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In the present study, cell-surface markers and cytokine gene expression of lymphocytes from the local lymph nodes were studied 9 days after primary infection with Ostertagia ostertagi in previously naive calves or in calves previously immunized with multiple, chemically attenuated infections. Changes in lymphocyte populations were assessed by flow cytometry utilizing monoclonal antibodies specific for bovine cell-surface markers. Changes observed in the percentages of lymphoid populations after challenge were similar in animals immunized by either three or five drug-attenuated infections. In both immunized groups, the CD4/CD8 ratio was significantly higher than in naive animals after challenge infections. In addition, both immunized groups showed significantly lower levels of Ig-bearing cells upon experimental challenge when compared to animals with a primary experimental infection. No differences were observed in the number of gd or interleukin 2 receptor (IL-2R) positive cells. The levels of mRNA for IL-4, IL-10, IL-15, IFN-g and TGF-b1 were examined by competitive RT-PCR. After challenge, the levels of these cytokines were lower in animals immunized by five drug-attenuated infections, and in the case of IL-4 and TGF-b1, these differences were statistically significant. These results indicate that animals exhibiting protection from reinfection with O. ostertagi do not show a shift to higher percentages of Ig cells characteristic of a primary infection. In addition, protected animals appear to show a decreased IL4 and TGF-b1 response upon challenge when compared to non-immune animals. Published by Elsevier Science B.V.