Nectins are adhesion molecules that participate in the organization of epithelial and endothelial junctions and serve as receptors for herpes simplex virus entry. They belong to the immunoglobulin superfamily, are homologues of the poliovirus receptor (PVR/CD155), and were also named poliovirus receptor-related (PRR) proteins. We identify a new member of the nectin family named nectin4. Peptide sequences of human and murine nectin4 share 92% identity, and as for other members, the ectodomain is made of three immunoglobulin-like domains of V, C, C types. In contrast to other nectin molecules, detection of nectin4 transcripts is mainly restricted to placenta in human tissues. Expression is broader in mouse, and interestingly nectin4 is detected at days 11, 15, and 17 during murine embryogenesis. Nectin4 interacts with afadin, a F-actin-associated molecule, via its carboxyl-terminal cytoplasmic sequence. Both molecules co-localize at cadherin-based adherens junctions in the MDCKII epithelial cell line. Nectins are homophilic adhesion molecules, and recently heterophilic interactions have been described between nectin3/nectin1 and nectin3/nectin2. We confirmed these trans-interactions and also described nectin3 as the PVR/CD155 ligand. By means of several approaches, we report on the identification of nectin4 as a new ligand for nectin1. First, a soluble chimeric recombinant nectin4 ectodomain (nectin4-Fc) trans-interacts with cells expressing nectin1 but not with cells expressing nectin2, nectin3, or PVR/CD155. Conversely, nectin1-Fc binds to cells expressing nectin4. Second, nectin1-Fc precipitates nectin4 expressed in COS cells. Third, reciprocal in vitro physical interactions were detected between nectin4-Fc and nectin1-Fc. The nectin4-Fc/nectin4-Fc interaction was detected suggesting that nectin4 exhibits both homophilic and heterophilic properties. Using the same approaches we demonstrate, for the first time, that the V domain of nectin1 acts as a major functional region involved in trans-heterointeraction with nectin4 and also nectin3.

DNAX accessory molecule 1 (DNAM-1; CD226) is a transmembrane glycoprotein involved in T cell and natural killer (NK) cell cytotoxicity. We demonstrated recently that DNAM-1 triggers NK cell–mediated killing of tumor cells upon engagement by its two ligands, poliovirus receptor (PVR; CD155) and Nectin-2 (CD112). In the present paper, we show that PVR and Nectin-2 are expressed at cell junctions on primary vascular endothelial cells. Moreover, the specific binding of a soluble DNAM-1–Fc molecule was detected at endothelial junctions. This binding was almost completely abrogated by anti-PVR monoclonal antibodies (mAbs), but not modified by anti–Nectin-2 mAbs, which demonstrates that PVR is the major DNAM-1 ligand on endothelial cells. Because DNAM-1 is highly expressed on leukocytes, we investigated the role of the DNAM-1–PVR interaction during the monocyte transendothelial migration process. In vitro, both anti–DNAM-1 and anti-PVR mAbs strongly blocked the transmigration of monocytes through the endothelium. Moreover, after anti–DNAM-1 or anti-PVR mAb treatment, monocytes were arrested at the apical surface of the endothelium over intercellular junctions, which strongly suggests that the DNAM-1–PVR interaction occurs during the diapedesis step. Altogether, our results demonstrate that DNAM-1 regulates monocyte extravasation via its interaction with PVR expressed at endothelial junctions on normal cells.

In T cells, the PI3K pathway promotes proliferation and survival induced by Ag or growth factors, in part by inactivating the FOXO transcription factor 1. We now report that FOXO1 controls the expression of L-selectin, an essential homing molecule, in human T lymphocytes. This control is already operational in unprimed T cells and involves a transcriptional regulation process that requires the FOXO1 DNA-binding domain. Using transcriptional profiling, we demonstrate that FOXO1 also increases transcripts of EDG1 and EDG6, two sphingosine-1-phosphate receptors that regulate lymphocyte trafficking. Additionally, FOXO1 binds the promoter of the cell quiescence and homing regulator Krüppel-like factor 2 and regulates its expression. Together, these results reveal a new function of FOXO1 in the immune system and suggest that PI3K controls a coordinated network of transcription factors regulating both cell quiescence and homing of human T lymphocytes.