R-Ras is required for murine dendritic cell maturation and CD4+ T-cell priming.

R-Ras is a member of the RAS superfamily of small GTP-binding proteins. The physiologic function of R-Ras has not been fully elucidated. We found that R-Ras is expressed by lymphoid and nonlymphoid tissues and drastically up-regulated when bone marrow progenitors are induced to differentiate into dendritic cells (DCs). To address the role of R-Ras in DC functions, we generated a R-Ras-deficient mouse strain. We found that tumors induced in Rras(-/-) mice formed with shorter latency and attained greater tumor volumes. This finding has prompted the investigation of a role for R-Ras in the immune system. Indeed, Rras(-/-) mice were impaired in their ability to prime allogeneic and antigen-specific T-cell responses. Rras(-/-) DCs expressed lower levels of surface MHC class II and CD86 in response to lipopolysaccharide compared with wild-type DCs. This was correlated with a reduced phosphorylation of p38 and Akt. Consistently, R-Ras-GTP level was increased within 10 minutes of lipopolysaccharide stimulation. Furthermore, Rras(-/-) DCs have attenuated capacity to spread on fibronectin and form stable immunologic synapses with T cells. Altogether, these findings provide the first demonstration of a role for R-Ras in cell-mediated immunity and further expand on the complexity of small G-protein signaling in DCs.