The factors determining the degree of hepatic inflammation and progression to cirrhosis in chronic HBV and HCV infections are not well-defined. Microbial translocation has been suggested as a contributing factor in some human studies and mouse models5–9,20
. However, the association of microbial translocation and its local and systemic effects with disease severity and prognosis has not been clearly established. We found that: (1) levels of sCD14, a marker of LPS bioreactivity, distinguished subjects with severe liver fibrosis and correlated with markers of hepatic inflammation and fibrosis; (2) enterocyte death, microbial translocation, LPS-induced monocyte activation and IL-6 levels were increased in HBV and HCV infections; (3) portal hypertension, reflected by low platelet counts, was associated with increased enterocyte necrosis; (4) levels of I-FABP, reflecting enterocyte necrosis, and IL-6, a marker of hepatic inflammation and regeneration, decreased with successful treatment of HBV or HCV infection; (5) high plasma sCD14 levels predicted disease progression in HBV and HCV infection independent of other markers of hepatic inflammation, fibrosis and disease progression; and (6) a high density of CD14+
cells in the liver was associated with hepatic fibrosis and predicted disease progression.
sCD14 levels correlated with ferritin, AST, GGT, AFP, APRI, Ishak score and HAI before and after antiviral therapy, suggesting an intimate association between LPS-induced monocyte activation and hepatic inflammation and fibrosis. Given the cross-sectional nature of these associations, the determination of cause and effect is difficult. Nonetheless, it is tempting to speculate and propose a model in which increased LPS in the portal circulation would activate Kupffer cells through CD14 and MD-2/TLR4 to produce cytokines such as IL-6, TNF and pro-fibrotic TGFβ, stimulating hepatic stellate cells to synthesize collagen, resulting in hepatic fibrosis. These effects would be enhanced in patients with more CD14+ cells and higher sCD14 levels. Hepatic fibrosis would result in portal hypertension, leading to enterocyte necrosis and further potentiating microbial translocation. Indeed, sickle cell disease patients with nodular regenerative hyperplasia (NRH) and portal hypertension (T.H., unpublished observations) have high sCD14 and LPS levels. In these patients, the hepatic venous pressure gradient, indicative of sinusoidal portal hypertension, correlated with low platelet counts and high sCD14 levels, which correlated with high ALP, a marker of NRH, suggesting that microbial translocation and LPS-induced monocyte or Kupffer cell activation perpetuate a cycle culminating in hepatic fibrosis and portal hypertension, even in the absence of viral infection. Thus, increased microbial translocation and the ensuing immune activation may be both a contributor to and a consequence of hepatic fibrosis.
Importantly, we found that high plasma levels of sCD14, reflective of the host response to LPS, rather than LPS itself, predict disease progression independent of Ishak score and markers of hepatic inflammation, fibrosis and disease progression. The association of sCD14 with disease progression is similar to observations in HIV/HCV co-infected patients, in whom increased sCD14 and LPS levels were associated with the development of cirrhosis34
. The detection of LPS can be complicated by its rapid clearance and by inhibitory plasma proteins11,35–38
, and not all bacteria produce bioactive LPS or LPS detectable by the Limulus assay39–41
. Thus, sCD14 may be a more relevant biomarker of disease progression as it reflects the host response to products of microbial translocation. Higher sCD14 levels may reflect more cells responding to LPS or a genetic predisposition towards increased LPS responsiveness. Indeed, high sCD14 levels in the setting of alcohol abuse42
or HCV infection HCV infection43
have been associated with a polymorphism in the promoter region of the CD14 gene (−159C/T). LPS-induced monocyte activation may increase systemic cytokine production, particularly as both circulating CD14+
monocytes and hepatic macrophages are increased in cirrhosis44
. High sCD14 levels may facilitate LPS-induced activation of endothelial14,45
and dendritic cells14
via the MD-2/TLR4 complex, leading to further production of pro-inflammatory cytokines. Thus, it is not the mere presence of the inflammatory stimulus that determines pathology but rather the host’s response to that stimulus. The association of sCD14 with worsening hepatic disease parallels findings in HIV infection in which sCD14, not LPS, predicts mortality, suggesting a common pathway in which activation of the immune system by microbial products accelerates the progression of a variety of diseases46
While previous studies have demonstrated that LPS induces liver injury based on portal vein concentrations of LPS47
, we detected E. coli proteins in the liver parenchyma, directly demonstrating that bacterial products drain into the liver in humans with chronic viral hepatitis infection. However, we found that the density of CD14+
cells in the liver, likely Kupffer cells which upregulate CD14 upon LPS stimulation20,33
, but not the density of E. coli proteins was associated with severity of fibrosis and disease progression. This finding supports the concept that the host response to the stimulus, rather than the stimulus itself, is associated with pathology.
Taken together, our data show that LPS-induced activation of both circulating monocytes and resident Kupffer cells is associated with severe hepatic fibrosis and failure to respond to therapy and predicts progression to end-stage liver disease independent of the degree of fibrosis. Thus, attenuation of microbial translocation and its inflammatory consequences may improve clinical outcomes in HBV and HCV infections.