In vivo tropisms and kinetics of rat theilovirus infection in immunocompetent and immunodeficient rats

Abstract

Rat theilovirus (RTV) is a cardiovirus related to Theiler's murine encephalomyelitis virus. While RTV is a prevalent viral pathogen of rats used in biomedical research, the pathogenesis and characterization of RTV infections is not well understood. In the studies reported herein, we used immunohistochemistry to identify viral antigens in enterocytes of the small intestines of Sprague-Dawley (SD) rats. Fecal viral shedding in immunocompromised and immunocompetent rats following oral gavage with RTV1 was high for the first 2 weeks of infection with persistent shedding of high viral loads being observed in immunocompromised nude rats but not in immunocompetent rats. RTV was also detected in mesenteric lymph nodes and spleen of immunocompromised rats but not immunocompetent rats. In addition, the magnitude of serum antibody responses differed between immunocompetent rat strains with Brown Norway and SD rats having a significantly higher antibody response than CD or Fischer 344 rats. These data suggest that RTV1 has a tropism for the epithelial cells of the small intestine, immunocompetent rats have differing serum antibody responses to RTV infection, and sustained fecal shedding and extraintestinal dissemination of RTV1 occurs in rats deficient in T cell-dependent adaptive immunity. RTV infection in immunocompromised and immunocompetent rats has merit as a model for further studies of theilovirus pathogenesis following oral viral exposure.

Figure 1

Representative photomicrograph of an immunohistochemically-stained section of proximal small intestine from an experimental Sprague Dawley rat. Chromogen deposition was apparent in the cytoplasm of isolated enterocytes (arrows) from all SD rats inoculated with 2.5 × 10⁶ PFU RTV1 by oral gavage (n=10) and probed with polyclonal anti-RTV antibodies and goat anti-rabbit labeled secondary antibodies (1000×). No chromogen deposition was apparent in SD rats sham inoculated with uninfected BHK cell lysates (n=6). No chromogen deposition was apparent in experimental rats inoculated with 2.5 × 10⁶ PFU RTV1 and probed with normal transudate from the preimmunized rabbit and goat anti-rabbit labeled secondary antibodies or probed with secondary antibodies alone (data not shown).

Figure 2

Detection of RTV1 in the spleen and mesenteric lymph nodes by RT-PCR. Brown Norway (BN) n=11, Fischer 344 (F344) n=11, Hsd:RH-Fox1^rnu (Hsd:rnu) n=10, and NTac:NIH-Whn (Tac:rnu) n=10 rats were inoculated with 2.5 × 10⁶ PFU RTV1 by oral gavage. Statistical analysis performed by Fischer’s Exact test with asterisk indicating statistical difference between immunocompromised and immunocompetent groups (P<0.05).

Figure 3

Detection of serum antibodies in rats inoculated with RTV1. Serum samples were tested in a microsphere fluorescent immunoassay using RTV1 as antigen to evaluate the magnitude of serologic response in Hsd:SD and CRL:CD (n=10 per group) and BN (n=11) and F344 (n=11) rats inoculated with RTV1 by oral gavage. Control rats were sham inoculated with uninfected BHK cell lysates (n=6 per group) and showed negligible activity. The relationships are based on a nonlinear mixed model. Solid lines represent mean fluorescent units on a log₂ fluorescence scale and shaded regions are the 95% confidence bands for a given group.