Sex Differences in Older Adults' Immune Responses to Seasonal Influenza Vaccination

Abstract

Background: Sex differences in immune responses to influenza vaccine may impact efficacy across populations. Methods: In a cohort of 138 older adults (50-74 years old), we measured influenza A/H1N1 antibody titers, B-cell ELISPOT response, PBMC transcriptomics, and PBMC cell compositions at 0, 3, and 28 days post-immunization with the 2010/11 seasonal inactivated influenza vaccine. Results: We identified higher B-cell ELISPOT responses in females than males. Potential mechanisms for sex effects were identified in four gene clusters related to T, NK, and B cells. Mediation analysis indicated that sex-dependent expression in T and NK cell genes can be partially attributed to higher CD4+ T cell and lower NK cell fractions in females. We identified strong sex effects in 135 B cell genes whose expression correlates with ELISPOT measures, and found that cell subset differences did not explain the effect of sex on these genes' expression. Post-vaccination expression of these genes, however, mediated 41% of the sex effect on ELISPOT responses. Conclusions: These results improve our understanding of sexual dimorphism in immunity and influenza vaccine response.

Keywords: elderly; immunity; influenza; influenza vaccine; sex differences; sexual dimorphism; systems biology; vaccinomics.

Figure 1

Memory B-cell ELISPOT responses to influenza virus differ by subject sex. Memory B cells capable of responding to vaccine strain influenza A/H1N1 were quantified using B-cell ELISPOT. Dot plots indicating individuals' responses are available in Supplementary Figure 1.

Figure 2

Subject sex does not significantly affect influenza A/H1N1 antibody titers. Box plots are shown for levels of influenza A/H1N1-reactive antibody in subject sera before and after vaccination using hemagglutination-inhibition and virus neutralization assays. Dot plots indicating individuals' responses are available in Supplementary Figure 2.

Figure 3

Mediation analysis indicates that the sex effect on B-cell ELISPOT responses is mediated by Day 3 and Day 28 gene expression in a small gene cluster of 135 genes highly enriched for B cell-related genes. The total sex effect represents the fraction of the standard deviation in ELISPOT response that can be explained by sex.

Figure 4

CD4+ and NK cell fractions in PBMCs differ between males and females. PBMC samples were harvested from our cohort subjects immediately prior to vaccination, and populations of immune cell subsets were quantified in each subject sample using fluorescent antibody staining for unique cellular markers and flow cytometry. (A) mean PBMC composition and 95% confidence intervals across 135 subjects. (B) Significantly different fractions of CD4+ T cells and NK cells were found in males (n = 45) vs. females (n = 90). Dot plots indicating individuals' responses are available in Supplementary Figure 3. (C) CD4+ T cell and NK cell fractions in each subject show weak correlation with one another. Source data may be found in Supplementary Table 5.

Figure 5

Mediation analysis indicates whether sex-related differences in gene expression of select gene clusters is mediated by observed sex differences in CD4+ cell (left column) and NK cell (right column) fractions of subject PBMCs.

Figure 6

Apparent mechanisms of sex effects on influenza vaccine responses. CD4+ T cell and NK cell numbers mediate part of the effect of subject sex on the expression levels of gene clusters involved in NK cell and T cell genes, with unknown impact on immune outcomes. Higher expression of 135 B cell genes in females was not found to relate to sex differences in immune cells; however, it was determined that these B cell genes were, however, statistical mediators of the higher B-cell ELISPOT responses to seasonal influenza vaccine found in females relative to males. ^*Gene expression heatmaps present the sex-differential expression of the 50 most representative genes of each sex-dependent gene cluster (i.e., most highly correlated with the cluster eigengene).