Inhibition of human anti-pig t cell responses by monoclonal antibodies to porcine cd45 molecules

P1057 Background: Targeting of pig cell surface molecules by monoclonal antibodies (mAb) offers the possibility of an organ-specific immunomodulation after pig to human xenotransplantation. We have previously shown that mAb directed to porcine CD45 molecules inhibit human anti-pig T cell responses in vitro without affecting T cell activation induced by alloantigen or mitogens. Aims: In the present study we analyzed the immunomodulatory potential of mAb directed to different isoforms of the porcine CD45 molecule. Furthermore, to elucidate the underlying mechanisms of CD45-mediated immunomodulation cellular and biochemical consequences were analyzed after binding of CD45 mAb to pig cells. Results: In the absence of CD45 mAb porcine stimulatory cells induced significant activation of human T cells as shown by proliferation, upregulation of the IL-2 receptor (CD25), and cytokine production. In the presence of mAb to porcine CD45RA isoforms human T cell responses were reduced by 40 - 70 %. The inhibitory effects were less pronounced when CD45 common mAb were used (10 – 25 %). MAb directed to CD45RC isoforms did not significantly influence the capacity of pig lymphocytes to induce human T cell activation. Binding of CD45 mAb changed the tyrosine phosphorylation pattern as detected by Western-Blot analysis of whole cell lysates of porcine stimulatory cells. Furthermore, treatment of porcine stimulatory cells with CD45 mAb induced strong cellular aggregation. Conclusions: Inhibition of human anti-pig T cell responses by porcine CD45 mAb may result from the induction of signals which change the adhesion properties of pig stimulatory cells. Alterations in adhesion could influence their capacity to interact with human T cells. Since CD45 molecules are not expressed on endothelial cells the usage of anti-pig CD45 mAb might be an approach to block the direct pathway of T cell activation initiated by porcine APC without affecting the endothelium of a graft. Supported by the Deutsche Forschungsgemeinschaft, FOR 535.