Under normal conditions, activation of the classical, alternative, or lectin complement pathways results in the formation of a C5 convertase enzyme capable of cleaving C5 to C5a and C5b.^1,2 C5b is a key constituent of the Terminal Complement Complex and has a variety of functions. C5a is a low molecular weight protein fragment of 74 amino acids.³ C5a is rapidly metabolized by the serum enzyme carboxypeptidase to a more stable, less active, 73 amino acid form, C5a des-Arg.^2,3 Both forms are measured by the C5a immunoassay. The C5a assay is highly specific and quantitative for measuring C5a levels which are elevated in conditions associated with complement pathway activation. C5a has numerous biologic functions including mast cell degranulation, chemotaxis, leukosequestration, as well as cellular activation via binding to the C5a receptor (C5aR or CD88). Elevated levels of C5a may be associated  with a variety of disease states including atypical hemolytic uremic syndrome, myocardial infarction, stroke, vascular leak syndrome and acute kidney injury.

The drug eculizumab, a terminal complement inhibitor, has been shown to inhibit hemolysis in patients with paroxysmal nocturnal hemoglobinuria (PNH) and to inhibit complement-mediated thrombotic microangiopathy in patients with atypical HUS (aHUS).^4,5 Eculizumab is a humanized monoclonal antibody that binds to the complement protein C5 with high affinity, thereby inhibiting its cleavage to C5a and C5b and preventing the generation of the cytolytic terminal complement complex, C5b-9. Elevated plasma levels of sC5b-9 may be an indicator of an insufficient level of eculizumab to maintain blocking the formation of the terminal attack complex.^6,7

Atypical HUS patients have been shown to display increased levels of C3a, C5a and C5b-9 in plasma prior to treatment.^8,9 Elevated plasma levels of the biomarkers C3a C5a may reflect ongoing activation even though full inhibition of the global activity (CH50 and sC5b-9) has been demonstrated.^10,11

REFERENCES:

1. Tack, Brian F., Sam C. Morris, and James W. Prahl. “Fifth component of human complement: purifi cation from plasma and polypeptide chain structure.” Biochemistry 18(8) (1979): 1490-1497.
2. Guo, Ren-Feng and Peter A. Ward. “Role of C5a in Infl ammatory Responses.” Annu. Rev. Immunol. 23 (2005): 821-852.
3. Hugli, T. E. “Biochemistry and biology of anaphylatoxins.” Complement 3(3) (1986): 111-127.
4. Hillmen P, Hall C, Marsh JC, et al.  Effect of eculizumab on hemolysis and transfusion requirements in patients with paroxysmal nocturnal hemoglobinuria.  N Engl J Med. 2004; 350:552-9.
5. Legendre, CM, Licht, C, Muus, P, et al.  Terminal complement inhibitor eculizumab in atypical hemolytic-uremic syndrome.  N Engl J Med 2013; 368:2169-81.
6. Prüfer, F, Scheiring, J, Sautter, S, et al.  Terminal complement complex (C5b-9) in children with recurrent hemolytic uremic syndrome.  Semin Thromb Hemost 2006; 32:121-7.
7. Zimmerhackl, LB, Hofer, J, Cortina G, et al.  Prophylactic eculizumab after renal transplantation in atypical hemolytic-uremic syndrome.  N Engl J Med. 2010; 362:1746-8.
8. Mache CJ, Acham-Roschitz B, Fremeaux-Bacchi V, Kirschfink M, Zipfel PF, Roedi S, Vester U, Ring E.  Complement inhibitor eculizumab in atypical hemolytic uremic syndrome Clin J Am Soc Nephrol 2009: 4: 1312-16.
9. Wehling C, Amon O, Hohenstein B, Pape L, Bommer M, Kirschfink M.  Eculizumab drug monitoring in serum and urine opens new insights into therapy of complement-medicated nephropathies.  Mol Immunol 2013; 56: 257.
10. Spero R, Cataland V, Holers M, Geyer S, Yang S, and Wu HM.  Biomarkers of the alternative pathway and terminal complement activity at presentation confirms the clinical diagnosis of aHUS and differentiates aHUS from TTP.  Blood 2014; 123: 3733-8.
11. Noris M, Galbusera M, Gastoldi S, Macor P, Banteria F, Bresin E, Tripodo C, Bettoni A, Donadelli R, Valoti E, Tedesco F, Amore A, Coppo R, Ruggenenti P, Gotti E and Remuzzi G.  Dynamics of complement activation in aHUS and how to monitor eculizumab therapy.  Blood 2014 124:1715-1726.