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The study of antibody production in cotton rats infected with Rickettsia prowazekii B and TB has revealed that R. prowazekii antigens, inducing the production of antibodies determined in the complement fixation, indirect hemolysis, and passive hemagglutination tests, are T-independent. The study of the nonspecific reactivity of T-lymphocytes in cotton rats infected with R. prowazekii TB has indicated that in case of the prolonged persistence of the infective agent in the animals no secondary immune deficiency develops.

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Antibody production has been studied in cotton B-rats and in CBA B-mice during immunization with chemical typhus vaccine (CTV) and during infection with Rickettsia prowazekii. Studies of the immune response to rickettsial antigen in T-deficient animals have shown a high immunogenicity of CTV and independence of antibody production on T-lymphocytes. Active antibody synthesis was also observed in B-rats and B-mice during Rickettsia prowazekii infection. The absence of T-lymphocyte dependence in experimental animals was tested by administration of sheep red blood cells (SRBC).

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The susceptibility of cotton rat macrophages to Rickettsia (R.) prowazekii, the percentage of the affected cells, and the intensity of damage to individual cells by rickettsiae were found to be much higher than those in guinea pig macrophages infected under similar conditions. At the same time, cotton rat macrophages proved to be more resistant to the effect of rickettsiae than guinea pig macrophages. Some common features of infection in cell culture and in animals have been observed. It is suggested that the outcome of interaction of rickettsiae with macrophages of one or another animal species may be important in generating acute or persistent infection.

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Adoptive transfer of T lymphocytes harvested from spleen of Rickettsia conorii-infected DBA/2 mice to intact and cyclophosphamide- (CPA-) pretreated syngeneic mice protected the latter from lethal infection caused by R, conorii. Protection from infection was not observed in recipient mice given immune serum or B lymphocytes and macrophages from the spleens of convalescent mice. The protective effect was most pronounced after intravenous transfer of lymphocytes obtained from donor mice on day 14 post infection. Lethal infection was not prevented by transfer of lymphocytes harvested on day 50 p. i., although at this interval the donor mice were still resistant to reinfection with F. conorii.

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The process of infection caused by R. prowazekii (strain Breinl) in guinea-pigs was accompanied by the development of delayed type hypersensitivity manifested by the macrophage migration inhibition test (MMIT). This reaction could be detected on week 2 after the subcutaneous inoculation of the infective agent, achieved its maximum on weeks 3-4 with individual fluctuations taken into account and decreased by week 5 remaining at a low level during the whole period of the study (63 days). The period when the MMIT was most pronounced correlated with the period of the maximum increase in the titers of complement-fixing antibodies and the presence of the infective agent in the animal body.

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The function of peritoneal macrophages from Rickettsia prowazeki-infected guinea pigs at various intervals of postinfection immunity was studied. The activity of macrophages in immunized animals was higher than in non-immunized ones; it was the highest in the period of the highest level of immunity and high levels of complement-fixing antibodies.

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Delayed type hypersensitivity (DTH) was studied in the macrophage migration inhibition test, antibody titers in the CFT and functional activity of spleen macrophages, obtained from guinea-pigs infected with R. conori, in cell cultures. No direct correlation between antibody synthesis and functional macrophage activity accompanied by the development of DTH was revealed. The most pronounced protective function of macrophages could be observed during the decrease of antibody titers and the extinction of DTH.

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The studies carried out on Rickettsia prowazekii (strain Breinl) have shown that these organisms can proliferate in the culture of guinea pig peritoneal macrophages, and the character of their development in macrophages depends mainly on the infective dose. Macrophages obtained from immune animals seem to have undergone functional transformation, thus becoming capable of destroying the infective agent. The process of the destruction of Rickettsia was more active in the presence of guinea pig immune serum.

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A definite correlation between the susceptibility of spleen macrophage cultures derived from a highly susceptible mouse line (C57Bl/6) and a line with low susceptibility (DBA/2) and the susceptibility of these mouse lines to infection with Rickettsia acari was established. Intensive replication of the rickettsiae in cultures from highly susceptible animals caused marked disorders in cell metabolism and eventually death of the culture. Cell cultures derived from resistant mice gradually eliminated the rickettsiae by intracellular digestion; necrobiotic changes in them were insignificant.

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