A Study on Immunity in Tsutsugamushi Disease

The immune response in human tsutsugamushi disease (scrub typhus) was studied. Antirickettsial activity of sera, peripheral mononuclear cells and their culture supernatants from patients on in vitro growth of Rickettsia tsutsugamushi proliferating in normal human peripheral macrophages was examined. The results obtained were as follows. 1) Sera from patients at the early convalescent stage, which exhibited high antibody titers against R. tsutsugamushi, effectively inhibited their growth in macrophages. 2) Sera from patients after a long period from the onset showed low antibody titers and did not inhibit rickettsial growth. 3) Mononuclear cells and T cell enriched fractions suppressed rickettsial growth when they were obtained from the patients at the early convalescant stage and even after as long as 3.5 years from the onset. 4) The culture supernatants of the T cell-enriched fractions which were collected from the patients mentioned above, exhibited a similar antirickettsial activity. These findings indicate that sensitized T lymphocytes and macrophages might play a fundamental role in immunological defense mechanism in tsutsugamushi disease. And the results obtained in our experiments are compatible with those previously reported in experimental scrub typhus in laboratory animals such as mice and monkeys.