DESCRIPTION: (provided by the applicant):Giardia lamblia is a protozoan parasite which replicates in the small intestine of many species of mammals, and infections with G. lamblia are one of the most common human infections in the word. Most infections are self-limiting and acquired immune responses are essential for controlling G. lamblia infections in humans and other host species. Understanding the immune response to G. lamblia is therefore essential for better control of this disease. We have recently shown that B cell, gamma-delta T cell, IL-4 and IFN-gamma deficient mice can control acute infections with G. lamblia. In contrast, CD4+, alpha-beta T cells are required to control infections. Thus, a T cell-dependent, but antibody-independent. mechanism exists which can control G. lamblia. We have also shown that mast cell-deficient mice and IL-6 deficient mice cannot control G. lamblia infections. Also, in vitro studies have shown that nitric oxide and anti-microbial peptides known as defensins can inhibit G. lamblia. Based on these findings, the following hypothesis has been formulated: CD4+ T cells activate mast cells to produce IL-6 during G. lamblia infections. IL-6 production then leads to epithelial cell production of nitric oxide and defensins that control acute G. lamblia infections. We will test this hypothesis with the following specific aims: 1.To determine the importance of IL-6 production during G. lamblia infections. We will confirm the importance of IL-6 by treating immnunodeficjent mice with recombinant IL-6 during infections. IL-6 production will also be measured in vivo during infections in wild type and immunodeficient mice using RT-PCR. 2. To determine the importance of mast cell IL-6 production during G. lamblia infections. We will confirm the role of mast cells during infections by measuring mast cell responses during infections, by immuno-depletion of mast cells, and by reconstitution of mast cell deficient mice. We will examine mast cell production of IL-6 using immunohistochemistry and adoptive transfers. 3. To determine the mechanism of mast cell activation during G. lamblia infections. We will examine cytokine production by T cells in vivo and in vitro. We will also examine infections in cytokine deficient mice. We will examine intestinal epithelial cell (IEC) production of stem cell factor in vitro and in vivo. 4. To determine the mechanisms by which IL-6 production leads to control of acute G. lamblia infections. We will examine production of defensins and nitric oxide by IEC in vivo using RT-PCR and in vitro by Northern blots and biochemistry. Mice deficient in defensin expression and nitric oxide production will then be infected to determine their importance in controlling infections. Successful completion of these experiments will give us insights into immunity to G. lamblia, as well as developing this organism as a model system for understanding mucosal immune responses.