CD100 expression is down-regulated on antigen-specific T cells after peptide stimulation but not after in vitro cytokine culture or HIV-1 infection. (A) CD100 gMFI after 7 days of culture in the absence or presence of NL4–3 infection. (B) CD100 gMFI after 24 hours of incubation in the presence of proinflammatory or homeostatic cytokines or anti-CD3 plus anti-CD28. (C) Representative dot plots illustrating IFN-γ expression in CD100⁻ CD8⁺ T cells after CEF or p24 peptide pool stimulation. (D) Percentage of CD100⁺ and CD100⁻ CD8⁺ T cells expressing IFN-γ after CEF or p24 stimulation. HCs were assessed for CD100 expression after CEF stimulation. HIV-1 infected subjects were assessed for CD100 expression after p24 or CEF stimulation. CEF and p24 responses in HIV⁺ subjects are not necessarily matched from the same individual. (E) CD100 frequency on CMV pentamer-specific CD8⁺ T cells (open circles) in HC and HIV-1 infected individuals and HIV-1 pentamer-specific CD8⁺ T cells (closed square) in HIV-1 infected individuals compared with IFN-γ producing cells after PBMC peptide-stimulation with the peptide corresponding to the pentamer within the same individual. Statistical analysis was performed using (D) Student t tests between CD100⁺ and CD100⁻ cells, and (E) 2-tailed, paired Student t tests (*P < .05 and **P < .01). CEF = pool of peptides from cytomegalovirus, Epstein-Barr, and influenza viruses.