Non-major histocompatibility complex-restricted cytotoxicity of bovine coronavirus-infected target cells mediated by bovine intestinal intraepithelial leukocytes.

Non-specific cellular mechanisms of defence against intestinal virus infections of cattle were investigated using bovine coronavirus (BCV) as a representative enteric virus. Since BCV infection is limited to the epithelial cells of the intestinal tract, defence mechanisms must be capable of acting at that site to be effective. Therefore, the intraepithelial leukocyte (IEL) population of the intestinal mucosa was chosen for initial study. Treatment of intestinal samples with DTT and EDTA in calcium- and magnesium-free buffers allowed recovery of viable IEL populations appropriate for further functional assessment. Studies of IELs isolated from neonatal calves revealed that non-major histocompatibility complex (MHC)-restricted cytotoxicity of BCV-infected target cells was more prevalent in calves with concurrent virus infection, suggesting in vivo activation of the cytotoxic response. Peripheral blood mononuclear cells from the same calves did not mediate cytotoxicity, emphasizing the difference in function of lymphocytes isolated from different anatomical sites. IELs from normal adult animals rarely showed spontaneous non-MHC-restricted cytotoxicity. However, interleukin-2 (IL-2) was a potent activator of IEL cytotoxicity in vitro, enhancing the killing of BCV-infected target cells after just 18 h of treatment. Incubation of IELs with interferon-gamma and tumour necrosis factor (TNF) did not induce cytotoxic activity, but TNF could augment the levels of IL-2-induced cytotoxicity. Although further analysis of the cytotoxic effector cells present in the intestinal epithelium is required, the present study indicates that the IEL population may play a role in enteric antiviral activity.