The variability of the serological response to SARS-corona virus-2: Potential resolution of ambiguity through determination of avidity (functional affinity)

Abstract

Data on the serological response toward severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in 16 recent reports were analyzed and a high degree of variability was shown. Immunoglobulin M (IgM) responses were either found earlier than IgG, or together with IgG, later than IgG, or were missing. Therefore, clear distinctions between early, intermediate, and past infections are obviously not possible merely on the basis of IgM and IgG determinations. A review of publications on the serology of other virus groups shows that variable IgM responses can be found as well and therefore are not unique for SARS-CoV-2 infections. A model to explain this variability is proposed. The inclusion of avidity determination into regular diagnostic procedures has allowed to resolve such "atypical" serological constellations. The potential use of avidity determination for the diagnosis of COVID-19, for risk assessment, epidemiological studies, analysis of cross reactions, as well as for the control of vaccination programs is suggested and discussed.

Keywords: IgG; SARS-CoV-2; affinity; avidity; serology; variability.

Figure 1

Variability of the serological response to SARS‐CoV‐2 infection. A, Schematic presentation of key findings presented in Long et al. The analysis of the variability of the serological response in 26 cases of polymerase chain reaction (PCR) confirmed cases of SARS‐CoV‐2 infection shows that in seven cases immunoglobulin M (IgM) preceded IgG (a), in nine cases IgM and IgG seroconversion was determined at the same time, (b) and in 10 cases the IgM response was detected after the IgG response (c). B, Schematic presentation of key findings presented in Zhang et al. The serological response in16 PCR‐confirmed cases of SARS‐CoV‐2 infections was analyzed on day 0 and day 5 after the onset of clinical symptoms. Group a: Eight cases showed positivity for IgM and IgG in parallel on day 0. Parallel positivity of IgG and IgM was maintained on day 5 except for one serum. Group b: Five cases showed IgG, but not IgM on day 0. In three of these cases, the IgM response was detectable on day 5 (“delayed IgM response”). In two cases, no IgM response was detectable at day 5, but acute infection was confirmed by the increase in the IgG response between day 0 and day 5. Group c: In three cases with confirmed infection, no serological response was detectable on day 0. Serum conversion both for IgM and IgG was detectable on day 5. The data from A and B demonstrate the high variability of the IgM and IgG response after acute SARS‐CoV‐2 infection. The number of cases with parallel or delayed IgM response (A) and the number of cases with parallel, delayed, negative IgM response (B) compared to the number of expected cases with preceding IgM (according to an outdated classical view) is highly significant (P = .0079 for A, .0005 for B, determined by the Yates continuity corrected χ ² test) [Color figure can be viewed at wileyonlinelibrary.com]

Figure 2

Variability of the immunoglobulin M (IgM) response: preceding IgM and parallel appearance of IgM and IgG A, “Classical picture” with IgM preceding IgG. An immune stimulus generates B cells that generate IgM toward a defined epitope (#1). Replication of this cell can lead to the generation of plasma cells and the massive production of free IgM (#2‐4), to apoptotic cell death (#5, 6) or to immunoglobulin class switch and the generation of IgG‐producing B cells (#7). These may either differentiate into plasma cells and generate low affinity IgG (#8‐10), die through apoptosis (#11) or contribute to the generation of IgG of higher affinity through clonal selection (#12‐14). The increase in affinity/avidity occurs in several steps (#15) and leads to the generation of memory cells (#16) that allow anamnestic responses. The setting shown in A leads to the classical picture of IgM preceding IgG in the serum. B, Parallel occurrence of IgM and IgG in the serum. Assuming that IgM‐producing plasma cells are generated later than shown under A (#1‐3), and the immunoglobulin class switch (#6) is occurring earlier than in the scenario described under A, the production of IgM and IgG through plasma cells (#3, #7) will occur in parallel and lead to the parallel detectability of IgM and IgG in the serum—despite the fact that IgM‐presenting B cells had been established before IgG‐presenting B cells [Color figure can be viewed at wileyonlinelibrary.com]

Figure 3

Variability of the immunoglobulin M (IgM) response: delayed and missing IgM responses. A, Delayed IgM response. A delayed differentiation of IgM‐presenting B cells to plasma cells (#1‐5), in combination with a fast immunoglobulin class switch (#8) and generation of IgG‐producing plasma cells (#9) explains the frequently observed phenomenon of delayed appearance of IgM in the serum, though IgM‐presenting B cells are always generated first. B, Missing IgM response in the serum. A low degree of replication of IgM‐presenting B cells and their early cell death (#3) prevents differentiation to IgM‐producing plasma cells and does not lead to detectable IgM in the serum, whereas IgG is produced and matures with respect to avidity. This scenario explains how IgG production depends on IgM‐presenting B cells, without IgM being detectable in the serum [Color figure can be viewed at wileyonlinelibrary.com]

Figure 4

Proposed resolution of serological ambiguity through determination of avidity (functional affinity). A, Possible serological immunoglobulin M (IgM)/IgG constellations following primary viral infection are schematically summarized. In serological practice, the “text book constellation” with IgM seroconversion preceding IgG seroconversion (a) is rather rare. Parallel determination of IgM and IgG usually represents the most frequent case (b). Delayed occurrence of IgM is less frequent (c). In several cases of acute infections, the detection of the IgM may be missed (d). This may be due to low expression of IgM response, problems of sensitivity of the assay used, competition of IgM by IgG during the assay or missing the right time point of positivity of the response. In few cases, IgM responses may persist for longer times after primary infection (e). As discussed in the text, constellations a‐d have been reported for severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2) infections. Based on the experience with SARS‐CoV‐1 and many other viral systems, constellation "e" can be predicted to occur as well and may be detected as soon as longer follow‐up studies will have been performed. Part A shows that the determination of IgM and IgG does not allow to draw an unequivocal conclusion on the time point of infection or beginning of clinical symptoms. B, The inclusion of IgG avidity allows an unambiguous determination of early, intermediate, and past infection, irrespective of the variability of the IgM response. The diagnostic power of avidity determination has been shown for many viral systems. It is suggested to include this general immunological feature into routine diagnostics of SARS‐CoV‐2 infections [Color figure can be viewed at wileyonlinelibrary.com]