B1a cells protect againstSchistosoma japonicum–induced liver inflammation and fibrosis by controlling monocyte infiltration

AbstractDuringSchistosomainfection, lack of B cells results in more severe granulomas, inflammation, and fibrosis in the liver, but the mechanisms underlying this pathology remain unclear. Thus, our aim was to clarify the mechanisms underpinning the immunomodulation of B cells in mice infected withSchistosoma japonicum. We found that B cell deficiency led to aggravated liver pathology, as demonstrated by increases in the size of the egg-associated granulomas, alanine transaminase levels, and collagen deposition. Compared with infected wild-type mice, infected B cell–deficient μMT mice showed increased infiltration of Ly6Chimonocytes and higher levels of proinflammatory cytokines (tumor necrosis factor alpha, interleukin 6, and interleukin 12) and chemokines ([C-C motif] ligands (CCL)2, CCL3, CCL4, and CCL5). The results of flow cytometric analysis and cell transfer experiments showed that B1a cells increased significantly in the liver followingS. japonicuminfection, with some of those cells deriving from the peritoneal cavity. We also found that secretion of IL-10 from hepatic B cells increased significantly in infected wild-type mice and that this IL-10 was mainly derived from B1a cells. In addition, adoptively transferring peritoneal cavity B cells purified from wild-type, but not from IL-10–deficient mice, to μMT mice significantly reduced liver pathology and liver infiltration of Ly6Chimonocytes. These reductions were accompanied by decreases in the expression levels of chemokines and inflammatory cytokines. Taken together, these data indicated that afterS. japonicuminfection, an increased number of hepatic B1a cells secrete IL-10, which inhibits the expression of chemokines and cytokines and suppresses the infiltration of Ly6Chimonocytes into the liver thereby alleviating liver early inflammation and late fibrosis. Understanding this immunomodulatory role of B1a cells in schistosomiasis may lead to the development of therapeutic strategies forSchistosoma-induced liver disease.Author summaryInfection withSchistosoma,a waterborne parasitic flatworm (trematode) commonly called a blood fluke, results in strong granulomatous inflammation caused by the deposition of eggs in the liver. A granuloma is a substantial immune cell infiltration around the eggs intermixed with liver cells that can protect the host against liver damage. However, excessive infiltration and inflammation can lead to severe liver injury and fibrosis. Here, we found that B1a cells accumulate in the liver of mice afterS. japonicum–induced infection and that these B1a cells release the anti-inflammatory cytokine interleukin 10 to regulate inflammation. The B1a cell–derived interleukin 10 inhibits the expression of chemokines (which attract cells such as monocytes to sites of infection or inflammation) and thus restrains excessive infiltration of Ly6Chimonocytes (which may have proinflammatory activity) into the liver, thereby alleviating early inflammation and later fibrosis. Our study provides insight into the immunomodulation of B1a cells in schistosomiasis and offers key information for the development of therapeutic strategies inSchistosoma-induced liver disease.