Bovine colostral CD8-positive cells are potent IFN-γ-producing cells

IFN-γ plays an important role in cellular immunity contributing to microorganism elimination. We have previously reported that bovine colostrum contains high levels of IFN-γ as well as immunoglobulin. Lymphocytes are potent producers of IFN-γ, so establishing the lymphocyte population in colostrum will help to identify the source of colostral cytokines. In this study, we used flow cytometric analysis to quantify the population of three types of lymphocytes found in colostrum; namely, CD4 (Th) cells, CD8 (cytotoxic T) cells, and γδ-T cells. We also quantified the concentration of colostral IFN-γ using ELISA. IFN-γ concentrations were measured in colostrum obtained from 96 healthy Holstein cows; the levels tended to decrease on the first day post-parturition. Flow cytometric analysis showed that many γδ-T- and CD8-positive cells were present in the colostrum, and that the CD4/CD8 ratio was low. The ratios of CD8- and γδ-T-positive cells to cells of other types decreased during the 5 days after parturition, but that of CD4-positive cells showed no change during the observation period. To identify IFN-γ-expressing colostral lymphocytes, we used magnetic separation technology to separate the lymphocytes (CD4, CD8 and γδ-T) from colostral cells, then examined them for IFN-γ mRNA expression with real-time PCR (RT-PCR). RT-PCR analysis revealed potent expression of the IFN-γ gene in CD8-positive cells, reaching higher levels than in CD4- or γδ-T-positive cells. These results suggest that the CD8-positive T cells in colostrum play a role as producers of IFN-γ.