Neutrophils are recruited from the blood to sites of inflammation, where they contribute to immune defense but may also cause tissue damage. During inflammation, neutrophils roll along the microvascular endothelium before arresting and transmigrating. Arrest requires conformational activation of the integrin lymphocyte function–associated antigen 1 (LFA-1), which can be induced by selectin engagement. Here, we demonstrate that a subset of P-selectin glycoprotein ligand-1 (PSGL-1) molecules is constitutively associated with L-selectin. Although this association does not require the known lectin-like interaction between L-selectin and PSGL-1, the signaling output is dependent on this interaction and the cytoplasmic tail of L-selectin. The PSGL-1–L-selectin complex signals through Src family kinases, ITAM domain–containing adaptor proteins, and other kinases to ultimately result in LFA-1 activation. The PSGL-1–L-selectin complex–induced signaling effects on neutrophil slow rolling and recruitment in vivo demonstrate the functional importance of this pathway. We conclude that this is a signaling complex specialized for sensing adhesion under flow.

Dendritic cells (DCs) have a key role in both the generation of the immune response and the induction of tolerance to self-Ags. In this work, the possible role of P-selectin glycoprotein ligand 1 (PSGL-1) on the tolerogenic activity of human DCs was explored. We found that the engagement of PSGL-1 by P-selectin on DCs induced the expression of c-Fos, IDO, IL-10, and TGF-β genes. Remarkably, stimulation of DCs through PSGL-1 with P-selectin enhanced their capability to generate CD4⁺CD25⁺Foxp3⁺ regulatory T cells, which expressed high levels of TGF-β1 mRNA, synthesized IL-10, and suppressed the proliferation of autologous CD4⁺CD25⁻ T cells. Accordingly, we found that DCs from PSGL-1^−/− mice expressed higher levels of MHC class II molecules, and exhibited an enhanced immunogenicity compared with wild-type mice. In addition, the percentage of CD4⁺CD25⁺Foxp3⁺ regulatory T cells in the thymus of PSGL-1-deficient animals was significantly reduced. Our data reveal an unexpected role of PSGL-1 on the tolerogenic function of DCs, and the regulation of the immune response.