T helper 2 (Th2)effector cells, through Th2 cytokine production, can mediate resistance to helminthic parasite infection. Understanding the cell and molecular interactions that lead to their differentiation is therefore important for the development of therapies and vaccines that promote host protective immune responses. We have begun to examine the innate response that initially develops following infection of mice with nematode parasites to identify cell populations or signals that may be important in promoting adaptive immunity that leads to the developing of IL-4producing Th2 cells. Global gene expression analyses of draining lymph nodes shortly after parasite infection showed pronounced increases in chemokines, including MCP-1 and CXCR3 ligands. Both chemokines are kn ow to recruit monocytes and granulocytes. Gr-1 is a cell surface marker shared by these cell populations and also plasmacytoid dendritic cells. Draining lymph nodes showed pronounced increases in Gr-1+ cells at 4 and 16 hours after inoculation with the intestinal nematode parasite, N. brasiliensis. Treatment with anti-GR-1 antibody caused marked increases in lymph node IFN-v expression at 18 hours after N. brasiliensis inoculation and decreased IL-4 expression and host resistance was observed at 7 days after inoculation. In the proposed studies, we will examine the function of Gr-1+ cells in promoting the development of Th2 effector cells in vivo. Specifically, we propose to identify the chemokine/chemokine receptors that mediate the recruitment of Gr-1+ cells to the peripheral lymph nodes at early stages of the immune response to N. brasiliensis. We will also elucidate the mechanism of how Gr-1+ cells suppress IFN-y elevations and promote Th2 cell development leading to host resistance by examining Gr-1+ cell interactions with developing Th2 cells in situ. Finally, we will address in two parasite models, N. brasiliensis and H. polygyrus, the role of Gr-1+ cells in the development of the host protective primary and memory response.