Production by murine spleen cells of an activity stimulating the PHA-responsiveness of thymus lymphocytes

Abstract

An increase in thymidine uptake was found when murine thymus cells were cultured together with spleen cells in the presence of phytohemagglutinin (PHA). The results of experiments with γ-irradiated and mitomycin C-treated cells indicated that this effect was due to enhancement of the PHA responsiveness of thymus cells by spleen cells (Thymocyte Stimulating Activity: TSA). TSA was found to be present in supernatants of cultures of spleen cells but not of bone marrow or thymus cells from normal or hydrocortisone-treated mice. Some TSA were present in cultures of lymph node cells. TSA did not enhance the PHA responsiveness of spleen or lymph node cells. The PHA-responsiveness of bone marrow cells was stimulated only to a small degree. PHA was a necessary requirement for the production of TSA. TSA was not mitogenic for thymus cells in the absence of PHA. The absence of adherent cells did not prevent production of TSA.