CD8+ T Cells Are Necessary for Recognition of Allelic, But Not Locus-Mismatched or Xeno-, HLA Class I Transplantation Antigens

Abstract

Although HLA transgenic mice (HLA TgM) could provide a powerful approach to investigate human MHC-specific T cell responsiveness, the extent to which these molecules are recognized by the mouse immune system remains unclear. We established TgM expressing HLA class I alleles A2, B7, or B27 in their fully native form (HLAnat) or as hybrid molecules (HLAhyb) of the HLA α1/α2 domains linked to the H-2Kb α3, transmembrane, and cytoplasmic domains (i.e., to maintain possible species-specific interactions). Comparison of each as xeno- (i.e., by non-TgM) vs allo- (i.e., by TgM carrying an alternate HLA allele) transplantation Ags revealed the following: 1) Although HLAhyb molecules induced stronger xeno-CD8+ T cell responses in vitro, additional effector mechanisms must be active in vivo because HLAnat skin grafts were rejected faster by non-TgM; 2) gene knockout recipients showed that xenorejection of HLAnat and, unexpectedly, HLAhyb grafts doesn’t depend on CD8+ or CD4+ T cells or B cells; 3) each HLAhyb strain developed tolerance to “self” but rejected allele- (-B27 vs -B7) and locus- (-B vs -A) mismatched grafts, the former requiring CD8+ T cells, the latter by CD8+ T cell-independent mechanisms. The finding that recognition of xeno-HLAhyb does not require CD8+ T cells while recognition of the identical molecule in a strictly allo context does, demonstrates an α1/α2 domain-dependent difference in effector mechanism(s). Furthermore, the CD8+ T cell-independence of locus-mismatched rejection suggests the degree of similarity between self and non-self α1/α2 determines the effector mechanism(s) activated. The HLA Tg model provides a unique approach to characterize these mechanisms and develop tolerance protocols in the context of human transplantation Ags.

Human MHC Class I Transgenic Mice Deficient for H2 Class I Expression Facilitate Identification and Characterization of New HLA Class I-Restricted Viral T Cell Epitopes

“…The HLA nat and HLA hyb class I Tg mouse strains, originally generated in (B6/SJL)F 2 , were backcrossed a minimum of 10 generations onto C57BL/6J (referred to as Tg HLA nat /B6 and HLA hyb /B6) and described in detail previously (15,16). C57BL/6J mice for breeding or experiments were from The Jackson Laboratory (Bar Harbor, ME).…”

“…C57BL/6J mice for breeding or experiments were from The Jackson Laboratory (Bar Harbor, ME). HLA Tg offspring were identified by tail DNA hybridization or by flow cytometry of PBLs (15,16). Although all Tg HLA class I molecules reacted with mAb B9.12.1, Tg HLA-B27 hyb , HLA-B27 nat , HLA-B7 hyb , and HLA-B7 nat could also be detected with mAb ME1, and Tg HLA-A2 hyb and A2 nat with mAb MA2.1 (15,16).…”

“…However, despite some efforts along these lines, the extent to which human MHC molecules function efficiently and with the identical specificity as in humans is not clear. Although some studies have suggested that fully native Tg HLA class I molecules are recognized poorly as restriction elements by Tg mouse T cells (11,12), other reports using apparently similar strains or additional engineered versions of this model suggest that Tg HLA molecules can be recognized by the mouse immune system essentially as alternate mouse H2 class I alleles (13)(14)(15)(16). It is possible that at least some of these discrepant results are due to allele-specific differences in the function of human MHC molecules in the mouse background.…”

“…To attempt to reduce this complexity with respect to HLA function and recognition, we and others have explored the possiblity that HLA class I molecules expressed in transgenic (Tg) 3 mice might provide a useful model for studying HLA-specific T cell recognition in vivo (11)(12)(13)(14)(15)(16). In particular, if human MHC class I alleles bind and present the same Ag-derived peptides to Tg T cells as they do in allele-matched humans, then a panel of Tg mice expressing different HLA alleles should facilitate studies in the context of the human Ag-presenting molecules of immunodominance.…”

“…We previously described a panel of HLA Tg mice on a wildtype (WT) C57BL/6 background that express the class I alleles HLA-A2, HLA-B7, or HLA-B27 as fully native molecules in association with human ␤ 2 -microglobulin (h␤ 2 m) or as human/ mouse hybrid molecules consisting of the HLA ␣1␣2 Ag-binding domains linked to the mouse ␣3, transmembrane, and cytoplasmic domains (16). Although such mice are useful for certain applications, coexpression of endogenous H2 class I may reduce the efficiency of detection of HLA-restricted Ag-specific responses.…”