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Several properties of virulent, avirulent, and interferon-resistant Rickettsia prowazekii strains were compared. All of the interferon-resistant rickettsial strains (which were derived from the avirulent Madrid E strain) resembled the virulent Breinl strain in that they grew well in untreated mouse macrophagelike RAW264.7 cells. In contrast, the avirulent Madrid E strain grew poorly in untreated RAW264.7 cells. Pretreatment of interferon-resistant rickettsiae or R. prowazekii Breinl with antirickettsial serum or immunoglobulin G suppressed the ability of the rickettsiae to grow in untreated RAW264.7 cells. Interferon-resistant R. prowazekii strains, like the Madrid E and Breinl strains, rapidly killed a substantial proportion of RAW264.7 cells that had been treated with gamma interferon or very high concentrations of alpha/beta interferon. Untreated infected RAW264.7 cells and interferon-treated mock-infected RAW264.7 cells were not killed during the same period. In cultures of RAW264.7 cells treated with either alpha/beta interferon (120 to 1,200 U/ml) or a subsaturating concentration of gamma interferon (0.5 U/ml), R. prowazekii Breinl organisms killed a higher percentage of the cells than did comparable numbers of R. prowazekii Madrid E organisms or interferon-resistant rickettsiae. Although R. prowazekii Breinl (like R. prowazekii Madrid E) was quite sensitive to gamma interferon in mouse L929 cells, the Breinl strain was resistant to murine alpha/beta interferon compared with the Madrid E strain and the two strains selected for resistance to murine gamma interferon. One of the interferon-resistant strains (strain 60P, which was selected for resistance to murine alpha/beta interferon) differed from the other R. prowazekii strains in that it induced little or no detectable interferon in L929 cell cultures.

Comparison of properties of virulent, avirulent, and interferon-resistant Rickettsia prowazekii strains