My research group is studying the molecular and cellular basis for homing of leukocytes to sites of inflammation. In particular, we study the biology of adhesion molecules responsible for the binding and emigration of leukocytes across vascular endothelial cells. These adhesion events are crucial to the processes of inflammation and atherosclerosis, as well as to both the afferent and efferent arms of acquired immunity.

We have identified and cloned PECAM-1, a molecule that is required for transendothelial migration of leukocytes. We have also cloned VE-Cadherin, an endothelial-specific cadherin that may play a regulatory role in leukocyte emigration. Most recently, we have identified CD99 as a molecule concentrated at endothelial cell borders and on the surfaces of leukocytes that also plays a critical role in transendothelial migration that is distinct from that of PECAM. The roles of these adhesion molecules are being tested using both in vitro and in vivo models.

We have already shown that anti-PECAM monoclonal antibodies or soluble recombinant PECAM blocks inflammation in several models of acute inflammation. These studies were the first to show that transendothelial migration is a distinct step in leukocyte recruitment, which can be dissected on the molecular level independently of earlier and later steps. Furthermore, we demonstrated that distinct molecular domains of PECAM are responsible for two separate steps in the emigration process: Homophilic interaction between immunoglobulin domains 1 and 2 of PECAM on leukocytes with the same domains of PECAM on endothelial cells is crucial for diapedesis. In contrast, an independent interaction of immunoglobulin domain 6 of leukocyte PECAM with components of the subendothelial basal lamina is crucial for migration of leukocytes across the basement membrane once they had passed through the interendothelial junction. Our work was the first to demonstrate that migration across the basal lamina was a distinct step of emigration with its own molecular controls.

In order to study the role of PECAM in chronic inflammatory states such as multiple sclerosis, arthritis, and atherosclerosis, we have generated transgenic mice expressing soluble murine PECAM behind both constitutive and inducible promoters. These mice will be mated into appropriate susceptible strains for long-term studies. We have recently obtained PECAM "knockout" mice, which will be valuable for complementary experiments in these models. We have recently cloned the murine form of CD99 and are presently generating reagents to test the role of CD99 in inflammation using murine models.

We are currently involved in experiments to understand the various signaling mechanisms by which PECAM, CD99 and other molecules function. In addition to intracellular signaling, we are looking at intercellular signaling, or "cross-talk" between leukocytes and endothelial cells during inflammation. We recently published (Nature Feb. 13, 2003) novel results demonstrating that transmigration requires regulated trafficking of PECAM-rich membrane vesicles within the endothelial cell. This membrane surrounds the transmigrating leukocyte providing increased surface area for its passage without disrupting the endothelial monolayer.

Leukocyte-endothelial cell interactions influence the subsequent phenotype and physiology of both cell types and are important determinants in the outcome of the inflammatory response. We have recently found that interaction of monocytes with endothelial cells directs the differentiation of monocytes into macrophages or dendritic cells. The signals involved in this critical immune regulation are under investigation.