Effect of normal mouse serum on mouse lymphocyte transformation in vitro

Mouse spleen cells were cultured in the presence or absence of one of the following mitogens: phytohemagglutinin (PHA), concanavalin A, allogeneic spleen cells or bacterial lipopolysaccharide. The addition of normal mouse serum (NMS) to the cultures usually depressed mitogen‐induced lymphocyte transformation as measured by DNA, RNA or protein synthesis, whether calculated as gross or net synthesis. The degree of depression was increased as the concentration of NMS serum increased. DNA and RNA synthesis in unstimulated cultures were also usually depressed by NMS. NMS added 24 hours or more after the start of cultures with PHA or allogeneic lymphocytes was less depressive than serum present from the start and in some cases, actually increased DNA synthesis. The presence of NMS in cultures containing supra‐optimal doses of concanavalin A could also increase DNA synthesis. Cells incubated with NMS for 24 hours survived as well as cells incubated without NMS, but subsequently responded less well to PHA. Cells passed through a column of glass wool had a lower baseline DNA synthesis, which was not inhibited by NMS; they responded less well to mitogens and this response was further depressed by NMS. It is suggested that NMS contains a factor which damps the proliferation of mouse T and B cells, without exerting an overt cytotoxic effect, and that it acts directly on lymphocytes.