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It has been proposed that HIV infection is associated with an imbalance in Th1 and Th2 subsets. Recent reports indicate that Th1 and Th2 effectors differ in their susceptibility to activation-induced apoptosis. To determine whether increased T cell apoptosis in HIV-infected patients contributes to alterations in cytokine synthesis, we performed single-cell analysis of type 1 and type 2 cytokine production by CD4 and CD8 T cells, simultaneously with detection of apoptosis. We demonstrate that a differential alteration in representation of Th1 subsets, rather than commitment of T cells to secrete Th2 cytokines, occurs throughout HIV infection. A significant decrease in the number of IL-2- or TNF-alpha-producing T cells was observed, whereas those producing IFN-gamma remained preserved. Furthermore, there is a gradient of susceptibility to activation-induced apoptosis (IL-2 < IFN-gamma < TNF-alpha) among the different Th1 subsets. This gradient was detected in both CD4 and CD8 subsets, as well as in control donors and HIV-infected patients, in whom the susceptibility to apoptosis of IL-2 and IFN-gamma producers was increased compared with controls. This differential intrinsic apoptosis susceptibility of Th1 effectors was found to be tightly regulated by Bcl-2 expression. In HIV-infected persons, disappearance of IL-2-producing T cells was a good indicator of disease progression and was correlated with the progressive shrinkage of the CD4+ CD45RA+ T cell compartment and a gradual increased susceptibility to activation-induced apoptosis of the IL-2-producing subset. This close relationship between the CD45RA/CD45R0 ratio, the level of type 1 cytokine production, and susceptibility to apoptosis should be considered in HIV-infected patients under antiviral or immune-based therapies.

We have previously reported significant alterations of gamma delta subset distribution in the peripheral blood of HIV-infected donors. These modifications are characterized by the depletion of the V delta 2 subset associated with a strong increase in peripheral V delta 1 T cells. In addition, the latter exhibit ex vivo an activated phenotype and show a restricted complementarity-determining region 3 (CDR3) repertoire. In the present report we first address the question of the origin of these expanded cells. The lack of expansion and the Gaussian complementarity-determining region 3 size distribution of lymph node V delta 1 T cells suggest that lymph nodes do not represent the site of specific activation of this subset. The function of blood V delta 1 T cells was also explored. We report that patients' V delta 1 T cells express high levels of perforin and display an in vitro cytotoxic activity, whose characteristics are different from those of NK and LAK cells. In addition, single cell analysis of cytokine production revealed that, in contrast to V delta 1 T cells from control donors, patients' V delta 1 T cells are primed in vivo for IFN-gamma and TNF-alpha production. Together, these results indicate that in the course of HIV infection, expanded blood V delta 1 T cells are differentiated into a functional subset and raise the question of the contribution of this subset to AIDS pathogenesis.

We analyzed the potential causes of the increased susceptibility to apoptosis of peripheral lymphocytes from a large cohort of HIV-infected persons that we followed during a 3-yr period. By using quantitative cytofluorometric methods, we demonstrate that all lymphocyte populations were contributing proportionally to the apoptotic population in both groups of donors, but the percentage of T and B cells involved in this cell death process was significantly increased in HIV-infected persons. To study the relationship between the increased apoptosis in HIV infection and the activation state of the immune system, we analyzed cell surface expression of activation markers on apoptotic and nonapoptotic cells. We found that in the chronic phase of HIV infection, 50 to 60% of the apoptotic cells exhibited an activated phenotype (they were HLA-DR+, CD38+, CD45RO+, and Fas+), and interestingly, the CD45RO+ subset appeared to be more prone to apoptosis in HIV-positive persons. This study also indicates that the activated phenotype found on apoptotic cells was not a distinctive feature of patients' lymphocytes since it was in similar proportion in apoptotic cells from control lymphocytes. However, a significant correlation was found between the intensity of anti-CD3-induced apoptosis in both CD4 and CD8 T cells from HIV-infected persons and their in vivo expression of CD45RO and HLA-DR molecules. Finally, a significant correlation was found between the intensity of spontaneous or anti-CD3-induced apoptosis in total lymphocytes and disease progression; this was confirmed when the CD4 and CD8 T cell subsets were analyzed separately. Altogether these observations indicate that the increased susceptibility to apoptosis of peripheral T cells from HIV-infected persons correlates with disease progression and support the hypothesis that the chronic activation of the immune system occurring throughout HIV infection is the primary mechanism responsible for this cell deletion process.