I‐region‐associated determinants: expression on mitogen‐stimulated lymphocytes and detection by cytotoxic T cells

We have studied the expression of Ia‐antigens, controlled by genes in the I‐region of the H‐2 complex, on phytohemagglutinin (PHA)‐stimulated lymph node cells and on lipopolysaccharide (LPS)‐stimulated spleen cells, and have compared these two types of cell populations as targets for killer cells in the cell mediated lympholysis (CML) assay. PHA targets are almost completely insensitive to complement‐mediated lysis by anti‐Ia sera while the majority of LPS targets are killed. T cell‐mediated lysis against I‐region determinants was also detected, and these determinants, in contrast to H‐2K and H‐2D CML determinants, seem to be much more strongly expressed on LPS‐stimulated cells. No differences in the kinetics of the response to K‐or I‐region CML determinants were observed. Lysis by killer cells can also be obtained against incompatibilities which do not give rise to strong skin graft rejection and against determinants which are most probably controlled by genes outside the K‐D interval.