Complement System Part II: Role in Immunity.

The complement system has been considered for a long time as a simple lytic cascade, aimed to kill bacteria infecting the host organism. Nowadays, this vision has changed and it is well accepted that complement is a complex innate immune surveillance system, playing a key role in host homeostasis, inflammation, and in the defense against pathogens. This review discusses recent advances in the understanding of the role of complement in physiology and pathology.

Inhibition of the mTORC pathway in the antiphospholipid syndrome.

Background: Although thrombosis is considered the cardinal feature of the antiphospholipid syndrome, chronic vascular lesions are common, particularly in patients with life-threatening complications. In patients who require transplantation, vascular lesions often recur. The molecular pathways involved in the vasculopathy of the antiphospholipid syndrome are unknown, and adequate therapies are lacking.

The interaction between factor H and VWF increases factor H cofactor activity and regulates VWF prothrombotic status.

Vascular endothelial cells (ECs) link hemostasis, thrombosis, and complement. ECs synthesize both the clotting initiator von Willebrand factor (VWF) and the complement regulator factor H (FH). VWF is stored in EC Weibel-Palade bodies (WPBs), but the intracellular location of FH is not well defined.

Complement activation by heme as a secondary hit for atypical hemolytic uremic syndrome.

Atypical hemolytic uremic syndrome (aHUS) is characterized by genetic and acquired abnormalities of the complement system leading to alternative pathway (AP) overactivation and by glomerular endothelial damage, thrombosis, and mechanical hemolysis. Mutations per se are not sufficient to induce aHUS, and nonspecific primary triggers are required for disease manifestation. We investigated whether hemolysis-derived heme contributes to aHUS pathogenesis.

A prevalent C3 mutation in aHUS patients causes a direct C3 convertase gain of function.

Atypical hemolytic uremic syndrome (aHUS) is a rare renal thrombotic microangiopathy commonly associated with rare genetic variants in complement system genes, unique to each patient/family. Here, we report 14 sporadic aHUS patients carrying the same mutation, R139W, in the complement C3 gene. The clinical presentation was with a rapid progression to end-stage renal disease (6 of 14) and an unusually high frequency of cardiac (8 of 14) and/or neurologic (5 of 14) events.

M-ficolin and leukosialin (CD43): new partners in neutrophil adhesion.

M-ficolin specificity for sialylated ligands prompted us to investigate its interactions with the main membrane sialoprotein of human neutrophils, CD43. rM-ficolin bound CD43 and prevented the access of anti-CD43 mAb. Moreover, rM-ficolin reacted exclusively with CD43 on Western blots of neutrophil lysate.

LPS-induced galectin-3 oligomerization results in enhancement of neutrophil activation.

Galectin-3 (Gal 3) is a glycan-binding protein that can be secreted by activated macrophages and mast cells at inflammation sites and plays an important role in inflammatory diseases caused by Bacteria and their products, such as lipopolysaccharides (LPS). Although it is well established that Gal 3 can interact with LPS, the pathophysiological importance of LPS/Gal 3 interactions is not fully understood. Data presented herein demonstrate for the first time that the interaction of Gal 3, either via its carbohydrate binding C-terminal domain or via its N-terminal part, with LPS from different bacterial strains, enhances the LPS-mediated neutrophil activation in vitro.

Functional complement C1q abnormality leads to impaired immune complexes and apoptotic cell clearance.

C1q plays a key role in apoptotic cell and immune complex removal. Its absence contributes to the loss of tolerance toward self structures and development of autoimmunity. C1q deficiencies are extremely rare and are associated with complete lack of C1q or with secretion of surrogate C1q fragments.

Alternative complement pathway assessment in patients with atypical HUS.

The atypical Hemolytic Uremic Syndrome (aHUS) is a rare thrombotic microangiopathy leading to end stage renal disease in approximately 60% of patients. Over the last decade, a clear link has been demonstrated between this disease and defective complement regulation. The hallmark of the aHUS is the association with mutations in complement alternative pathway genes.

Complement alternative pathway acts as a positive feedback amplification of neutrophil activation.

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.

Human neutrophil integrin alpha9beta1: up-regulation by cell activation and synergy with beta2 integrins during adhesion to endothelium under flow.

Neutrophil beta1 integrin expression and contribution to cell adhesion were revisited in this study. alpha9beta1 and alpha5beta1 appeared here as the main beta1 integrins expressed on the membrane of resting platelet-depleted neutrophils-alpha6beta1 representing <15% and alpha2beta1 undetectable. Neutrophil activation slightly enhanced alpha5 expression, did not change alpha6, but resulted in a two- to threefold increase of alpha9beta1, which then became the major beta1 integrin of the neutrophil membrane.

Hyperfunctional C3 convertase leads to complement deposition on endothelial cells and contributes to atypical hemolytic uremic syndrome.

Complement is a major innate immune defense against pathogens, tightly regulated to prevent host tissue damage. Atypical hemolytic uremic syndrome (aHUS) is characterized by endothelial damage leading to renal failure and is highly associated with abnormal alternative pathway regulation. We characterized the functional consequences of 2 aHUS-associated mutations (D(254)G and K(325)N) in factor B, a key participant in the alternative C3 convertase.

The macrophage-derived neutrophil chemotactic factor, MNCF: a lectin with TNF-alpha-like activities on neutrophils.

The macrophage-derived neutrophil chemotactic factor (MNCF) is an alpha-galactoside-binding lectin, known to induce dexamethasone-insensitive neutrophil recruitment. We further characterized MNCF effects on neutrophils and showed that it shares with TNF-alpha the ability to delay apoptosis and to trigger degranulation. MNCF and TNF-alpha effects show similar kinetics and involve Src kinases and MAPKinases dependent pathways.

The cleavage of neutrophil leukosialin (CD43) by cathepsin G releases its extracellular domain and triggers its intramembrane proteolysis by presenilin/gamma-secretase.

The highly negatively charged membrane sialoglycoprotein leukosialin, CD43, is shed during neutrophil activation. This is generally thought to enhance cell adhesion. We here describe two novel consequences of this shedding, during neutrophil activation by phorbol esters or by chemoattractants after TNF-alpha priming.

Increased membrane expression of proteinase 3 during neutrophil adhesion in the presence of anti proteinase 3 antibodies.

We investigated membrane proteinase 3 (mPR3) expression during TNF-alpha-induced adhesion of neutrophils in the presence of anti-PR3 antibodies, a situation occurring during anti-neutrophil cytoplasmic autoantibodies (ANCA)-associated vasculitis. Three increasing levels of mPR3 expression were observed on the mPR3(+) neutrophil subset after stepwise cell activation. TNF-alpha activation without adhesion, TNF-alpha-induced adhesion, and adhesion in the presence of anti-PR3 mAb or human anti-PR3 ANCA resulted, respectively, in a two-, seven-, and 24-fold increase of mPR3 levels.

Distinct signaling pathways are involved in leukosialin (CD43) down-regulation, membrane blebbing, and phospholipid scrambling during neutrophil apoptosis.

Although leukosialin (CD43) membrane expression decreases during neutrophil apoptosis, the CD43 molecule, unexpectedly, is neither proteolyzed nor internalized. We thus wondered whether it could be shed on bleb-derived membrane vesicles. Membrane blebbing is a transient event, hardly appreciated during the asynchronous, spontaneous apoptosis of neutrophils.

TNF-induced beta2 integrin activation involves Src kinases and a redox-regulated activation of p38 MAPK.

We previously demonstrated that the TNF-alpha-induced inside-out signaling leading to beta(2) integrin activation is redox regulated. To identify kinases involved in this pathway, the effects of kinase inhibitors on the expression of beta(2) integrin activation neoepitope (clone 24) were investigated. We show that both p38 MAPK (inhibited by SB203580) and Src kinases (inhibited by PP2) are involved in beta(2) integrin activation by TNF and oxidants in human neutrophils.

Long-term reversal of established autoimmunity upon transient blockade of the LFA-1/intercellular adhesion molecule-1 pathway.

Transgenic models and administration of mAbs directed against the LFA-1/intercellular adhesion molecule 1 (ICAM-1) pathway have shown that these costimulatory molecules play a key role in generating effector cells mediating inflammatory responses. In this report, durable remission of recent diabetes in nonobese diabetic (NOD) mice was induced by transient expression of an immunoadhesin gene encoding the soluble form of ICAM-1 (sICAM-1/Ig). A single i.