Identification of a Novel Mode of Complement Activation on Stimulated Platelets Mediated by Properdin and C3(H2O)

Abstract

Artículo de publicación ISIElevated numbers of activated platelets circulate in patients with chronic inflammatory diseases, including atherosclerosis and coronary disease. Activated platelets can activate the complement system. Although complement activation is essential for immune responses and removal of spent cells from circulation, it also contributes to inflammation and thrombosis, especially in patients with defective complement regulation. Proinflammatory activated leukocytes, which interact directly with platelets in response to vascular injury, are among the main sources of properdin, a positive regulator of the alternative pathway. The role of properdin in complement activation on stimulated platelets is unknown. Our data show that physiological forms of human properdin bind directly to human platelets after activation by strong agonists in the absence of C3, and bind nonproportionally to surface CD62P expression. Activation of the alternative pathway on activated platelets occurs when properdin is on the surface and recruits C3b or C3(H2O) to form C3b,Bb or a novel cell-bound C3 convertase [C3(H2O),Bb], which normally is present only in the fluid phase. Alternatively, properdin can be recruited by C3(H2O) on the platelet surface, promoting complement activation. Inhibition of factor H–mediated cell surface complement regulation significantly increases complement deposition on activated platelets with surface properdin. Finally, properdin released by activated neutrophils binds to activated platelets. Altogether, these data suggest novel molecular mechanisms for alternative pathway activation on stimulated platelets that may contribute to localization of inflammation at sites of vascular injury and thrombosis

Properdin binding to complement activating surfaces depends on initial C3b deposition

“…In contrast to our findings with exogenous ligands, previously shown with zymosan and E. coli (35) and in the present work with N. meningitidis, there is evidence of C3-independent properdin binding to ligands on endogenous cells, such as DNA on late apoptotic or necrotic cells (26), and presumably various glycosaminoglycans on early apoptotic T cells, but not necrotic T cells (25), activated platelets (27), and renal tubular epithelial cells (39,40). To the best of our knowledge, no data for endothelial cells and properdin binding have been reported previously, even though endothelial cells are a source of serum properdin and can express heparin sulfate proteoglycans, which have been shown to bind properdin (39,48).…”

“…direct recognition by properdin for AP complement activation. Reported patterns include exogenous microorganisms (7,24), endogenous cells (25)(26)(27), and various biological substrates (9,28,29); however, many of these experiments were performed in systems permitting either C3 activation with initial C3b deposition or in buffer systems with purified properdin. In the presence of intact C3, it is virtually impossible to demonstrate whether properdin acts in a recognition manner or subsequently binds to C3b.…”

Monomeric C-reactive protein inhibits renal cell-directed complement activation mediated by properdin

“…Since properdin bound to mCRP is still capable of binding C3b, this indicates that mCRP notably targets the pattern recognition ability of properdin, independent of complement activation. The amounts of properdin used for experiments were within the normal physiological levels of properdin in the circulation, which are ϳ4 -25 g/ml (22,38). The level of 3.3 g/ml mCRP induced significant inhibition of properdin binding.…”

“…Both cell culture mediums used contained calcium as mentioned above. A fixed concentration of human properdin (5 or 10 g/ml, both within the serum physiological normal range of 4 -25 g/ml) (38) or medium alone was preincubated in the presence or absence of normal human serum (NHS) or pCRP or mCRP for a minimum of 30 min at 4°C. Cells were washed using their respective medium and exposed to their respective preincubated mix for 60 min at 4°C.…”

“…It has been demonstrated that human serum can inhibit the ability of properdin to interact with platelets; however, the exact mechanism is still unclear (38). Intriguingly, during an acute phase response, properdin levels decrease in the circulation in contrast to various other serum proteins, such as C-reactive protein (CRP), which show an increase (36).…”