The purpose of this proposal is to characterize interactions between autologous lymphocytes and macrophages in the absence of antigen. Our hypothesis is that autologous stimulation to T cells by only the anti-Ia portion of the T cell receptor is an important homeostatic regulatory mechanism that results in: 1) maintenance of the T cell repertoire by low level polyclonal stimulation; and 2) a population of "primed" macrophages that have received the first signal for activation and only require a second signal (ng amounts of LPS, for example) for full bactericidal and tumoricidal activity. Preliminary experiments indicate that co-culture of resident peritoneal macrophages and autologous Thy 1 positive, L3T4+ positive T cells results in increased phagocytic activity by the macrophages as measured by uptake of colloidal carbon. This response is abolished in the presence of anti-Ia. The experiments described below are designed to define the conditions required for priming of macrophages by autologous interactions, to study the cellular mechanisms of the activation, to initiate studies of the function of macrophages activated by autologous stimulation and to determine if the T cell repertoire is lost more rapidly in the absence of autologous stimulation. Using appropriate monoclonal antibodies, the contribution of Ia and of L3T4 to the autologous interaction of macrophages and T cells will be evaluated. In addition, congenic mice will be used to determine if the recognition by T cells is clonally distributed. The requirement for cell contact will be evaluated in studies of the morphology of co-cultures under activating or non-activating conditions. A role for cytokines in syngeneic activation of macrophages will be evaluated in diffusion chamber experiments and by testing supernatent fluids from the co-cultures for the ability to activate resident peritoneal macrophages. The state of activation of macrophages stimulated by the autologous co- cultures will be evaluated by measuring their ability to kill bacteria or tumors in the presence or absence of a second activating signal, LPS. Finally, maintenance of the T cell repertoire will be evaluated in F1 into P radiation chimeras by comparing the response of the F1 T cells to antigen-pulsed recipient parental macrophages with their response to antigen- pulsed macrophages from the other parent.