CD4 is involved in a post-binding event in the cytolytic reaction mediated by human CD4+ cytotoxic T lymphocyte clones

CTL lyse their target cells in discrete phases. First, the CTL bind to the target cell in a Mg2+-dependent manner followed by a Ca2+-dependent cytolytic phase. In the present study, we investigated the role of CD4 in the different phases of the cytolytic reaction mediated by human CD4+ class II MHC-specific CTL clones by using a single cell assay. It was found that the anti-CD4+ mAb OKT4A, which blocks cytotoxic reactions by CD4+ CTL clones as measured with a 51Cr release assay, only marginally affects the formation of conjugates. It appeared that OKT4A more strongly blocked the post-binding phase of the cytolytic reaction. In contrast, anti-leukocyte function-associated mAb strongly blocked the formation of conjugates but not the subsequent lytic phase. As was found previously with CD8+ CTL clones, anti-TCR mAb generally did not affect the formation of conjugates. One exception was noted. The activity of a CD4+ CTL clone, HY-640, could not be blocked by OKT4A, but was affected by an anti-TCR mAb. This anti-TCR mAb could partly reduce the formation of conjugates between HY-640 cells and their specific target cells. These results suggest that this clone has a high affinity TCR, which can contribute to the formation of conjugates. Although preincubation of the CTL clones with OKT4A only marginally affects the number of conjugates upon subsequent mixture with target cells, it was observed that incubation at 37 degrees C of preformed conjugates with OKT4A markedly reduced the number of conjugates. This dissociation of preformed conjugates was optimal only after 2 h of incubation. In contrast, an anti-leukocyte function-associated mAb induced a much more rapid dissociation of preformed conjugates