THE ROLE OF DIFFERENTIAL CLASS II ANTIGEN EXPRESSION IN STIMULATION OF ALLOGENEIC MIXED LYMPHOCYTE REACTIONS BY HUMAN MONOCYTE HYBRIDOMAS

Abstract

The recognition of foreign class II antigens on accessory cells is the crux of an alloreactive immune response. This phenomenon is clearly demonstrated in the primary mixed lymphocyte reaction, which correlates with the type and density of expressed gene products of the HLA-D region. We have generated a series of human monocyte hybridomas by fusing monocytes with the hypoxanthine guanine phosphoribosyl transferase (HGPRT)-deficient, HLA-D antigen-negative U937 histiocytic cell line. Clones bearing combination of HLA-DQ, -DP with or without HLA-DR have been isolated, allowing for the functional assessment of these molecules. In contrast to the U937 cells, the HLA-DR+DQ+DP+ clone 16.1 was capable of stimulating a primary allogeneic MLR. Interestingly, the DR- but DP+DQ+ clones 13 and 15 were also capable of stimulating alloreactive T cells, and the addition of anti-DQ or -DP but not -DR was associated with significant inhibition of the MLR response. Furthermore, gamma-IFN was found to have diverse effects on class II antigen expression in the U937 cells and the hybrids. gamma-IFN down-regulated the expression of HLA-DQ, -DP without altering -DR on clone 16.1, and this was associated with a significant reduction in its MLR stimulatory capacity. The MLR generated by this gamma-IFN-stimulated hybridoma (HLA-DR+DQ-DP-) was now unaffected by the addition of anti-DQ or -DP mAbs. In contrast, up-regulation of DQ and DP antigens on the U937 cells by gamma-IFN now rendered these cells stimulatory in MLR. These data are consistent with the concept that DQ and DP are both important allostimulatory determinants. Our results stress the potential importance of all D-region molecules in acute allograft rejection or successful engraftment.