Thymocyte-stimulating factor from peripheral blood cells.

Human peripheral blood leukocytes (HPBL) produce a thymocyte-stimulating factor (TSF-HPBL) that enhances the phytohemagglutinin (PHA) and concanavalin A (Con A) responsiveness of murine thymocytes. This activity is considerably specific for thymocytes. TSF-HPBL is not mitogenic by itself. Experiments with cell cultures pretreated with carbonyl iron particles showed that phagocytic cells are not involved in the production of mouse and rat TSF but are involved in the production of TSF-HPBL. The dose-response profile to PHA of murine thymocytes cultured in the presence of TSF-containing supernatants is similar to that of mature, immunocompetent spleen cells. TSF-HPBL, however, does not enhance the PHA responsiveness of murine thymocytes at low (<0.25 μg/microwell) concentrations of mitogen. TSF enhances the PHA and Con A responsiveness of the high-density subpopulations of thymocytes isolated on a Ficoll-Hypaque gradient. In general, the enhancing effect of TSF-HPBL on these subpopulations of thymocytes is smaller than that exerted by TSF. While supernatants containing TSF confer to thymocytes the ability to participate in a mixed lymphocyte reaction, this effect is not exerted by supernatants containing TSF-HPBL. A factor enhancing the PHA and Con A responsiveness of murine thymocytes is also produced by murine peripheral blood leukocytes (TSF-MPBL). This factor, similarly to TSF-HPBL, is produced by phagocytic cells and does not confer to murine thymocytes the ability to participate in a mixed lymphocyte reaction. Human T-cell lines do not enhance the PHA or Con A responsiveness of murine thymocytes. TSF-HPBL has a molecular weight of about 30,000 daltons, as measured by Sephadex filtration. Its half-time of inactivation as 56 °C is 162 ± 8 min.