The ability of CD8+ T lymphocytes to suppress the transcription and replication of HIV-1 is well documented. We have demonstrated that the factor(s) responsible for the suppression of HIV-1 LTR-mediated gene expression are not the CC chemokines RANTES, MIP-1alpha, and MIP-1beta. Interestingly, these and other chemokines and cytokines are produced by both CD8+ and CD4+ T lymphocytes. On the presumption that CD4+ T lymphocytes may also be able to modulate HIV-1 expression in vitro we assessed the LTR-modulatory effects of a panel of culture supernatants derived from stimulated CD4+ T lymphocytes from HIV-positive patients and uninfected controls. Supernatants of both CD4+ and CD8+ T cells mediated a suppression of LTR-driven gene expression in Jurkat T cells and an enhancement of gene expression in U38 monocytic cells. On the basis of these results, and using a herpesvirus saimiri (HVS)-transformed CD4+ T lymphocyte clone (HVSCD4), we demonstrate that both suppressive and enhancing effects are dose dependent. Furthermore, we have shown that supernatants of both HVSCD4 and HVSCD8 cells suppress LTR-mediated gene expression and HIV-1 replication in transfected/infected T cells. In U1 monocytic cells, supernatants of both CD4+ and CD8+ lymphocytes from an HIV-1-infected individual enhanced LTR-mediated gene expression, HIV-1 replication, and TNF-alpha production. However, only these effects as induced by CD8+ T cells were sensitive to the G protein inhibitor pertussis toxin. These results indicate that factors produced by both CD4+ and CD8+ T cells exert dichotomous effects on HIV-1 gene expression and replication in T cells and monocytes.