GITRL (Glucocorticoid-Induced TNF receptor family-Related Ligand) triggering up-regulates VCAM-1 and ICAM-1 and promotes leukocyte adhesion.

The interaction of Glucocorticoid-Induced TNF receptor-family Related (GITR) protein with its ligand (GITRL) modulates different functions including immune/inflammatory response. These effects are consequent to intracellular signals activated by both GITR and GITRL. Previous results suggest that lack of GITR expression, in GITR-/- mice, decreases the number of leukocytes within inflamed tissues. We performed experiments to analyze whether GITRL/GITR system modulates leukocyte adhesion and extravasation. For that purpose, we first evaluated the capability of murine splenocytes to adhere to endothelial cells (EC). Results indicated that adhesion of GITR-/- splenocytes to EC was reduced as compared to wild type cells suggesting that GITR plays a role in adhesion and that this effect may be due to GITRL-GITR interaction. Moreover, adhesion was increased when EC were pre-treated with an agonist GITR-Fc fusion protein thus indicating that triggering of GITRL plays a role in adhesion by EC regulation. In a human in vitro model, the adhesion to human EC of HL-60 cells differentiated towards the monocytic lineage was increased by EC pre-treatment with agonist GITR-Fc. Conversely, antagonistic anti-GITR and anti-GITRL Ab decreased adhesion thus further indicating that GITRL triggering increases the EC capability to support leukocyte adhesion. EC treatment with GITR-Fc favored extravasation, as demonstrated by a transmigration assay. Notably, GITRL triggering increased ICAM-1 and VCAM-1 expression and anti-ICAM-1 and anti-VCAM-1 Abs reversed GITR-Fc effects. In conclusion, our study demonstrates that GITRL triggering in EC increases leukocyte adhesion and transmigration, suggesting new anti-inflammatory therapeutic approaches based on inhibition of GITRL/GITR interaction.