Repeated exposure induces periportal fibrosis in Schistosoma mansoni-infected baboons: role of TGF-beta and IL-4.

Recently, we observed that repeated Schistosoma mansoni infection and treatment boost Th2-associated cytokines and TGF-beta production in baboons. Other studies have shown that some chronically infected baboons develop hepatic fibrosis. Because TGF-beta, IL-2, and IL-4 have been shown to participate in development of fibrosis in murine schistosomiasis, the present study examined whether repeated exposure stimulates hepatic fibrosis in olive baboons. To test this hypothesis, animals were exposed to similar numbers of S. mansoni cercariae given once or repeatedly. After 19 wk of infection, animals were cured with praziquantel and reinfected once or multiple times. Hepatic granulomatous inflammation and fibrosis were assessed from serial liver biopsies taken at weeks 6, 9, and 16 after reinfection and egg Ag (schistosome egg Ag)-specific cytokine production by PBMC were measured simultaneously. Periportal fibroblast infiltration and extracellular matrix deposition (fibrosis), angiogenesis, and biliary duct hyperplasia developed in some animals. The presence and amount of fibrosis directly correlated with the frequency of exposure. Fibrosis was not associated with adult worm or tissue egg burden. The amount of fibrosis correlated with increased schistosome egg Ag-driven TGF-beta at 6, 9, and 16 wk postinfection (rs = 0.9, 0.8, and 0.54, respectively, all p < 0.01) and IL-4 production (p = 0.02) at 16 wk postinfection and not IFN-gamma, IL-2, IL-5, or IL-10. These data suggest that repeated exposure is a risk factor for periportal fibrosis by a mechanism that primes lymphocytes to produce increased levels of profibrotic molecules that include TGF-beta and IL-4.