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Peripheral monocyte-derived CX3C chemokine receptor 1 positive (CX3CR1(+)) cells play important roles in tissue homeostasis and gut repopulation. Increasing evidence also supports their role in immune repopulation of the brain parenchyma in response to systemic inflammation. Adoptive bone marrow transfer from CX3CR1 fluorescence reporter mice and high-resolution confocal microscopy was used to assess the time course of CX3CR1(+) cell repopulation of steady-state and dextran sodium sulfate (DSS)-inflamed small intestine/colon and the brain over 4 weeks after irradiation. CX3CR1(+) cell colonization and morphologic polarization into fully ramified cells occurred more rapidly in the small intestine than in the colon. For both organs, the crypt/mucosa was more densely populated than the serosa/muscularis layer, indicating preferential temporal and spatial occupancy. Repopulation of the brain was delayed compared with that of gut tissue, consistent with the immune privilege of this organ. However, DSS-induced colon injury accelerated the repopulation. Expression analyses confirmed increased chemokine levels and macrophage colonization within the small intestine/colon and the brain by DSS-induced injury. Early increases of transmembrane protein 119 and ionized calcium binding adaptor molecule 1 expression within the brain after colon injury suggest immune-priming effect of brain resident microglia in response to systemic inflammation. These findings identify temporal differences in immune repopulation of the gut and brain in response to inflammation and show that gut inflammation can impact immune responses within the brain.

Calcium is an essential second messenger in endothelial cells and plays a pivotal role in regulating a number of physiologic processes, including cell migration, angiogenesis, barrier function, and inflammation. An increase in intracellular Ca(2+) concentration can trigger a number of diverse signaling pathways under both physiologic and pathologic conditions. In this review, we discuss how calcium signaling pathways in endothelial cells play an essential role in affecting barrier function and facilitating inflammation. Inflammatory mediators, such as thrombin and histamine, increase intracellular calcium levels. This calcium influx causes adherens junction disassembly and cytoskeletal rearrangements to facilitate endothelial cell retraction and increased permeability. During inflammation endothelial cell calcium entry and the calcium-related signaling events also help facilitate several leukocyte–endothelial cell interactions, such as leukocyte rolling, adhesion, and ultimately transendothelial migration.

Neutrophil (PMN) infiltration of the intestinal mucosa is a hallmark of gastrointestinal inflammation, with significant implications for host defense, injury and repair. However, phenotypic and mechanistic aspects of PMN recruitment in inflamed intestines have not been explored in vivo. Using novel epithelial/PMN fluorescence reporter mice, advanced intravital imaging and 3D reconstruction analysis, we mapped the microvasculature architecture across the intestinal layers and determined that in response to Salmonella/endotoxin-induced inflammation, PMN transendothelial migration (TEM) was restricted to submucosal vessels. PMN TEM was not observed in villus or crypt vessels, proximal to the epithelium that underlies the intestinal lumen, and was partially dependent on (C-X-C motif) ligands 1 (CXCL1) and 2 (CXCL2) expression, which was found to be elevated in the submucosa layer. Restricted PMN extravasation at the submucosa and subsequent PMN interstitial migration may serve as a novel regulatory step of PMN effector function and recruitment to the luminal space in inflamed intestines.

The movement of leukocytes across endothelium [referred to as diapedesis or transendothelial migration (TEM)] is a critical step in the inflammatory process. Recently, it was demonstrated that treatment of endothelial cells and monocytes with antibodies against poliovirus receptor (PVR; CD155) and DNAX-associated molecule-1 (DNAM-1; CD226) arrested monocytes over endothelial junctions and prevented TEM, suggesting that these molecules are involved in diapedesis. However, nothing was known about the mechanism by which PVR and DNAM-1 work in TEM. Herein, we show that, similar to endothelial PECAM interacting with leukocyte PECAM, activation of endothelial PVR with anti-PVR antibodies or interaction with its ligand, DNAM-1, results in recruitment of the tyrosine phosphatase Shp-2, and this process is dependent on Src kinases. Furthermore, differential and sequential treatment with blocking antibodies directed against PVR, DNAM-1, PECAM, and CD99 showed that endothelial PVR and monocyte DNAM-1 interact at and regulate a step between those regulated by PECAM and CD99. Further studies demonstrate that PVR resides in the recently identified lateral border recycling compartment, similar to PECAM and CD99. These findings suggest that the localization of adhesion/signaling molecules to the lateral border recycling compartment and the recruitment of Shp-2 may be common mechanisms for the regulation of TEM by endothelial cells.