expression of appropriatge human MHC has recently been reported to afford significant protection against human NK cell mediated lysis.

In vivo analysis of the human anti-pig cellular immune response has been hampered by the lack of suitable models. A limited number of studies using nonhuman primates as recipients of neovascularized islet xenografts have been reported. Cellular rejection of these porcine islets appears to be a T-cell dependent process, also characterized by striking numbers of eosinophils and macrophages in cellular infiltrates. Other investigators have turned to the use of immunodeficient rodents reconstituted with human cells to study the human anti-pig response. Although these models indicate that hu-scid or hu-RAG-1 deficient mice can mount immune responses against porcine islets or skin xenografts, described defects in the T-cell repertoire of reconstituted immunodeficient mice raise questions regarding the applicability and relevance of observations in these models.

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