C1.7 Antigen expression on CD8⁺ T cells is activation dependent: Increased proportion of C1.7⁺CD8⁺ T cells in HIV-1-infected patients with progressing disease

The C1.7 Ag is a surface marker previously shown to be expressed on all NK cells and on a subset of CD8⁺ T cells. We report in this study that C1.7 Ag expression on peripheral blood-derived CD8⁺ T cells overlaps with activation markers S6F1(high) and CD29(high) and is reciprocally expressed with CD62L. C1.7 Ag expression can be induced in vitro on CD8⁺ T cells by anti-CD3 cross-linking, suggesting that C1.7 Ag is activation dependent. In contrast to NK cells, C1.7 Ag does not signal on CD8⁺ T cells, nor does it induce redirected lysis upon ligation. The proportion of C1.7 Ag⁺CD8⁺ T cells is increased in HIV-infected patients compared with healthy donors. In 69 HIV-infected patients, we observed a significant inverse correlation between the percentage of C1.7 Ag-expressing CD8⁺ T cells and the absolute CD4⁺ T cell count. Two-year clinical follow-up of patients with initial CD4⁺ T cell count of >400 cells/mm³ and a normal proportion of C1.7 Ag⁺CD8⁺ T cells revealed that these patients were clinically stable with minimal HIV-associated symptoms. In contrast, 10 of 12 patients with CD4⁺ T cell counts of >400 cells/mm³ and an elevated proportion of C1.7 Ag⁺CD8⁺ T cells were symptomatic. ANOVA analysis of patients indicates that C1.7 Ag is a better predictor of disease progression than CD4 count. Overall, our findings indicate that C1.7 Ag is the first described marker for activated/memory CD8⁺ T cells and a useful parameter for evaluating the level of CD8⁺ T cell activation in vivo.