IMPAIRED PLASMA-MEMBRANE TARGETING OF GRB2-MURINE SON OF SEVENLESS (MSOS) COMPLEX AND DIFFERENTIAL ACTIVATION OF THE FYN-T CELL-RECEPTOR (TCR)-XI-CB1 PATHWAY MEDIATE T-CELL HYPORESPONSIVENESS IN AUTOIMMUNE NONOBESE DIABETIC MICE

Nonobese diabetic (NOD) mouse thymocytes are hyporesponsive to T cell antigen receptor (TCR)-mediated stimulation of proliferation, and this T cell hyporesponsiveness may be causal to the onset of autoimmune di abetes in NOD mice. We previously showed that TCR-induced NOD T cell h yporesponsiveness is associated with a block in Ras activation and def ective signaling along the PKC/Ras/MAPK pathway. Here, we report that several sequential changes in TCR-proximal signaling events may mediat e this block in Ras activation. We demonstrate that NOD T cell hypores ponsiveness is associated with the (a) enhanced TCR-beta-associated Fy n kinase activity and the differential activation of the Fyn-TCR-zeta- Cbl pathway, which may account for the impaired recruitment of ZAP70 t o membrane-bound TCR-zeta; (b) relative inability of the murine son of sevenless (mSOS) Ras GDP releasing factor activity to translocate fro m the cytoplasm to the plasma membrane; and (c) exclusion of mSOS and PLC-gamma 1 from the TCR-zeta-associated Grb2/pp36-38/ZAP70 signaling complex. Our data suggest that altered tyrosine phosphorylation and ta rgeting of the Grb2/pp36-38/ZAP70 complex to the plasma membrane and c ytoskeleton and the deficient association of mSOS with this Grb2-conta ining complex may block the downstream activation of Ras and Ras-media ted amplification of TCR/CD3-mediated signals in hyporesponsive NOD T cells. These findings implicate mSOS as an important mediator of downr egulation of Ras signaling in hyporesponsive NOD T cells.