Role of three quantitatively dominant endogenous peptides from HLA-DRB1 ∗0401 molecules in class II specific alloreactivity

Abstract

Alloreactivity remains an important barrier to organ transplantation and is caused by T cell recognition of foreign histocompatibility antigens (HAg) in two ways: (1) indirect recognition, in which processed HAg peptides are presented by self MHC like any other foreign antigen, and (2) direct recognition, where the foreign MHC itself is recognized in contravention of the T cell recognition rule of self restriction. Whereas the role of endogenous peptides in direct MHC class I specific recognition is now established, their role in class II specific direct alloreactivity remains controversial, since no defined endogenous peptide has been shown to be required for alloreactivity. That mutations resulting in defective antigen processing impair class II specific allostimulation, however, suggests that the endogenous pathway is important for class II as well as class I alloreactivity. We attempted to establish the importance of endogenous peptides for alloreactivity by identifying common sequences of peptides bound by DR molecules of an HLA-DRB1 ★0401 homozygous B cell line. Peptides corresponding to three of these (calreticulin, HLA class I and an unidentified molecule) were used to restimulate established allospecific HLA-Dw4 reactive T cell clones, as well as to sensitize allogeneic T cells de novo in vitro. Xenogeneic chinese hamster ovary (CHO) cells coexpressing the relevant DR allele together with CD80 were used as antigen presenting cells. The role of CD80 could be determined on these cells because (1) they are xenogeneic and (2) they do not express B7 family members bound by CTLA-4Ig. However, we were unable to obtain evidence for allorecognition of any of these three peptides, although alloreactive T cell clones could be obtained only following stimulation of naive T cells by CHO cells in the presence of synthetic peptide, and not after stimulation by DR/CD80-bearing transfectants alone. Therefore, it seems unlikely that the most common endogenous peptides bound to DR molecules of B cell lines are directly relevant to allorecognition and, consequently, that attempts to influence alloreactivity by peptide blockade are unlikely to succeed.