key: cord-0751179-j92d1gge
authors: Knies, Andrea; Ladage, Dennis; Braun, Ralf J.; Kimpel, Janine; Schneider, Miriam
title: Persistence of humoral response upon SARS‐CoV‐2 infection
date: 2021-06-30
journal: Rev Med Virol
DOI: 10.1002/rmv.2272
sha: 4cab12f41910e3e1544005167459a770cf5ddfb7
doc_id: 751179
cord_uid: j92d1gge

SARS‐CoV‐2 continues to leave its toll on global health and the economy. Management of the pandemic will rely heavily on the degree of adaptive immunity persistence following natural SARS‐CoV‐2 infection. Along with the progression of the pandemic, more literature on the persistence of the SARS‐CoV‐2‐specific antibody response is becoming available. Here, we summarize findings on the persistence of the humoral, including neutralizing antibody, response at three to eight months post SARS‐CoV‐2 infection in non‐pregnant adults. While the comparability of the literature is limited, findings on the detectability of immunoglobulin G class of antibodies (IgG) were most consistent and were reported in most studies to last for six to eight months. Studies investigating the response of immunoglobins M and A (IgM, IgA) were limited and reported mixed results, in particular, for IgM. The majority of studies observed neutralizing antibodies at all time points tested, which in some studies lasted up to eight months. The presence of neutralizing antibodies has been linked to protection from re‐infection, suggesting long‐term immunity to SARS‐CoV‐2. These neutralizing capacities may be challenged by emerging virus variants, but mucosal antibodies as well as memory B and T cells may optimize future immune responses. Thus, further longitudinal investigation of PCR‐confirmed seropositive individuals using sensitive assays is warranted to elucidate the nature and duration of a more long‐term humoral response.

transmembrane proteins M and envelope (E), the nucleoprotein (N) and the trimeric transmembrane glycoprotein spike (S). In particular, the S protein is of high interest, as it is responsible for attachment, fusion, and entry of SARS-CoV-2. The S protein is composed of an Nterminal S1 subunit and a C-terminal S2 subunit. S1 contains the receptor-binding domain (RBD) and an N-terminal domain (NTD). 6 The virus gains entrance into cells via the S1, which binds to the angiotensin-converting enzyme 2 (ACE2) receptor through RBD.

Receptor binding triggers cleavage of S at the S2' site by cellular proteases such as TMPRSS2 either at the cell surface or within the endosome. This allows insertion of the fusion peptide into the cellular membrane and subsequent conformation changes in S2 which enable the fusion between the viral envelope and cellular membranes and, finally, the entry into the target cells. 2, [6] [7] [8] The viral load has been reported to peak in coronavirus disease 2019 (Covid-19) patients at symptom onset or shortly thereafter, before it begins to slowly decrease. The adaptive immune response is initiated as soon as the virus replicates, leading to the generation of cellular responses and antibody production, in most SARS-CoV-2 infected patients. 9, 10 Antibodies generally play a crucial role in dealing with viral infection by different mechanisms such as neutralizing incoming virus, tagging viral antigens on the surface of infected cells, as well as modulating the activity of further immune components (e.g., phagocytes or natural killer cells). 2, 11 For diagnostic purpose, the viral N and S protein are important for antibody detection, as they are targeted by most commercial serological asiksays. Findings reported on the early immune response indicate that SARS-CoV-2 induced a similar humoral response compared to SARS-CoV. 7 While the antibody response following SARS-CoV-2 infection in humans has been well documented for the initial phase, literature on antibody persistence beyond three months post infection is still scarce. 9, 12 Several studies suggest an early decline of antibody production within weeks to months. [13] [14] [15] [16] However, the decline of antibody production does not follow a linear pattern and thus cannot be predicted from earlier time points. 17 Preclinical findings 18, 19 and rare reports of confirmed re-infections with SARS-CoV-2 in humans support the notion that immunity may last for some time after infection, but the persistence of immunity remains uncertain. 3, 9, 20, 21 Hence, a better understanding of the kinetics and the persistence of the humoral response is of high importance. This is especially true for the long-term persistence of NAbs, which are considered an important correlate of immunity. 3, 5 Despite a plethora of research focussing on the short-term immune response after SARS-CoV-2infection, data on the specific duration of immunity will only become available within the following years. Along with the pro- infection in non-pregnant adults. We provide an overview of study populations, assays used, targeted antigens and time points that yielded positive/negative findings for total immunoglobins, the major immunoglobulin classes (IgG, IgM, IgA) and NAbs.

Upon invasion of an infectious agent such as SARS-CoV-2 and its antigens, the host elicits a humoral response by producing antibodies, or immunoglobulins (Ig) from plasma cells. This immune response is orchestrated by an interplay of various antibody classes including immunoglobulins IgM, IgD, IgG, IgA and IgE. Each immunoglobulin class has specific constant regions, which have distinct biophysical qualities, functions, distributions and half-lives. The antigen binding sites can be found in the highly variable regions of the immunoglobulins; they are located at the top of the two arms of the Y-shaped antibodies. 2, 22 Immunoglobulins that have been most frequently reported to be involved in the humoral response following SARS-CoV-2 infection are IgM, IgA and IgG. The first class to be activated through the entry of an infectious agent are immunoglobulins IgM, followed by IgA and IgG. 23 Dimeric IgA can be found on mucosal surfaces and in secretions, such as saliva, breast milk and nasal mucus. Hence, IgA also plays a crucial role for mucosal immunity. 2 

All studies testing total Ig, employing pan-immunoglobulin assays 

The majority of studies included in this review focused on the persistence of IgG. This is not surprising as IgGs have been described to represent the immunoglobulin class with the most consequential implications for serological testing and humoral responses including its capacity for viral neutralization and its involvement in immunity persistence after infection or vaccination. 2 

IgM immunoglobulins are the first immunoglobulin class produced following SARS-CoV-2 infection. 2, 23 IgM has been reported to become detectable at a median of 7 days post infection, peaks at a median of 20 days and starts to decline as early as a median of 27 days. 23 Notably, recent studies also indicate a crucial role for IgM in neutralization capacities of SARS-CoV-2, as strong correlations between neutralization potency and the presence of RBD-specific

IgM have been reported. 15, 60 Compared to IgG, the persistence of IgM was examined by a much smaller number of reviewed studies. The majority of studies reported durability of IgM responses for up to 30-31 weeks, the longest time point studied. 15, 20, 31, 32, 37, 38, 44, 45, 56, 59 Five studies reported initially positive findings followed by negative ones 21, 36, 43, 54, 59 detected as early as 14-15 weeks 21 One study did not report any IgM response within the time scope targeted by our review 28 and two studies reported negative findings at early time points, followed by positive results later on. 31, 42 In at least one of the two studies, the initial negative finding is most likely caused by the circumstance that the subject who tested positive at later time points was not tested at the earlier time point. 31 In addition, recent findings regarding the potential insufficient performance of IgM and IgA assays should be taken into consideration. 24 Overall, it appears that more studies are needed to evaluate the specific long-term kinetics of IgM at this point.

The limited number of studies we identified to examine IgA is in line with a general scarcity of literature on IgA production following the initial phase of SARS-CoV-2 infection. 2, 9, 61 More literature is needed

given the crucial function of IgA to act as a neutralizing barrier to infectious agents invading the respiratory and digestive system, as it has been described that secretory dimeric IgA may neutralize SARS-CoV-2 before binding epithelial cells. 61 IgA positivity decay, that is, a change from initially positive to negative findings at later test time points, was reported by three studies. 42, 43, 48 One of these studies found highly variable results which appear to be associated with study cohort characteristics and targeted antigen. 48 For instance, whereas no anti-S1/anti-RBD IgAs to differences in study cohort or targeted antigen, although the reported insufficient performance of IgM and IgA assays 24 may also be a contributing factor that should be taken into consideration.

While and potentially the only, target for NAbs and the same circumstance has been suggested for SARS-CoV-2. 52 Within the S protein the RBD and the adjacent NTD have been identified to contain the major epitopes for NAbs, 3, 63 although also the fusion peptide in S2 has been discussed to be targeted by some NAbs. 64 RBD appears to be particularly vulnerable because blocking the binding of RBD to the ACE2 receptor represent a major mechanism of neutralization. 62, 65 Current neutralization studies employ a wide variety of different SARS-CoV-2 neutralization assays with different methodological approaches, and it is crucial to bear in mind those differences when interpreting results.

Conventional virus neutralization tests that use live virus, and hence require biosafety level 3 laboratories, are still considered the gold standard of neutralization testing. 5 However, those assays are labour intense and not always readily available, so that alternative tests such as pseudovirus virus-neutralizing tests (pVNTs) have been developed and validated 2,5 Additionally, more recently surrogate neutralization assays (sVNTs) have been described. 66 These assays are based on an antibody-mediated blockage of RBD/ACE2 binding.

The early timeline of the NAb response upon SARS-CoV-2 infection has been systematically reviewed by Arkhipova-Jenkins et al. 23 They report that NAbs can be detected as early as a median of 6 days PSO or post PCR, peak at a median of 31 days and start declining around the same time when peak prevalence is observed, approximately at a median of 30 days. Notably, the authors report a lower confidence in these findings compared to IgG or IgM reports, due to a low number of studies, the high variance in neutralization test used and test time points used. Our findings on neutralizing humoral response are shown in Table 3 . Analogously to KNIES ET AL.

T − Ab levels found negative by assay and positivity threshold used in study. 67 This finding may be dependent on the dimeric, form of IgA, as it was reported to have a higher potency than its respective monomer against authentic SARS-CoV-2. 68 In contrast, another study reported a strong correlation between neutralization potency and the presence of RBD-specific IgM. 60 Nevertheless, despite the growing evidence for a correlation between binding and NAbs, neutralizing assays are still considered the gold standard for assessing potential immunity in patients. 5

Here, we reviewed current peer-reviewed literature on humoral and NAb response at 3+ months PSO in non-pregnant adults. Available studies including such more long-term time points of sampling remain limited and, due to considerable methodological heterogeneity, their findings generally lack comparability.

Most consistent are outcomes on the IgG response, with positive antibody titres reported for up to 8 months post infection, 40, 55 regardless of targeted antigens and assays used across studies. Given those consistent findings, it seems likely that future studies may establish IgG responses at even later time points. Likewise, IgA, even though it was studied by a much smaller number of studies, was reported to remain fairly persistent with time points later than 7 months PSO, 55 similar to IgG these findings were observed despite different assays and antigens tested. Findings on the IgM response were much more inconsistent, and more research is needed.

Consistent findings, with few exceptions, were also reported for the persistence of NAbs which have been shown so far in some studies to persist up to 32-33 weeks. 17, 26, 40 Regarding results for IgM and IgA titres it is important to note that while most IgG/pan-Ig assays have been found to show a reliable performance, IgM and IgA test performance was recently reported to be poor for most assays. 24 A key issue in the current state of the pandemic is the question about the potential strength and duration of immunity after SARS-CoV-2 infection and vaccination, as these factors will strongly impact decisions on current restrictions. 16 Regarding the question of long-term immunity to SARS-CoV-2, seropositivity and in particular the presence of NAbs has been linked to several months of protection from re-infection. Within a study describing a SARS-CoV-2 outbreak at a summer school retreat, no individual in a sub-group of 24 persons, who had been seropositive three months prior to the retreat, developed any symptoms. 69 A prospective study in the United Kingdom found among a large sample of 12,541 healthcare workers that those who were anti-S IgG positive at baseline (n = 1265) were also less likely to have a preceding or succeeding positive PCR test result. 46, 70 Another factor that should be taken into consideration when speculating about potential long-term immunity, are the recently Research available for SARS-CoV suggests that NAbs can still be detected up to 17 years later 74, 75 and only a few cases of confirmed SARS-CoV-2 re-infections were reported. 76, 77 Similarly, to other human coronaviruses, this leaves room for hope that re-infections 12 of 15may follow milder disease trajectories than the initial infection. 48, 78, 79 Finally, waning serological antibodies may not equate to loss of immunity. 48 Mucosal antibodies located in the respiratory tract may prevent SARS-CoV-2 infection. Moreover, memory B and T cells stimulated during the initial infection may optimize future response to the virus. 48 At this point, further longitudinal investigation of PCR-confirmed seropositive individuals using sensitive assays is warranted to elucidate the nature and duration of a more longterm humoral response.

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We wish to thank Titas Saha for her technical assistance, Dr. OliverHarzer for his scientific advice and comments during the design of the study.

We declare no competing interests.

Not applicable. 

Data sharing not applicable to this article as no datasets were generated.

https://orcid.org/0000-0001-6393-6103Dennis Ladage https://orcid.org/0000-0002-4512-0917Miriam Schneider https://orcid.org/0000-0003-2689-8317