Induction of apoptosis in B lineage cells by activin A derived from macrophages.

A factor produced by P388D1 cell line murine macrophages showed a profound suppressive effect on the in vitro proliferation of B lineage cells. It was purified to homogeneity from conditioned media of P388D1 cells stimulated with phorbol 12-myristate 13-acetate for 48 h by a three-step procedure. The purified factor gave a single band of protein with a molecular mass of 16 kD on SDS-polyacrylamide gel electrophoresis. We show here that exposure of B lineage cells to this factor results in the induction of a cytotoxic effect and a significant increase in the proportion of fragmented DNA. DNA fragmentation was detected in B lineage cells after 3 h culture with the factor in the quantitative colorimetric determination. The mechanism of cell death was characterized by a ladder-like electrophoretic pattern of degraded chromosomal DNA, indicating that the factor induces apoptosis. The NH2-terminal amino acid sequence of this factor was identical with that of activin A over the 26 amino acid residues identified. We sought to determine whether apoptosis could be modulated by two kinds of inhibitor of protein kinases, H7 and HA1004, in concentrations that are below their toxicity limits. Apoptosis induced by the factor was suppressed by H7 but was relatively unaffected by HA1004. These findings suggest that the signals by protein kinases may regulate apoptotic B cell death by the factor activin A, derived from macrophages.