Xenogeneic β2-Microglobulin Substitution Affects Functional Binding of MHC Class I Molecules by CD8+ T Cells

Abstract

NK cells and CD8+ T cells bind MHC-I molecules using distinct topological interactions. Specifically, murine NK inhibitory receptors bind MHC-I molecules at both the MHC-I H chain regions and β2-microglobulin (β2m) while TCR engages MHC-I molecules at a region defined solely by the class I H chain and bound peptide. As such, alterations in β2m are not predicted to influence functional recognition of MHC-I by TCR. We have tested this hypothesis by assessing the capability of xenogeneic β2m to modify the interaction between TCR and MHC-I. Using a human β2m-transgenic C57BL/6 mouse model, we show that human β2m supports formation and expression of H-2Kb and peptide:H-2Kb complexes at levels nearly equivalent to those in wild-type mice. Despite this finding, the frequencies of CD8+ single-positive thymocytes in the thymus and mature CD8+ T cells in the periphery were significantly reduced and the TCR Vβ repertoire of peripheral CD8+ T cells was skewed in the human β2m-transgenic mice. Furthermore, the ability of mouse β2m-restricted CTL to functionally recognize human β2m+ target cells was diminished compared with their ability to recognize mouse β2m+ target cells. Finally, we provide evidence that this effect is achieved through subtle conformational changes occurring in the distal, peptide-binding region of the MHC-I molecule. Our results indicate that alterations in β2m influence the ability of TCR to engage MHC-I during normal T cell physiology.