Release of mitogenic factor by mouse lymph node cells stimulated with PHA in vitro. Inhibition of this phenomenon by the addition of thymocytes.

Thymidine uptakes of mouse lymph node cells and thymocytes in response to phytomitogen lectins were investigated in vitro. It was observed that mixtures of lymph node cells and thymocytes yielded higher 3H-TdR incorporations than expected when exposed to concanavalin A (Con A) or pokeweed mitogen (PWM), but lower incorporations than anticipated with phytohemagglutinin (PHA) as a stimulant. The observed enhancement of the response could be explained by factors which are stimulatory for thymocytes released by the lymph node cells. However, such mitogenic factors (MF) were released not only by Con A and PWM exposed cells but also by PHA expose lymph node cells. Further experiments showed that the admixture of thymocytes inhibited the release of MF by lymph node cells exposed to PHA but not to Con A or PWM. Since medullary thymocytes did not exhibit any inhibitory activity, it is likely that cortical thymocytes are responsible for this effect. The mechanism by which thymocytes inhibit MF production of lymph node cells exposed to PHA is not clear. MF was efficiently produced by lymph node cells cultured with PHA in "thymocyte-conditioned" medium and the MF activity was only marginally decreased by absorption with thymocytes. The results are discussed in the light of recent findings showing that thymocytes may both enhance and depress the immune response of other lymphocytes.