T cell responses to multiple minor histocompatibility antigens are governed by the complex phenomenon of immunodominance, as demonstrated clearly in the generation of CTL in the C57BL/6By (B6) anti-BALB.B strain combination. Immunodominance has also been found in lethal graft-versus-host disease (GVHD) responses directed to BALB.B minor histocompatibility antigens, after transplantation of B6 T cells and T cell-depleted bone marrow to irradiated (825 cGy) recipients of either the BALB.B or CXB recombinant inbred strains. However, previous results indicated that the hierarchy of immunodominance in GVHD differed from that predicted from the in vitro CTL studies. Lethal GVHD was observed in BALB.B, CXBE, CXBI, and CXBJ recipients, but not in CXBG and CXBK recipients, the latter 2 strains expressing immunodominant antigens for CTL generation. A major hypothesis to account for these discordant observations is that GVHD reflected the activity of CD4+, but not CD8+, T cell subsets, in contrast to only the in vitro cytolytic potential of CD8+ T cells. Therefore, the current study was undertaken to assess the GVHD potential of both T cell subsets in the B6 → BALB.B, CXBE, CXBI, and CXBJ strain combinations and to analyze the early GVHD responses in the B6 → CXBG and CXBK strains. The results indicate that lethal GVHD responses in the B6 → BALB.B combination can be mediated by either CD4+ T cells or CD4-dependent CD8+ T cells; a similar observation was made with the B6 → CXBI strain combination. Lethal GVHD in the B6 → CXBE strain combination is mediated only by CD4-dependent CD8+ T cells, whereas GVHD in the B6 → CXBJ combination involves either CD4+ T cells alone or CD4-independent CD8+ T cells. In the B6 → CXBG and CXBK recipients, which do not develop lethal GVHD, the early phases of a GVHD response was detected with involvement of both CD4+ and CD8+ T cells. These results indicate that CD8+ T cells are active at some level in all of the strain combinations tested, that CD4+ T cells do not account for the GVHD immunodominant response in the CXBE recipients, and that the failure to obtain extensive clinical disease in the CXBG and CXBK strains is not due to a lack of a graft-versus-host response.
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