Changes in the population of lymphocytes and their response to mitogens during the growth of a Simian virus 40‐induced fibrosarcoma in hamsters

Changes have been studied occurring in spleen and thymus lymphocytes, as well in their response to different mitogens, during the growth of Simian virus 40‐induced fibrosarcomas in hamsters. During tumor growth, the spleens of tumor‐bearing animals became considerably enlarged, and their content in lymphoid cells increased by 100%. Spleen lymphocytes responded normally or slightly higher than did normal controls to concanavalin A (Con A) stimulation when tumors were small, but their response gradually decreased to 30–40% of normal in animals bearing large tumors. These same cells, when cultured with normal lymphocytes in the presence of Con A, were without effect, or were mildly suppressive of the normal cellular response to Con A. The response to Iipopoly‐saccharide (LPS) decreased slightly. The thymus, on the other hand, became smaller and depleted of cells, and here too, the response to Con A gradually diminished. Macrophages were found in increased numbers in the enlarged spleens of tumor‐bearing animals. It is suggested that they played a role in the suppression of lymphocyte response to mitogens. Elimination of macrophages from cultures by the addition of carageenan resulted in an enhanced response of lymphocytes from tumor‐bearing animals to Con A. In addition, we found that carrageenan acts as a mitogen, most probably for B cells, and that when added to cultures containing LPS, a potentiation of the mitogenic response to LPS was obtained.