Rap1-mediated lymphocyte function-associated antigen-1 activation by the T cell antigen receptor is dependent on phospholipase C-gamma1.

The small GTPase, Rap1, is a potent activator of leukocyte integrins and enhances the adhesive activity of lymphocyte function-associated antigen-1 (LFA-1) when stimulated by the T cell receptor (TCR) or chemokines. However, the mechanism by which Rap1 is activated remains unclear. Here, we demonstrate that phospholipase C (PLC)-gamma1 plays a critical role in the signaling pathway leading to Rap1 activation triggered by the TCR. In Jurkat T cells, TCR cross-linking triggered persistent Rap1 activation, and SDF-1 (CXCL12) activated Rap1 transiently. A phospholipase C inhibitor, U73122, abrogated Rap1 activation triggered by both the TCR and SDF-1 (CXCL12). PLC-gamma1-deficient Jurkat T cells showed a marked reduction of TCR-triggered Rap1 activation and adhesion to intercellular adhesion molecule-1 (ICAM-1) mediated by LFA-1. In contrast, SDF-1-triggered Rap1 activation and adhesion were not affected in these cells. Transfection of these cells with an expression plasmid encoding PLC-gamma1 restored Rap1 activation by the TCR and the ability to adhere to ICAM-1, accompanied by polarized LFA-1 surface clustering colocalized with regulator of adhesion and polarization enriched in lymphoid tissues (RAPL). Furthermore, when expressed in Jurkat cells, CalDAG-GEFI, a calcium and diacylglycerol-responsive Rap1 exchange factor, associated with Rap1, and resulted in enhanced Rap1 activation and adhesion triggered by the TCR. Our results demonstrate that TCR activation of Rap1 depends on PLC-gamma1. This activity is likely to be mediated by CalDAG-GEFI, which is required to activate LFA-1.

Development of both human connective tissue-type and mucosal-type mast cells in mice from hematopoietic stem cells with identical distribution pattern to human body.

The transplantation of primitive human cells into sublethally irradiated immune-deficient mice is the well-established in vivo system for the investigation of human hematopoietic stem cell function. Although mast cells are the progeny of hematopoietic stem cells, human mast cell development in mice that underwent human hematopoietic stem cell transplantation has not been reported. Here we report on human mast cell development after xenotransplantation of human hematopoietic stem cells into nonobese diabetic severe combined immunodeficient (NOD/SCID)/gamma(c)(null) (NOG) mice with severe combined immunodeficiency and interleukin 2 (IL-2) receptor gamma-chain allelic mutation. Supported by the murine environment, human mast cell clusters developed in mouse dermis, but they required more time than other forms of human cell reconstitution. In lung and gastric tract, mucosal-type mast cells containing tryptase but lacking chymase located on gastric mucosa and in alveoli, whereas connective tissue-type mast cells containing both tryptase and chymase located on gastric submucosa and around major airways, as in the human body. Mast cell development was also observed in lymph nodes, spleen, and peritoneal cavity but not in the peripheral blood. Xenotransplantation of human hematopoietic stem cells into NOG mice can be expected to result in a highly effective model for the investigation of human mast cell development and function in vivo.

RAPL, a Rap1-binding molecule that mediates Rap1-induced adhesion through spatial regulation of LFA-1.

The small GTPase Rap1 is a potent activator of leukocyte integrin. However, the regulatory mechanism involved is unknown. Here, we identify the Rap1 effector, RAPL, as an essential regulator in this activation. RAPL was enriched in mouse lymphoid tissues and associated with Rap1 after stimulation by the T cell receptor and with chemokine CXCL12. Human RAPL stimulated lymphocyte polarization and the patch-like redistribution of lymphocyte-function-associated antigen 1 (LFA-1) to the leading edge, resulting in enhanced adhesion to intercellular adhesion molecule 1 (ICAM-1). Triggered by activated Rap1, RAPL associated with LFA-1 and rapidly relocated to the leading edge and accumulated at immunological synapses. Thus, RAPL regulates lymphocyte adhesion through the spatial distribution of LFA-1.

Rap1 translates chemokine signals to integrin activation, cell polarization, and motility across vascular endothelium under flow.

Chemokines arrest circulating lymphocytes within the vasculature through the rapid up-regulation of leukocyte integrin adhesive activity, promoting subsequent lymphocyte transmigration. However, the key regulatory molecules regulating this process have remained elusive. Here, we demonstrate that Rap1 plays a pivotal role in chemokine-induced integrin activation and migration. Rap1 was activated by secondary lymphoid tissue chemokine (SLC; CCL21) and stromal-derived factor 1 (CXCL4) treatment in lymphocytes within seconds. Inhibition of Rap1 by Spa1, a Rap1-specific GTPase-activating protein, abrogated chemokine-stimulated lymphocyte rapid adhesion to endothelial cells under flow via intercellular adhesion molecule 1. Expression of a dominant active Rap1V12 in lymphocytes stimulated shear-resistant adhesion, robust cell migration on immobilized intercellular adhesion molecule 1 and vascular cell adhesion molecule 1, and transendothelial migration under flow. We also demonstrated that Rap1V12 expression in lymphocytes induced a polarized morphology, accompanied by the redistribution of CXCR4 and CD44 to the leading edge and uropod, respectively. Spa1 effectively suppressed this polarization after SLC treatment. This unique characteristic of Rap1 may control chemokine-induced lymphocyte extravasation.