- Objective To study the role of cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4) during HIV infection. Methods Intracellular CTLA-4 expression, determined by flow-cytometry, and proliferative responses to HIV antigens, were studied in peripheral blood mononuclear cells (PBMC) from 93 HIV-1-infected [HIV(+)] patients and 40 HIV-1 seronegative controls. Results The proportions of CTLA-4 expressing CD4+ T cells were: (1) significantly higher in HIV(+) patients, 10.95 ± 0.66%, than in controls, 6 ± 0.45% (P < 0.0001); (2) inversely correlated to CD4+ counts (r = −0.67, P < 0.005, n = 16, drug-naive patients;r = −0.57, P < 0.0001, n = 77, HAART-treated patients); and (3) positively correlated to proportion of activated (HLA-DR+CD3+) (r = 0.53, P < 0.0001) and memory (CD45RO+CD4+) T cells (r = 0.46, P < 0.001). CD28 median fluorescence intensity in CTLA-4- cells was twice that in CTLA-4+ cells (140 ± 5.3 versus 70 ± 2.28, P < 0.00001), whereas cells low in CD28 and CD4, expressed more CTLA-4 (P < 0.0001). Higher proportion of CTLA-4+CD4+ cells expressed CCR5 and Ki-67, in comparison with CTLA-4-CD4+ cells, (65 ± 11.9 and 25 ± 7.5% versus 27 ± 8.9 and 3.7 ± 2%, P < 0.0001 and P < 0.01, respectively). Among HAART-treated patients, with viral load below detectable levels, CD4+ cells increase was inversely correlated to %CTLA-4+CD4+ cells (r = −0.5, P = 0.003, n = 39). Proliferation of PBMC to anti-CD3, gp-120 depleted HIV-1 antigen or HIV-1 p24 stimulation was inversely correlated with CTLA-4 levels (r = −0.68, P = 0.0035;r = −0.38, P = 0.04; and r = −0.43, P = 0.028, respectively). Conclusions (1) CTLA-4 is upregulated during HIV infection and may therefore account for CD4 T-cell decline and anergy in HIV-1 infection. (2) Increased levels of CTLA-4 may undermine immune responses and in the HAART-treated patient-immune reconstitution. (3) Blocking of CTLA-4 may offer a novel approach for immune-based therapy in HIV infection.