- Previous studies suggested that peripheral blood monocytes (Mo) from HIV-infected patients were poor accessory cells (AC), although most of these studies were limited by using autologous T cells as responders. Using allogeneic T cells from uninfected volunteers as responders, the current studies demonstrate that Mo from infected individuals were comparable to Mo from uninfected volunteers as AC in Con A and pokeweed mitogen-stimulated lymphocyte proliferation assays, but were inferior to normal Mo in stimulating a mixed leukocyte reaction. This deficiency was not explained by HIV Mo-induced suppression of lymphoproliferation or by death of responding CD4 lymphocytes induced by HIV transmission from infected Mo in 6-day MLR cultures. Mo from HIV-infected patients retained the ability to stimulate mumps-specific T cell lines in response to antigen, demonstrating that Mo from these individuals could process and display antigen on their cell surface in association with a functional DR molecule. Taken together these results suggest that Mo from HIV-infected patients (i) retain the ability to act as AC in T cell responses to mitogenic signals or to stimulate already activated antigen-specific T cells, but (ii) fail to stimulate resting and/or unprimed T cells in response to alloantigen and perhaps de novo antigen exposure. It is possible this Mo defect may have an adverse effect on the immune responsiveness of HIV-infected individuals.