A DOMINANT-NEGATIVE MUTANT OF THE RAB5 GTPASE ENHANCES T-CELL SIGNALING BY INTERFERING WITH TCR DOWN-MODULATION IN TRANSGENIC MICE

TCR triggering results in the down-modulation of engaged receptors by endocytosis, As a result of this process, Ag-binding sites are deplete d from the surface and signaling responses should be attenuated, To te st the importance of TCR down-regulation on T cell signaling, we gener ated mice expressing a dominant-negative form of Rab5 (Rab5N133I) in T cells, Rab5, a monomeric GTPase of the pas superfamily, has been impl icated in the regulation of early steps in the endocytic pathway. In R ab5N133I mice, mature thymocytes developed, but the absolute number of CD4(+)CD8(+) double positive thymocytes was reduced. Fluid phase endo cytosis was severely impaired in the transgenic thymocytes, In periphe ral T cells, the kinetics and rate of ligand-induced TCR down-modulati on were delayed and reduced, These effects were correlated with enhanc ed early and late signaling responses. Analysis of thymocyte developme nt in doubly transgenic mice for Rab5N133I and a lymphocytic choriomen ingitis virus (LCMV) peptide-specific TCR demonstrated that TCR signal ing was enhanced by dominant inhibition of Rab5 function, resulting in altered thymic selection, These findings suggest that TCR endocytosis is an important regulatory component of TCR signaling and that defect s in this regulation can result in prolonged signaling and alter thymi c development.