Complement Alternative Pathway Activation in Human Nonalcoholic Steatohepatitis

Abstract

The innate immune system plays a major role in the pathogenesis of nonalcoholic steatohepatitis (NASH). Recently we reported complement activation in human NASH. However, it remained unclear whether the alternative pathway of complement, which amplifies C3 activation and which is frequently associated with pathological complement activation leading to disease, was involved. Here, alternative pathway components were investigated in liver biopsies of obese subjects with healthy livers (n = 10) or with NASH (n = 12) using quantitative PCR, Western blotting, and immunofluorescence staining. Properdin accumulated in areas where neutrophils surrounded steatotic hepatocytes, and colocalized with the C3 activation product C3c. C3 activation status as expressed by the C3c/native C3 ratio was 2.6-fold higher (p<0.01) in subjects with NASH despite reduced native C3 concentrations (0.94±0.12 vs. 0.57±0.09; p<0.01). Hepatic properdin levels positively correlated with levels of C3c (rs = 0.69; p<0.05) and C3c/C3 activation ratio (rs = 0.59; p<0.05). C3c, C3 activation status (C3c/C3 ratio) and properdin levels increased with higher lobular inflammation scores as determined according to the Kleiner classification (C3c: p<0.01, C3c/C3 ratio: p<0.05, properdin: p<0.05). Hepatic mRNA expression of factor B and factor D did not differ between subjects with healthy livers and subjects with NASH (factor B: 1.00±0.19 vs. 0.71±0.07, p = 0.26; factor D: 1.00±0.21 vs. 0.66±0.14, p = 0.29;). Hepatic mRNA and protein levels of Decay Accelerating Factor tended to be increased in subjects with NASH (mRNA: 1.00±0.14 vs. 2.37±0.72; p = 0.22; protein: 0.51±0.11 vs. 1.97±0.67; p = 0.28). In contrast, factor H mRNA was downregulated in patients with NASH (1.00±0.09 vs. 0.71±0.06; p<0.05) and a similar trend was observed with hepatic protein levels (1.12±0.16 vs. 0.78±0.07; p = 0.08). Collectively, these data suggest a role for alternative pathway activation in driving hepatic inflammation in NASH. Therefore, alternative pathway factors may be considered attractive targets for treating NASH by inhibiting complement activation.

The Role of Complement in Liver Injury, Regeneration, and Transplantation

“…C1q has been associated with HCC development through activation of the collagen receptor discoidin domain receptor 1 . Thus, NASH and related HCC could be decreased by inhibition of a constantly activated complement system, for instance, by targeting the alternative pathway …”

Nonalcoholic Steatohepatitis in Children: The Modern Day Pediatric Epidemic

“…Several components of the classical and lectin pathways, including C1q, MBL, and C4d, were also found to accumulate in the liver of subjects with NAFLD. However, C3 activation was not accompanied by C1q, MBL, or C4d deposition in all patients, suggesting that the alternative pathway could also be involved in complement activation in NAFLD [18] . Steatotic liver is susceptible to the action of next insults that may exacerbate steatosis and promote NASH.…”

What is the influence of complement factor C3 and lipid metabolism of adipose tissue on hepatic steatosis in mice?

“…NAFLD can however progress in nonalcoholic steatohepatitis (NASH), a more detrimental and advanced stage of fatty liver disease, characterized by hepatic inflammation. Patients with NASH can develop hepatic fibrosis, cirrhosis and finally hepatocellular carcinoma, which will lead to increased mortality [3].…”

“…The complement system is a subset of the innate immune system [9]. Preliminary data suggest that complement activation may have a significant role in the pathogenesis and progression of NAFLD and NASH [3,8]. Complement activation occurs through three pathways, the classical, alternative and mannose binding lectin pathway.…”