Physiological Contribution of CD44 as a Ligand for E-Selectin during Inflammatory T-Cell Recruitment

Abstract

Endothelial selectins guide the migration of inflammatory T cells to extralymphoid tissues. Whereas Pselectin glycoprotein ligand-1 (PSGL-1) functions as the exclusive ligand for P-selectin, it acts in coordination with additional glycoproteins to mediate Eselectin binding. CD44 can act as one such ligand in neutrophils, but its contribution in inflammatory T lymphocytes remains unexplored. We have used realtime in vivo imaging of the cremasteric and dermal microcirculations to explore the dynamics of leukocyte recruitment, as well as the physiological contribution of CD44 in a model of Th1-driven inflammation. CD4؉ T-cell rolling frequency and kinetics, as well as arrest, were dependent on endothelial selectins and were markedly altered under inflammatory conditions. CD44 extracted from Th1 cells bound to soluble E-selectin in vitro and cooperated with PSGL-1 by controlling rolling velocities and promoting firm arrest. Using several competitive recruitment assays in a delayed-type hypersensitivity model, we show that the combined absence of CD44 and PSGL-1 impairs inflammatory T-cell recruitment beyond that of PSGL-1 alone. Differential expression of leukocyte fucosyltransferases in these cells may account for the differential use of E-selectin ligands relative to neutrophils. Our results identify additional mechanisms by which CD44 modulates the inflammatory response. The trafficking of T lymphocytes is regulated by their repertoire of adhesion and chemotactic receptors. Naïve T cells recirculate through secondary lymphoid organs in search of their cognate antigen, a process facilitated by their specific binding to high endothelial venules through L-selectin, ␤2 integrins, and the chemokine receptor CCR7. Once activated, effector and memory T cells switch their adhesive and signaling repertoire to one that allows immune surveillance of tissues. In particular, skin and inflamed areas are dynamically infiltrated by activated T cells, a process that underlies a number of pathogenic inflammatory diseases.1 It is generally accepted that this infiltration is initiated by extravasation from the blood microvasculature through a multistep cascade similar to that undergone by neutrophils, including initial tethering and rolling events followed by firm arrest and diapedesis.2 Tethering and rolling are initiated by labile interactions mediated by endothelial P-and E-selectins (encoded by Selp and Sele genes, respectively) expressed constitutively in the skin microvasculature 3 and induced in other tissues during inflammation.2 Induction of ligands for endothelial selectins is therefore critical for the acquired migrating properties of inflammatory lymphocytes. 4 Two glycoproteins expressed on Th1 lymphocytes, Pselecting glycoprotein ligand-1 (PSGL-1; encoded by Selplg) and the sialomucin CD43, have been shown to

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“…However, studies have identified differences in homing molecule expression on CD4 + T cells activated in vivo versus ex vivo, indicating that assessment of endogenous T cells may provide a more accurate analysis of the molecular basis of recruitment of activated T cells (25). To this end, more recently, rapid frame-rate multichannel imaging has allowed analysis of endogenous CD4 + T cell rolling in the inflamed skin, revealing the contributions of endothelial selectins and their ligands to CD4 + T cell rolling at the peak of the CS response (29,30). However, these studies did not extend to analysis of Tregs or CD8 + T cells or examine T cell interactions throughout the entire CS response.…”

“…Although we identified PSGL-1 as the dominant functional P-selectin ligand on Tregs, consistent with observations in other leukocyte subsets (55,56), identifying the functional E-selectin ligands on Tregs is likely to be more challenging. Several leukocyte-expressed E-selectin ligands, including PSGL-1, E-selectin ligand-1, CD44, and CD43, have been identified (9,30,(56)(57)(58). Expression of E-selectin ligand-1 has not been reported on lymphocytes.…”

“…1B). Inflammation-associated changes in rolling in the dermal microvasculature can be manifest as reductions in leukocyte rolling velocity (30,40). Therefore, we also examined the effect of CS on rolling velocity.…”

“…administered Abs against subset-restricted markers to differentiate CD4 + T cells from neutrophils and demonstrated that CD4 + T cells undergo infrequent but consistent interactions in activated postcapillary venules. Nacher et al (30) found CD4 + T cell rolling in inflamed skin to be entirely dependent on endothelial selectins and leukocyte-expressed selectin ligands. Extending this approach to Tregs has been hampered by the lack of a specific cell surface marker for this subset.…”

“…An alternative approach has been the examination of transgenic mice expressing GFP in CD4 + and CD8 + T cells (27,28). The advent of rapid frame rate, multichannel forms of in vivo imaging has facilitated the analysis of the interactions of multiple low-frequency endogenous leukocyte subsets in the microvasculature (29)(30)(31). These studies used i.v.…”