Accessory cell function of rabbit alveolar macrophages

Alveolar macrophages (AM) are the first cells to contact inhaled antigen. Accessory cell function of AM would modulate the immunologic response to antigenic exposure. We sought to examine the capability of AM to act as accessory cells for proliferation of lymphocytes using conditions that are best suited for expression of this function. We used rabbit popliteal lymph node cells, both before and after depletion of macrophages by 2 successive Sephadex G-10 columns that decreased nonspecific esterase positive cells from 2.56 to < 0.1%, and abolished or markedly decreased concanavalin A-induced lymphocyte proliferation. AM were lavaged from rabbit lungs and treated with mitomycin C. We found that AM enhanced proliferation of 5 x 10(4) and 7 x 10(4) unseparated, lymph node cells 5- to 15-fold, but had no effect on 2 x 10(5) cells. The optimal ratio of macrophages to lymphocytes (AM:L) was between 1:2 and 2:1. An AM:L value of 4:1 suppressed proliferation. The addition of AM to macrophage-depleted lymph node cells enhanced proliferation of 5 x 10(4), 7 x 10(4), and 2 x 10(5) lymphocytes 15- to 50-fold (AM:L, 1:2 to 2:1). Excessive numbers of macrophages (AM:L, 4:1) suppressed proliferation 5- to 50-fold. Rabbit AM can act as accessory cells in assays of mitogen-induced lymphocyte proliferation with maximal effect at AM:L values of 1:2 to 2:1. Suppression can occur as a result of excessive numbers of macrophages.