(P382) Treponema pallidum-specific memory T cells persist in skin after secondary syphilis

Background: There is increased interest in developing a vaccine to address the ongoing syphilis epidemic. Prior work showed that Treponema pallidum subsp pallidum (Tp) bacteria are cleared by antibody mediated opsonophagocytosis by IFN-γ-activated macrophages. Antigen-specific T cells likely produce IFN-γ and support the development of opsonic antibodies. Although many Tp B cell epitopes are described in the literature, little is known about the human T cell response to Tp infection. To address this knowledge gap regarding T cell immunity to syphilis, we sought to identify human Tp-specific T cell antigens in healing secondary syphilis lesions as well as circulating PBMCs.

Methods: Peripheral blood mononuclear cells (PBMCs) and skin biopsies were collected from volunteers with secondary syphilis, after obtaining informed consent. T cells were expanded non-selectively from biopsies of skin lesions. The resultant polyclonal T cell lines (TCL) were probed with a library of recombinant syphilis proteins in a proliferation assay weighted to CD4 T cell responses. Responses were validated to the epitope and HLA restriction levels with synthetic peptides, including IFN-γ secretion as an endpoint. Tp-specific CD4 T cells were separately enriched from PBMCs by Activation-Induced-Marker sorting and polyclonal expansion, and TCL similarly assessed for reactivity to the syphilis protein library.

Results: Tp-protein T cell antigens were identified in T cell lines derived from blood and skin, including Tp0548, 0856, 0858 and Tp0870. Remarkably, Tp0858-reactive T cells were detectable in both skin and PBMC-derived TCL immediately after penicillin treatment and persisted at both sites at days 30 and 180 post treatment. Additional T cell antigens identified from PBMCs include Tp0136, Tp0326 and Tp0772.

Conclusions: These investigations provide the first known Tp antigens that stimulate Th1, IFN-γ producing CD4 T cells in infected humans. We have shown that Tp-specific T cells persist in previously infected skin as tissue resident memory (T_RM) T cells for at least 180 days after treatment. If Tp-specific CD4 T cells can be conditioned by vaccination to persist in skin, they may increase the efficacy of opsonophagocytosis of treponemes in mucocutaneous sites. Such a vaccine could reduce intrahost dissemination and potentially interhost transmission of the syphilis spirochete.