MPXV H3L elicits broadly directed CD4 T cell responses in mpox patients and MVA vaccinees.

Data on MPXV-specific T-cell responses at the single-peptide level remain limited in patients with mpox and in MVA-BN vaccinees. Here, we characterized the breadth and specificity of MPXV-specific T-cell responses at the single-peptide level after in vitro expansion with overlapping 20-mer peptides spanning the MPXV proteins H3L, A35R, and B6R. The study included 28 adult males: 15 with a history of mpox (including 6 with additional MVA-BN vaccination), 7 MVA-BN-only vaccinees, and 6 unexposed participants. All mpox and MVA-BN participants responded to at least one H3L peptide, indicating broad immunogenicity, while responses to A35R and B6R were more common in the mpox group. Notably, the breadth of B6R-specific CD4+ T-cell responses correlated with hybrid immunity (r = 0.6; p = 0.02). Interestingly, MVA-BN and mpox individuals demonstrated distinct immunodominant patterns: H3L_251-270 and H3L_211-230 were mainly recognized among mpox individuals, whereas MVA-BN recognized H3L_221-240 more frequently. High-affinity HLA binding to multiple H3L peptides suggests broad population coverage. Additional immunogenetic analysis revealed a shared TRBV15 clonotype in about 50% of mpox cases. In summary, these findings highlight H3L as a potential vaccine target, guiding the development of next-generation multi-antigen MPXV vaccines to elicit comprehensive T-cell immunity.