Transglutaminase 2 is needed for the formation of an efficient phagocyte portal in macrophages engulfing apoptotic cells

Transglutaminase 2 (TG2), a protein cross-linking enzyme with many additional biological functions, acts as coreceptor for integrin beta(3). We have previously shown that TG2(-/-) mice develop an age-dependent autoimmunity due to defective in vivo clearance of apoptotic cells. Here we report that TG2 on the cell surface and in guanine nucleotide-bound form promotes phagocytosis. Besides being a binding partner for integrin beta(3), a receptor known to mediate the uptake of apoptotic cells via activating Rac1, we also show that TG2 binds MFG-E8 (milk fat globulin EGF factor 8), a protein known to bridge integrin beta(3), to apoptotic cells. Finally, we report that in wild-type macrophages one or two engulfing portals are formed during phagocytosis of apoptotic cells that are characterized by accumulation of integrin beta(3) and Rac1. In the absence of TG2, integrin beta(3), cannot properly recognize the apoptotic cells, is not accumulated in the phagocytic cup, and its signaling is impaired. As a result, the formation of the engulfing portals, as well as the portals formed, is much less efficient. We propose that TG2 has a novel function to stabilize efficient phagocytic portals. The Journal of Immunology, 2009, 182: 2084-2092

Nanometer-scale organization of the alpha subunits of the receptors for IL2 and IL15 in human T lymphoma cells\ud

Interleukin 2 and interleukin 15 (IL2 and IL15, respectively) provide quite distinct contributions to T-cell-mediated immunity, despite having similar receptor composition and signaling machinery. As most of the proposed mechanisms underlying this apparent paradox attribute key significance to the individual -chains of IL2 and IL15 receptors, we investigated the spatial organization of the receptors IL2R and IL15R at the nanometer scale expressed on a human CD4+ leukemia T cell line using single-molecule-sensitive near-field scanning optical microscopy (NSOM). In agreement with previous findings, we here confirm clustering of IL2R and IL15R at the submicron scale. In addition to clustering, our single-molecule data reveal that a non-negligible percentage of the receptors are organized as monomers. Only a minor fraction of IL2R molecules reside outside the clustered domains, whereas 30% of IL15R molecules organize as monomers or small clusters, excluded from the main domain regions. Interestingly, we also found that the packing densities per unit area of both IL2R and IL15R domains remained constant, suggesting a `building block' type of assembly involving repeated structures and composition. Finally, dual-color NSOM demonstrated co-clustering of the two -chains. Our results should aid understanding the action of the IL2R-IL15R system in T cell function and also might contribute to the more rationale design of IL2R- or IL15R-targeted immunotherapy agents for treating human leukemia. \u