Cytotoxic T lymphocytes specific for I region determinants do not require interactions with H-2K or d gene products*

CTL specific for determinants coded for by the I region of the H-2 complex can be generated in vitro by sensitization with cells carrying allogeneic I region determinants. The detection of/region specific CTL is dependent upon testing target cells which express sufficient levels of these determinants, thus we employed LPS blasts as target cells. When analyzing allogeneically induced CTL, the clones reactive with K and D region products greatly exceed those generated to Ia antigens. It has been demonstrated by genetic analysis and by antibody inhibition that I region determinants are recognized independently of H-2K and H-2D gene products. CTL directed to the I region determinants lyse all targets bearing the appropriate Ia antigens even if the target cells differ at H-2K and/or D from the stimulator. An antiserum specific for I region antigens significantly inhibits target lysis by CTL, while addition of specific antiserum directed to the H-2D d products of the target has little or no affect on the ability of /-specific CTL to lyse appropriate Ia bearing targets. Ia antigens co-cap, redistribute, and immunoprecipitate independently of K and D molecules (17). It now appears that functionally as well, Ia antigens act separately from K and D antigens in their ability to generate specific CTL. Since by all biochemical and functional criteria these Ia antigens are not associated with K and D coded molecules, they appear to represent the products of a third transplantation locus in the mouse MHC as has been suggested by the graft rejection studies of Klein et al. (10, 12). In light of the findings of Bevan (8) and Gordon, et al. (9), that CTL stimulated by antigens coded for by minor histocompatibility loci or the H-Y locus require presentation of these antigens on targets syngeneic at H-2K or D to the responders, I region determinants clearly do not act as other minor histocompatibility loci in this respect. The H-2 complex thus codes for at least three molecules which can act as independent CTL stimulating antigens. Viruses, tumor-specific transplantation antigens, chemicals, and minor H antigens appear to behave as modifiers of H-2K and D determinants, making them recognizable to syngeneic CTL. Studies are now in progress to test whether syngeneic I region determinants can also be chemically altered and stimulate CTL in systems which have shown such modifications and activities for K and D molecules. Whether there exists a special regulatory role for the relatively few I region reacting clones remains unclear. Within the I region, there are genes which regulate many immune responses. The existence of a small number of clones of lymphocytes with receptors which are committed to recognizing I region products independently of H-2K and D products, is suggestive of a possible regulatory role for these cells.