Immunological Studies of Experimental Tsutsugamushi Disease in Congenitally Athymic (Nude) Mice *,†

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Abstract
Athymic mice were taken ill and died from infection with the high virulence as well as the low virulence strains of Rickettsia tsutsugamushi, and they did not improve in spite of tetracycline therapy. Moreover, neither 7S nor IgM antibody was detected by immunofluorescent antibody method in serum samples of athymic mice infected with the high virulence strain. Although immune serum-transfer exhibited some protective effect in athymic mice infected with the high virulence strain, it was far lower than in euthymic mice. Although both athymic and euthymic mice having received non-immune T-lymphocytes were taken ill and died, the mice having received immune T-lymphocytes survived infection with the high virulence strain. This protective capacity of T-lymphocytes was weak by 10 days after immunization of donor mice, became firm after a month and lasted as long as 12 months without decay. For athymic mice infected with the low virulence strain, not only immune but also non-immune T-lymphocytes from euthymic mice exhibited significant protective effect. By treatment of immune T-lymphocytes with anti-Thy-1.2 or anti-Lyt-1.2 alloserum, the protective capacity was lost entirely, and considerably diminished by treatment with anti-Lyt-2.2 alloserum in a homologous system using the high virulence strain. The results show that the inhibition of progress of tsutsugamushi disease is principally dependent on cellular immune mechanism(s) and that the production of antibody against R. tsutsugamushi is thymus-dependent.