Modified Rickettsia prowazekii strains have been derived from the avirulent Madrid E strain by passage in the lungs of white mice (strain EVir) or by selection for resistance to gamma interferon (IFN-gamma) (strains 427-19 and 87-17) or alpha/beta interferon (IFN-alpha/beta) (strains 83-2P, 60P, 103-2P, and 110-1P). Compared with the Madrid E strain, strain EVir has increased virulence (N. M. Balayeva and V. N. Nikolskaya, J. Hyg. Epidemiol. Microbiol. Immunol. 17:11-20, 1973) and a different lysine methylation profile in its surface protein antigen (A. V. Rodionov, M. E. Eremeeva, and N. M. Balayeva, Acta Virol. 35:557-565, 1991). The other six strains differ from the Madrid E strain in their resistance to IFN and their ability to grow well in untreated macrophagelike RAW264.7 cells. In the present study, to determine which properties are shared by these strains, we examined R. prowazekii EVir for the following: (i) the sensitivity of its growth in L929 cells to the cytokines IFN-alpha/beta, IFN-gamma, tumor necrosis factor alpha (TNF-alpha), and IFN-gamma plus TNF-alpha; (ii) the ability to grow in untreated RAW264.7 cells; and (iii) the ability to induce interferon in L929 cell cultures; we also evaluated strains 83-2P and 87-17 for lysine methylation. Multiplication of strain EVir in growing L929 cells was not markedly inhibited by either IFN-alpha/beta or IFN-gamma. In X-irradiated L929 cells, growth of strain EVir was slightly inhibited (11%) by TNF-alpha alone, somewhat inhibited (38%) by IFN-gamma alone, and markedly inhibited (87%) by IFN-gamma plus TNF-alpha. Nitrite production was induced in X-irradiated, strain EVir-infected L929 cell cultures treated with TNF-alpha alone or IFN-gamma alone; however, more nitrite was produced in infected cultures treated with IFN-gamma plus TNF-alpha. Nitrite production, the dramatic inhibitory effect of IFN-gamma plus TNF-alpha, and the modest inhibitory effect of IFN-gamma on the growth of strain EVir in X-irradiated L929 cells were all alleviated by the addition of the nitric oxide synthase inhibitor NG-methyl-L-arginine. Strain EVir grew very well in untreated macrophagelike RAW264.7 cells and appeared defective in the ability to induce IFN in L929 cell cultures. All strains grown in L929 cells in the presence of radiolabeled lysine had similar percentages of their radioactivity as methylated lysines.(ABSTRACT TRUNCATED AT 400 WORDS)