In our previous studies, we have demonstrated that during Trichinella spiralis infection, T helper (Th) 2 cells contribute to the development of intestinal muscle hypercontractility and worm expulsion from the gut via signal transducer and activator of transcription factor 6 (Stat6). In addition, we have linked the altered muscle contractility to the eviction of the parasite, and thereby to host defense. However, the initial events linking infection to the development of muscle hypercontractility are poorly understood. In this study, we examined the contribution of CD40-CD40 ligand interaction in the development of intestinal muscle hypercontractility, monocyte chemoattractant protein-1 (MCP-1) production and in the Th2 response in CD40 ligand deficient (CD40L -/-) mice infected with T. spiralis. Expulsion of intestinal worms was substantially delayed in CD40L -/- mice as compared to the wild-type mice following T. spiralis infection. Consistent with delayed worm expulsion, there was a significant attenuation of intestinal muscle contractility in CD40L-/- mice. Infected CD40L -/- mice also exhibited marked impairment in production of MCP-1, IL-4, IL-13, IgG1, IgE, mouse mucosal mast cell protease 1 (MMCP-1), and in goblet cell response. These results demonstrate that CD40- CD40 ligand interaction plays an important role in MCP-1 production, Th2 response, intestinal muscle hypercontractility, and worm expulsion in nematode infection. The present data suggest that the early events leading to the generation of Th2 response include CD40-CD40 ligand interaction, which subsequently influences the production of Th2 cytokines most likely via up-regulation of MCP-1.