Peptide ligand structure and I-Aq binding avidity influence T cell signaling pathway utilization

The role of Syk in peripheral T cells

The aim of this study was to understand how Syk affects peripheral T cell function. T cells from Syk^-/- chimeric mice and DR1 Syk^fl/fl CD4^cre conditional mice gave strong CD3-induced Th1, Th2, and Th17 cytokine responses. However, an altered peptide ligand (APL) of human CII (256-276) with two substitutions (F263N, E266D), also called A12, elicited only Th2 cytokine responses from Syk^fl/fl T cells but not Syk^fl/fl-CD4^cre T cells. Western blots revealed a marked increase in the phosphorylation of Syk, JNK and p38 upon A12/DR1 activation in WT or Syk^fl/fl T cells but not in Syk^fl/flCD4^-cre cells. We demonstrate that Syk is required for the APL- induction of suppressive cytokines. Chemical Syk inhibitors blocked activation of GATA-3 by peptide A12/DR1. In conclusion, this study provides novel insights into the role that Syk plays in directing T cell activity, and may shape therapeutic approaches for autoimmune diseases.

Characterization of the Syk-Dependent T Cell Signaling Response to an Altered Peptide

Rheumatoid arthritis is an autoimmune disorder characterized by T cell dysregulation. We have shown that an altered peptide ligand (A9) activates T cells to use an alternate signaling pathway that is dependent on FcRγ and spleen tyrosine kinase, resulting in downregulation of inflammation. In the experiments described in this study, we have attempted to determine the molecular basis of this paradox. Three major Src family kinases found in T cells (Lck, Fyn, and Lyn) were tested for activation following stimulation by A9/I-A^q Unexpectedly we found they are not required for T cell functions induced by A9/I-A^q, nor are they required for APL stimulation of cytokines. On the other hand, the induction of the second messenger inositol trisphosphate and the mobilization of calcium are clearly triggered by the APL A9/I-A^q stimulation and are required for cytokine production, albeit the cytokines induced are different from those produced after activation of the canonical pathway. DBA/1 mice doubly deficient in IL-4 and IL-10 were used to confirm that these two cytokines are important for the APL-induced attenuation of arthritis. These studies provide a basis for exploring the effectiveness of analog peptides and the inhibitory T cells they induce as therapeutic tools for autoimmune arthritis.