Helminths such as Schistosoma mansoni are remarkably efficient at establishing chronic infections with limited inflammatory pathology in some of their hosts, while in other infected people they can cause severe morbidity and they are a major health problem worldwide. Our long-term goal is to understand how liver and intestinal inflammation is regulated during S. mansoni infection. Perhaps the balance between pathogenesis and symbiosis is dependent on the nutritional status of infected individuals. However, we have a very limited understanding of the molecular mechanisms that underpin the relationship between helminth infections, nutritional status and inflammatory responses. Micronutrients such as vitamin A may play a critical role in regulating the inflammatory immune response during helminth infection. Recently, vitamin A metabolism and retinoic acid (RA) has emerged to be important in regulating mucosal immunity. Aldh1a2 is the major enzyme regulating RA production by antigen presenting cells in the gut. Alternatively activated macrophages (AAM) induced by IL-4 have been shown to play an important role in regulating liver and intestinal inflammation during S. mansoni infection, although the mechanism of regulation is not known. Here, we hypothesize that RA production via Aldh1a2 activity in AAM plays an important role in promoting Th2 responses and hence regulating liver and intestinal inflammation during S. mansoni infection. We have preliminary results showing that Aldh1a2 is upregulated by AAM in vitro and in vivo during helminth infection. By infecting vitamin A deficient mice with S. mansoni, we have found that Th2 responses are strikingly reduced, whereas surprisingly, we see an increase in regulatory T cells. We have previously shown that AAMs could bias T helper cell differentiation pathways from Th1 to Th2 cells and RA might therefore provide a mechanism. Since the Aldh1a2 deficient mice are embryonically lethal, we have also designed and generated a transgenic reporter mouse with an inducible deletion system. We are also generating mice with cell type specific (macrophage and dendritic cell) deletions of the Aldh1a2 gene. In this proposal, our specific aims are: (1) to determine if RA production by AAM plays a role in enhancing Th2 responses during in vivo S. mansoni infection; (2) to determine if AAM can influence T helper cell differentiation and function through RA in vitro; (3) to characterize new mouse strains that will provide genetic loss of function evidence for cell specific roles of Aldh1a2 in promoting Th2 responses during S. mansoni infection. The results of these studies will provide us with a framework to devise new ways of regulating inflammation through manipulating the capacity of macrophages to control vitamin A metabolism.