Chlamydia trachomatis induces low-frequency, sustained CD4 T cell responses in most women, predominantly targeting chlamydial protease-like activity factor, CPAF.

BACKGROUND: Chlamydia trachomatis (CT) is a globally prevalent sexually transmitted infection (STI) that can result in pelvic inflammatory disease, ectopic pregnancy and infertility in women. Currently, there is no prophylactic vaccine. METHODS: This study examined T cell immunity in a cohort of women recently infected with CT. Participants were screened against peptides spanning 33 of 894 possible CT proteins, either ex vivo or using short-term cell lines (STCL). CT-specific T cells were characterized by IFN-γ ELISpot and flow cytometry. RESULTS: Ex vivo CT-specific T cells were rarely detected; however, following in vitro expanded CT-specific T cells were detected by IFN-γ ELISpot in 90% (27/30) of participants. Notably, over 50% of participants had T cell responses targeting chlamydial protease-like activity factor (CPAF). T cell epitopes were dispersed across the CPAF protein. Flow cytometry analysis of STCL found CT-specific cells, were mainly CD4+, produced IFN-γ and TNF-α and were sustained over 12 months. Ex vivo analysis suggested CT-specific T cells mostly exhibited a central memory phenotype. CONCLUSION: Our results indicate that CT infection elicits low-frequency, persistent CD4 T cell responses in most women and that the secreted protein, CPAF, is an immunoprevalent CT antigen. Altogether, these data support development and testing of CT vaccines that enhance CD4 T cells against CPAF.

CD3 downregulation identifies high-avidity human CD8 T cells.

CD8 T cells recognize infected and cancerous cells via their T-cell receptor (TCR), which binds peptide-MHC complexes on the target cell. The affinity of the interaction between the TCR and peptide-MHC contributes to the antigen sensitivity, or functional avidity, of the CD8 T cell. In response to peptide-MHC stimulation, the TCR-CD3 complex and CD8 co-receptor are downmodulated. We quantified CD3 and CD8 downmodulation following stimulation of human CD8 T cells with CMV, EBV, and HIV peptides spanning eight MHC restrictions, observing a strong correlation between the levels of CD3 and CD8 downmodulation and functional avidity, regardless of peptide viral origin. In TCR-transduced T cells targeting a tumor-associated antigen, changes in TCR-peptide affinity were sufficient to modify CD3 and CD8 downmodulation. Correlation analysis and generalized linear modeling indicated that CD3 downmodulation was the stronger correlate of avidity. CD3 downmodulation, simply measured using flow cytometry, can be used to identify high-avidity CD8 T cells in a clinical context.