Podoplanin discriminates distinct stromal cell populations and a novel progenitor subset in the liver

Podoplanin/gp38+ stromal cells present in lymphoid organs play a central role in the formation and reorganization of the extracellular matrix and in the functional regulation of immune responses. Gp38+ cells are present during embryogenesis and in human livers of primary biliary cirrhosis. Since little is known about their function, we studied gp38+ cells during chronic liver inflammation in models of biliary and parenchymal liver fibrosis and steatohepatitis. Gp38+ cells were analyzed using flow cytometry and confocal microscopy, and the expression of their steady state and inflammation-associated genes was evaluated from healthy and inflamed livers. Gp38+ cells significantly expanded in all three models of liver injury and returned to baseline levels during regression of inflammation. Based on CD133 and gp38 expression in the CD45−CD31−Asgpr1− liver cell fraction, numerous subsets could be identified that were negative for CD133 (gp38hiCD133−, gp38lowCD133−, and gp38−CD133−). Moreover, among the CD133+ cells, previously identified as progenitor population in injured liver, two subpopulations could be distinguished based on their gp38 expression (gp38−CD133+ and CD133+gp38+). Importantly, the distribution of the identified subsets in inflammation illustrated injury-specific changes. Moreover, the gp38+CD133+ cells exhibited liver progenitor cell characteristics similar to the gp38−CD133+ population, thus representing a novel subset within the classical progenitor cell niche. Additionally, these cells expressed distinct sets of inflammatory genes during liver injury. Our study illuminates a novel classification of the stromal/progenitor cell compartment in the liver and pinpoints a hitherto unrecognized injury-related alteration in progenitor subset composition in chronic liver inflammation and fibrosis.