3.5
TITLE: THE FAS:FASL LYTIC PATHWAY IS INHIBITED BY HLA CLASS I VIA A MECHANISM INVOLVING KIR RECEPTORS

Zacharie Brahmi, Hui Lin Chua.

Medicine and Microbiology–Immunology, Indiana University School of Medicine, Indianapolis, IN

The down–regulation of natural killer (NK) cells cytotoxicity by HLA Class I–specific inhibitory receptors is now firmly established. The Class I inhibitory receptors are classified as C–type and immunoglobulin–like inhibitory receptors (KIR), each recognizing a unique Class I ligand. Most studies to date, however, have concentrated only on the inhibition of cytotoxicity mediated by the perforin/granzyme pathway. To our knowledge, the effect of HLA Class I on the NK Fas:FasL pathway has not been examined. Because perforin, granzyme B and FasL are stored within the same granules, we thought it was important to establish if HLA Class I also regulates Fas:FasL lytic activity mediated by NK cells. As effector cells we used a transfectant (YT/C143), NK cells and LAK cells. In each case, Fas:FasL activity was inhibited by HLA Class I–expressing targets but not by the negative controls. Next, we wanted to determine the mechanism that led to this inhibition. We stimulated NK cells with 2B4 and LFA1, 2 novel, potent upragulators of FasL surface expression and of FasL mRNA, and we studied these 2 markers using real time PCR. Surprisingly, the urpregulation of FasL was inbited by KIR but not by CD94/NKG2A, comfirming that these pathways are differently regulated. %Thus: (1) the peforin/granzyme as well as the Fas:FasL pathways are inhibited by HLA Class I, indicating that tumor cells expressing HLA Class I can thrive even in the presence of fully functional NK cells; (2) in addition to the activation of FcRgIIIA (CD16) receptor , we have shown for the first time that the simultaneous activation of 2B4 and LFA1 receptors leads to activation–induced cell death (AICD) of NK cells.