Transendothelial Migration of Leukocytes: Developing New Paradigms in Health and Disease

Transendothelial migration (TEM), or diapedesis, is the step in which leukocytes squeeze between tightly apposed endothelial cells that line the post-capillary venules at sites of inflammation. Most of the good, the bad, and the ugly of inflammation occurs after leukocytes cross blood vessels. A thorough understanding of the molecules and mechanisms that regulate TEM should therefore enhance our ability to control the process therapeutically. Therefore, my lab has been studying this process for 30 years. We have made some of the seminal discoveries in the field, including the identification and discovery of: 1.Platelet/endothelial cell adhesion molecule-1 (PECAM) and CD99 as major selective regulators of TEM and 2. their downstream signaling pathways leading to TEM; 3.The lateral border recycling compartment (LBRC), an interconnected reticulum of tubule-vesicular membrane that recycles locally along the endothelial cell borders; 4.That the surface molecules involved in TEM (PECAM, CD99, etc.) work sequentially in the process as the leukocyte passes through the endothelial cell border; 5.That the act of TEM promotes differentiation of some monocytes into dendritic cells; 6. That paracellular and transcellular TEM of leukocytes use the same machinery and mechanisms. Whether we approached TEM from the standpoint of the leukocyte or the endothelial cell, the surface adhesion/signaling molecules, the intracellular signaling pathways, membrane dynamics, or endothelial cell ultrastructure, we consistently and independently converged on a final common mechanism regardless of whether we were studying neutrophils, monocytes, or T cells; regardless of the inflammatory conditions or models studied: TEM required the targeted movement of the LBRC along microtubules to the site at which the leukocyte was migrating. All of the molecules that we studied worked to activate this mechanism. Anything that inhibited this “targeted recycling of the LBRC” inhibited TEM by 80-90% in vitro and in vivo. The biggest shortcoming of existing anti-inflammatory therapies is that they also block beneficial inflammation. We have developed biochemical and genetic tools to selectively block targeted recycling of the LBRC and hence TEM in multiple in vivo models of inflammatory disease. Since these reagents and inducible EC-selective knockout mice only affect EC, all other aspects of the innate and adaptive inflammatory responses remain intact. Since we are only able to block TEM by 80–90%, the 10–20% of leukocytes that escape blockade enter the tissues able to mount a normal inflammatory response. Our preliminary data show that we can diminish the intensity of maladaptive inflammation without interfering with the ability of the host to mount desirable inflammatory responses and remain healthy. We will test this hypothesis in a number of models of acute and chronic inflammation. We will also use our ability to selective block TEM at selected time points to study the role of TEM in the initiation, progression, and resolution of disease. The molecules and mechanisms that regulate TEM in the pulmonary vascular bed are unknown. We will identify them and compare to the systemic circulation.