Complement C3a

Activation of the complement system plays an important role in our natural ability to ward off infection and in the pathogenesis of infection and inflammation.^1-6 Anaphylatoxins produced as the result of complement activation play a role in a number of infectious and inflammatory conditions, including sepsis, ischemia-reperfusion injury, immune complex diseases, and hypersensitivity diseases like asthma.¹ Anaphylatoxins are also thought to be important in the pathogenesis of allergy, autoimmunity, neurodegenerative diseases, and cancer.^2,3

Complement activation can occur through three separate mechanisms. The first mechanism to be discovered, referred to as the classical complement cascade, is activated by antigen-antibody complexes.^4-6 This was the basis for the name of this system as they serve to "complement" humoral immunity. Alternatively, the complement cascade can be activated directly by contact with bacterial cell surface molecules, including lipopolysaccharide from gram-negative outer membranes, teichoic acid from gram-positive cell walls, zymosan from fungal and yeast cell walls, and some parasite surface molecules. Recently, a third activation mechanism has been characterized in which mannose-binding lectin synthesized by the liver in response to inflammatory macrophage cytokines stimulates the activation of complement.

Complement cascade activation results in the formation of complement split products C₃a, C₄a, and C₅a.⁴ These proteins, referred to as anaphylatoxins, facilitate the phagocytosis of immune complexes, viral particles, toxic cell debris, and apoptotic corpses. Anaphylatoxins promote an inflammatory response by binding to complement receptors on granulocytes and macrophages.² Anaphylatoxins also bind to receptors on mast cells that trigger the release of histamine, increasing blood vessel permeability and smooth muscle contraction.⁶ They control the local inflammatory response through activation of leukocytes and stimulating their chemotaxis to the site of infection.⁴

Complement C₃a levels can become increased in any condition associated with inflammation.^2,3 Normal human pregnancy is associated with evidence of complement activation, with an increase in concentrations of the anaphylatoxins C₃a, C₄a, and C₅a in the maternal circulation.⁷ Gloor and associates found that plasma levels of complement C₃a measured daily during the first week after onset of symptoms of acute pancreatitis represent highly specific and sensitive parameters for the prediction of severe acute pancreatitis.⁸ Levels of anaphylatoxins C₃a and C₄a have also been found to be elevated in patients with antiphospholipid syndrome relative to healthy controls.⁹

Complement C₃ has been implicated in the development of allergic inflammation associated with asthma.¹⁰ Nakano and coworkers found that plasma C₃a concentrations were significantly higher in patients seeking emergency treatment for severe asthma exacerbations than in control subjects with stable asthma.¹¹ They found that C₃a concentrations were highest among patients who failed to improve with treatment with inhaled bronchodilators and intravenous corticosteroids.¹¹ They also observed that levels of plasma C₃a in admitted asthmatic patients decreased significantly within a week after admission.¹¹

Complement activation split products are present only in trace amounts in normal plasma in vivo.¹² It is crucial that samples be collected and stored properly in order to avoid in vitro activation.¹² Blood must be drawn directly into tubes containing EDTA at a final concentration of at least 10 mM.¹² Citrate and heparin do not block complement activation efficiently and should not be used.¹² The addition of nafamostat mesilate (Futhan, FUT-175) further reduces in vitro complement activation.¹³