Addition of autologous alveolar or peritoneal macrophages to spleen cell suspensions in vitro led to an inhibition of DNA synthesis of control and antigen-stimulated cells. The inhibition was studied using alveolar macrophages and did not appear to depend on the release of a soluble factor, was serum independent, only occurred when both cell types were in intimate contact, and was not the result of consumption or degradation of essential medium ingredients. Neither sensitized macrophages nor polymorphs responded to secondary challenge by antigen, or treatment with phytohemagglutinin, in vitro. No inhibition of DNA synthesis resulted when autologous polymorphs were added to spleen cell cultures. Although homogenized alveolar macrophages were not inhibitory, attempts to obtain immunogenic material from macrophages preincubated with antigen were unsuccessful. Destruction of macrophages already present in the spleen cell suspensions by the addition of silica resulted in a small stimulation of DNA synthesis.

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