Should an anti-complement agent prove necessary, e.g., if acute humoral xenograft rejection develops, then early evidence from allotransplantation suggests that the monoclonal antibody, eculizumab, might prove successful (52,53). An unexpected bonus of these genetic manipulations is the observation that the absence of expression of Gal and the expression of a hCRP CB2R-IN-1 renders the T cell response to the graft weaker. reduced by the presence of a graft from a pig transgenic for a mutant (human) class II transactivator gene, resulting in downregulation of SLA class II expression, or from a pig with local vascular endothelial cell expression of an immunosuppressive gene, e.g., CTLA4-Ig. The immunomodulatory efficacy of regulatory T cells or mesenchymal stromal cells has been demonstrated 2012; 19:311-316 [33] with permission.) Open in a separate window Figure 2 Thrombin does not upregulate SLA class I or II expression on GTKO porcine aortic endothelial cells (pAEC)GTKO pAEC were activated using thrombin (40U/mL), pIFN- (40U/mL), or hIFN- (200U/mL) for 24h. SLA class II expression was upregulated only after pIFN- activation, but not after thrombin or hIFN- activation. There was no change in SLA class I expression after activation. Data are representative of three different experiments. (Reproduced from Ezzelarab C, et al, 2012; 19:311-316 [33] with permission). There is also an inflammatory response to the graft, which may also contribute towards the xenoreactive immune response appearing stronger. There are data that indicate that there is considerable cross-talk between the innate and adaptive responses and between those responses CB2R-IN-1 and the factors responsible for coagulation dysfunction and inflammation (34,35). Together, these observations indicate that the immune response to a pig xenograft cannot be considered in isolation, and will not be controlled simply by suppression of T cell activity (as is generally possible in allotransplantation). Equal attention needs to be directed to the innate immune, coagulation, and inflammatory responses. (Of interest, a process by which an innate immune response is induced by the formation of thrombi inside blood vessels C in pathologic conditions unassociated with xenotransplantation – has recently been recognized and termed immunothrombosis [36].) Suppression of the immune responses CACN2 The prevention or reduction of the innate immune response has been approached by the genetic engineering of the organ-source pig rather than by the administration of immunosuppressive agents, which are largely ineffective. In this respect, the transgenic expression of one or more human complement-regulatory proteins (hCRPs), e.g., CD46 (hMCP), CD55 (hDAF), CD59, contributes significant protection (37). Similarly, the introduction of pigs in which the gene for 1,3-galactosyltransferase has been deleted (GTKO pigs) (38-40), thus preventing the expression of the important galactose-1,3-galactose (Gal) antigen, which is the major target for primate anti-pig antibodies (41,42), was a major advance (29,43). GTKO pigs expressing one or more hCRPs provide increased protection than either manipulation alone (44,45). Building on this genetic background, an increasing number of genetically-engineered pigs are becoming available for transplantation studies in NHPs (reviewed by Ekser 2012 [2]). Although various techniques, e.g., plasmapheresis, extracorporeal immunoadsorption (46), the administration of natural or synthetic oligosaccharides (28,47,48) that prevent anti-Gal antibody binding to the graft, proved valuable in early studies, the value of these has largely been negated by the availability of GTKO pigs. Similarly, although cobra venom factor and other anti-complement agents were administered successfully (49,50), these are no longer necessary when the organ-source pig expresses one or more hCRP. Indeed, there are reasons to believe that cobra venom factor could be detrimental in some respects, as it can result in the release of the anaphylatoxin C5a, which contributes to innate and adaptive immune responses (51). Should an anti-complement agent prove necessary, e.g., if acute humoral xenograft rejection develops, then early evidence from allotransplantation suggests that the monoclonal antibody, eculizumab, might prove successful (52,53). An unexpected bonus of these genetic manipulations is the observation that the absence of expression of Gal and the expression of a hCRP renders the T cell response to the graft weaker. Studies by Wilhite et al have demonstrated a significant reduction in the primate mixed lymphocyte reaction to pig ECs when Gal is absent or an hCRP is expressed (54,55) (Figure 3). Although there CB2R-IN-1 is a definite reduction in the human T cell proliferative response to GTKO/hCRP pig cells in vitro, it is.
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