Mechanism of gamma sigma T-cell-mediated inhibition of stem cell differentiation in vitro: possible relevance for myelosuppression in HIV-infected individuals. 
We investigated whether gamma delta T cells contribute to the suppression of myelopoiesis in HIV infection.  Freshly isolated gamma delta T cells from HIV seropositive patients suppressed CFU-GM growth in vitro.  Preactivation of gamma delta T cells with IL-2 and/or IL-15 further reduced the number of CFU-GM.  Natural killer cells and to a lower extent CD4+ and CD8+ cells also inhibited CFU-GM growth.  In contrast to gamma delta T cells, this effect was not dependent on IL-15 or IL-2 preactivation.  Moreover, no enhanced inhibitory effect of CD56+ and CD4+ cells was observed in HIV+ subjects compared to HIV- donors.  The myelosuppressive effect of supernatants of gamma delta T cells could be inhibited by antibodies against IFN-gamma or TNF-alpha.  Accordingly, we found increased numbers of TNF-alpha or IFN-gamma-secreting CD8+ gamma delta T cells in HIV+ patients.  We conclude that the increased fraction of activated gamma delta T cells producing myelosuppressive cytokines might contribute to the dyshematopoiesis frequently observed in HIV-infected individuals.