key: cord-008761-b36x05fn
authors: Billiau, A.
title: The interferon system as a basis for antiviral therapy or prophylaxis
date: 2012-02-26
journal: Antiviral Res
DOI: 10.1016/s0166-3542(85)80020-6
sha: 
doc_id: 8761
cord_uid: b36x05fn

nan

The interferon system is part of the so-called aspecific defense mechanism of vertebrates against virus infections. The term "aspecific" mainly refers to the fact that the system is operative against all taxonomic classes of viruses. It consists of a set of evolutionary related proteins, interferons, which possess the ability to tune the synthetic biochemistry of the cell so that various processes which are essential for virus replication are impaired. Under "normal" conditions the organism synthesizes or releases little if any of the interferons.

Only under attack from viruses or other forms of biological stress is the interferon system triggered. Hence, the two sets of fundamental questions which have governed interferon research : (i) the physiology and biochemistry of induction (1,2) and (ii) the physiology and biochemistry of action (3) . Possessing satisfactory answers to these questions is essential, not only to understand the natural role of the interferon system in virus disease but also to assess possible applications in medicine, in particular the use of interferons or interferoninducing substances as antiviral drugs.

A striking feature brought to light by virtually each sector of interferon research is the pleomorphism of the system. Thus, each animal species possesses a family of genes for interferon, and in some instances a single gene can give rise to mUltiple molecular forms. Also there are several ways in which production of these interferons can be induced. Finally, the antiviral effect is not due to a single cellular modification, blocking the virus at one particular junction in its replicative cycle. Rather, virus replication is counteracted at different steps. Another feature worth mentioning is that the interferon system is leaky blocking of virus replication or of the effects of viruses on the host is neither complete nor permanent. Therefore, if the virus host is placed under the protective influence of interferon, some virus replication still occurs, and as the interferon effect wanes, residual virus can again multiply at full strength unless this would be blocked by some other mechanism (e.g. specific immunity) which has been generated in the. meantime.

Currently, interferons are defined as endogenous proteins which exert virusnonspecific, antiviral activity at least in homologous cells through cellular metabolic processes involving synthesis of both RNA and protein (4) . One of the main points in the definition is that it excludes all natural proteinaceous factors which inhibit virus infection at extracellular steps, e.g. adsorption. A limitation of this definition is that it would also call "interferon" any protein produced by cells which induces production of a known interferon. For instance, a monokine related to interleukin-I, has been shown to induce production of interferon-B by cells and thereby to exert an antiviral effect.

In mammalians three (sero)types of interferon have been described : IFN-a, IFN-B and IFN-y. Subtypes, reflecting the presence of multiple, slightly different genes, may occur. Thus, in man there are several subtypes of IFN-a (HuIFN-al, -a2, -a3, •.. ), but only one IFN-B and -yo Types of interferon (a, Band y) are distinguished on the basis of reactivity with polyclonal antisera. Antiserum against a given type neutralizes biological activity of all its sUbtypes but does not react with the other types. Antigenic differences between the types reflect differences in primary structures. However, some homology in sequence exists between IFN-a and IFN-B. On the other hand, the eequence of IFN-y is completely different from that of either IFN-a or -B.

The structural similarities and differences between the IFN types are reflected by the fact that IFN-a and -B, in acting on cells, share a common membrane receptor, while IFN-y uses a different receptor (5) .

Interferons for animal experiments or for clinical trials are prepared in various ways. At present it is common and useful to make a distinction between preparations of so-called natural interferons and preparations of recombinant DNA-derived interferon (rDd-interferons). Among the natural interferon preparations, those most commonly used are as follows.

Interferon prepared from mouse cell lines infected with Newcastle disease virus is a mixture of a and B (6) . The two types can be separated but this has seldomly if ever been done for preparations destined to perform in vivo experiments. Thus, literature data are representative for a mixture of IFN-a and -B.

Interferon prepared from suspensions of mouse splenocytes incubated in the presence of lymphocyte mitogens (concanavalin A, phytohemagglutinine, Staphylococcus enterotoxin, ..• ) contain IFN-y. It should be mentioned that, depending on the degree of purity, these preparations also contain variable numbers and quantities of monokines and lymphokines such as interleukins, colony stimulating factors, etc. Hence, all data obtained so far with these preparations represent effects of total lymphokine rather than of IFN-y preparations.

The following interferons are currently available for clinical studies in Immune interferon is produced from suspensions of blood leukocytes induced with mitogens. The antiviral activity present in these preparations is mainly due to IFN-y. Small quantities of IFN-~ and -S may be present. However, the main contaminants are several lymphokines, some of which may indirectly affect viral disease, for instance by modulating the immune system.

The prototype of rOd-interferon is human IFN-a2 prepared from E.coli or yeasts containing an adequate expression plasmid into which the interferon genes have been inserted (7) . In their crude form these preparations do not contain any products of human origin other than the interferons . Bacterial products are completely removed by the purification process. The main difference between these products and their natural-source-derived counterparts is that they contain only a single subtype, namely a2' While it is known that different subtypes of HuIFN-~ may differ in their antiviral potency depending on the cells on which they are tested, it is not yet known whether this has any repercussion for the clinical effects. Another currently available rOd-interferon is HuIFN-~ produced in E.coli. This interferon differs from the natural counterpart by the absence of carbohydrate side-chains. Although these side-chains do not playa significant role in the effects of the IFN-S on cells, the pharmacokinetic behavior of this rDd-IFN-S was found to be rather different from that of natural fibroblast-derived interferon (8) . rDd-HuIFN-S possessing carbohydrate side-chains can be obtained by insertion and expression of the gene in yeasts or mammalian cells. Finally, a rDd-HuIFN-y is available, which is produced from E.coli or mammlian expression systems.

rDd-Interferons for experiments in animals are becoming available at a much slower pace and in lesser quantities than those necessary for experiments in man.

The genes for several animal interferons have been isolated and brought to expression in E.coli, in yeasts or in mammalian cells. This should allow to produce large quantities without great technical difficulty. Especially in the case of MuIFN-y, this will be a great advantage over the cumbersome "natural" production method which depends on the availability of fresh mouse splenocytes.

Pharmacokinetic studies have been done with human interferons-a and -So In as far as they were done in man, they are of course quite relevant to the design of therapeutic trials. Studies with these interferons in animals are of lesser value, especially as it has become clear that comparative studies between interferon-a and -S may yield quite different conclusions depending on the animal species used (9).

One of the key questions is to know whether interferon given by any route is concentrated in some organ. This question has so far not been studied in man although, with current techniques of production and radioactive scanning, it seems quite approachable. That the question is not trivial may be apparent from the observation that human interferon-a and -S, when injected by the intramuscular route, cause comparable increases in NK-cell activity, although blood levels with interferon-a are 10-to 100-fold higher (10) . Similarly, studies in mice have revealed that human interferon-a and -S given intraperitoneally, yield different blood levels but quite similar tissue levels (II).

There is evidence that some organs, in particular the brain, are rather difficult to penetrate by interferons. Thus, during the initial phases of experimental mengo virus infection in mice, interferon levels in serum being quite high, no interferon was detectable in the brain (12) . Brain interferon became detectable only in later stages of the infection when full-blown virus replication had started in the brain itself. HuIFN-S on the other hand undergoes rapid uptake, suggesting that desialylation, wherever it may take place, is a first step toward degradation of HuIFN-S. The kidney was shown to playa key role in that interferons seem to pass easily through the glomerular sieve and to be taken up and degraded by tubular cells.

Of particular importance are pharmacokinetic studies with rDd-interferons.

Since these interferons often differ from the natural ones . bY , the absence of carbohydrate side-chains it may be expected that their pharmacokinetics will be quantitatively different from those of their natural counterparts. It is not unthinkable that some of the artificial interferons . (e.g. those obtained by sitespecific mutagenesis) may be degraded slowly by the organism, allowing to decrease the doses. However, there is nothing that would lead one to think that site-specific mutagenesis . may enable one to target an interferon to a specific , The main lesson to be learned from these experiments is that interferon therapy is unlikely to achieve protective effects unless started before the major viral replication burst in the organism. In general one can say that this means that therapy has to be started before the first symptoms.

As already mentioned, mouse model systems for chronic virus infections are scarce. One example is experimental viral leukemia. It has been known for a long time that continuous interferon administration started before or shortly after infection can significantly delay the development of the disease and can also prolong survival time (16) . This finding may be expected to receive renewed in- (17, 18, 19) .

The question whether interferon therapy may be able to control chronic or recurrent disease at the level of the whole body, is confounded by the fact that much of the symptoms of these chronic virus diseases are not directly due to viral replication or cytopathogenicity but result from immunological and inflammatory reactions. Therefore, it is not clear whether alleviation of symptoms by interferon is due to antiviral activity or to interference with secondary effects.

Thus, in the aforementioned example of Friend leukemia in mice, it is not clear whether delay in splenomegaly development is due to reduction in viral load or to antimitotic or immunoregulatory effects of interferon. Recurrent herpetic keratitis (25) can benefit from topical interferon therapy if it is given in conjunction with debridement and/or with some of the older or newer antiherpetics (25) .

Acute rhinopharyngitis due to rhinovirus or coronavirus infections can favorably be influenced if rather high doses of HuIFN-a (but not -S) (23) are given and if treatment is started early, preferentially before infection. It has been envisaged to use daily instillations of interferon during autumn and winter periods as a means of prophylaxis against common cold. However, it is now being suggested that such long-term instillations cause damage to the nasal mucosa.

Condyloma accuminatum is amenable to treatment with ointments or gel lies containing interferon (26) . A problem in interpreting these results is the possible confounding, especially in non-placebo-controlled trials, of the mere effects of hygiene. Daily care of the lesions may by itself contribute to regression of the condylomas. However, the interpretation that the effects are largely due, indeed, to the action of the interferon is corroborated by the observation that other manifestations of HPV infections are also amenable to treatment with interferon.

Thus, skin warts can be made to disappear by repeated injections of fibroblast interferon in the perilesional skin (27) .

Beginning or established acute (primary or recurrent) infections. Systemic administration of interferons (a-or 8-type) has been found to favorably affect the course of a number of acute (prima ry or recurrent) virus infections : varicella in imrnunocompromised patients (28) , zoster in cancer patients (29) . The practical importance of these observations is rather minor, in that beginning or established HSV and VZV infections can now effectively be treated with nucleoside analogs.

Prevention of acute infections. Systemic treatment with interferon can prevent herpes recurrence after surgical interventions on the trigeminal nerve ganglion (operation for "tic douloureux" ) (30) . Interferon therapy has a l so been considered as a possible means to cope with the increased incidence and severity of viral infections in renal transplant patients (31) ; it appears that interferon therapy reduces virus-shedding as well as the severity of symptoms of CMV infections.

Chronic active virus infections . The following chronic active virus infections are currently investigated as pos s ible targets for systemic interferon therapy : persistent infections with HB virus, chronic infections with HPV (warts and wartlike diseases), chronic infection with leukemia viruses, e.g. HTLV. The effects of interferon therapy on persistence of HB virus has rather extensively been investigated. In some patients transient and sometimes definitive disappearance of viral infectivity markers, accompanied by clinical improvements, were seen. It is difficult, however, to certify tha t these were not spontaneous cures or placeboeffects (32) . The need for properly controlled trials, expressed early on by several investigators, has at this time still not been satisfied. Therefore it remains impossible to even approximately assess the therapeutic potential of interferon in chronic hepatitis.

The situation with chronic HPV infections is quite different. Thus it is now generally recognized that juvenile laryngeal papilloma, a rather rare but serious complication of infection with certain HPV types, is quite amenable to control by therapy with various preparations of HuIFN-a (33) . The therapy has to be given continuously for months before recurrence of the laryngea l warts is brought under control. Arrest of treatment entails recurrence, but it is hoped that suitable low-dose maintenance treatments can be designed, by which protections can be prolonged to the age where the papillomas tend to regress spontaneously.

Retroviruses have only recently been implicated as etiologic fa c tors in human diseases, namely human T-cell leukemia (HTLV-I and -II) and AIDS (LAV or HTLV-III). Interferon therapy has already been investigated as a means to control AIDS and AIDS-associated malignancies (Kaposi sarcoma), and some encouraging results have been reported in that the symptoms of the disease can in some patients be alleviated (34) . A cure of AIDS however, has not been seen.

Mechanisms of Production and Action

Mechanisms of Production and Action

Proceedings of Symposium on Clinical Use of Interferon