Insight from Michael HickeyIt has long been understood that the endothelium is not simply a passive barrier to leukocyte transendothelial migration (TEM). Complex cellular responses within endothelial cells involving signaling, membrane trafficking, and remodeling of cell--cell junctions are necessary for leukocytes to cross the endothelium and exit the vasculature. Inhibition or absence of junctional molecule CD99 has been shown to trap leukocytes midway through TEM, indicating an essential role for this molecule. However, the mechanisms by which CD99 contributes to TEM have been unclear. In this Issue, Watson et al. describe the signaling pathway activated by CD99 and demonstrate an important role for soluble adenylyl cyclase (sAC) in TEM.

Using an elegant imaging-based assay, Watson et al. show that CD99 inhibition prevents movement of the lateral border recycling compartment (LBRC) to the site of TEM, whereas CD99 cross-linking can restore this response, providing evidence of a role for CD99 in signaling for LBRC recycling. Using a comprehensive array of in vitro and in vivo analyses, the authors show that CD99 forms a complex at the endothelial cell junction with sAC, the scaffolding protein ezrin, and protein kinase A (PKA). Engagement of CD99 leads to activation of sAC and release of cAMP, which subsequently activates PKA, promoting LBRC recycling and facilitating leukocyte TEM. This is the first demonstration of a role for sAC in TEM and of the signaling pathway activated by CD99.

A model of how CD99 signals during TEM. Under resting conditions, CD99, sAC, PKA, and ezrin form a signaling complex at endothelial junctions. During TEM, homophilic engagement of endothelial CD99 with leukocyte CD99 (1) signals through soluble adenylyl cyclase (2) to elevate cAMP (3), to activate PKA, which works through an unknown mechanism (4) to induce LBRC membrane trafficking to sites of leukocyte--endothelial contact (5).Adapted from [@bib1]The key question to emerge from this study is: can this information be useful therapeutically? Inhibition of leukocyte recruitment is now established as a viable therapeutic approach for several inflammatory diseases, although it is not without risks in terms of susceptibility to infection. CD99 inhibition may be worth investigating as an additional strategy for inhibition of leukocyte recruitment. However, it remains to be determined how universal this mechanism is within the microvasculature. The current studies were performed using human umbilical vein endothelial cells and mouse dermal microvessels. It is now clear that the mechanisms of leukocyte recruitment can differ markedly in different microvascular beds. Detailed imaging-based analysis of important vascular beds such as the gastrointestinal tract, lung, and brain will be necessary to determine whether CD99 retains this function throughout the microvascular system. In addition, the mechanism whereby PKA promotes LBRC recycling remains to be determined. Unquestionably, the molecular complexity of transmigration will continue to engage the efforts of researchers for many years to come.
