Mast Cell-Independent Mechanisms of Immediate Hypersensitivity: A Role for Platelets

Fluorescent labeling of endogenous platelets for intravital microscopy: Effects on platelet function

OBJECTIVE

Monitoring endogenous platelets during intravital microscopy often involves two approaches: fluorescently labeled antibodies or genetic models of platelet-specific fluorescent protein expression. Due to limited data available on platelet functional changes induced by these methods, we compared functional effects of these methods on platelets.

METHODS

Platelet aggregation to collagen and thrombin, and collagen matrix-mediated platelet adhesion/aggregation under flow were tested. We assessed platelets from mice expressing EYFP on platelets (Cre(+)), littermate controls (Cre(-)), C57BL/6 mice, and platelets from vehicle control and x-488 treatment. We utilized intravital microscopy to monitor platelets in vivo using Cre(+) mice and x-488 treatment.

RESULTS

Both genetic and antibody-based approaches yielded substantial platelet-specific fluorescence. Platelets from Cre(+) and Cre(-) mice behaved similarly in aggregation and adhesion/aggregation under flow. However, they exhibited significantly enhanced aggregation and higher adhesion/aggregation as compared to platelets from C57BL/6 mice. Compared to vehicle control, x-488 platelet labeling did not induce significant functional changes in vitro. Both methods of platelet labeling provided satisfactory platelet detectability in vivo.

CONCLUSIONS

x-488 antibody labeling of platelets induced less alteration of platelet function than genetic approaches under our experimental conditions and seems more suitable for monitoring of endogenous platelets.

The role of platelets in autoimmunity, vasculopathy, and fibrosis: Implications for systemic sclerosis

INTRODUCTION

Systemic sclerosis (SSc) is an autoimmune disease characterized by vasculopathy, autoimmunity, and widespread dermal and visceral fibrosis. This article summarizes the current knowledge about the potential contribution of platelets in the disease process and the rationale of targeting platelets as an adjunct treatment for SSc.

METHODS

We performed an electronic search (Medline) using the keywords platelets, systemic sclerosis, autoimmunity, fibrosis, Raynaud, and pulmonary arterial hypertension.

RESULTS

The link that connects vasculopathy, autoimmunity, and fibrosis in SSc remains obscure. Experimental data suggest that platelets are not solely cell fragments regulating hemostasis but they have a pleiotropic role in several biologic processes including immune regulation, vasculopathy, fibrosis, and all key features of SSc. Platelets interplay with the impaired endothelium, can interact with immune cells, and they are storages of bioactive molecules involved in tissue injury and remodeling. The potential role of platelets in the pathogenesis of SSc is further supported by experimental data in animal models of SSc. Platelet-derived serotonin represents a novel target in SSc and serotonin blockade is currently being tested in clinical trials.

CONCLUSION

Platelets may be actively involved in the pathogenesis of SSc by activating immune responses and facilitating the fibrotic process. However, definite conclusions cannot be drawn until more data from both basic and clinical research are available.

Platelets in Pulmonary Immune Responses and Inflammatory Lung Diseases

Platelets are essential for physiological hemostasis and are central in pathological thrombosis. These are their traditional and best known activities in health and disease. In addition, however, platelets have specializations that broaden their functional repertoire considerably. These functional capabilities, some of which are recently discovered, include the ability to sense and respond to infectious and immune signals and to act as inflammatory effector cells. Human platelets and platelets from mice and other experimental animals can link the innate and adaptive limbs of the immune system and act across the immune continuum, often also linking immune and hemostatic functions. Traditional and newly recognized facets of the biology of platelets are relevant to defensive, physiological immune responses of the lungs and to inflammatory lung diseases. The emerging view of platelets as blood cells that are much more diverse and versatile than previously thought further predicts that additional features of the biology of platelets and of megakaryocytes, the precursors of platelets, will be discovered and that some of these will also influence pulmonary immune defenses and inflammatory injury.

Food components and the immune system: from tonic agents to allergens

The intestinal mucosa is the major site of contact with antigens, and it houses the largest lymphoid tissue in the body. In physiological conditions, microbiota and dietary antigens are the natural sources of stimulation for the gut-associated lymphoid tissues (GALT) and for the immune system as a whole. Germ-free models have provided some insights on the immunological role of gut antigens. However, most of the GALT is not located in the large intestine, where gut microbiota is prominent. It is concentrated in the small intestine where protein absorption takes place. In this review, we will address the involvement of food components in the development and the function of the immune system. Studies in mice have already shown that dietary proteins are critical elements for the developmental shift of the immature neonatal immune profile into a fully developed immune system. The immunological effects of other food components (such as vitamins and lipids) will also be addressed. Most of the cells in the GALT are activated and local pro-inflammatory mediators are abundant. Regulatory elements are known to provide a delicate yet robust balance that maintains gut homeostasis. Usually antigenic contact in the gut induces two major immune responses, oral tolerance and production of secretory IgA. However, under pathological conditions mucosal homeostasis is disturbed resulting in inflammatory reactions such as food hypersensitivity. Food allergy development depends on many factors such as genetic predisposition, biochemical features of allergens, and a growing array of environmental elements. Neuroimmune interactions are also implicated in food allergy and they are examples of the high complexity of the phenomenon. Recent findings on the gut circuits triggered by food components will be reviewed to show that, far beyond their role as nutrients, they are critical players in the operation of the immune system in health and disease.

Role of reactive oxygen and nitrogen species in the vascular responses to inflammation

Inflammation is a complex and potentially life-threatening condition that involves the participation of a variety of chemical mediators, signaling pathways, and cell types. The microcirculation, which is critical for the initiation and perpetuation of an inflammatory response, exhibits several characteristic functional and structural changes in response to inflammation. These include vasomotor dysfunction (impaired vessel dilation and constriction), the adhesion and transendothelial migration of leukocytes, endothelial barrier dysfunction (increased vascular permeability), blood vessel proliferation (angiogenesis), and enhanced thrombus formation. These diverse responses of the microvasculature largely reflect the endothelial cell dysfunction that accompanies inflammation and the central role of these cells in modulating processes as varied as blood flow regulation, angiogenesis, and thrombogenesis. The importance of endothelial cells in inflammation-induced vascular dysfunction is also predicated on the ability of these cells to produce and respond to reactive oxygen and nitrogen species. Inflammation seems to upset the balance between nitric oxide and superoxide within (and surrounding) endothelial cells, which is necessary for normal vessel function. This review is focused on defining the molecular targets in the vessel wall that interact with reactive oxygen species and nitric oxide to produce the characteristic functional and structural changes that occur in response to inflammation. This analysis of the literature is consistent with the view that reactive oxygen and nitrogen species contribute significantly to the diverse vascular responses in inflammation and supports efforts that are directed at targeting these highly reactive species to maintain normal vascular health in pathological conditions that are associated with acute or chronic inflammation.

Intravital microscopy to study leukocyte recruitment in vivo

The intravital microscopy is a valuable tool to capture images of cells in living organisms and to make studies of molecular determinants of leukocyte trafficking easier. Using this technique, we can directly visualize and measure each step of the leukocyte recruitment paradigm, including leukocyte rolling flux, rolling velocity, adhesion, and emigration. Thus, it is possible to understand the process involved in leukocyte homing as well as the cell recruitment to inflammatory tissues. Nowadays, two types of intravital microscopy are used routinely. The light microscopy is used to assess migration of intravascular cells in thin, tissues which must be sufficiently translucent. Epifluorescence microscopy allows the visualization of the microcirculation while permitting the distinction of leukocyte subpopulations in solid organs.

Safety and tolerability of infliximab therapy: suggestions and criticisms based on wide clinical experience

Infliximab, an IgG1 monoclonal chimeric antibody against tumor necrosis factor (TNF)-alpha, represents the main biological drug employed for the treatment of several immuno-mediated inflammatory disorders. Infliximab infusion can be complicated by clinically heterogeneous adverse reactions, potentially interfering with the course of treatment. We analysed the adverse events recorded in 49 patients affected by different chronic inflammatory disorders (rheumatoid arthritis, seronegative spondyloarthritis, Behçet's disease, Wegener's granulomatosis, Churg-Strauss syndrome, Cogan's disease) who were receiving a total of 709 infliximab infusions, in order to correlate the development of infliximab reactions and their features to some potential risk factors. We displayed a lower frequency of infusion reactions (1.5 percent; 11 out of 709 infusions) than those previously reported. However, patients suffering from rheumatoid arthritis and/or patients who underwent re-treatment after a long period, showed a higher prevalence of infliximab-related reactions. In conclusion, in our experience infliximab treatment is rarely complicated by adverse reactions which are, more importantly, almost always mild. Some good clinical practices, such as the low rate of infusion, pre-treatment with anti-histamine and prednisone in all patients, chronic immunosuppressive therapy and avoidance of long intervals between infusions may represent a combined useful strategy to reduce the frequency of infliximab reactions and to increase safety.

The roles of mast cells and Kupffer cells in rat systemic anaphylaxis

Mast cells and other cells such as macrophages have been shown to mediate systemic anaphylaxis. We determined the roles of mast cells and Kupffer cells in hepatic and systemic anaphylaxis of rats. Roles of mast cells were examined by using the mast cell-deficient white spotting (Ws/Ws) rat; the Ws/Ws and wild type (+/+) rats were sensitized with ovalbumin (1 mg). Roles of Kupffer cells were examined by depleting Kupffer cells using gadolinium chloride or liposome-encapsulated dichloromethylene diphosphonate in the Ws/Ws and Sprague-Dawley rats. An intravenous injection of 0.6 mg ovalbumin caused substantial anaphylactic hypotension in both the Ws/Ws and +/+ rats; however, the occurrence was delayed in the Ws/Ws rats. After antigen, portal venous pressure increased by 13.1 cmH2O in the +/+ rats, while it increased only by 5.7 cmH2O in the Ws/Ws rats. In response to antigen, the isolated perfused liver of the Ws/Ws rats also showed weak venoconstriction, the magnitude of which was one tenth as large as that of the +/+ rats, indicating that hepatic anaphylaxis was primarily due to mast cells. In contrast, Kupffer cell depletion did not attenuate anaphylactic hepatic venoconstriction in isolated perfused livers. In conclusion, mast cells are involved mainly in anaphylactic hepatic presinusoidal portal venoconstriction but only in the early stage of anaphylactic systemic hypotension in rats. Macrophages, including Kupffer cells, do not participate in rat hepatic anaphylactic venoconstriction.

Mast cells and neutrophils proteolytically activate chemokine precursor CTAP-III and are subject to counterregulation by PF-4 through inhibition of chymase and cathepsin G

The CXC chemokines platelet factor 4 (PF-4/CXCL4) and connective tissue-activating peptide III (CTAP-III) are released by activated human platelets in micromolar concentrations. So far, neutrophils have been recognized to cleave the precursor CTAP-III to form the active chemokine neutrophil-activating peptide 2 (NAP-2/CXCL7) through limited proteolysis by membrane-associated cathepsin G. Here we show for the first time that activated human skin mast cells (MCs) convert CTAP-III into biologically active NAP-2 through proteolytic cleavage by released chymase. A direct comparison on a cell number basis revealed that unstimulated MCs exceed the CTAP-III–processing potency of neutrophils about 30-fold, whereas MCs activated by IgE cross-linking exhibit even 1000-fold higher CTAP-III–processing capacity than fMLP-stimulated neutrophils. Intriguingly, PF-4 counteracted MC- as well as neutrophil-mediated NAP-2 generation at physiologically relevant concentrations. Addressing the underlying mechanism, we obtained evidence that PF-4 acts as an inhibitor of the CTAP-III–processing enzymes cathepsin G and chymase without becoming cleaved itself as a competitive substrate. Because cleavage of the CTAP-III–unrelated substrate substance P was also affected by PF-4, our results suggest a regulatory role for PF-4 not only in NAP-2 generation but also in neutrophil- and MC-mediated processing of other physiologically relevant inflammatory mediators.

Activation of connective tissue-type and mucosal-type mast cells in compound 48/80-induced airway response

The pathology of non-immunological airway contraction is not well understood. To define the activation of different phenotypes of mast cells, a rat non-immunological asthmatic model was prepared. Airway contraction in rats was measured by an unrestrained whole-body plethysmographic system following a 10-min inhalation challenge with a 5% solution of compound 48/80. Histamine, leukotrein C(4) (LTC(4)) and tumor necrosis factor (TNF)-alpha levels in bronchoalveolar lavage fluid, as well as tissue histamine content were quantified. Mast cells and eosinophils were detected by histology. Both the early and late phase of airway responses were induced by inhalation of compound 48/80. Histamine and TNF-alpha levels increased significantly 30 min after challenge, but no increases were detected at either 8 or 24 h after challenge. A high LTC(4) level was detected in 30 min and 8 h after challenge. Tissue histamine content decreased at 30 min after challenge and returned to the unstimulated level by 8 h. Connective tissue mast cells in rat trachea showed a degranulation response. Along with the increase in numbers of mucosal mast cells, rat mast cell protease II at both mRNA and protein levels in the trachea epithelial layer was also increased significantly at 30 min after challenge. We conclude that compound 48/80 inhalation causes both the early and late phase of airway contraction in rats. Mast cell degranulation is responsible for the early phase of airway response, which subsequently triggers the late phase of airway response.