The xenogeneic effect. III. Induction of cell-mediated cytotoxicity by alloantigen-stimulated thymocytes in the presence of xenogeneic reconstitution factor.

A supernatant from human mixed leukocyte cultures (MLC), Xenogeneic Reconstitution Factor (XRF), was added to one-way murine MLC. This supernatant greatly enhances the generation of cytotoxic cells from C57BL/6 or DBA/2 responder thymocytes, as assayed by a standard 51Cr-release assay. This enhancement is shown 1) to be dependent upon the presence of BDF1 semi-allogeneic stimulator cells and 2) to result in the generation of cytotoxic cells that are specific for the H-2 type of the stimulating alloantigen. In some experiments, alloantigen-stimulated murine spleen cells, when cultured with XRF showed increased induction of cell-mediated cytotoxicity. These experiments show that XRF contains a cytotoxic-cell activating factor(s) that is functionally similar to those found in murine-derived supernatants, and that this factor can participate in the generation of cytotoxic cells from precursors present in the murine thymus.