Antigen-presenting capabilities of human monocytes correlates with their expression of HLA-DS, an Ia determinant distinct from HLA-DR.

Utilizing a monoclonal antibody (Mac-120) specific for 40 to 60% of peripheral blood adherent mononuclear cells (M phi), we were able to separate M phi into two populations based on their reactivity with the antibody. Both populations, Mac-120+ and Mac-120- cells, were then compared for a) their ability to present antigen to T cells, b) their display of HLA-DR determinants, c) their ability to stimulate in an autologous and allogeneic mixed lymphocyte reaction, and d) their display of an Ia molecule, HLA-DS, which is distinct from HLA-DR and which is homologous with murine I-A. Our findings indicate that a) only Mac-120+ cells can present antigen, b) Mac-120+ and Mac-120- cell populations are equivalent in terms of the number of HLA-DR+ cells and in the mean density of HLA-DR determinants per cell, c) although Mac-120+ and Mac-120- cells are equivalent in their ability to serve as stimulators in an allogeneic mixed lymphocyte reaction, Mac-120+ cells are better stimulators in an autologous mixed lymphocyte reaction, and d) only Mac-120+ cells display HLA-DS. These studies demonstrate that peripheral adherent mononuclear cells exhibit heterogeneity with regard to their display of Ia antigens. Furthermore, they provide functional data to support the existence of a human Ia determinant, HLA-DS, which is distinct from HLA-DR and which is important in antigen presentation and stimulation in the autologous mixed lymphocyte reaction.