Chronic lymphocytic leukemia (CLL) is characterized by the accumulation of noncycling B cells in lymphatic and extralymphatic tissues. In the present study we investigated the possible contribution of TGF-beta, as secreted by CLL-B cells, on this low proliferative state. CLL-B cells were shown to express TGF-beta RNA and to release bioactive TGF-beta into culture supernatants. Antibody neutralization of endogenously secreted TGF-beta increased the proliferation of CLL-B cells as cultured in the presence of IL-2 or IL-4 or in direct contact with activated CD4+ T cells. In these culture systems, addition of exogenous TGF-beta downregulated basal and cytokineinduced proliferation of CLL-B cells. In contrast, neither neutralization of endogeneous TGF-beta, nor addition of exogeneous TGF-beta changed the proliferation of CLL-B cells as cultured in the CD40 system. In order to further explore this differential antiproliferative effect of TGF-beta, cytokine secretion of B cells and of CD4+ T cells as well as surface marker expression of CD4+ T cells were assessed in relation to TGF-beta: There was no negative effect of TGF-beta on autocrine secretion of TNF-alpha or sCD23 by CLL-B cells. Unlike tonsillar B cells, CLL-B cells cultured alone or in the CD40 system did no release significant amounts of IL-6 or IL-8 into supernatants. Secretion of IL-2 or IL-4 by activated CD4+ T cells was higher, when T cells were cocultured with normal tonsillar B cells than with CLL-B cells. The amount of IL-2 or IL-4 released by CD4+ T cells cocultured in direct contact with tonsillar or CLL-B cells was not consistently influenced either by neutralization of endogenous TGF-beta or by addition of TGF-beta. Exogenous TGF-beta did not downregulate expression of CD40L, CD27, CD28, CD54 or mTNF-alpha by T helper cells activated with anti-CD3 or PHA. In conclusion, autocrine secretion of TGF-beta exhibits an antiproliferative effect on CLL-B cells. This effect is most relevant in B cells cultured in direct contact with activated CD4+ T cells suggesting an indirect mode of action.