The duration of time that a T cell interacts with an antigen-presenting cell influences the activation of the T cell. T cells have an α4β2 integrin called LFA-1 that contributes to the adhesiveness and motility of T cells. Integrin affinity for ligands is increased by “inside-out” signaling. Raab et al. investigated the molecular events by which activated T cell receptors (TCRs) decrease T cell motility through inside-out signaling to LFA-1. It was known that T cells defective in the interaction between the guanosine triphosphatase Rap1 and its effector RapL, as well as T cells deficient in the adaptor SKAP1, exhibit defective adhesion, but whether Rap1-RapL and SKAP1 function in the same pathway was unknown. Raab et al. found that whereas RapL translocated to the membrane (cell fractionation) and the Rap1-RapL complex (immunoprecipitation) formed in T cells in which the TCR was activated, these two events failed to occur in T cells isolated from Skap1–/– mice. Further investigation of the involvement of SKAP1 in the formation of the Rap1-RapL complex was performed in transfected Jurkat cells, which showed that a constitutively activated mutant Rap1, but not an inactive mutant, formed a SKAP1-dependent complex with RapL and that two other adaptor proteins that interact with SKAP1, ADAP and SLP-76, were not necessary. Analysis of mutant forms of SKAP1 and RapL, by coimmunoprecipitation from transfected cells or in glutathione S-transferase pull-down experiments, revealed that the N-terminal region of SKAP1 was required for the interaction and that the coiled-coil domain, called SARAH, of RapL was required. Furthermore, a L224A mutation in the SARAH domain of RapL disrupted the interaction with SKAP1 but did not prevent the interaction with the kinase MST-1. Coimmunoprecipitation experiments showed that in transfected 293 cells SKAP1 competed with MST-1 for binding to RapL. Although Rap1, RapL or the L224A mutant RapL, and SKAP1 were present in clusters in resting cotransfected Jurkat cells, the molecules did not colocalize in the clusters. In cells also expressing SKAP1, TCR stimulation increased the colocalization of Rap1 and RapL but not the colocalization of Rap1 with the L224A mutant. RapL colocalized with the α subunit of LFA-1 and coimmunoprecipitated with the β subunit in TCR-stimulated Jurkat cells cotransfected with SKAP1, RapL, and activated Rap1. This interaction with LFA-1 was also detected in stimulated primary T cells and was absent in Skap–/– T cells. T cell–stimulated Jurkat or T8.1 cells transfected with SKAP1 and RapL exhibited increased binding of the cells to the adhesion protein ICAM1 (Jurkat cells) or an increase in the interaction between T cell and antigen-presenting cell (T8.1 cells) compared to cells transfected with the L224A mutant RapL or control cells. With an ex vivo lymph node T cell imaging system, the authors showed that RapL, but not the L224A mutant, reduced the motility of T cells in the lymph node and increased the contact times of T cells with antigen-presenting cells. Thus, the authors propose a model in which the TCR stimulates the formation of a SKAP1-requiring Rap1-RapL complex that regulates LFA-1–mediated adhesion.
M. Raab, H. Wang, Y. Lu, X. Smith, Z. Wu, K. Strebhardt, J. E. Ladbury, C. E. Rudd, T cell receptor “inside-out” pathway via signaling module SKAP1-RapL regulates T cell motility and interactions in lymph nodes. Immunity 32, 541–556 (2010). [PubMed]