It should further be noted that beside help, CD4+ T cells might also directly contribute, by nonperforin nongranzymes pathways, to skin rejection as shown in the anti-HY TCR-transgenic model [[26, 27]]. Such direct participation would account for the fact that depletion of DBA/2 mHfe KO mice in CD4+ T cells resulted in more complete graft protection than depletion in CD8+ T cells. That other MHC class Ib molecules could directly stimulate αβ T lymphocytes and behave autonomously as transplantation antigens has been shown with TL-transgenic mice

[[28]]. However, the TL-encoding transgene T3b was placed under the control of an H-2 MHC class MG132 Ia promoter and, consequently, tissue expression of TL was considerably broadened. Thus, all MHC class Ib molecules might have the intrinsic potential to behave as autonomous histocompatibility antigens. However, this potential should be modulated by the molecular topology of their polymorphic or mutated residues, their tissue distribution and the ICG-001 mw level of their cell surface expression. Could other mutated forms of HFE also behave as autonomous histocompatibility antigens? There are two other frequent mutated forms of human HFE molecules (H63D, S65C) that

are associated with human hereditary hemochromatosis, albeit loosely [[29, 30]]. However, unlike the C282Y mutated molecule, these variant forms of HFE are cell-surface expressed [[31, 32]]. Furthermore, the H63 and S65 mutated residues are part of an external loop joining two β strands of the floor of the HFE groove and are distant from the area (top of MHC α helices and aa of the presented peptide) of conventional MHC class Ia molecules contacted by αβ TCRs [[33]]. Assuming that MHC class Ib molecules are similarly contacted by αβ TCRs, it seems unlikely that these structural differences of HFE would, at least directly, stimulate conventional T lymphocytes. Considering the rapidity with which mHFE+ skin grafts were rejected by anti-mHFE TCR-transgenic

mice (whether mHfe KO or mutated) and the efficacy with which anti-mHFE TCR-transgenic CD8+ T Fenbendazole cells differentiated in CTL when in vitro stimulated, without CD4+ T cell help in both cases, the absence of GVHD following injection of a large number of anti-mHFE TCR-transgenic CD8+ T cells in Rag 2 KO DBA/2 mHFE+ mice was surprising. However, a similar observation has been reported in the anti-HY TCR transgenic model, where the transferred T cells in male recipient mice, following transient activation, disappeared after a few days [[34]]. In the present anti-mHFE TCR-system, disappearance is even more rapid, suggesting that the anti-mHFE CD8+ T cells are eliminated through apoptosis.