Cell-mediated immune response and T-like cells in thymectomized Oncorhynchus mykiss (Walbaum) infected with or vaccinated against the pathogenic haemoflagellate Cryptobia salmositica Katz, 1951.

T-cell-mediated delayed-type hypersensitivity (TDTH) reaction was detected in Cryptobia salmositica-infected intact and thymectomized (2 months post-thymectomy) Oncorhynchus mykiss (Walbaum). Both groups of fish showed significant induration at the site of C. salmositica antigen injection at 8, 12 and 16 weeks post-infection. A significant difference was not observed in TDTH reactions between the thymectomized and intact (control) infected fish. The total numbers of circulating leucocytes detected in infected thymectomized fish were significantly lower than those found in infected shamthymectomized fish. The numbers of T-like cells determined (using alpha-naphthyl acid esterase assay) in thymectomized fish (9 months post-thymectomy) were similar to those seen in intact fish prior to and at 4 weeks after vaccination with an avirulent strain of C. salmositica. At 2 weeks after challenge with the pathogen the numbers of T-like cells in intact vaccinated fish increased significantly (P < 0.01) and remained high for the duration of the study (15 weeks). However, in vaccinated thymectomized fish the numbers of T-like cells remained low after parasite challenge. These results suggest that thymectomy in adult rainbow trout did not lower T-cell-mediated delayed-type hypersensitivity; however, it reduced the numbers of circulating leucocytes and retarded the proliferation of T-like cells after antigenic stimulation.