Platelets are necessary for airway wall remodeling in a murine model of chronic allergic inflammation

Platelet P-selectin is required for pulmonary eosinophil and lymphocyte recruitment in a murine model of allergic inflammation

Platelets are necessary for lung leukocyte recruitment in a murine model of allergic inflammation, and platelet–leukocyte aggregates are formed in circulating blood of patients with asthma after allergen exposure. However, it is unknown how platelets induce pulmonary leukocyte recruitment in asthma. Here, we have investigated the importance of platelet adhesion molecule expression on pulmonary eosinophil and lymphocyte recruitment and on leukocyte CD11b and very late antigen (VLA)–4 expression in mice. Pulmonary leukocyte recruitment in platelet-depleted mice (sensitized and exposed to ovalbumin) transfused with fixed, unstimulated platelets (FUSPs) was abolished, whereas transfusion with platelets stimulated and fixed (FSPs), expressing P-selectin and P-selectin glycoprotein ligand-1 (PSGL-1), restored eosinophil and lymphocyte recruitment. Transfusion with platelets from P-selectin–deficient mice, or with FSPs stimulated in the presence of a blocking anti–P-selectin antibody, were unable to restore pulmonary leukocyte recruitment. Flow cytometric analysis revealed increased expression of CD11b and VLA-4 on leukocytes attached to platelets after allergen exposure, and CD11b expression on leukocytes was suppressed in thrombocytopenic mice but was restored with the transfusion of FSPs, but not FUSPs, a phenomenon concurrent with the formation of platelet–leukocyte complexes. P-selectin expression on the surfaces of platelets is a major requirement for pulmonary eosinophil and lymphocyte recruitment, allowing circulating platelets to bind to and stimulate leukocytes for endothelial attachment.

Airway remodeling: lessons from animal models

Airway remodeling, an array of persistent tissue structural changes that occurs through a process of injury and dysregulated repair linked to airway chronic inflammation, is presently believed to largely account for the disease mechanisms of asthma. Increases in airway smooth muscle mass are probably the main mechanism causing airway hyperresponsiveness, and changes in the extracellular matrix may stimulate smooth muscle growth and contribute to the mechanics of airway obstruction. The various components of airway remodeling described inhuman asthma have been successfully reproduced in animal models of several species. Most of the data have been contributed by rat models of allergic sensitization and repeated challenge,transgenic mouse models of cytokine overexpression localized to the lung and, more recently, allergen-driven mouse models using wild-type inbred strains. Overall, animal model shave provided significant insights into the mechanisms of airway remodeling and recent technological developments allow us to exploit these models in new directions. However, the challenge of finding new therapeutic strategies that prevent or control airway remodeling,thus providing etiopathogenically oriented treatments for asthma, still stands. Experimental airway remodeling in animals should be an essential tool for treatment discovery in the near future.

Blood and endothelium in immune complex-mediated tissue injury

Antigen-antibody complexes can be formed both intravascularly and perivascularly and damage tissues by inducing inflammatory mechanisms. Recent studies have characterized a definite sequence of steps involved in these inflammatory mechanisms, and identified the predominance of particular chemical mediator(s) in each step. The lesions associated with this type of inflammation are characterized by the early development of plasma leakage, followed by the recruitment of polymorphonuclear leukocytes mediated by chemokines generated by FcgammaR-dependent mechanisms. The development of these lesions is modulated by endothelial cell-derived paracrine mediators, and activation of the coagulation system can ensue. The activation of platelets and coagulation, if not properly counterbalanced by fibrinolysis, might be a major factor for the late development of fibrotic changes and organ remodeling.

Human eosinophil chemotaxis and selective in vivo recruitment by sphingosine 1-phosphate

Sphingosine 1-phosphate (S1P) is a sphingolipid mediator that is involved in diverse biological functions. Local administration of S1P causes inflammation coupled to a large eosinophil (EO) recruitment in the rat-paw tissue. The inflammatory response is accompanied by an increase in S1P receptors, namely S1P(1), S1P(2), S1P(3), and by an enhanced expression of CCR3, which is the main chemokine receptor known to be involved in EO function. Human EOs constitutively express S1P(1) and, at a lower extent, S1P(2), S1P(3) receptors. S1P in vitro causes cultured human EO migration and an increase in S1P receptor mRNA copies and strongly up-regulates CCR3 and RANTES (regulated on activation, normal T cell-expressed and secreted) message levels; in particular CCR3 is up-regulated 18,000-fold by S1P. A blocking anti-CCR3 Ab inhibits S1P-induced chemotaxis, implying that S1P acts as specific recruiting signal for EOs not only through its own receptors but also through CCR3. These results show that S1P is involved in EO chemotaxis and contribute to shed light on the complex mechanisms underlying EO recruitment in several diseases such as asthma and some malignancies.