Gut intraepithelial lymphocytes induce immunity against Cryptosporidium infection through a mechanism involving gamma interferon production.

Immunological control of infection with cryptosporidia in mice is dependent on CD4+ T cells and the production of gamma interferon (IFN-gamma), but to date, the mucosal T cells which produce IFN-gamma local to the infection have not been characterized. We previously showed that immunity against the gastric parasite Cryptosporidium muris could be adoptively transferred to adult SCID (severe combined immunodeficiency) mice with small intestinal intraepithelial lymphocytes (IEL) from previously infected immunocompetent mice, but only if the donor CD4+ T cells were intact. The present investigation examined whether IFN-gamma was important in the effector mechanisms mediated by immune IEL in SCID mice. The development of resistance against C. muris infection in SCID mice given immune IEL was prevented by treatment with a hamster anti-mouse IFN-gamma-neutralizing monoclonal antibody, but following cessation of antibody treatment, the mice recovered from infection. In further experiments, an enzyme-linked immunospot (ELISPOT) technique was used to compare frequencies of IFN-gamma-producing cells in activated T-cell populations from C. muris-immune and naive donor mice. Stimulation with concanavalin A or a rat anti-mouse CD3 monoclonal antibody resulted in detection of greater numbers of cells producing IFN-gamma from immune than naive IEL populations. Small numbers of IEL from C. muris-immune mice, but not from naive mice, also produced IFN-gamma when cultured with soluble oocyst antigen, but this occurred only if gamma-irradiated spleen cells were cocultured with the immune IEL. These results suggested that IEL were important in the generation of immunity to Cryptosporidium and that one of their crucial functions was to produce IFN-gamma at the site of infection.