SARS-CoV-2 XBB.1.5 mRNA booster vaccination elicits limited mucosal immunity

Abstract

Current COVID-19 vaccines provide robust protection against severe disease but minimal protection against acquisition of infection. Intramuscularly administered COVID-19 vaccines induce robust serum neutralizing antibodies (NAbs), but their ability to boost mucosal immune responses remains to be determined. In this study, we show that the XBB.1.5 messenger RNA (mRNA) boosters result in increased serum neutralization to multiple severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants in humans, including the dominant circulating variant JN.1. In contrast, we found that the XBB.1.5 mRNA booster did not augment mucosal NAbs or mucosal IgA responses, although acute SARS-CoV-2 XBB infection substantially increased mucosal antibody responses. These data demonstrate that current XBB.1.5 mRNA boosters substantially enhance peripheral antibody responses but do not robustly increase mucosal antibody responses. Our data highlight a separation between the peripheral and mucosal immune systems in humans and emphasize the importance of developing next-generation vaccines to augment mucosal immunity to protect against respiratory virus infections.

Figure 1.. Key spike protein mutations in the Omicron variant, XBB subvariants, and the currently circulating JN.1 SARS-CoV-2 variant.

Spike protein mutations in JN.1 and other SARS-CoV-2 variants are shown. Substitutions in the BA.1, BA.2, BA.5, XBB.1.5, EG.5.1, FL.1.5.1, HV.1, HK.3, BA.2.86, and JN.1 SARS-CoV-2 variants relative to the Wuhan/WIV04/ reference strain (https://gisaid.org/WIV04/) are indicated. Amino acid substitutions are indicated in red tiles, and deletions in blue tiles. NTD, N-terminal domain; RBD, receptor binding domain.

Figure 2.. Peripheral and mucosal neutralizing antibody responses following XBB.1.5 mRNA boosting.

(A) Serum neutralizing antibody (NAb) titers after XBB.1.5 mRNA boosting against SARS-CoV-2 WA1/2020, BA.1, BA.5, XBB.1.5, EG.5.1, FL.1.5.1, HV.1, HK.3, and JN.1 variants by luciferase-based pseudovirus neutralization assays at baseline prior to boosting and at week 3 after boosting (n=31). NT₅₀, half-maximal neutralizing titers. Median values are shown at the top. (B) Longitudinal paired analysis of NAb titers are shown. Median fold-change values 3 weeks after XBB.1.5 mRNA vaccination are indicated numerically at the top. (C) Serum NAb titers in participants who did not receive the XBB.1.5 mRNA boosters (n=27) presented as in (A). (D) Paired analysis of NAb titers and their fold-change values during the 3-week follow-up in participants without XBB.1.5 mRNA boosting as in (B). (E and F) Mucosal NAb titers in nasal swabs against SARS-CoV-2 WA1/2020, BA.5, XBB.1.5, HV.1, and JN.1 variants in participants with XBB.1.5 mRNA boosting (E) and without boosting (F) by luciferase-based pseudovirus neutralization assays. Median values are shown at the top. For all panels, the dotted lines reflect the limits of quantitation, and red bars reflect median responses. P values were calculated using a two-tailed Mann-Whitney test; *P≤0.05, **P≤0.01, ****P≤0.0001, and ns = not significant.

Figure 3.. Peripheral and mucosal binding antibody responses following XBB.1.5 mRNA boosting.

(A and B) Serum IgG binding antibody titers against the RBD of SARS-CoV-2 WA1/2020, BA.1, BA.5, XBB.1.5, and JN.1 were measured at 3 weeks after XBB.1.5 mRNA boosting (A) or without XBB.1.5 mRNA boosting (B) by ELISA. (C and D) Mucosal IgG and IgA binding antibody titers in nasal swabs against the RBD of SARS-CoV-2 WA1/2020 and XBB.1.5 were measured at 3 weeks after XBB.1.5 mRNA boosting (C) or without XBB.1.5 mRNA boosting (D) by ELISA. The dotted lines reflect the limits of quantitation. Red bars reflect median responses and are shown numerically at the top. P values were calculated using a two-tailed Mann-Whitney test; **P≤0.01, ****P≤0.0001, and ns = not significant.

Figure 4.. Cellular immune responses following XBB.1.5 mRNA vaccination.

(A) Spike protein-specific CD4⁺ and CD8⁺ T cell responses to SARS-CoV-2 WA1/2020, BA.5, XBB.1.5, and JN.1 following XBB.1.5 mRNA vaccination. Samples were stimulated with pooled peptides and IFN-γ production was measured by intracellular cytokine staining (ICS). (B) Spike protein-specific T cell responses to SARS-CoV-2 WA1/2020, BA.5, XBB.1.5, and JN.1 following XBB.1.5 mRNA vaccination were measured by IFN-γ ELISPOT assays after pooled peptide stimulation. Data are presented as as spot-forming cells (SFCs) per million peripheral blood mononuclear cells (PBMCs). (C and D) The ICS assay (C) and ELISPOT assay (D) were also conducted in participants without XBB.1.5 mRNA boosting and are shown as in (A) and (B), respectively. The dotted lines reflect the limits of quantitation. The red bars reflect median responses and are shown numerically at the top of each panel.