A monokine which binds to class II histocompatibility proteins.

There is a marked decrease in the viability, in vitro, of dense, immature rat thy mocytes over a 4 h incubation period. The addition of a monokine of relative molecular weight 36 000 derived from cultured rat peritoneal macrophages which had been previously stimulated with lipopolysaccharide prevented these cells from dying. The early release of this factor, together with preliminary results of its physical and functional properties suggest that it is distinct from the well characterized monokines. The macrophage factor was found to bind in competition with a monoclonal antibody directed against a common determinant on the la antigen complex, but also to bind non-competitively with a monoclonal antibody to a strain-specific epitope on Ia. Based on the results of experiments using a fractionated thymic cell model, it would appear that this monokine binds to the la protein complex on the cell surface of thymic epithelial cells, causing them in turn to release an activity which is responsible for the survival of thymocytes. [ABSTRACT FROM AUTHOR]