CELL-MEDIATED IMMUNE-RESPONSE AND T-LIKE CELLS IN THYMECTOMIZED ONCORHYNCHUS-MYKISS (WALBAUM) INFECTED WITH OR VACCINATED AGAINST THE PATHOGENIC HEMOFLAGELLATE CRYPTOBIA-SALMOSITICA KATZ, 1951

T-cell-mediated delayed-type hypersensitivity (T-DTH) reaction was det ected in Cryptobia salmositica-infected intact and thymectomized (2 mo nths post-thymectomy) Oncorhynchus mykiss (Walbaum). Both groups of fi sh showed significant induration at the site of C. salmositica antigen injection at 8, 12 and 16 weeks post-infection. A significant differe nce was not observed in T-DTH reactions between the thymectomized and intact (control) infected fish. The total numbers of circulating leuco cytes detected in infected thymectomized fish were significantly lower than those found in infected sham-thymectomized 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 se en in intact fish prior to and at 4 weeks after vaccination with an av irulent strain of C. salmositica. At 2 weeks after challenge with the pathogen the numbers of T-like cells in intact vaccinated fish increas ed significantly (P < 0.01) and remained high for the duration of the study (15 weeks). However, in vaccinated thymectomized fish the number s of T-like cells remained low after parasite challenge. These results suggest that thymectomy in adult rainbow trout did not lower T-cell-m ediated delayed-type hypersensitivity; however, it reduced the numbers of circulating leucocytes and retarded the proliferation of T-like ce lls after antigenic stimulation.