Complement C4d – A Renewed Interest In This Classical Marker of Complement Activation

The complement system is part of the body's innate immune system, which helps to identify and remove pathogens. The complement system comprises a group of around 30 proteins whose interactions have been studied in depth over many years. Complement activation is a critical process in the innate immune response and generates three different types of effector molecule:

1) Anaphylatoxins – C3a and C5a which attract and activate leucocytes to sites and promote inflammation

2) Opsonins – C3 fragments that decorate surfaces and promote the removal of cells or immune complexes by phagocytes

3) The terminal Membrane Attack Complex is comprised of C5b-C9, which directly lyses opsonised bacteria or infected host cells.

The details of the complement system have been widely reviewed and are well covered in any good immunology textbook. The review by Noris and Remuzzi (2013) provides a succinct overview (1)

A marker of particular interest concerning the complement system is the C4d fragment, which acts as a sensitive and long-lived marker of complement activation. C4d is a fragment of the C4 protein that is produced during complement activation by the classical or lectin pathways. The C1r/C1s complex (classical pathway) or MBL-associated serine proteases (lectin pathway) cleave complement 4 (C4) to form C4a and C4b. C4b contains a highly reactive thioester motif, which binds to nearby proteins or cell surfaces. Further cleavage of C4b alpha by Factor I forms C4d and C4c, where C4d remains bound close to the site of C4b binding. C4d thus acts as a marker of complement activation that can be localised to cell surfaces, such as vascular endothelial cells. It remains long after antibodies have been cleared (2).

C4d has attracted much interest as a marker of rejection in solid organ transplants as a marker of complement activation. As well as kidney tissues, this marker has been used successfully in heart, lung, and pancreas tissues (3). The presence of C4d in renal peritubular capillaries has been found to be a key indicator for acute antibody-mediated rejection (4).  

C4d has also found application in assessing kidney damage in the autoimmune condition Systemic Lupus Erythematous (SLE). Studies have shown that immunohistological staining of kidney biopsies can predict episodes of SLE-related nephritis, thereby offering essential insights into clinical management (5).

References 

1)     Noris,M. and Remuzzi,G. (2013)  Overview of Complement Activation and Regulation. Semin. Nephrology 33:479

2)     Sacks,S.H. and Chowdhury,P. (2002) Footprints of humoral rejection  Curr. Opin. Nephrol. Hyper. 11:627

3)     Colvin, R.B. and Smith, R.N. (2005). Antibody-mediated organ-allograph rejection. Nat Rev Immunol 5:807

4)     Collins, A.B. et al. (1999). Complement activation in acute humoral renal allograft rejection: diagnostic significance of C4d deposits in peritubular capillaries. J Am Soc Nephrol. 10:2208

5)     Ding,Y. et al (2021) The Spectrum of C4d Deposition in Renal Biopsies of Lupus Nephritis Patients Front. Immunol. 12:654652