Cryptosporidium parasites are pathogens of human intestinal epithelial cells. To determine which genes are regulated during early infection, human ileal mucosa cultured as explants was infected with C. parvum or C. hominis, and gene expression was analyzed by microarray. The gene for osteoprotegerin (OPG) was up-regulated by both parasites. OPG mRNA was also significantly increased in biopsy specimens obtained from a volunteer experimentally infected with C. meleagridis, compared with levels in a prechallenge biopsy specimen. After in vitro infection of HCT-8 cells, there was an early peak in production of OPG mRNA protein. Treatment of infected cells with the OPG ligand tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) induced epithelial cell apoptosis and reduced parasite numbers, and recombinant OPG blocked these effects. These results suggest a novel TRAIL-mediated pathway for elimination of Cryptosporidium infection and a role for OPG in modulating this host response. © 2008 by the Infectious Diseases Society of America. All rights reserved.