Neutrophils recruited by chemoattractants in vivo induce microvascular plasma protein leakage through secretion of TNF.

Microvascular plasma protein leakage is an essential component of the inflammatory response and serves an important function in local host defense and tissue repair. Mediators such as histamine and bradykinin act directly on venules to increase the permeability of endothelial cell (EC) junctions. Neutrophil chemoattractants also induce leakage, a response that is dependent on neutrophil adhesion to ECs, but the underlying mechanism has proved elusive. Through application of confocal intravital microscopy to the mouse cremaster muscle, we show that neutrophils responding to chemoattractants release TNF when in close proximity of EC junctions. In vitro, neutrophils adherent to ICAM-1 or ICAM-2 rapidly released TNF in response to LTB4, C5a, and KC. Further, in TNFR(-/-) mice, neutrophils accumulated normally in response to chemoattractants administered to the cremaster muscle or dorsal skin, but neutrophil-dependent plasma protein leakage was abolished. Similar results were obtained in chimeric mice deficient in leukocyte TNF. A locally injected TNF blocking antibody was also able to inhibit neutrophil-dependent plasma leakage, but had no effect on the response induced by bradykinin. The results suggest that TNF mediates neutrophil-dependent microvascular leakage. This mechanism may contribute to the effects of TNF inhibitors in inflammatory diseases and indicates possible applications in life-threatening acute edema.

Chemokines and their receptors in allergic disease.

Mechanisms of chemoattraction underlie the spatial organization of the cells of the immune system under basal conditions and the localization of these cells to sites of inflammation. The chemokines, a family of around 50 small proteins, play a major role in these processes. Leukocytes are equipped with cell-surface sensors for chemokines. There are 19 such receptors that are differentially expressed on leukocytes: the repertoire of receptor expression depending on the type of leukocyte and its stage in maturation. From observations in animal models, clinical studies, in vitro cell biology, and molecular analysis, a working hypothesis has been established to explain the cellular interactions underlying allergic responses and the chemokines-chemokine receptors involved. Chemokines signal through G protein-coupled receptors that are used typically for sensory functions (eg, detection of olfactory signals in the nose). This type of receptor can be blocked selectively by small-molecule antagonists. This provides the opportunity for the development of therapeutic compounds designed to suppress the recruitment of particular leukocyte types in allergic reactions.

Regulation of human eotaxin-3/CCL26 expression: modulation by cytokines and glucocorticoids.

Eotaxin-3 (CCL26) is a CC chemokine that signals exclusively via the CCR3 receptor and has eosinophil-selective chemoattractant activity. Comparison of Eotaxin-1 (CCL11) and Eotaxin-2 (CCL24), demonstrates differences in their expression profiles, cell specificity and effector kinetics, implying distinct biological actions. But little data in this regard have been reported for Eotaxin-3. We aimed to analyse the effect of Th2 cytokines and glucocorticoids on Eotaxin-3 mRNA expression in human lung epithelial cells and dermal fibroblasts; cells implicated in the pathogenesis of allergic asthma and allergic dermatitis respectively. Eotaxin-3 mRNA levels in primary dermal fibroblasts and NCI-H727 lung epithelial cells were determined by Northern hybridization. In contrast to Eotaxin-1, Eotaxin-3 mRNA expression was not detected in unstimulated cells. The Th2 cytokines IL-4 and IL-13 induced Eotaxin-3 expression in a time and dose dependent manner, with IL-4 demonstrating a 100-fold greater potency. Unlike Eotaxin-1, Eotaxin-3 mRNA expression was not induced by either tumour necrosis factor (TNF)-alpha or interleukin (IL)-1 beta alone. Both IL-4 and IL-13 acted synergistically with TNF-alpha in superinducing Eotaxin-3 mRNA expression. Dexamethasone pre-treatment diminished induction of Eotaxin-3 mRNA expression. We conclude that modulation of Eotaxin-3 mRNA expression by Th(2) cytokines is different from that of Eotaxin-1 and Eotaxin-2, further supporting a distinct biological role for Eotaxin-3.

CCR3 functional responses are regulated by both CXCR3 and its ligands CXCL9, CXCL10 and CXCL11.

The chemokine receptor CXCR3 is predominantly expressed on T lymphocytes, and its agonists CXCL9, CXCL10 and CXCL11 are IFN-gamma-inducible chemokines that promote Th1 responses. In contrast, the CCR3 agonists CCL11, CCL24 and CCL26 are involved in the recruitment of cells such as eosinophils and basophils during Th2 responses. Here, we report that although CCL11, CCL24 and CCL26 are neither agonists nor antagonists of CXCR3, CCL11 binds with high affinity to CXCR3. This suggests that, in vivo, CXCR3 may act as a decoy receptor, sequestering locally produced CCL11. We also demonstrate that the CXCR3 ligands inhibit CCR3-mediated functional responses of both human eosinophils and CCR3 transfectants induced by all three eotaxins, with CXCL11 being the most efficacious antagonist. The examination of CCR3-CCR1 chimeric constructs revealed that CCL11 and CXCL11 share overlapping binding sites contained within the CCR3 extracellular loops, a region that was previously shown to be essential for effective receptor-activation. Hence, eosinophil responses mediated by chemokines acting at CCR3 may be regulated by two distinct mechanisms: the antagonistic effects of CXCR3 ligands and the sequestration of CCL11 by CXCR3-expressing cells. Such interplay may serve to finely tune inflammatory responses in vivo.