The dynamic interdependence of amebiasis, innate immunity, and undernutrition

Entamoeba histolytica, the protozoan parasite that causes amebic dysentery, greatly contributes to disease burden in the developing world. Efforts to exhaustively characterize the pathogenesis of amebiasis have increased our understanding of the dynamic host-parasite interaction and the process by which E. histolytica trophozoites transition from gut commensals to invaders of the intestinal epithelium. Mouse models of disease continue to be instrumental in this area. At the same time, large-scale studies in human populations have identified genetic and environmental factors that influence susceptibility to amebiasis. Nutritional status has long been known to globally influence immune function. So it is not surprising that undernutrition has emerged as a critical risk factor. A better understanding of how nutritional status affects immunity to E. histolytica will have dramatic implications in the development of novel treatments. Future work should continue to characterize the fascinating host-parasite arms race that occurs at each stage of infection.

Leptin protects host cells from Entamoeba histolytica cytotoxicity by a STAT3-dependent mechanism

The adipocytokine leptin links nutritional status to immune function. Leptin signaling protects from amebiasis, but the molecular mechanism is not understood. We developed an in vitro model of ameba-host cell interaction to test the hypothesis that leptin prevents ameba-induced apoptosis in host epithelial cells. We demonstrated that activation of mammalian leptin signaling increased cellular resistance to amebic cytotoxicity, including caspase-3 activation. Exogenous expression of the leptin receptor conferred resistance in susceptible cells, and leptin stimulation enhanced protection. A series of leptin receptor signaling mutants showed that resistance to amebic cytotoxicity was dependent on activation of STAT3 but not the Src homology-2 domain-containing tyrosine phosphatase (SHP-2) or STAT5. A common polymorphism in the leptin receptor (Q223R) that increases susceptibility to amebiasis in humans and mice was found to increase susceptibility to amebic cytotoxicity in single cells. The Q223R polymorphism also decreased leptin-dependent STAT3 activation by 21% relative to that of the wild-type (WT) receptor (P = 0.035), consistent with a central role of STAT3 signaling in protection. A subset of genes uniquely regulated by STAT3 in response to leptin was identified. Most notable were the TRIB1 and suppressor of cytokine signaling 3 (SOCS3) genes, which have opposing roles in the regulation of apoptosis. Overall apoptotic genes were highly enriched in this gene set (P < 1E-05), supporting the hypothesis that leptin regulation of host apoptotic genes via STAT3 is responsible for protection. This is the first demonstration of a mammalian signaling pathway that restricts amebic pathogenesis and represents an important advance in our mechanistic understanding of how leptin links nutrition and susceptibility to infection.