (82) (83) . Wben CS7BL/6 mice were infected with tbe mutant viruses, they mounted CTL responses to subdominant epitopes including epitope(s) in the viral polymerase wbich are not readily apparent during tbe response to wild-type virus, but the responses driven by the in vitro-generated escape viral variants controlled virus replication less efficiently than the response to wild-type virus (58, 59) . These experiments indicate tbat although tbe immnne response is extremely plastic, its capacity to evolve to efficiently contain the replication of escape mutant viruses may be limited, particularly in inbred mice where the diversity of MHC alleles is more restricted than in the outbred human population. The evolution of escape viral variants may thus make a significant contribution to viral persistence during some infections by driving a suboptimal immune response.

For a CTL escape viral variant to emerge, it must tip the balance between virus replication and the host antiviral immune response at least somewhat in favor of the virus. This replicative advantage can in itself affect the course of a virus infection, as was seen in tbe experiments originally demonstrating the in vivo selection of CTL escape viral variants in LCMV epitope-specific TCR transgenic mice: these animals developed a persistent infection following inocnlation with a dose of virns whicb non-transgenic mice were rapidly able to clear (10). However, in tbis system, the TCR repertoire was artificially restricted, leaving open the question of whether CTL escape viral variants may also have biologically significant effects on the course of infection under more natural circumstances, where the immune system has the capacity to respond to a mucb broader range of viral epitopes and, as discussed above, the CTL response may co-evolve with the viral quasispecies. Mutations which affect epitope-specific CTL lysis in vitro bave been described in a number of different viruses; here, their likely significance in some of these virus infections is discnssed.

Mouse hepatitis virus-strain JHM (MHV-JHM) is a coronavirus which produces an acute fatal encephalitis in most inbred strains of mice. If C57BL/6 mice are infected wbilst suckling on dams previously immunized to the virus, tbey are protected from tbe acute encephahtis; a proportion of mice then control the infection, but the majority develop a persistent infection associated with chronic demyelinating encephalomyehtis (84), Tbe virusspecific CD8+ T-cell response is important in controlling this infection; in C57BL/6 mice this is directed against two epitopes in the viral surface GP (S), an immunodominant epitope at S 510-518, and a subdominant epitope at S 598-60S (85), A recent study showed that mutations which cause a loss of recognition are present in tbe immnnodominant epitope sequence in almost all virus sampled from symptomatic mice, but not in other T-cell epitopes (86) , Mutations in this epitope were not detected in mice witb acute encephalitis (whicb die very soon after the antiviral T-cell response is mounted) and were fonnd at only low frequency in the residual viral RNA in the central nervous system of animals which had cleared infectious virus and remained asymptomatic at late times after infection. Further, when MHV-JHM variants bearing mutations in epitope S 510-518 isolated from infected mice were used to infect naive mice, they produced an increased morbidity and mortahty (87) . This is thus a convincing example of an infection in wbicb CTL escape variant selection does appear to be a key factor influencing virus pathogenesis. This infection may represent a situation where the host's abihty to control virus rephcation is tenuous (CD8"^ T-cell control of virus replication in the central nervous system may not be tbat efficient), and the rephcative advantage conferred on the virus by escape mutations may tip the balance firmly in favor of the virus.

EBV is a human gamma herpesvirus that is carried by the majority of individuals as a hfelong asymptomatic infection, but has oncogenic potential and is implicated in the pathogenesis of a range of malignancies. The increased risk of development of EBV-positive mahgnancies associated with immune suppression illustrates the importance of immune control in maintaining a non-pathogenic equilibrium between this potentially oncogenic virns and its host; the potent antiviral CTL response is thought to play an important role in controlling virus replication and spread and the development of disease (20, 88, 89) . The abihty of tbis virus to evade clearance from the infected host has been attributed to a number of different mechanisms (reviewed in (20)), but CTL escape variant selection is not thought to play a major role. Unlike tbe other viruses discussed in this section, FBV is a genetically stable DNA virus; mutations whicb may confer escape from the prevailing host CTL response are tbus hkely to arise too infrequently for escape variant generation and selection to constitute an efficient means ofimmune evasion within a given host.

Mutations have been documented in the virus population prevaihng in certain areas wbich confer escape from recognition by CTL directed against epitopes whicb are presented by HLA alleles possessed by a high proportion of the local populace. Tbe best known example involves HLA-A11 -restricted CTL epitopes in the EBNA3B protein which are conserved in tbe majority of type 1 EBV isolates from Caucasian and African populations, where the HLA-A 11 allele frequency is low, but are mutated in virtually all isolates from highly All-positive ImmunologicoJ Reviews 164/1998 populations in China and coastal New Guinea (19, 90) . The majority of these mutations are located within anchor residues and abrogate epitope binding to the HLA-Al 1 molecule; as discussed earher, such mutations would provide escape from epilope-specific CTL regardless of their TCR usage, suggesting that they may have conferred a selective advantage on the virus in the A11 -positive populations from which they were isolated. However, FBV isolates from South-East Asia differ from Caucasian and African isolates at a number of loci, and other polymorphisms have been shown to affect the antigenicity of epitopes that are not present at high frequency in South-East Asian populations (91) , raising the possibility that the changes observed in the Al 1-restricted epitopes may be coincidental. Support for the hypothesis of specific epitope loss was not obtained when EBV isolates from a highly HLA-B35-positive African population were sequenced across a B35-restricted epitope-containing region of the EBNA3A protein (92), However, the HLA-B35 frequency in this population was only about half that of the HLA-A11 frequency in the South-East Asian populations where A11 epitope mutations were detected; further, the HLA-B3 5-restricted CTL response to EBV is not usually as strong as that restricted by HLA-A 11 (92) . Whether the EBVspecific CTL response does in fact select for viral variants with a growth advantage within human populations bearing certain HLA alleles thns remains an unresolved issue.

HBV is a hepadnavirus which, although it has a DNA genome, replicates via reverse transcription from a pregenomic RNA and thus has a high mutation rate. This virus produces acute and chronic infections in man during which the CD8"^ CTL response plays a key role both in virus clearance and in the pathogenesis of the associated liver disease (93) . During acute HBV infection, most patients develop a strong polyclonal CTL response against multiple epitopes in different viral proteins (93) . As discussed above, the likelihood of selection of escape mutant viruses under these conditions is prohably low, because a mutation which provides escape from CTL of just a single specificity may not confer a significant selective advantage on the virus bearing it in the presence of strong CTL responses to many other epitopes. In contrast, the CTL response is usually much weaker during chronic HBV infection (93) . In some patients it may also be more ohgospecific, providing greater opportunity for the selection of epitope-specific escape mutations. Indeed, there is one report (94) of two patients who showed strong HLA-A2-restricted CTL responses narrowly focused on an epitope in the HBV core protein at amino acids 18-27 and failed to respond to any of the other HLA-A2-restricted CTL epitopes in HBV that are frequently recognized in acutely infected patients. When the persisting virus in these patients was sequenced, the quasispecies was found to be dominated by variant virus carrying mutations within the HBV core 18-2 7 epitope that affected epitope recognition by the patients' CTL and could antagonize the response of certain CTL clones to the index epitope (30). Although conversion from a wild-type to the mutant sequence was not demonstrated in these patients, it is very likely that this does represent an example where escape variants were selected by the strong epitopespecific CTL response.

CTL responses restricted by other HLA alleles were not analyzed in these patients, and no information is available about how the CTL repertoire may have evolved in response to the emergence of the mutant viruses. It is thus difficult to assess how the escape variant may have impacted on the overall efficiency of CTL-mediated control of virus rephcation in these individuals. Other reports suggest that the emergence of virus variants bearing CTL escape mutations may not be a common event during chronic hepatitis B infection: no escape mutations were identified in a stndy where the CTL response against eight different HLA-A2-restricted epitopes defined in patients with acute HBV infection and the sequence of these regions in the in vivo viral quasispecies were analyzed in parallel in 12 patients chronically infected with HBV (95) . Virus-specific CTL responses were undetectable in eight of these patients and weak in the other four; CTL-mediated pressure may thus have been too low to select for escape variants. Whether selection of escape mutations plays a critical role in HBV persistence is thus questionable (96) ; it is likely that the development of a weak antiviral immune response is ol much greater importance in determining the course of this infection (93) .

HCV is a positive-stranded RNA virus, assigned to a new genus in the Flaviviridae, which is well adapted for persistence in its human host: at least 60% of infected individuals develop a chronic infection that is frequently associated with clinical hepatitis (97) , As in HBV infection, the virus-specific CDS-' CTL response is thought to play an important role in limiting virus replication, and simultaneously to contribute to disease pathogenesis. However, unlike the situation in HBV infection, where cell-mediated immune responses to the virus are generally low/undetectable in chronically infected individuals, HCV persistence in the presence of strong virus-specific CTL responses has been observed in both humans and experimentally infected chimpanzees (reviewed in (98) ). The highly mutable nature of this virus's RNA genome raises the possibility that antigenic variation may he one of the mechanisms via which it aciiieves persistence in the face of the antivirai immune response. Indeed, there is one report that in a chimpanzee which became persistentiy infected with HCV tiie virai quasispecies underwent compiete repiacement with virai variants hearing a conservative amino acid suhstitution at residue 4 of an epitope in non-structural protein 3 (NS3) recognized hy iiver-derived CTL iines from this animal; this mutation ahrogated recognition of the epitope by the animai's CTL (99) . The CTL response in this animai recognized epitopes in mtiltipie virai proteins (iOO); whetiier mutations were seiected for in any of the other epitopes, how epitope specificity/immunodominance may iiave ciianged over the course of infection in response to virai variation, and what impact escape variant seiection may have had on the overali efficiency of controi of virus repiication were not determined. However, tiiis study iliustrates that even though the CTL response to HCV is frequentiy poiycionai, escape virai variants can be seiected during HCV infection. As discussed beiow for HIV, where more information ahout the co-evoiution of the virai quasispecies and tiie immune response is avaiiahie, escape mutations iikeiy do compromise immune control of HCV infection, and are probabiy one of the factors which contrihute to the persistence of this virus in its human host (98) .

HIV ehcits strong CTL responses in most infected individuals. The eariy CD8+ CTL response is temporally coincident with the reduction in acute plasma viremia (63, 64); as discussed ahove, virus-specific CTL Iikeiy expand to extremeiy iiigh frequencies during the primary immune response (53, 73) . The frequency of virus-specific CTL then deciines somewhat; however, high ieveis of activated CTL are commoniy detected in chronicaiiy infected asymptomatic individuais, even in the setting of an extremeiy iow virai ioad (77, i 01 -104). As disease progression occurs, virus-specific CTL frequencies may initialiy increase in response to increasing viral ioad, but generaily faii to imdetectabie ieveis in the end stages of the infection (104) . A number of hnes of both direct and indirect evidence (discussed in (53) ) indicate that the CD8+ CTL response does piay an important role in controiiing virus repiication in HIV-1 -infected individuais, and a variety of mechanisms have been proposed to contribute to virus persistence in the face of the host immune response (105) (106) (107) (108) (109) (110) (111) . We focus here just on the role that the selection of CTL escape virus variants may piay during this infection.

The high virai turnover during HIV-1 infection, rapid mutation rate of this virus and strong virus-specific CTL responses provide conditions under which CTL escape mutants wouid be iikeiy to emerge. Indeed, as indicated earher. there have been numerous reports of variant viruses within the quasispecies in different patients w-hich are able to escape recognition by epitope-specific CTL. Escape virus variants may be selected at different stages of infection and correspondingly impact on the virus-host baiance in different ways. During primary HIV-1 infection, when the virus undergoes an intensive hurst of rephcation and CTL directed against dominant epitopes may reach extremeiy high frequencies, escape variant seiection may be particuiariy favored. We have observed repiacement of the virai quasispecies hy variants with mutations in an epitope targeted by tiie primary CTL response in the eariy stages of infection in two patients anaiyzed (53) , and there has aiso been another study documenting CTL escape variant emergence in a seroconverter (54) . Interestingly, the two patients in whom we observed eariy seiection of CTL escape variant viruses botii underwent rapid disease progression: studies of larger numbers of patients are required to reveai how frequentiy eariy seiection of mutations in epitopes recognized during the primary CTL response in fact occurs, and whether this phenomenon is aiways associated with rapid progression to AIDS. The emergence of virai variants with mutations that confer resistance to controi by dominant CTL responses in the eariy stages of infection may potentialiy impact on the suhsequent course of infection in two ways. First, as a virai variant wiii oniy be seiected for if it has a rephcative advantage, the emergence of CTL escape variants must he associated with an increase in eariy virus rephcation and spread. Consequences of this may inciude increased ioss of CD4+ T cells and estabhshment of a lareo er pooi of iatentiy infected ceiis or a higiier setpoint ievei of persisting virus: the iatter has been demonstrated to be a good predictor of the subsequent rate of disease progression (112) . Second, because (as discussed above) mutations in immunodominant CTL epitopes may promote evoiution of the immune response to recognize subdominant epitopes, the host may he left after tiie acute phase of the infection not oniy with a higher Ievei of persisting virus, but aiso with a CTL response that Is iess adequate to contain it.

In mid-HIV infection, mutations which affect recognition by epitope-specific CTL have been demonstrated in the persisting virai quasispecies in a numher of studies (e.g. (15. 77, 113)); however, clear examples of such variants being selected for and fixed in the virai quasispecies over time are difficuit to find. This is hkeiy hecause CTL responses to muitipie virai epitopes are frequendy present and, as discussed earher, under such conditions a complex cycie of fluctuations in the domi-nance of CTL responses directed against different viral epitopes and corresponding shifts in the epitopic composition of the viral quasispecies may occur (78, 79) , There are also examples of strong CTL responses being exhibited over long periods of time in the absence of appearance of mutations in the epitopes to which they are directed {76, 77). CTL epitope mutation during chronic HIV infection tbus does not seem to be essential for the continued persistence of virus; however, when it does occnr it may allow higher levels of virus replication, and bence promote disease progression. In the later stages of the infection, when the immune system is failing, the impact of viral variation on immune control may become more pronounced. The capacity of the immune response to evolve to focus on tmmutated epitopes as escape variants are selected may be more limited at this time. Simultaneously, the rehance of the host on CTL lysis as a means of controlling virus replication and spread may be increasing, particularly if syncytium-inducing viruses resistant to control by chemokines such as RANTES, MIEla and MIP1P appear. CTL escape variants may thus grow out and contribute to tbe escalating viremia. A recent paper described two examples of patients where mutations conferring evasion from a previously stable immunodominant CTL response became fixed in the viral quasispecies in the late stages of the infection (80), providing a clear illustration that tbis can occur CTL escape viral variants may thus influence the immune system's abihty to control virus replication at all stages of HIV infection, and likely do bave an important impact on the overall disease course in at least a proportion of infected individuals.

The preceding discussion illustrates tbat viral variants whicb are able to escape recognition by host CTL emerge during a number of different virus infections, and clearly have a biologically significant impact on tbe balance between virus replication and its control by the immune response in at least some of these cases. The latter include several infections that are of clinical significance in humans; it is thus important that immunebased propbylactic and therapeutic strategies to combat these infections should be designed to minimize the likelihood of CTL escape variant selection. There are examples in the literature of both prophylactic (114) and therapeutic strategies (115) wbich have led to the selection of viral variants able to escape recognition by the CTL response wbich was intended to mediate a beneficial effect. As described in this review, studies in both human virus infections and murine model systems have illustrated the conditions under which CTL escape viral variants are likely to emerge: this information should be considered in future vaccine/therapy design to prevent a similar outcome from occurring.

Given that escape-conferring mutations will not be selected for if they have detrimental effects on virus replication, and that escape mutations emerge most rapidly wben CTL pressure is predominantly focused onto a single bighly immunodominant epitope, vaccines should aim to induce broad immune responses that recognize multiple co-dominant viral epitopes, at least some of which should lie in conserved regions of viral proteins on which there are stringent functional constraints. Eeptide or minigene-based vaccination strategies are probably not the optimal choice for use in such infections: even if they include a carefully chosen cocktail of epitopes that can be recognized in association with multiple HLA alleles, there is a bigh probability that they will induce mono-or oligospecific immune responses in individuals of certain HLA types. Particularly if the infecting virus shows sequence variation compared to the antigens used for vaccination, there will be a danger that the vaccine might prime for an immune response that could be more detrimental than the immune response that may have been ehcited naturally following infection. Vaccines that inclnde multiple viral antigens are preferential immunogens for the induction of beneficial multispecific antiviral immune responses. 

Viral pathogenesis. Philadelphia: Lippincott-Raven Publishers

One-step ahead of the gameviral immunomodulatory molecules

Viral pathogenesis. Philadelphia: Lippincott-Raven Publishers; I 997

Specific cytotoxic T cells eliminate cells producing neutralizing antibodies

Virus persistence in acutely infected immunocompetent mice by exhaustion of antiviral cytotoxic effector T cells

Overview of herpesvirus latency

Viral pathogenesis. Philadelphia: Lippincott-Raven Publishers

Viruses use stealth technology lo escape from the host immune system

Nevirapine-resist ant human immunodeficiency virus: kinetics of replication and estimated prevalance in untreated patients

Ordered accumulation of mutations in HIV protease confers resistance to ritonavir

Combination therapy with zidovudine and didanosine selects for drugresistant human immunodeficiency virus type-1 strains with unique patterns of poi gene-mutations

Emergence of human immunodeficiency virus type-1 variants with resistance to multiple dideoxynucleosides in patients receiving therapy with dideoxynucleosides

Recovery of replicationcompetent HIV despite prolonged suppression of plasma viremia

Identification of a reservoir for HIV-1 in patients on highly active antiretroviral therapy

Lymphocytic choriomeningitis virus

Reduction of HIV concentration duiing acute infection: independence from a specific immune response

Antiviral pressure exerted by HIV-1 -specific cytotoxic T lymphocytes (CTLs) during primary infection demonstrated by rapid selection of CTL escape virus

Positive selection of HIV-1 cytotoxic T lymphocyte escape variants during primary infection

Massive expansion of an tig en-specific CDS -I-T cells during an acute virus infection

Counting antigenspecific CDS T cells; a reevaluation of bystander activation during viral infection

Direct visualisation of antigen-specific CD8+ T cells during the primary response to Epstein-Barr virus in vivo

CTL escape virai variatits II. Biologic activity in vivo

Optimal lymphocytic choriomeningitis virus sequences restricted by H-2Db major histocompatibihiy complex class I molecules and presented to cytotoxic T lymphocytes

Identification of Db and Kb-restricted subdominant cytotoxic Tcell responses in lymphocytic choriomeningiiis virus-infected mice

Virus-specific CD8+ cytotoxic T-Iymphocyte activity associated with control of viremia in primary human immunodeficiency virus type 1 infection

Temporal association of cellular immune responses with the initial control of viremia in primary human immunodeficiency virus type I syndrome

Contribution of proteasome-mediated proteolysis to the hierarchy of epitopes presented by major histocompatibility class I molecules

Peptide iranslocation by variants of the transporter associated with antigen processing

TAP-1-dependent peptide translocation in vitro is ATP dependent and peptide selective

Determinant selection of major histocompatibility complex class I-restricted antigenic peptides is explained by class Ipeptide affinity and is strongly influenced by non-dominant anchor residues

The hfe span of major histocompatibility complex-peptide complexes influences the efficiency of presentation and immunogenicity of two class I-restricted cytotoxic T lymphocyte epitopes in the Epstein Barr virus nuclear antigen 4

Different MHC class I alleles compete for presentation of overlapping viral epitopes

Siliciano RE, Soloski MJ, MHC class Irestricted processing of transmembrane proteins -mechanism and biological significance

Major expansion of CD8-I-T cells with a predominant Vb usage during the primary immune response to HIV

Patterns of immunodominance in HIV-1 -specific cytotoxic T lymphocyte responses in two human histocompaiibiliCy leukocyte antigens (HLA)-identical siblings with HLA-A*0201 are influenced by epitope mutation

Sequence constraints and recognition by CTL of an HLA-B27-restricted HIV-1 gag epitope

In vivo persistence of a HIV-1 -encoded HLA-B2 7 -restricted cytotoxic T lymphocyte epitope despite specific in vitro reactivity

Cytotoxic T lymphocytes in asymptomatic long-term non progressing HIV-1 infection

Antigenic oscillations and shifting immunodominance in HIV-1 infections

Late escape from an immunodominant cytotoxic T-lymphocyte response associated with progression to AIDS

In vitro selection of lymphocytic choriomeningitis virus escape mutants by cytotoxic T lymphocytes

CTL escape viral variants, I. Generation and molecular characterization

Discriminated selection among viral peptides with the appropriate anchor residues: implications for the size of the cytotoxic T-lymphocyte repertoire and control of viral infection

Late onset, symptomatic, demyelinating encephalomyelitis in mice infected with MHV-JHM in the presence of maternal antibody

CD8+ T cell epitopes within the surface glycoprotein of a neurotropic coronavirus and correlation with pathogenicity

Cytotoxic T cell-resistant variants are selected in a virus-induced demyelinating disease

CTL escape mutants cause increased mortality and morbidity Ln mice infected with mouse hepatitis virus, strain JHM. Keystone Symposium on Molecular Aspects of Viral Immunity

Cytotoxic T lymphocyte responses to Epstein-Bair virus

Human cytotoxic T lymphocyte responses to Epstein-Barr virus infection

BofTow & Shaw • In vivo importance of CTL escape viruses

T cell responses and virus evolution: lossof HLA Al 1-restricted CTL epitopes in Epstein-Barr virus isolates from highly Al 1 -positive populations by selective mutation of anchor residues

UnusuaEy high frequency of Epstein-Barr virus genetic variants iji Papua New Guinea that can escape cytotoxic T-cell recognition: implications for virus evolution

Epstein-Barr virus isolates with the major HLA B35.01-restricted cytotoxic T lymphocyte epitope are prevalent in a liighly B35.01-positive African population

Hepatitis B virus immunopathogenesis

Cytotoxic T lymphocyte response to a wild-type hepatitis B virus epitope in patients chronically infected by variant viruses carrying substitutions within the epitope

Chisari FV Hepatitis B virus (HBV) sequence variation in cytotoxic T lymphocyte epitopes is not common in patients with chronic HBV infection

Chisari FV Is antigenic variability a strategy adopted by hepatitits B virus to escape cytotoxic T-lymphocyte surveillance?

Hepatitis C viruses. In: Fields BN, ed. Fields virology

Cytotoxic T-lymphocyte responses to the hepatitis C virus in humans and chimpanzees

Persistent hepatitis C virus infection in a chimpanzee is associated with emergence of a cytotoxic T lymphocyte escape variant

Hepatitis C virus-specific CTL responses in the liver of chimpanzees with acute and chronic hepatitis C

HIV specific cytotoxic T lymphocytes in seropositive individuals

AIDS virus specific cytotoxic T lymphocytes in lung disorders

High levels of anti-human immunodeficiency virus type 1 (HIV-1) memory cytotoxic T-lymphocyte activity and low viral load are associated with lack of disease in HIV-1 -infected long-term nonprogressors

Kinetics of Gag-specific cytotoxic T lymphocyte responses during the clinical course of HIV-1 infection: a longitudinal analysis of rapid progressors and long-term asymptomatics

Why can't cytotoxic T cells handle HIV?

HIV-!-specific cytotoxic T lymphocytes and the control of HIV-1 reph cation

Evidence for rapid disappearance of initially expanded HIVspecific CD8-(-T cell clones during primary HIV infection

Vigorous HIV-1-specific CD4-I-T cell responses associated with control of viremia

HIV versus cytotoxic T lymphocytes -the war being lost

What can we learn about human immunodeficiency virus infection from a study of lymphocytic choriomeningitis virus? ImmunoiRev

HIV-1 Nef protein protects infected primary cells against killing by cytotoxic T lymphocytes

Prognosis in HIV-1 infection predicted by the quantity of virus iJi plasma

Human immunodeficiency virus genetic variation that can escape cytotoxic T cell recognition

Transfer of HIV-1 -specific cytotoxic T lymphocytes to an AIDS patient leads to selection for mutant HIV variants and subsequent disease progression

Bourgault-Villada L Selection of virus variants and emergence of virus escape mutants after immunization with an epitope vaccine

The authors are supported by grant numhers RO I AI37430-03 (PB)and UO1 AI41530 (PB and GMS) from the NIH and by core funding from the Edward Jenner Institute for Vaccine Research