Surprisingly, we observed an IgE-mediated immune response to both SARS-CoV-2 infection and vaccination using ChAdOx1, CoronaVac and BNT162b2 vaccines. Plüme et al. and Giménez-Orenga reported the presence of IgE antibodies following natural infection in their manuscripts,16,17 which was particularly evident in severe cases, leading them to propose that IgE-mediated mechanisms might contribute to inflammation in COVID-19. However, participants in our cohort were asymptomatic or experienced mild symptoms and exhibited detectable IgE antibodies as well. It is noteworthy that to detect IgE, which is less abundant than IgM and IgG, the assay’s sensibility needs to be enhanced – following Plume and collaborators,16 we used sera samples diluted at 1:5 ratio, secondary antibody specific to ε chain from IgE and TMB, a sensitive colorimetric substrate. To gain specificity, we calculated the cutoff using a 99% confidence interval;18 moreover, appropriate controls were employed throughout all experimentation, and results were confirmed by the lack of reactivity for heterologous antigen and by the agreement with IgG-depleted samples’ results (Fig. 6).
Atopic individuals are known to exhibit increased IgE secretion,19 however, our study suggests that a history of immune-mediated disease did not contribute to elevated IgE index values. This may be attributed to the small number and heterogeneity of immune-mediated conditions among the individuals in our cohort (n = 7), and larger study populations are warranted to address this point with greater confidence. In addition, no subjects of this study population reported hypersensitivity after having COVID-19 or being vaccinated, although there are studies showing otherwise.20,21
A relationship between respiratory viral infections and atopy has been suggested before. An interesting investigation in experimental model demonstrated FcεR expression by dendritic cells in mice’s lung after viral infection with Sendai virus, which was followed by mucous cell metaplasia, consistent with asthma presentation. These findings would support the hypothesis that respiratory viral infections in individuals genetically predisposed Th2 responses may facilitate asthma.22 In the case of SARS-CoV-2, we could not find mechanistic studies, but it was verified that patients with chronic rhinosinusitis took double of the time to clear SARS-CoV-2 when compared with matched controls,23 and elevation in total IgE was observed in pediatric patients with long-COVID; also, most of them were sensitized against aeroallergens.24 Other allergy-related symptoms following COVID-19, such as skin lesions, were reported during the pandemic but it seems that they were either unspecific to the infection or the causality relationship remains elusive.25 Even though results from our cohort do not support an association between atopy and enhancement of IgE-anti RBD levels, evidences suggesting the contrary led us to believe that more investigations are needed to elucidate this subject.
IgG4 and IgE are located on the upstream region of the Ig locus and B cells require prolonged antigenic stimulation to undergo class-switching to these antibodies, supported by Th2 cytokines.1 Aluminum hydroxide is known to induce a strong Th2 environment which supports this response;26 but in our cohort, ChAdOx1, which lacks Alum, induced even higher IgE and IgG4 indexes than CoronaVac, an Alum-adjuvanted vaccine. This suggests a role of the active immunogen in eliciting IgE and IgG4 responses. Similarly, other studies have demonstrated IgE-responses independent of aluminum salts, as seen with Flu and MMR vaccines.14,15 The overall IgE levels seems to reflect the IgG response observed in the cohort from where the samples of this study were obtained:27 subjects with COVID-19 history before vaccination showed slightly elevated antibodies levels after the two ChAdOx1 or CoronaVac doses; ChAdOx1 induced more antibodies than CoronaVac, suggesting to be more immunogenic; and a BNT162b2 booster increased antibody levels in all groups (with or without COVID-19, ChAdOx1 or CoronaVac).
The role of IgG avidity has been investigated in COVID-19. It was proposed that high affinity antibodies would be beneficial to interfere in the RBD-ACE-2 binding, thus neutralizing the virus,7 which was supported by laboratory results.8 A comparison between infection and vaccination responses showed that the latter induces IgG of higher avidity, suggesting better functionality,9 a result we also observed for IgE. Unfortunately, we could not find more studies about IgE avidity to SARS-CoV-2 or other pathogens to discuss this aspect much further. However, it is important to highlight that, for allergenic responses, higher affinity to the epitope may predict FcεR activation and IgE-mediated function.10 This would be especially relevant in inflammatory contexts of health and disease.
Provided that IgE specific to SARS-CoV-2 antigens has not been extensively investigated, it would be interesting to add related antibodies for a more comprehensive analysis. From what is known from allergy models – the main font of IgE investigations – IgG4 could be a related antibody.1,4 As IgE, both are located at distant downstream regions of the Ig locus and most antigens that trigger IgE, trigger IgG4.4 In the context of COVID-19, IgG4 was suggested as a marker of severe disease24,28,29 and, in respect of immunization, IgG4 levels were detected after booster doses of mRNA vaccines,30–32 agreeing with our results. IgG4 is mostly non-inflammatory, so the main worry about it would be the lack of Fc mediated-functions, such as antibody dependent- cellular phagocytosis (ADCP), complement deposition (ADCD) and cellular cytotoxicity (ADCC), conferred by IgG1 and IgG3 subclasses.33 IgG4 could contribute to protection neutralizing SARS-CoV-2, so further investigations are warrant to elucidate this issue,34,35 which is supported by the generally excellent binding affinity of IgG4.4 Our results shows only a modest correlation (r = 0.3408) between IgE and IgG4 levels, so we are not able to infer that the induction of both isotypes happened due to a similar microenviroment. Thus, cellular and genetic profiling would be required to better support this assumption.4
The literature reports several virus-induced IgE responses to viral infections or vaccines,11–14,16,17 whether these antibodies participate in the viral immune response or have any pathogenic role remains to be elucidated. As discussed above, viral infections and atopy might influence one another, exacerbating symptoms.22,24 On the other hand, anti-HIV IgE antibodies, even not demonstrating neutralizing activity; were able to inhibit HIV proliferation in vitro, finding that the authors hypothesized to be mediated by cytotoxicity.36 More recently, IgE-mediated cytotoxicity has been shown to induce the death of pancreatic cancer cells,37 taken together, these manuscripts suggest a potentially protective role for IgE unrelated with parasites.
Unfortunately, our study did not include similar functional assays; but the ELISA index of IgE-RBD antibodies exhibited a significant correlation with neutralizing index and the vaccine-induced IgE had mostly high avidity, suggesting a good binding strength to SARS-CoV-2. The neutralizing antibodies data we had available employed an assay that does not distinguish antibody classes, so we assume it would be worthwhile investigating whether IgE antibodies contribute to neutralization or participate in other immunological mechanisms that could provide protection against SARS-CoV-2. Given the potency of IgG for neutralization, this assumption could only be confirmed by a controlled experiment, purifying IgE from the sera and testing its ability to neutralize SARS-CoV-2 alone, as it has been done for IgG and IgM.38
The study population was female, which may limit the generalization of the findings. The literature measuring total IgE levels according to sex is inconclusive,39 some evidence pointing to higher IgE in men compared to women at different ages.40,41 We tested samples of 3 male subjects of the larger cohort that originated the population of this study, but we did not use such data for analysis to avoid biases due to the low n. These individuals were 38, 51 and 54 years old; 2/3 had comorbidities (both had hypertension and obesity), none of them presented immune-mediated disease, only one had COVID-19 before vaccination, and all three received 2 doses of CoronaVac and one BNT162b2 booster. None presented positive IgE before the vaccines, even the one that had COVID-19 (mean value 0.678 ± 0.021). After two doses of CoronaVac the mean IgE index was 1.323 (± 0.071) and, upon BNT162b2, it increased to 3.817 (± 1.054). Although the IgE index after two CoronaVac doses was somewhat lower than the female median, after booster it was similar to what we observed for women. Further studies with a proportional female/male ratio could support these observations.
Our study has many limitations: we evaluated a relatively small sample size, and only the female population was assessed, therefore, the data should be interpreted with caution. Moreover, we were unable to measure total IgE levels in the samples or conduct an in-depth functional characterization for IgE, describing virus neutralization and potential to induce cytoxicity, for example, besides avidity. However, it is important to note that samples were thoroughly characterized using molecular and various serology assays, control tests were performed and this is not the first report to demonstrate IgE antibodies specific to viral antigens. Finally, we encourage further investigation into the potential roles of IgE in non-allergic and non-parasitic immune responses.