Bessell et al. identified an epitope expressed on commensal bacteria (SVY) with homology to a model tumor antigen (SIY) capable of eliciting cross-reactive T cell responses. Despite different TCR-pMHC affinities between the two epitopes, T cells expanded against SVY or SIY cross-reacted with the other and shared T cell clones. Mice exposed to commensal bacteria expressing SVY, which varied by commercial source (or in some experiments, by co-housing), had slower B16-SIY tumor growth and could generate stronger SVY-specific T cell responses with altered clonal composition. Adoptively transferred SVY-specific T cells slowed B16-SIY growth.
Contributed by Alex Najibi
ABSTRACT: Recent studies show gut microbiota modulate antitumor immune responses; one proposed mechanism is cross-reactivity between antigens expressed in commensal bacteria and neoepitopes. We found that T cells targeting an epitope called SVYRYYGL (SVY), expressed in the commensal bacterium Bifidobacterium breve (B. breve), cross-react with a model neoantigen, SIYRYYGL (SIY). Mice lacking B. breve had decreased SVY-reactive T cells compared with B. breve-colonized mice, and the T cell response was transferable by SVY immunization or by cohousing mice without Bifidobacterium with ones colonized with Bifidobacterium. Tumors expressing the model SIY neoantigen also grew faster in mice lacking B. breve compared with Bifidobacterium-colonized animals. B. breve colonization also shaped the SVY-reactive TCR repertoire. Finally, SVY-specific T cells recognized SIY-expressing melanomas in vivo and led to decreased tumor growth and extended survival. Our work demonstrates that commensal bacteria can stimulate antitumor immune responses via cross-reactivity and how bacterial antigens affect the T cell landscape.