The transmembrane glycoprotein CD83 is rapidly upregulated on murine and human DC upon maturation and therefore a costimulatory function for T cell activation has been suggested. Studies employing human APC indeed showed that CD83 expression was positively correlated to the stimulatory capacity of the APC. Murine APC that were CD83 deficient however, did not display a reduced capacity to activate T cells. To elucidate this contradiction, we thoroughly compared the stimulatory capacity of CD83-overexpressing and CD83-deficient APC. Here we show that CD83 expression levels on APC did not affect the capacity of the APC to activate CD8(+) T cells. CD83 expression levels did not significantly affect CD4(+) T cell activation in vivo, but a weak positive correlation of CD83 expression with CD4(+) T cell activation was observed in vitro under suboptimal stimulation conditions. As CD83 expression also positively correlated with MHC-II but not with MHC-I expression, this differential stimulation specifically of CD4(+) T cells could be explained by a higher density of MHC-II peptide complexes on the APC surface. Taken together, our results strongly suggest that CD83 does not deliver crucial costimulatory signals to murine T cells.
