PECAM-1/CD31 trans-homophilic binding at the intercellular junctions is independent of its cytoplasmic domain; evidence for heterophilic interaction with integrin alphavbeta3 in Cis.

PECAM-1/CD31 is a cell adhesion and signaling molecule that is enriched at the endothelial cell junctions. Alternative splicing generates multiple PECAM-1 splice variants, which differ in their cytoplasmic domains. It has been suggested that the extracellular ligand-binding property, homophilic versus heterophilic, of these isoforms is controlled by their cytoplasmic tails. To determine whether the cytoplasmic domains also regulate the cell surface distribution of PECAM-1 splice variants, we examined the distribution of CD31-EGFPs (PECAM-1 isoforms tagged with the enhanced green fluorescent protein) in living Chinese hamster ovary cells and in PECAM-1-deficient endothelial cells. Our results indicate that the extracellular, rather than the cytoplasmic domain, directs PECAM-1 to the cell-cell borders. Furthermore, coculturing PECAM-1 expressing and deficient cells along with transfection of CD31-EGFP cDNAs into PECAM-1 deficient cells reveal that this PECAM-1 localization is mediated by homophilic interactions. Although the integrin alphavbeta3 has been shown to interact with PECAM-1, this trans-heterophilic interaction was not detected at the borders of endothelial cells. However, based on cocapping experiments performed on proT cells, we provide evidence that the integrin alphavbeta3 associates with PECAM-1 on the same cell surface as in a cis manner.

A chimeric cell adhesion molecule mediates homing of lymphocytes to vascularized tumors.

To facilitate tumor colonization by adoptively transferred cells of the immune system, we created a chimeric cell adhesion molecule that mediates tumor-specific homing by binding to the integrin alpha(v)beta3 on angiogenic endothelial cells. A high-affinity cell adhesion molecule for integrin alpha(v)beta3 was generated by fusing the disintegrin kistrin to the transmembrane adhesion molecule CD31/PECAM-1. This chimeric cell adhesion molecule, termed KISS31, mediates adhesion of lymphoid cells to soluble recombinant integrin alpha(v)beta3 and to endotheliomal monolayers in vitro. KISS31-expressing lymphoid cells accumulate in angiogenic tumors in two in vivo models, in B16/129 melanoma xenografts on the chick chorioallantois and in s.c. growing Lewis lung carcinoma in mice. Our data indicate that expression of KISS31 on lymphoid cells confers tumor-specific homing. This is, to our knowledge, the first example of an experimental mechanism that targets living cells to tumors by redirecting their homing pattern.

Current concepts in lymphocyte homing and recirculation.

The immune system consists of a complex collection of leukocytes and dendritic cells that surveys most tissues in the body for the appearance of foreign antigens. For an efficient immune response, the interaction and co-localization of antigen-presenting cells, costimulatory helper cells and effector cells are crucial parameters. Therefore, the migration routes of antigen-presenting cells and potential antigen-specific lymphocytes merge in secondary lymphoid organs in order to increase the likelihood and speed of a lymphocyte finding its cognate antigen. Additionally, antigen-primed effector cells are directed to the tissue where they are most likely to encounter their cognate antigen. This highly organized and efficient antigen encounter is based on a continuous recirculation of antigen-specific lymphocytes between blood, peripheral tissue, and secondary lymphoid organs. Moreover, the efficacy of the immune system is further increased by the ability of different lymphocyte subsets to recirculate only through distinct tissues. The scope of this review is to outline the concept and mechanisms of lymphocyte homing and recirculation and to discuss the significance for the immune defense. Current models in leukocyte homing and recirculation and the underlying molecular functions of implicated cell adhesion molecules, chemokines, and chemokine receptors are discussed.

Integrin alpha(v)beta(3) expression in colon carcinoma correlates with survival.

Integrin alpha(v)beta(3) is expressed by newly formed blood vessels in diseased and neoplastic tissue and can therefore be used as a marker for angiogenesis. We investigated its expression on the vasculature of 40 colon carcinomas using the anti-alpha(v)beta(3)-specific monoclonal antibody LM609. The average relapse-free interval and overall survival in patients suffering from colon carcinomas with high vascular expression of alpha(v)beta(3) integrin was significantly reduced compared with that in patients with low alpha(v)beta(3) integrin expressing tumor vasculature. Moreover, the expression level of alpha(v)beta(3) integrin correlated with the presence of liver metastases. In conclusion, we propose vascular expression of alpha(v)beta(3) integrin as a prognostic indicator for colon carcinoma.

Superactivation of integrin alphavbeta3 by low antagonist concentrations.

Integrins are implicated in cell adhesion, migration and homeostasis. An important feature is their ability to adopt different affinity states that can be regulated by a variety of intra- and extracellular factors. To study affinity modulation of the integrin ectodomain by extracellular factors, we produced a soluble recombinant form of mouse integrin alphavbeta3 in a mammalian expression system and isolated it to purity. We show that the two transmembrane truncated integrin subunits stably associate to form a functional receptor, soluble recombinant alphavbeta3. The affinity of this receptor for its ligands vitronectin, fibronectin and fibrinogen can be modulated by the divalent cations magnesium, calcium and manganese. Most importantly, we found that a cyclic RGD-peptide has a biphasic effect on rsalphavbeta3and native purified alphavbeta3, with an antagonistic phase at high concentrations, and an agonistic phase at low concentrations. This integrin superactivation by low antagonist concentrations is shown in binding of sralphavbeta3 to immobilized ligands by ELISA, and in adhesion of cells that express the chimaeric integrin ligand KISS31 to immobilized rsalphavbeta3 and native purified alphavbeta3. Our results indicate that low concentrations of the ligand mimetic cyclo-RGD can result in superactivation of the extracellular domain of integrin alphavbeta3 to a comparable level as activation by manganese.