Peptide binding affinity and pH variation establish functional thresholds for activation of HLA-DQ-restricted T cell recognition

Destabilization of peptide:MHC interaction induces IL-2 resistant anergy in diabetogenic T cells

Autoreactive T cells are responsible for inducing several autoimmune diseases, including type 1 diabetes. We have developed a strategy to induce unresponsiveness in these cells by destabilizing the peptide:MHC ligand recognized by the T cell receptor. By introducing amino acid substitutions into the immunogenic peptide at residues that bind to the MHC, the half life of the peptide:MHC complex is severely reduced, thereby resulting in abortive T cell activation and anergy. By treating a monoclonal diabetogenic T cell population with an MHC variant peptide, the cells are rendered unresponsive to the wild type ligand, as measured by both proliferation and IL-2 production. Stimulation of T cells with MHC variant peptides results in minimal Erk1/2 phosphorylation or cell division. Variant peptide stimulation effectively initiates a signaling program dominated by sustained tyrosine phosphatase activity, including elevated SHP-1 activity. These negative signaling events result in an anergic phenotype in which the T cells are not competent to signal through the IL-2 receptor, as evidenced by a lack of phospho-Stat5 upregulation and proliferation, despite high expression of the IL-2 receptor. This unique negative signaling profile provides a novel means to shut down the anti-self response.

HLA class II peptide-binding and autoimmunity

The HLA class II locus is located in the 6p21.3 region on the short arm of chromosome 6 and encompasses approximately 700 kb. It consists of over 30 gene loci including the major class II structural genes DP, DQ and DR. While autoimmune disease correlates to specific DP, DQ or DR alleles have been documented, due to the strong linkage disequilibrium between the different HLA alleles, especially between the DR and DQ, the precise identification of susceptible MHC alleles for a number of autoimmune diseases remains elusive.

Antigen presenting cells integrate opposing signals from CD4+ and CD8+ regulatory T lymphocytes to arbitrate the outcomes of immune responses

An individual's set of polymorphic HLA class II and I molecules is known to select the T cell repertoire in the thymus and to present processed antigenic peptides (pAg) to mature peripheral CD4+ T helper (Th) and CD8+ T cytotoxic (Th) cells in the periphery. This review highlights new studies which address how antigen presenting cells (APC) integrate the responses of cognate Th and T suppressor (Ts) cells to determine the outcome of immune responses. Together with other findings, these studies emphasize that understanding the mechanism of immune processes requires consideration of HLA molecules in the context of the peptides they bind, the antigen presenting cells (APC) that express them and the T lymphocytes that recognize them. The activities of lymphocyte and APC surface structures are becoming integrated into a physiological understanding of the cellular interactions between regulatory and effector T cells with APC.