A monoclonal antibody (IgG 1) (designated as MAb-001) was produced against the pathogenic haemoflagellate Cryptobia salmositica Katz. The antibody agglutinated live patasites under in vitro conditions. Live C. salmositica, incubated with MAb-001 at 10 °C, did not multiply and were dead within 4 weeks in culture. About 50% of juvenile rainbow trout, Oncorhynchus mykiss (Walbaum), inoculated intraperitoneally with C. salmositica, incubated in MAb-001 prior to inoculation, did not become infected, while in adult rainbow trout, the peak parasitaemia was reduced. These results indicate that MAb-001 is a protective monoclonal antibody and the antigen it recognizes is locared on the surface membrane of C. salmositica. The antibody also inhibits multiplication and affects viability of the parasite under in vitro conditions.

No abstract

The monoclonal antibody (MAb‐001), which was produced against a surface membrane glycoprotein on C. salmositica, significantly inhibited the activities of the intracellular proteases of the parasite. The total activity in the partially purified metallo‐protease, and about 80% of activity in the partially purified cysteine protease, were inhibited by the antibody (at 10 μg protein ml–1). The inhibitory effects of the antibody on the proteases were also demonstrated using haemoglobin (substrate)‐SDS‐PAGE. The activities of the metallo‐protease band (200 kDa) and the three cysteine protease bands (66, 70 and 97 kDa) were inhibited by MAb‐001, but the activity of the fourth cysteine protease band (49 kDa) was not affected. Similar inhibitory effects of the antibody were also found in the crude protease extract (parasite lysate), except that more antibody was required to obtain the same level of inhibition. The metallo‐protease band was more sensitive than the cysteine protease bands to the antibody.

We recently produced a species-specific monoclonal antibody (MAb-001) against a surface antigen of Cryptobia salmositica Katz 1951 When a single dose (4 % body of weight in ml) of MAb-001 was injected into fish infected with C. salmositica, subsequent parasitaemias were significantly (p < 0.05) reduced at 48 h after treatment. Parasitaemias were still significantly lower than in control fish (injected with cold-blooded vertebrate Ringer's solution, CBVR) 1 wk after injection of MAb-001. There was no significant difference in parasitaemias between injection of a single dose or a double dose of MAb-001. Injection of MAb-001 into fish before infection and at 1 wk after infection did not prevent infection. Parasitaemias in fish which received MAh on Days 0, 1, 2, 3, 4 , 5, 6 (daily injected flsh) were sign~flcantly h~gher than in fish which received CBVR. The production of complement-fixing antibody against C salmositica in MAb daily injected fish was significantly lower than in those injected with CBVR. Antibodies against MAb-001 were detected in fish injected daily with MAb-001 These results suggest that a single dose of MAb-001 is therapeutic and perhaps can be used to prevent mortality of infected fish. Injection of MAb-001 every day in the first week after infection induced significant antigenic competition between parasites and MAb-001 Consequently, these fish produced less complement-fixing antibody against C. salmositica and this resulted in higher parasitaemias than in CBVR-injected fish.

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.