Macrophage-lymphocyte interactions mediated by soluble factors.

Adherent-cell-depleted primed rabbit splenocytes were capable of mounting an in vitro response to SRBC. The addition of alveolar macrophages (AM) to adherent-cell-depleted or unseparated lymphoid cell populations resulted in significant suppression of the PFC response. Suppressive activity was limited to AM and dependent on the presence of a ratio of 1 AM:20 lymphocytes. The cell-mediating suppression was found to be resistant to irradiation and antithymus globulin but sensitive to heat, freeze-thawing, and treatment with iodoacetamide. Suppression was mediated by a soluble factor (MW greater than 10,000 daltons) that required an AM-lymphocyte interaction for its production. Suppression appeared to be achieved through the inhibition of proliferation of antigen sensitive cells although the effect of AM could possibly be exerted on an early event in the immune response. AM were capable of enhancing the proliferative responses of rabbit lymphoid cells to PHA and Con A. Enhancing and suppressing activities of AM were abolished by inhibition of RNA synthesis but unaffected by inhibitors of DNA and protein synthesis.