Differential Recognition and Activation Thresholds in Human Autoreactive GAD-Specific T-Cells

Abstract

The activation requirements of autoreactive CD4+ T-cells were investigated in GAD65-specific HLA-DR0401-restricted clones derived from a diabetic patient using major histocompatibility complex (MHC) class II tetramers (TMrs) as stimulating agents. Despite the fact that TMrs loaded with an immunodominant-altered GAD peptide (TMr-GAD) bound a limited number of T-cell receptors, they were capable of efficiently delivering activation signals. These signals ranged from the early steps of phospholipase C (PLC)-γ1 phosphorylation and Ca2+ mobilization to more complex events, such as CD69 upregulation, cytokine mRNA transcription and secretion, and proliferation. All the effects triggered by TMr-GAD were dose dependent. On the contrary, [3H]-thymidine incorporation decreased at high TMr-GAD concentrations because of activation-induced cell death (AICD) after initial proliferation. Lower-avidity clones (as defined by TMr-GAD binding) were less sensitive to activation as well as less susceptible to AICD compared with higher-avidity clones. Induction of apoptosis is a potential Immunomodulatory target for therapeutic applications of MHC class II multimers, but the relative resistance of low-avidity T-cells may limit its benefits.