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Complement alternative pathway plays an important, but not clearly understood, role in neutrophil-mediated diseases. We here show that neutrophils themselves activate complement when stimulated by cytokines or coagulation-derived factors. In whole blood, tumor necrosis factor/formyl-methionyl-leucyl-phenylalanine or phorbol myristate acetate resulted in C3 fragments binding on neutrophils and monocytes, but not on T cells. Neutrophils, stimulated by tumor necrosis factor, triggered the alternative pathway on their surface in normal and C2-depleted, but not in factor B-depleted serum and on incubation with purified C3, factors B and D. This occurred independently of neutrophil proteases, oxidants, or apoptosis. Neutrophil-secreted properdin was detected on the cell surface and could focus “in situ” the alternative pathway activation. Importantly, complement, in turn, led to further activation of neutrophils, with enhanced CD11b expression and oxidative burst. Complement-induced neutrophil activation involved mostly C5a and possibly C5b-9 complexes, detected on tumor necrosis factor- and serum-activated neutrophils. In conclusion, neutrophil stimulation by cytokines results in an unusual activation of autologous complement by healthy cells. This triggers a new amplification loop in physiologic innate immunity: Neutrophils activate the alternative complement pathway and release C5 fragments, which further amplify neutrophil proinflammatory responses. This mechanism, possibly required for effective host defense, may be relevant to complement involvement in neutrophil-mediated diseases.

Thrombotic thrombocytopenic purpura (TTP) is a life-threatening disease that occurs mainly in young adults. Acquired cases are usually a result of antibodies directed against ADAMTS13 (a disintegrin-like and metalloprotease [reprolysin type] with thrombospondin type 1 motif 13), a protease that cleaves the von Willebrand factor multimers. Prognosis has been improved by plasma therapy, but some acute severe forms are refractory to this treatment and achieving a sustained remission is still a challenge in chronic relapsing forms. We therefore conducted a multicentric open-label prospective trial to test the efficacy of rituximab, an anti–B-cell monoclonal antibody, as a curative and prophylactic treatment in patients with TTP as a result of anti-ADAMTS13 antibodies. Six patients were included during an acute refractory TTP episode. Five patients with severe relapsing TTP and persistent anti-ADAMTS13 antibodies were prophylactically treated during remission. All patients received 4 weekly infusions of rituximab. The target of treatment was to restore a significant ADAMTS13 plasma activity (> 10%). Treatment with rituximab led to clinical remission in all cases of acute refractory TTP. In all patients, anti-ADAMTS13 antibodies disappeared, and a significant (18%-75%) plasma ADAMTS13 activity was detected following treatment. Tolerance of rituximab was good. Rituximab is a promising first-line immunosuppressive treatment in patients with acute refractory and severe relapsing TTP related to anti-ADAMTS13 antibodies.

Proteinase 3 (PR3), which is also called myeloblastin, the target autoantigen for antineutrophil cytoplasmic antibodies (ANCA) in Wegener’s granulomatosis, is a serine proteinase stored in azurophil granules of human neutrophils. We have previously shown that, in contrast to elastase or myeloperoxidase, PR3 is also expressed at the plasma membrane of a subset of unactivated neutrophils and that a high proportion of neutrophils expressing membrane PR3 is a risk factor for vasculitis. The present study demonstrates that the association of PR3 with the plasma membrane is not an ionic interaction and seems to be covalent. Fractionation of neutrophils shows that, besides the azurophil granules, PR3 could be detected both in specific granules and in the plasma membrane-enriched fraction containing secretory vesicles, whereas elastase and myeloperoxidase were exclusively located in azurophil granules. Electron microscopy confirms that PR3 is present along with CR1 in secretory vesicles as well as in some specific granules. In neutrophils stimulated with an increasing dose of FMLP, membrane PR3 expression increased with the degranulation of secretory vesicles, followed by specific granules, and culminated after azurophil granules mobilization. The presence of a readily plasma membrane-mobilizable pool of PR3 contained in the secretory vesicles might play a relevant role in the pathophysiological mechanisms of ANCA-associated vasculitis.

Using affinity-purified 125l-F(ab')2anti-human C3, we have investigated the ability of various leukemic cells to activate complement. Lymphocytes from patients with chronic lymphocytic leukemia (CLL) activated the alternative pathway, but cells from patients with other forms of leukemia or normal lymphocytes did not do so. The amount of C3 deposited on the CLL cells was significantly higher in patients with organomegaly (i.e., splenomegaly and/or hepatomegaly). Activation of complement by CLL cells as assessed by C3 deposition on the membrane occurred both in vivo and in vitro and was not related to the N-acetylneuraminic acid content of the membrane.