SHORT COMMUNICATION Mixed Lymphocyte Culture Reactivity and H-2 Histocompatibility Loci Diff erencesl”

The relationship between the major histocompatibility systems as defined serologically and by the mixed leukocyte culture (MLC) test is one which has received attention in both man (1) and mouse (2). In man, studies within families suggest that cells of siblings who inherit the same HL-A chromosomes from their parents and thus are serologically identical at HL-A do not stimulate in the MLC test. Cells of siblings who inherit different HL-A chromosomes do stimulate. This, combined with the finding that cells of all of several hundred unrelated pairs stimulate in the MLC test, suggests that a single genetic system controls reactivity in MLC, that this system is highly polymorphic and either the same as that coding for the HL-A antigens or very closely linked to the HL-A (1). Similarly, differences at the H-2 system in the mouse result in MLC activation (2). The H-2 system in the mouse can be divided into two separate regions, H-2K and H-2D (3). Recently Rychlikova et al. (4) studied a series of mouse recombinant strains that differed only for the H-2D region or only for the H-2K region and other inbred strains that differed at both the H-2D and the H-2K regions. Their findings suggested that differences for H-2K alone or for both H-2K and H-2D resulted in MLC activation. Differences for the H-2D region alone. however, did not result in MLC activation in their study. We have repeated these studies using a number of recombinant strains tested in