Analysis of xenoreactivity in a transgenic mouse model: Evidence of indirect xenorecognition and involvement of antibody response

Background. The role of T cells in graft rejection in xenotransplantation is unclear. We have reported that skin from HLA-DR1 transgenic mice is rejected by control littermates and T cells from rejecting mice proliferate to donor cells but do not exert cytotoxicity (Transplantation, July, 1999). We are now analyzing the mechanisms responsible for xenoreactivity. Materials and Methods. Control littermates were immunized with HLA-DR1 transgenic spleen cells and tested for cytokines and antibodies production against donor cells. Results. T cells from immunized mice proliferate and produce IFN? in response to HLA-DR1 transgenic antigen presenting cells with a peak response at 45 days after immunization. However, T cells from immunized mice do not exert cytotoxicity against HLA-DR1 transgenic spleen cells. Indirect xenorecognition is showed by the finding that proliferation and IFN? production by T cells from mice immunized with HLA-DR1 transgenic cells is induced by antigen presenting cells from HLA-DR1 tg mice, but not by an HLA-DR1 Lymphoblastoid B cell line, is inhibited by mAbs to mouse MHC class II molecules and is induced by antigen presenting cells from control mice pulsed with purified HLA-DR1 molecules. IgM antibodies anti-HLA-DR1 are detected at 15 days after immunization, while IgG anti-HLA-DR1 peak at 30 days. Conclusions. An HLA-DR1 transgene induces a xenoresponse characterized by IFN? and antibody production but absence of specific cytotoxicity. Moreover, indirect xenorecognition of the HLA-DR1 transgene appears the main mechanism.