To understand how virulent mycobacteria subvert host immunity and establish disease, we examined the differential response of mice to infection with various human outbreak Mycobacterium tuberculosis clinical isolates. One clinical isolate, HN878, was found to be hypervirulent, as demonstrated by unusually early death of infected immune-competent mice, compared with infection with other clinical isolates. The differential effect on survival required lymphocyte function because severe combined immunodeficiency (SCID) mice infected with HN878 or other clinical isolates all died at the same rate. The hypervirulence of HN878 was associated with failure to induce M. tuberculosis-specific proliferation and IFN-␥ production by spleen and lymph node cells from infected mice. In addition, 2-to 4-fold lower levels of tumor necrosis factor-␣ (TNF-␣), IL-6, IL-12, and IFN-␥ mRNAs were observed in lungs of HN878-infected mice. IL-10, IL-4, and IL-5 mRNA levels were not significantly elevated in lungs of HN878 infected mice. In contrast, IFN-␣ mRNA levels were significantly higher in lungs of these mice. To further investigate the role of Type 1 IFNs, mice infected with HN878 were treated intranasally with purified IFN-␣͞␤. The treatment resulted in increased lung bacillary loads and even further reduced survival. These results suggest that the hypervirulence of HN878 may be due to failure of this strain to stimulate Th1 type immunity. In addition, the lack of development of Th1 immunity in response to HN878 appears to be associated with increased induction of Type 1 IFNs.

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Clonal isolates of mouse 3T3 cells and primary rat embryo cells, recovered nonselectively after infection by simian virus 40 (SV40), have been tested for tumorigenicity in the immune-deficient nude mice in order to determine the cellular growth properties in vitro specifically correlated with neoplastic growth in vivo. In addition, mouse 3T3 cells transformed by murine sarcoma virus (MuSV, Kirsten strain), and revertants isolated from cells fully transformed by either SV40 or MuSV were also studied. Results suggest that the single cellular property consistently associated with tumorigenicity in nude mice is the acquisition by virus-transformed cells of the ability to proliferate in vitro in the absence of anchorage. Other cellular parameters of virusinduced transformation, such as lack of sensitivity to high cell density and the capacity to grow in low serum concentration, are dissociable from cellular tumorigenicity. This conclusion is supported further by the demonstration that specific selection in vivo for tumorigenic cells from anchorage-dependent cells results in the isolation of anchorage-independent cells. Conversely, a single-step selection in vitro for anchorage-independent cells from nontumorigenic cells results in a simultaneous selection of highly tumorigenic subclones. Infection of susceptible animal cells in vitro by tumor viruses usually results in a spectrum of stable alterations in cellular growth properties, as well as in the appearance of virus-specific antigens in the transformed cells (1). In particular, division in populations of untransformed cells is inhibited by any of the following three environmental constraints: extensive cell-cell contact (2), reduction of serum concentration (3, 4), or deprivation of a solid substrate for cell anchorage (5, 6).Recent results have demonstrated that cellular responses to the experimental parameters which differentiate the normal cell from its transformed counterpart are not coordinately controlled (7,8). Each constraint is the source of a selective assay that yields a different class of transformed cell line. Nonselective transformations of 3T3 mouse cells and of primary rat embryo cells by simian virus 40 (SV40) yielded lines displaying many different transformed phenotypes. While some lines were fully insensitive to each of the three constraints, most transformed lines lost only one or two of these constraints and remained normal for the others. Negative selection of revertant cell lines from a fully transformed 3T3 cell also dissociated these three parameters of growth control (9, 10).These observations suggested to us that not all of the altered cellular growth properties commonly associated with Abbreviations: SV40, simian virus 40; MuSV, murine sarcoma virus, Kirsten strain; RE, rat embryo; ME, mouse embryo. t Present address:

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