1. Introduction
The introduction of anti-SARS-CoV-2 vaccines has substantially modified the pandemic picture, inducing effective protection in the population. Several vaccines have been developed and administered, which have shown heterogeneous efficacy and immunogenicity against the different variants of SARS-CoV-2 with wide inter-individual differences, so that booster doses were required. The Pfizer-BioNTech vaccine BNT162b2, based on lipoic nanoparticles containing the mRNA for the spike (S) protein [
1], will be introduced at the end of 2020. However, it is well known that the immunogenicity and effectiveness of a vaccine can be impacted by different factors, and differential patterns of vaccine effectiveness have been observed in diverse populations, resulting from the complex interplay among the host, pathogens, and environmental factors [
2,
3]. Concerning the host factors, gender, aging, and genetic factors, such as the frequencies of specific genetic variants of immune inflammatory genes [
4], have been largely documented. As a result, genome-wide association studies (GWAS) and other genetic investigations have focused on large panels of genes, including genetic variants of HLA, PAMPs, cytokines, chemokines, and receptor molecules, as possible causes of variability in the response to antiviral vaccines against hepatitis B, measles, rubella, influenza A, smallpox, anthrax, and mumps [
5,
6,
7,
8,
9,
10,
11,
12]. Similar results were obtained studying the association of antiviral vaccine response with polymorphic variants in cytokine genes, such as TNF-α, IL-2, IL-4 IL-10, IL-28, IFNγ [
13,
14,
15,
16].
However, there is currently limited evidence on factors influencing individual responses to anti-SARS-CoV-2 vaccines. A wide range of sociodemographic, biological, clinical, and nutritional factors have been reported to influence the different production of anti-spike antibodies and their titers after vaccination in people of different ages and ethnicities [
17]. Therefore, individual genetic background, which influences the intensity and quality of the immune and inflammatory response, could also be implicated in the regulation of the vaccine-induced anti-SARS-CoV-2 immune response. Based on such evidence, it might be useful to evaluate the impact of cytokine gene variability on vaccine efficacy.
Consistent with this aim, in this study we aimed to detect the biological effect of single nucleotide polymorphisms (SNPs) in cytokine genes involved in the regulation of inflammation and antibody production on the magnitude and duration of the humoral immune response induced by vaccination against SARS-CoV-2. The association of functionally relevant genetic variants in cytokine genes with anti-S1/S2 IgG antibody levels measured at 31 and 105 days after administration of the second dose of Pfizer-BioNTech’s Comirnaty BNT162b2 was evaluated.
We evaluated polymorphisms of the following IL-1 superfamily genes: IL-1A rs1800587 [
18], IL-1B rs1143634 [
19] and rs16944 [
20], IL-18 rs187238 [
21] and rs1946518 [
22], which influence cytokine levels and production, IL-1RN rs315952, whose alleles modulate efficiency of inflammation control [
23], and IL-1R1 rs2234650, whose alleles create two alternative putative binding sites for two different transcription factors and activation of different metabolic pathways [
24]. In addition, IL-6 rs1800795 [
25] and TNFA rs1800629 [
26] polymorphisms, known to be involved in regulating the production of these two key cytokines for the inflammatory response, were typed. Considering the key role of IL-10 in regulating inflammation and antibody production, the IL-10 SNPs rs1800896, rs1800872, and rs3021097, which regulate IL-10 cytokine production [
27], were also typed. Finally, the association of functional polymorphisms of key Th1 and Th2 cytokines (IL-4 rs2243250 [
28], IL-13 rs1800925 [
29], IFNG rs2430561 [
30], and IFNGR2 rs2834213 [
31]) with vaccination-induced anti-S1/S2 IgG antibody levels was evaluated.
4. Discussion
SARS-CoV-2 infection can result in considerable variability in the severity of clinical symptoms, from asymptomatic infection to acute respiratory distress syndrome, with a clear age- and gender-dependent trend, with men older than 65 years accounting for approximately 80% of hospitalizations for severe COVID-19 [
35]. Accordingly, in early 2021, COVID vaccinations were offered first to people over the age of 80, their caregivers, and sanitary staff professionally in contact with vulnerable people. Considering the impact of age and gender on severe COVID-19 susceptibility, we have evaluated the biological effect of these variables on the antibody response to the Pfizer- BNT162b2 vaccination. Our results indicate that subjects with higher antibody titers were significantly younger than those with lower titers, both at 31 and 105 days after the second vaccine dose. The same significant difference in age distribution was observed among uninfected and infected subjects at day 105. Actually, SARS-CoV-2-infected subjects were older than uninfected individuals. These results are in good agreement with data reported by other groups showing a reduction of anti-SARS-CoV-2 spike-specific IgG and neutralization titers in older subjects [
36,
37].
About 25% of the subjects recruited for this study were infected by SARS-CoV-2 between 31 and 105 days after administration of the second dose of vaccine, and age seemed to correlate with susceptibility to infection in vaccinated subjects.
The differential gender-related susceptibility to COVID-19 is well known [
38,
39]. However, our data indicate that gender does not influence antibody titers or the likelihood of vaccinees acquiring a breakthrough SARS-CoV-2 infection. Banki and coworkers [
40] had found no gender correlation between SARS-CoV-2-specific T- and B-cell responses, at 35 ± 8 and 215 ± 7 days after the second dose in 600 subjects who participated in a rapid mass vaccination against SARS-CoV-2 with BNT162b2 in Austria.
Although the development of a high titer of anti-S antibodies cannot be considered the only suitable biomarker to predict protection against SARS-CoV-2 after vaccination [
41,
42], a lower antibody titer could be considered predictive of a higher susceptibility to infection in vaccinees [
43]. Our data seem to confirm the latter, as at day 31, the antibody titers of vaccinated subjects who later developed SARS-CoV-2 infection were significantly reduced compared to those of subjects still uninfected at day 105. From our results, reduced anti-S IgG titers in subjects older than 50 years, one month after the second dose of vaccination, might be a marker for assessing the risk of SARS-CoV-2 infection. However, these findings should be confirmed in a larger group of subjects, also evaluating the cell-mediated response to anti-SARS-CoV-2 vaccines.
A key mechanism for inducing an efficient response to RNA based vaccines is the induction of a sustained inflammatory response induced by lipidic envelopment [
44]. Some authors indicated that anti-S IgG and neutralizing antibody titers resulting from the second BNT162b2 dose were significantly associated with fever, a reaction in which proinflammatory cytokine activation is a mandatory mechanism [
45]. Therefore, a genetically determined increase in proinflammatory cytokine production could be relevant to achieving a satisfactory rate of protection. RNA vaccines activate an inflammatory pathway mediated by IL-1 [
44], which is an essential modulator in the natural and induced immune responses. We found no association between the genotypic frequency of IL-1 and an increase or decrease in the antibody titer in response to the BNT162b2 vaccine. However, a significant finding relates to the
IL-1R rs2234650 polymorphism, as
rs2234650T carrier genotypes correspond to the highest neutralizing Ab titers at days 31 and 105, while the homozygous CC condition is associated with the lowest antibody titers. In a 2015 study, using in silico analyses, Vasilyev and coworkers [
24] suggested that
rs2234650 alleles create two alternative putative binding sites for two different transcription factors. In silico structure associated to the C allele might bind yin yang 1 transcription factor (YY1), whereas the T allele structural model resulted in a binding site for the activation of protein 1 (AP-1) transcriptional factor. YY1 is involved in inflammation and tumorigenesis [
26,
46]. In particular, YY1 is known to be involved in viral diseases, playing a role in the activation of HIV-1 replication [
47] and in the promotion of oncogenic HPV [
48]. AP-1 modulates proliferation, differentiation, apoptosis, and inflammation [
49] and has a central role in the induction of IL-1β production and secretion mediated by nuclear translocation of activated AP-1 [
50,
51]. The conformational change associated with the T allele might facilitate increased production of IL-1, which is essential to obtain protective anti-S IgG and neutralizing antibody titers toward RNA vaccines [
44]. Accordingly, our data demonstrate that IL-1R1 rs2234650 T-positive genotypes are significantly associated with higher titers of anti-S IgG at both day 31 and day 105.
Antibody production by B cells is influenced by a concert of signal-specific coactivators, including inflammasome activators and IL-1 linking to the IL-1 receptor, all converging on MyD88 and associated signaling adapters. The latter lead to the activation of the NF-κB and release of trapped NF-κB/Rel transcription factors into the nucleus, resulting in the alteration of the expression of hundreds of target genes, including immunoglobulins genes [
52].
IL-6 is involved in both activating inflammation [
53] and stimulating immunoglobulin production [
54,
55] by driving helper lymphocyte differentiation [
56]. The best known and most studied polymorphism of
IL-6 is located in the promoter, the G/C substitution at position -174 from the transcription start site (
rs1800795). The major
-174G allele is associated with increased mRNA expression, up to 2.4-fold following IL-1 stimulation, whereas the C allele is associated with genotypes with low production of IL-6 [
23,
57]. In spite of the well-known role of IL-6 in the cytokine storm induced by SARS-CoV-2 infection [
58,
59], no clear evidence has been presented on the role of IL-6 SNPs as susceptibility or protective factors in COVID-19. In a recent study, a haplotype common in Asia, C-T-T, represented by variant alleles of
rs1800796, rs1524107, and
rs2066992 SNPs, associated with a reduced expression of IL-6 following inflammatory stimuli, was identified as a protective genetic background associated with a better outcome of SARS-CoV-2 infection [
60].
IL-6 promoter SNPs
rs1800795 is polymorphic almost exclusively in Caucasians, and it has been suggested that genotype
rs1800795GG, associated with high IL-6 production, might be protective against severe COVID-19 [
61,
62]. Our data seem to indicate that the IL-6 rs1800795GG genotype is strongly associated with higher anti-spike antibody titers, at least at day 31 after the second dose of the BNT162b2 vaccine. On the other hand, proliferation of B-cells and their differentiation into antibody-secreting plasma cells, the key mechanism for a successful vaccine response, are sustained by type 2 T helper (Th2) cytokines such as IL-4, IL-10, and IL-6 itself [
63]. In this view, a genetically determined high IL-6 production might be useful to determine a favorable environment to reach a high rate of antibody protection against SARS-CoV-2 infection, as demonstrated by other groups for vaccination against the H1N1 flu virus [
64].
Our observation that the
IL-4 rs2243250T positive genotype (heterozygous C/T genotype) is associated with high levels of anti-spike antibodies prompt us to hypothesize that the presence of
IL-1R1 rs2234650T,
IL-6 rs1800795G, and
IL-4 rs2243250T positive genotypes, which appear to be associated with an increased production of the respective cytokines [
65,
66], tagged the maintenance of optimal antibody production in subjects receiving the second dose of anti-SARS-CoV-2 mRNA vaccine.
As is well known, IFNγ is the key cytokine for IgG isotype switching, whereas IL-4 stimulates the proliferation, maturation, and differentiation of B lymphocytes in plasma cells actively secreting IgE and IgG4 [
67]. In addition, IL-4 is essential to maintain naïve B cells and the production of memory B cells after exposure to an antigen or vaccination [
68]. Therefore, a genetically determined increased release of IL-4 might be involved in maintaining optimal IgG production after IFNγ-mediated isotype switch.
On the other hand, when the cytokine genotype assets of subjects who had been infected after the second dose of mRNA vaccine were compared to those of uninfected subjects, we found a higher frequency of
IL-4 rs2243250 T genotypes (associated with increased cytokine production). As is well known, IL-4, as well as IL-13, is the key cytokine in the induction of Th2- and macrophage 2 (M2)-mediated inflammation [
69]. In COVID-19 patients, significantly higher IL-4 lung tissue expression and M2 macrophages were observed [
70], and the prevalence of IL-4 Th2-mediated lung damage was a characteristic of the ineffective immune response elicited by SARS-CoV-2 [
69]. In addition, it has been reported that in COVID-19 patients, the virus activates apoptosis by stimulating JAK-STAT6 signaling pathway through increased Th2 and IL-4 expression [
71]. Therefore, it is possible to speculate that a genetic asset that favors high IL-4 production [
28] may have different pleiotropic effects, such as favoring immunoglobulin production after vaccine immune stimulation or, conversely, being a susceptibility factor for COVID-19 in vaccinated subjects.
Interestingly, we found that the frequencies of
IL-1R1 rs2234650CT and
IL-6 rs1800795C positive genotypes were significantly increased in the group of subjects infected with SARS-CoV-2 after administration of the second dose of vaccine. The opposite genetic asset was observed in the group of subjects with the higher anti-S antibody titer at day 31, and allowed speculating that the simultaneous presence of
IL-1R1 rs2234650TT and
IL-6 rs1800795GG genotypes may be protective against the SARS-CoV-2 infection in vaccinated patients. Finally, a higher frequency of the homozygous AA genotype, which is characterized by increased transcriptional expression [
24] of
TNFA rs1800629, was detected in breakthrough infections. The role of TNF-α in worsening the clinical picture of COVID-19, ARDS, and systemic inflammation, is well known [
72]. In addition, some research groups have identified
TNFA rs1800629A as a marker of susceptibility to COVID-19 [
73]. TNFA rs1800629G/A minor alleles seem to be associated with increased risk and severity of other viral respiratory infections, such as respiratory syncytial virus bronchiolitis and pneumonia [
74]. Overall, our data highlight the complex role of genetic background in the humoral immune response against SARS CoV-2 and vaccine antigens and suggest further studies to evaluate the role of polymorphic variants following the booster vaccination cycle and in a larger population sample.
A limitation of this study is the lack of data on the cellular immune response to vaccination with BNT162b2 (e.g., evaluation of IFNγ levels by COVID-19-specific Quantiferon assay [
75]), so we cannot analyze at this moment the effect of genetic background on the specific cellular immune response. Further studies are warranted to address this important topic.