In a recent paper, Myriam Martin and colleagues have studied C4d as a potential biomarker for lupus nephritis. C4d is part of the complement cascade, which in turn is an important part of innate immunity where it is involved in phagocytosis, inflammation and membrane attacks.
Complement activation has been used as a measure in SLE patients for decades. However, the levels of traditional markers, such as C3 and C4, are not exclusively dependent on activation, but also the rate of synthesis in the body, making them less reliable as activation markers. C4d, on the other hand, is produced exclusively upon complement activation, which at least in theory would make it a good activation marker.
In the paper, the authors analyzed plasma levels of C4d using Svar’s assay and found that they were significantly elevated in SLE patients compared to controls. In addition, they found that the levels were significantly higher in SLE patients suffering from lupus nephritis than in non-lupus nephritis SLE patients.
The sensitivity, specificity and accuracy of the assay was further improved when C4d/C4 ratios were measured compared to measuring C4d alone. Furthermore, the authors found that plasma C4d levels declined significantly after treatment in patients that experienced favorable clinical and histopathological response.
Taken together, these results indicate that C4d can be used as a biomarker to discriminate between lupus nephritis and non-lupus nephritis SLE patients, as well as for monitoring treatment response.
Read the paper