- Factor B
- Factor H
- Factor I
- C3 Nephritic Factor
- Factor H Autoantibody
Thrombotic microangiopathy (TMA) is a category of diseases linked by a common pathomechanism of endothelial injury leading to microvascular thrombosis and fibrin deposition, aggregation of platelets on the damaged endothelium, and organ dysfunction related to microvascular injury. This category includes Shiga toxin-producing E. coli associated hemolytic uremic syndrome (STEC-HUS), atypical hemolytic uremic syndrome (aHUS), and thrombotic thrombocytopenic purpura (TTP), among others. Atypical HUS is associated with defects in the regulation of the alternative complement pathway leading to uninhibited formation of the C3 convertase C3bBb on the endothelium and subsequent microvascular injury, whereas TTP is caused by the inability of ADAMTS13 to cleave ultra large multimers of von Willebrand Factor (vWF), with resulting adhesion of these ultra large vWF multimers to the endothelial surface under shear stress leading to microvascular thrombosis and injury. The evaluation of thrombotic microangiopathy includes testing the regulatory components of the alternative complement pathway for aHUS and the activity of ADAMTS13.
Please see Cincinnati Children's Hospital comprehensive platform of molecular and cellular diagnostic testing for thrombotic microangiopathies (TMA).
The complement system can be activated via three reaction pathways: the classical pathway, which is triggered primarily by cell-bound immune complexes; the mannan-binding lectin pathway, which is triggered by specific carbohydrate groups on microorganisms; and the alternative pathway, which is activated constitutively on all cell surfaces especially microorganisms. The complement component C3 is a key protein in all three reaction pathways, and complement activation is associated with consumption of C3 and a reduction in serum concentration. Diminished serum concentrations of C3 is observed through activation of the alternative pathway, and may be seen in atypical hemolytic uremic syndrome (aHUS) and forms of membranoproliferative glomerulonephritis (MPGN). Diminished serum concentrations of C3 is also observed through activation of the classical pathway (typically in combination with diminished serum levels of complement component C4) in active systemic lupus erythematosus (SLE), in some forms of membranoproliferative glomerulonephritis and in other immune complex diseases.
Diminished serum concentrations of C4 is observed primarily in activation of the classical pathway by immune complexes such as in active systemic lupus erythematosus (SLE), in forms of membranoproliferative glomerulonephritis (MPGN), and in other immune complex diseases (e.g. “shunt nephritis” and serum sickness), and is useful in distinguishing systemic activation of the classical pathway versus activation of the alternative complement pathway as seen in atypical hemolytic uremic syndrome (aHUS) and dense deposit disease/MPGN type II.
Factor B Level
Factor B(CFB) is a single-chain glycoprotein which provides the catalytic subunit of the C3/C5 convertases of the alternative complement pathway. Assembly of the C3 convertase (C3bBb) requires binding of Factor B to C3b followed by cleavage to Bb mediated by Factor D. This C3 convertase provides a positive amplification loop for the classical and alternative complement pathways. Gain of function mutations in Factor B causing enhanced binding to C3b and have been associated with atypical hemolytic uremic syndrome (aHUS). Due to consumption by formation of C3 convertase, serum levels of intact Factor B may be decreased in aHUS. The Bb fragment of Factor B is the serine protease element of this convertase, and elevations of Factor Bb may be regarded as an indicator of the alternative pathway of complement activation.
The complement Factor Bb level provides a quantitative assessment of the extent of activation of the alternative pathway of complement. Factor B, when bound to C3b, is cleaved by Factor D to yield fragments Ba and Bb and the resulting complex C3bBb is the alternative pathway C3 convertase. This complex is capable of cleaving additional C3 or forming a C5 convertase, cleaving C5 into its active fragments, C5a and C5b-9 (membrane attack complex). Failure to regulate the activation of this pathway has been associated with complement-mediated diseases including atypical hemolytic uremic syndrome and membranoproliferative glomerulonephritis type II/dense deposit disease. Since the Bb molecule is unique to the alternative pathway of complement, elevated levels of Bb as measured by ELISA may provide a marker for complement activation via this pathway.
Factor I Level
Factor I (CFI) is a serine protease that is essential for regulating the complement system by cleaving and inactivating C4b and C3b, and thus preventing the assembly of the C3 and C5 convertases. Mutations in CFI have been associated with a predisposition to atypical hemolytic uremic syndrome (aHUS), due to a failure to inactivate C3b by cleavage to iC3b, with subsequent unregulated formation of the alternative pathway C3 convertase C3bBb and ultimately membrane attack complex C5b-9 on the endothelial cell surface leading to thrombotic microangiopathy. Decreased serum levels of Factor I have been identified in some patients with mutations in CFI leading to aHUS.
Factor H Level
Factor H (CFH) is a regulatory protein of the alternative pathway of the complement system. Factor H is a serum glycoprotein that binds to both C3b and sialic acid/glycosaminoglycans, which enables it to act as co-factor for the Factor I-mediated proteolytic inactivation of C3b and accelerate the decay of the alternative pathway C3-convertase (C3bBb), thus regulating complement activation both in the fluid phase and on cellular surfaces. Decreased serum levels of Factor H due to mutations in Factor H or autoantibodies against Factor H may lead to subsequent unregulated formation of the alternative pathway C3 convertase C3bBb, and can be seen in some patients with aHUS as well as dense deposit disease/ membranoproliferative glomerulonephritis (MPGN) type II/Dense Deposit Disease.
C3 Nephritic Factor
An autoantibody that depresses the serum level of C3 and, secondarily, other complement components. Among the hypocomplementemic glomerulonephritides, nephritic factor is absent in acute post-streptococcal glomerulonephritis and lupus nephritis. It is present in most hypocomplementemic patients with membranoproliferative glomerulonephritis and also those with partial lipodystrophy. Although the antibody may be instrumental in producing the nephritis, its level is not closely associated with the clinical course.
Factor H Autoantibody
Factor H is a complement regulatory protein, which main function is to bind to C3b deposited on the surface of endothelial cells and facilitate its cleavage to an inactive form, iC3b, by Factor I. Failure to facilitate this cleavage lead to unregulated formation of the C3 convertase C3bBb on the surface of the cell, and ultimately unregulated formation of the membrane attack complex C5b-9 on the cell surface leading to cell lysis. Inhibitory autoantibodies directed against Factor H may reduce serum levels of Factor H and dysregulate the alternative pathway of the complement system, and have been identified in ~5% of patients with atypical hemolytic uremic syndrome (aHUS). Removal of anti-factor H antibodies from the bloodstream by plasmapheresis or the use of immunosuppressive drugs to eliminate the autoantibody production has shown benefit in the outcome of these diseases.