Complement Factor C4d Is a Common Denominator in Thrombotic Microangiopathy

Area of Science:

• Nephrology
• Immunology
• Pathology

Background:

• Complement activation significantly contributes to thrombotic microangiopathy (TMA), a critical condition with diverse clinical presentations.
• Recent advancements in terminal complement-inhibiting drugs necessitate reliable biomarkers for patient stratification.
• Understanding complement pathway involvement in TMA is crucial for targeted therapeutic strategies.

Purpose of the Study:

• To investigate the prevalence and kidney localization of complement factor C4d in patients with TMA.
• To identify the specific complement pathways responsible for C4d deposition in TMA.
• To assess the presence of the terminal complement complex (C5b-9) as a consequence of complement activation in TMA.

Main Methods:

• Analysis of 42 renal tissue sections from patients with histologically confirmed TMA.
• Histopathological scoring of C4d, mannose-binding lectin, C1q, IgM, and C5b-9 deposits in glomeruli, peritubular capillaries, and arterioles.
• Comparison of complement deposition patterns between TMA cases and control subjects (n=53).

Main Results:

• C4d deposits were identified in 88.1% of TMA cases, with distinct patterns across different renal vasculature compartments.
• Classical pathway activation was evident in 90.5% of TMA cases.
• Terminal complement complex (C5b-9) deposits were found in 78.6% of TMA cases, compared to 39.6% in controls, with differing staining patterns.

Conclusions:

• C4d is a frequent biomarker in TMA, irrespective of the underlying etiology.
• The high prevalence of C5b-9 in TMA samples suggests potential therapeutic benefit from terminal complement inhibitors.
• C4d and C5b-9 warrant further investigation as diagnostic biomarkers for complement-mediated TMA.