Complement system assays

Complement system assays assess the functional integrity, individual component concentrations, regulatory protein activities, and active consumption states across the classical, alternative, and lectin complement pathways.

1. Clinical Indications for Testing

• Evaluation of Primary Immunodeficiencies
  
  

  • Recurrent, severe pyogenic infections caused by encapsulated bacteria (such as Streptococcus pneumoniae or Haemophilus influenzae).

  • Recurrent or invasive Neisseria meningitidis or Neisseria gonorrhoeae infections, suggestive of terminal complement deficiencies ($C5$ through $C9$) or properdin deficiency.

• Autoimmune and Immune-Complex Diseases
  
  

  • Diagnosis, disease activity monitoring, and flare detection in systemic lupus erythematosus (SLE), lupus nephritis, and systemic vasculitis.

• Hematologic and Renal Pathologies
  
  

  • Investigation of atypical hemolytic uremic syndrome (aHUS), C3 glomerulopathy (C3G), and paroxysmal nocturnal hemoglobinuria (PNH).

• Angioedema Assessment
  
  

  • Differentiation between hereditary angioedema (HAE) types and acquired angioedema (AAE).

• Therapeutic Drug Monitoring
  
  

  • Assessing pathway blockade during treatment with complement inhibitors (e.g., eculizumab, ravulizumab).

2. Functional Hemolytic and Pathway Screening Assays

1. Classical Pathway Screening ($CH_{50}$)

   • Principle: Measures the functional capability of serum to lyse sensitized target red blood cells. Sheep erythrocytes coated with rabbit anti-sheep erythrocyte antibodies (hemolysin) are incubated with serial dilutions of patient serum.

   • Endpoint: The $CH_{50}$ titer represents the reciprocal of the serum dilution capable of lysing $50\%$ of the antibody-sensitized sheep red blood cells.

   • Pathway Integrity: Requires intact activity of all classical pathway components ($C1q$, $C1r$, $C1s$, $C4$, $C2$) and all terminal pathway components ($C3$, $C5$, $C6$, $C7$, $C8$, $C9$).

   • Interpretation: A value approaching zero indicates complete absence (genetic deficiency) of a required component or profound in vitro denaturation. Reduced but non-zero values reflect immune consumption.

2. Alternative Pathway Screening ($AH_{50}$ / $AP_{50}$)

   • Principle: Assesses alternative pathway function using unsensitized rabbit or guinea pig red blood cells, which inherently trigger alternative pathway activation without antibodies.

   • Buffer Specifics: Performed in the presence of ethylene glycol tetra-acetic acid (EGTA) and magnesium ($Mg^{2+}$). EGTA selectively chelates calcium ($Ca^{2+}$), preventing $C1$ complex assembly and lectin pathway activation, while $Mg^{2+}$ permits alternative pathway C3 convertase ($C3bBb$) assembly.

   • Pathway Integrity: Requires functional Factor D, Factor B, Properdin, C3, and terminal components ($C5$ through $C9$).

   • Interpretation: Depressed in Factor B, Factor D, or properdin deficiencies, as well as terminal pathway deficiencies; normal in isolated classical pathway defects.

3. Lectin Pathway Functional Assays

   • Principle: Microtiter plates coated with mannan or acetylated bovine serum albumin (BSA).

   • Mechanism: Patient serum is added under high-salt conditions to inhibit classical pathway C1q binding. Mannose-binding lectin (MBL) or ficolins bind mannan, activating MBL-associated serine proteases (MASPs).

   • Detection: Deposition of split products (e.g., C4 or $C5b\text{-}9$) is detected using enzyme-linked immunosorbent assay (ELISA).

4. Enzyme-Linked Functional Multiplex Assays (Wieslab System)

   • Principle: A standardized three-well microtiter assay where each well contains a selective pathway activator:

   • IgM for the classical pathway

   • Lipopolysaccharide (LPS) for the alternative pathway

   • Mannan for the lectin pathway

   • Readout: Detection of the terminal complement neo-epitope ($C5b\text{-}9$ / Membrane Attack Complex) via labeled monoclonal antibodies, offering automated, non-erythrocyte-dependent pathway activity scoring.

3. Quantitative and Antigenic Assays

1. Nephelometric and Turbidimetric Assays

   • Target Proteins: Primarily C3 and C4, representing the most common routine clinical tests.

   • Methodology: Light scattering (nephelometry) or light transmission reduction (turbidimetry) caused by immune complexes formed between the analyte and specific polyclonal antisera.

   • Clinical Utility:

   • Low C3 with Low C4: Classical pathway consumption driven by immune complexes (e.g., active lupus nephritis, cryoglobulinemic vasculitis).

   • Low C3 with Normal C4: Alternative pathway consumption (e.g., post-streptococcal glomerulonephritis, C3 glomerulonephritis).

   • Normal C3 with Low C4: Characteristic of early classical pathway abnormalities, silent C4 null alleles, or uninhibited C1s activity in hereditary angioedema.

2. Radial Immunodiffusion (RID) and Direct ELISA

   • Used to measure individual complement proteins ($C1q$, $C1r$, $C1s$, $C2$, $C5$, $C6$, $C7$, $C8$, $C9$, Factor B, Factor H, Factor I) when a specific deficiency is suspected based on abnormal screening hemolytic assays.

4. Complement Split Products and In Vivo Activation Markers

Antigenic measurements of native components can remain normal during ongoing activation due to compensatory acute-phase hepatic synthesis. Split products directly demonstrate active in vivo cleavage:

• Anaphylatoxins ($C3a$, $C4a$, $C5a$)
  
  

  • Measured by ELISA or radioimmunoassay (RIA). Unstable in circulation with short half-lives; quickly desarginated by carboxypeptidase N to their more stable forms ($C3a\text{-desArg}$, $C5a\text{-desArg}$).

• Opsonins and Degradation Fragments ($C3d$, $C3dg$, $C4d$)
  
  

  • Long plasma half-lives make them sensitive markers for persistent classical ($C4d$) or common pathway ($C3d$) activation.

  • $C4d$ tissue deposition serves as an established immunohistochemical marker for antibody-mediated organ transplant rejection.

• Alternative Pathway Activation Fragments ($Ba$ and $Bb$)
  
  

  • Generated during Factor B cleavage by Factor D. Elevated levels confirm alternative pathway turnover independently of classical activation.

• Soluble Terminal Complement Complex ($sC5b\text{-}9$ / sMAC)
  
  

  • Measures fluid-phase terminal complexes bound to endogenous regulatory proteins (vitronectin and clusterin). Indicates complete assembly of the lytic terminal cascade in plasma.

5. Regulatory Proteins and Specialized Diagnostic Assays

1. C1-Esterase Inhibitor (C1-INH) Assays

   • Indicated for suspected hereditary or acquired angioedema.

   • Antigenic Level: Quantified by nephelometry or RID.

   • Functional Activity: Quantified using a chromogenic substrate assay (cleavage of a synthetic peptide substrate by residual active C1s or plasma kallikrein) or functional ELISA.

   • Classification:

   • Type I HAE ($85\%$ of cases): Low C1-INH antigen and low C1-INH function.

   • Type II HAE ($15\%$ of cases): Normal or elevated C1-INH antigen with low C1-INH function.

   • Acquired Angioedema (AAE): Low C1-INH antigen, low function, accompanied by low $C1q$ levels.

2. Autoantibodies Against Complement Proteins

   • C3 Nephritic Factor ($C3NeF$): Autoantibody directed against the alternative pathway convertase ($C3bBb$). Stabilizes the enzyme against decay by Factor H, causing uncontrolled C3 cleavage; detected via hemolytic decay-acceleration assays or ELISA.

   • Anti-Factor H Autoantibodies: Detected via ELISA; associated with atypical hemolytic uremic syndrome (aHUS) and autoimmune dense deposit disease.

   • Anti-C1q Autoantibodies: Measured by ELISA; correlated with severe active proliferative lupus nephritis and hypocomplementemic urticarial vasculitis syndrome (HUVS).

6. Pre-Analytical Requirements and Handling

Complement proteins are heat-labile and susceptible to spontaneous in vitro activation and degradation, requiring strict collection protocols:

• Sample Collection: Serum (without separator gel) or plasma anticoagulated with EDTA. EDTA rapidly chelates divalent cations ($Ca^{2+}$ and $Mg^{2+}$), immediately arresting further ex vivo complement activation (preferred for measuring split products like $C3a$, $sC5b\text{-}9$, and $Ba$).

• Processing: Samples must be placed immediately on wet ice, centrifuged at $4\,^\circ\text{C}$ within $30\text{ to }60\,\text{minutes}$ of collection.

• Storage: Aliquots must be frozen immediately at $-70\,^\circ\text{C}$ or $-80\,^\circ\text{C}$. Freeze-thaw cycles degrade native factors and generate artifactual split products.

7. Diagnostic Interpretation Patterns

• Low $CH_{50}$, Normal $AH_{50}$
  
  

  • Isolated deficiency of a classical pathway component: $C1q$, $C1r$, $C1s$, $C4$, or $C2$.

• Normal $CH_{50}$, Low $AH_{50}$
  
  

  • Isolated deficiency of an alternative pathway component: Factor D, Factor B, or Properdin.

• Low $CH_{50}$, Low $AH_{50}$
  
  

  • Shared component deficiency (terminal pathway: $C3$, $C5$, $C6$, $C7$, $C8$, or $C9$), severe secondary consumption across both cascades, or improper sample transport/handling.

• Normal $CH_{50}$, Normal $AH_{50}$ with Systemic Symptoms
  
  

  • Suggests MBL deficiency, localized activation, or complement-mediated disease driven by endothelial-bound deregulation rather than broad systemic depletion.

The $CH_{100}$ assay refers to a total hemolytic complement assay designed to evaluate classical and terminal complement pathway function by measuring the capability of serum to achieve complete ($100\text{\%}$) lysis of antibody-sensitized erythrocytes (such as sheep red blood cells coated with anti-sheep hemolysin), or by expressing classical pathway activity relative to a normal reference serum calibrator defined as $100\text{\%}$. While conceptually linked to the standard $CH_{50}$ screening assay, $CH_{100}$ differs substantially in its kinetics, precision, and clinical utility. Hemolytic Kinetics and the Von Krogh Equation: Complement-mediated red blood cell lysis exhibits an S-shaped sigmoidal response governed by the Von Krogh relationship $x = K \times \frac{y}{1 - y}^{1/n}$, where $y$ is the proportion of lysed erythrocytes, $x$ represents the complement serum concentration, and $n$ is an empirical slope parameter (typically near $0.2$). The slope of this curve reaches its steepest gradient at $50\text{\%}$ lysis ($y = 0.5$ ). Consequently, the $CH_{50}$ assay operates where analytical sensitivity is maximized: small shifts in complement factor concentrations produce substantial, quantifiable differences in hemolysis. Conversely, at $100\text{\%}$ lysis ($y = 1.0$), the curve approaches an asymptotic plateau where marked alterations in complement factor concentrations result in virtually indistinguishable changes in measurable lysis. Diagnostic Limitations: Because of the flat kinetic plateau at $100\text{\%}$ lysis, a $CH_{100}$ endpoint possesses low diagnostic sensitivity for identifying partial complement consumption or heterozygous component deficiencies. For instance, in patients experiencing active immune-complex consumption during systemic lupus erythematosus flares or acute glomerulonephritis, serum may retain sufficient residual complement to achieve $100\text{\%}$ lysis under insufficiently diluted conditions, yielding false-negative screening outcomes. For this reason, $CH_{50}$ remains the preferred functional hemolytic standard. Practical Applications: Where $CH_{100}$ terminology or endpoints are utilized clinically, they typically represent qualitative or semi-quantitative single radial hemolysis (SRH) gel diffusion plates where clear lysis zones are inspected, or automated immunoassays reporting pathway activity as a normalized percentage relative to a $100\text{\%}$ healthy reference control.