Platelet activation leads to activation and propagation of the complement system

ESSENTIAL ROLE FOR PLATELETS IN ORGAN INJURY AND INFLAMMATION IN RESUSCITATED HEMORRHAGIC SHOCK

Platelets are known to contribute to ischemia/reperfusion in several organs, but their role in inflammation and organ injury after hemorrhagic shock (HS) has not been examined. To address this issue, we rendered mice thrombocytopenic (20% of normal platelet count) by treatment with pOp3, a rat monoclonal antibody against platelet glycoprotein Ibalpha, 24 h before subjecting them to either a standard HS or sham protocol. Liver apoptosis increased 3- to 5-fold (P<0.05), and focal liver necrosis increased 11-fold (P<0.01) in placebo-treated shock mice compared with sham; these increased indices of liver injury were completely prevented by pOp3 pretreatment. Neutrophils infiltrating the liver increased nearly 3-fold in placebo-treated shock mice versus sham (P<0.05); this shock-induced increase in neutrophil infiltration was also eliminated by pretreatment with pOp3. Alveolar cross-sectional area, used to histologically assess interstitial lung edema and cellular infiltration, was reduced by 25% in pOp3-treated shock mice versus placebo-treated shock mice (P<0.05). Similar to the results in liver, pOp3 pretreatment decreased neutrophil infiltration in the lung after HS. Thus, platelets contribute to the inflammatory injuries of the liver and lung after HS, in part, perhaps by facilitating neutrophil infiltration into tissues.

Blood platelets activate the classical pathway of human complement

OBJECTIVE

Activation of the complement system plays a key role in inflammation associated with vascular injury. Recently, platelet P-selectin was shown to activate C3 via the alternative pathway of human complement. As platelets also posses binding sites for C1q, the recognition unit of the classical complement pathway, the present study examined classical pathway activation on platelets.

METHODS

Complement activation was assessed by either a solid phase enzyme-linked immunosorbent assay (ELISA) or flow cytometry.

RESULTS

Using the ELISA approach, 2- to 10-fold increases (P < 0.001) in C1q and C4d deposition were demonstrated on adherent platelets following exposure (60 min 37 degrees C) to diluted (1/10) human plasma or serum. Similar results were obtained by flow cytometry using activated platelets in suspension. C1q and C4d deposition on platelets was accompanied by an approximately 4-fold increase in fluid phase C4d and C3a generation. Consistent with activation of the classical complement pathway, C4 cleavage failed to occur in serum depleted of C1q but was unchanged in factor B deficient serum. C4 activation was enhanced by platelet stimulation using chemical (SFLLRN peptide) or mechanical (shear) means, and decreased following platelet exposure to plasmin. These treatments were accompanied by changes in platelet surface gC1qR/p33 expression, a cellular C1q binding protein. In purified systems, recombinant gC1qR/p33 supported C4 activation, in a C1q dependent manner.

CONCLUSION

These data provide the first evidence for C1q dependent classical complement pathway activation on platelets, and support a role for gC1qR/p33 in this process. However, monoclonal antibodies (mAb) to gC1qR/p33 produced only modest (20% +/- 8%, mean +/- SD, n = 5) reductions in C4 activation on platelets. Thus, further studies are required to investigate the involvement of additional platelet membrane constituents in classical complement pathway activation.

C3a levels and occurrence of subdermal vascular thrombosis are age-related in deep second-degree burn wounds

BACKGROUND

After second-degree burns, thrombosis of the subdermal vascular plexus may occur, necessitating epifascial necrectomy instead of tangential excision to ensure split skin graft healing. Until now, no parameter has been obtained to explain this phenomenon.

METHODS

Thirty-four patients with deep second-degree burn wounds were divided into 2 groups. In group 1, patients' age was < 60 years, in group 2, > 60 years. In each patient, 2 microdialysis catheters were introduced into subdermal tissue of deep second-degree thermal wounds immediately after admission. Another 2 catheters were introduced in control tissue. We measured biochemical parameters (lactate, glycerol and glucose) and complement 3a (C3a) until surgery was performed. The surgically removed tissue was examined histologically.

RESULTS

In thermal wounds of both groups, glucose levels fell, but lactate and glycerol levels rose compared to healthy tissue. Within the first 24 hours after trauma, C3a levels were significant higher in both groups, compared with controls (P < .01). After 24 hours the levels in group 1 had fallen to nonsignificant values, while in group 2 these levels remained high until surgery was performed (P < .001). We found significantly more thrombotic blood vessels in deep dermal tissue of group 2 (P < .005). Abbreviated burn severity index score was comparable in both groups.

CONCLUSIONS

C3a is continuously elevated in deep second-degree burned wounds in patients > 60 years. This finding may be related to the occurrence of significantly more thrombotic blood vessels in deep dermal tissue of elderly patients. Microdialysis therapy is a useful tool to measure metabolic and immunologic parameters in thermally damaged tissue.

Regulation of platelet functions by P2 receptors

The main role of blood platelets is to ensure primary hemostasis, which is the maintenance of vessel integrity and cessation of bleeding upon injury. While playing a major part in acute arterial thrombosis, platelets are also involved in inflammation, atherosclerosis, and angiogenesis. ADP and ATP play a crucial role in platelet activation, and their receptors are potential targets for antithrombotic drugs. The ATP-gated cation channel P2X(1) and the two G protein-coupled ADP receptors, P2Y(1) and P2Y(12), selectively contribute to platelet aggregation and formation of a thrombus. Owing to its central role in the growth and stabilization of a thrombus, the P2Y(12) receptor is an established target of antithrombotic drugs such as clopidogrel. Studies in P2Y(1) and P2X(1) knockout mice and selective P2Y(1) and P2X(1) antagonists have shown that these receptors are also attractive targets for new antithrombotic compounds. The potential role of platelet P(2) receptors in the involvement of platelets in inflammatory processes is also discussed.

Survey of the year 2005 commercial optical biosensor literature

We identified 1113 articles (103 reviews, 1010 primary research articles) published in 2005 that describe experiments performed using commercially available optical biosensors. While this number of publications is impressive, we find that the quality of the biosensor work in these articles is often pretty poor. It is a little disappointing that there appears to be only a small set of researchers who know how to properly perform, analyze, and present biosensor data. To help focus the field, we spotlight work published by 10 research groups that exemplify the quality of data one should expect to see from a biosensor experiment. Also, in an effort to raise awareness of the common problems in the biosensor field, we provide side-by-side examples of good and bad data sets from the 2005 literature.

Cutaneous vasculitis

Vasculitis is defined as inflammation of blood vessels and can affect multiple organs. Several classification systems exist to categorize vasculitis such as vessel size, presence of anti-neutrophil cytoplasmic antibody, pathogenesis of the inflammation, and type of inflammatory cell infiltrate. Cutaneous vasculitis occurs as a manifestation of many diseases including rheumatologic diseases, hypersensitivity syndromes, infections, and malignancies. The diagnosis of the cutaneous vasculitis and the underlying cause requires a complete history and physical exam and usually a biopsy or angiogram. The treatment depends on the etiology of the inflammation and includes immunosuppression, withdrawal of the offending agent, antibacterial/antiviral agents, and chemotherapies. A clear understanding and approach to this condition will improve the physician's ability to provide optimal patient care.

Defective thrombus formation in mice lacking coagulation factor XII

Blood coagulation is thought to be initiated by plasma protease factor VIIa in complex with the membrane protein tissue factor. In contrast, coagulation factor XII (FXII)–mediated fibrin formation is not believed to play an important role for coagulation in vivo. We used FXII-deficient mice to study the contributions of FXII to thrombus formation in vivo. Intravital fluorescence microscopy and blood flow measurements in three distinct arterial beds revealed a severe defect in the formation and stabilization of platelet-rich occlusive thrombi. Although FXII-deficient mice do not experience spontaneous or excessive injury-related bleeding, they are protected against collagen- and epinephrine-induced thromboembolism. Infusion of human FXII into FXII-null mice restored injury-induced thrombus formation. These unexpected findings change the long-standing concept that the FXII-induced intrinsic coagulation pathway is not important for clotting in vivo. The results establish FXII as essential for thrombus formation, and identify FXII as a novel target for antithrombotic therapy.