The present study characterizes accessory cell dependency of mitogenic responses to PHA and Con A of murine spleen, lymph node, and peripheral blood lymphocytes. Depletion of macrophages by carbonyl iron from peripheral blood and lymph node lymphocytes remarkably reduced their mitogenic response, whereas that of spleen cells after iron phagocytosis was occasionally reduced, but generally enhanced or not changed. However, the further purification of spleen cells by nylon-filtration always reduced the mitogenic response. The adherent but not phagocytic cells (NPAC), which were purified from the macrophage-depleted spleen cells by the adherent techniques, effectively restored the mitogenic response of highly purified spleen, as well as lymph node and peripheral blood lymphocytes, as did the purified peritoneal macrophages. These results indicate that the major accessory cells in lymph node and peripheral blood were phagocytic macrophages, while the spleen contained a population of adherent but not phagocytic cells which play a role as an accessory cell in the mitogenic response. The Con A response of purified lymphocytes either from spleen, lymph node, or peripheral blood was fully restored by addition of 1 or 2% peritoneal macrophages. However, when mixed with a larger number of macrophages, the mitogenic responses of spleen and lymph node cells did not further increase. Addition of more than 3% macrophages remarkably suppressed the response but had little effect on peripheral blood lymphocytes. Thus, the mitogenic response of spleen and lymph node cells appears to be suppressed by macrophages more easily than that of peripheral blood cells. The heterogeneity of accessory cells and different susceptibility to suppression by macrophages may account for many of the observed variations in mitogen responsiveness of different lymphoid tissues in mice.
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