Modulation of resistance to Salmonella typhimurium infection in mice by mouse hepatitis virus (MHV)

Monoclonal antibody solution hybridization assay for detection of mouse hepatitis virus infection

A monoclonal antibody solution hybridization (MASH) assay was developed to detect fecal excretion of mouse hepatitis virus (MHV). The assay used a biotinylated cDNA probe to detect viral RNA target sequences by hybridization in solution, capture of hybrids on the solid phase with antibiotin antibody, and immunoassay with an enzyme-labelled monoclonal antibody specific for DNA-RNA hybrids. The MASH assay was used to monitor the time course of enterotropic MHV excretion after oronasal inoculation. Infectivity of the inoculated mice was simultaneously monitored with sentinel animals. The MASH assay detected MHV excretion in all inoculated mice, with the highest mean excretion levels occurring from day 3 through day 9 postinoculation. Mean excretion then decreased gradually to below detection limits by day 21 postinoculation. Sentinels became infected on exposure to inoculated mice up to but not after day 21 postinoculation. Infected sentinel mice showed a time course of virus excretion similar to that of inoculated mice. These results indicate that the MASH assay is useful for rapid, sensitive, and specific detection of MHV in clinical specimens from laboratory mice.

Interleukin-1 administration to C3H/HeJ mice after but not prior to infection increases resistance to Salmonella typhimurium

Interleukin-1 (IL-1) treatment of C3H/HeN and C3H/HeJ mice prior to infection with Salmonella typhimurium increased the survival fraction only in C3H/HeN mice. IL-1 administration after infection resulted in a significant increase in mean survival time in C3H/HeJ but not C3H/HeN mice. Bacterial growth in IL-1-treated C3H/HeJ mice was less than that in control mice.

The Salmonella typhimurium locus mviA regulates virulence in Itys but not Ityr mice: functional mviA results in avirulence; mutant (nonfunctional) mviA results in virulence

The virulent Salmonella typhimurium strain WB600 carries the mviA allele of the gene mouse virulence A. As shown here, the virulent phenotype of WB600 is the result of a nonfunctional mviA gene. As compared to the functional allele mviA+, mviA increases virulence in Itys mice, but not in Ityr mice. A specific BglII site, mviA4185, between osmZ and galU, located at approximately 35 min on the salmonella chromosome, was within mviA. Insertion of an antibiotic cassette in the mviA4185 site of mviA+ or the homologous mviA4093 site of mviA DNA resulted in virulence when either cassette was recombined into the chromosome. When mviA and mviA+ were both expressed in the same strain with one carried in the chromosome and the other on a plasmid, avirulence was dominant. Replacement of the mviA allele of strain WB600 using P22 transductions of linked antibiotic cassettes cloned into the chromosome of virulent S. typhimurium strains (SR-11, TML, SL1344, C5, ATCC14028, W118-2, and WB600) showed that all but WB600 contained the avirulent mviA+ allele. Southern hybridizations provided no evidence for a second mviA allele anywhere in the genome of the six non-WB600 strains.

Induction of self-reactive T cells after murine coronavirus infection

We studied the mechanism of in vitro spontaneous lymphokine production by spleen cells from mice injected intraperitoneally with murine coronavirus stain JHM 1 month after infection, when infectious virus had already been cleared from the spleens. Removal of either CD4+ T cells or Ia+ antigen-presenting cells (APC) from the spleen cells abrogated interleukin-2 (IL-2) production. Addition of anti-CD4 or anti-Iad monoclonal antibodies to the culture suppressed IL-2 production. These results suggest that the response involved typical receptor-mediated activation of T cells. Surprisingly, reciprocal mixing experiments with a coculture of T cells from infected mice and APC from either infected or naive mice resulted in the production of IL-2. The absence of viral antigens in spleen cells 1 month after infection, as indicated by their inability to induce the proliferation of T-cell clones specific for the viral antigens, suggest that the T cells from mice 1 month after infection were not responding to the viral antigens. The inoculum components other than the virus did not induce this immune response. We also found that the frequency of self-reactive but not alloreactive IL-2-producing T cells in the spleens of infected mice was 3- to 10-fold higher than that in naive mice. These findings suggest that an increased frequency of self-reactive T cells which secrete IL-2 occurs following murine coronavirus infection. This may have important implications in the development of autoimmunelike phenomena following murine coronavirus infection.

Mouse hepatitis virus strain UAB infection enhances resistance to Salmonella typhimurium in mice by inducing suppression of bacterial growth

We have previously shown that intranasal infection of mice with mouse hepatitis virus (MHV) strain UAB (MHV-UAB) increases their resistance to Salmonella typhimurium injected intravenously 6 days later. To study how salmonella resistance was induced, BALB/cAnNCr mice were infected with salmonella strains carrying specific genetic alterations. One set of studies compared the effect of MHV infection on subsequent salmonella infections with AroA- (avirulent) and Aro+ (virulent) salmonellae. Unlike its effect on Aro+ salmonellae, MHV failed to reduce the number of AroA- salmonellae recovered from mice. Because AroA- S. typhimurium shows almost no growth in vivo, this failure indicated that the effect of MHV on salmonella resistance required growth of the infecting salmonellae. In other studies, the effect of MHV infection on both growth and killing were monitored simultaneously in mice with growing salmonellae carrying a single copy of the temperature-sensitive pHSG422 plasmid, which is unable to replicate in vivo. MHV infection reduced salmonella growth but caused no increase in salmonella killing. MHV infection of mice given wild-type salmonellae also resulted in no increase in salmonella killing 4 h after salmonella challenge. These studies demonstrate that MHV-UAB infection increases host resistance to salmonellae by enhancing suppression of bacterial growth instead of by increasing the amount of salmonella killing.