Interference of natural mouse hepatitis virus infection with cytokine production and susceptibility to Trypanosoma cruzi

Abstract

Mouse hepatitis virus (MHV) infection can have a pronounced impact on several investigation areas. Reports on natural MHV outbreaks are rare and most studies have been conducted by deliberately infecting mice with MHV laboratory strains that cause moderate to severe disturbances to the immune system. We have investigated the effects of a natural acute outbreak of MHV in our otherwise specific-pathogen-free (SPF) inbred mouse colonies, and of enzootic chronic MHV infection on cytokine production and resistance to the intracellular pathogen Trypanosoma cruzi. We found that BALB/c and/or C57BL/6 SPF mice that had been injected with T. cruzi blood trypomastigotes from recently MHV-contaminated (MHV+) mice developed significantly higher parasite blood counts, accelerated death, and showed higher IL-10 production by spleen cells than their counterparts whose T. cruzi inoculum was derived from MHV-negative (MHV-) donors. Interferon-gamma (IFN-gamma) production by MHV+ and MHV- mice was not significantly different. In contrast, T. cruzi infection of chronically MHV-infected mice did not result in major changes in the course of infection when compared with that observed in mice from MHV- colonies, although a trend to higher parasitaemia levels was observed in BALB/c MHV+ mice. Nevertheless, both BALB/c and C57BL/6 T. cruzi-infected MHV+ mice had diminished IFN-gamma production to parasite-antigen stimulation in comparison with similarly infected MHV- mice. Interleukin-10 (IL-10) production levels by spleen cells did not differ between chronic MHV+ and MHV- mice, but IFN-gamma neutralization by monoclonal antibody treatment of anti-CD3-stimulated spleen cell cultures showed higher levels of IL-10 synthesis in MHV+ BALB/c mice.

Figure 1

Blood parasitaemia of BALB/c and C57BL/6 (B6) mice, serologically negative for MHV, infected with T. cruzi blood trypomastigotes obtained from mice that had been recently contaminated with MHV (MHV⁺) or from mice that were MHV⁻. Infection with 500 forms (a) or with 5000 forms (b). Arithmetic means, n=5. Significant differences (P < 0·05) between parasite counts from recipients of MHV⁺ versus MHV⁻ parasite donors on all infection days for mice injected with 500 forms and on days 7, 8 and 9 for mice injected with 5000 forms. Representative of two experiments.

Figure 2

Interferon-γ production by Con A-stimulated spleen cell cultures from BALB/c and C57BL/6 (B6) mice coming from chronically MHV-infected or from MHV⁻ colonies. The neutralizing anti-IL-10 mAb, 2A5, was added at the beginning of the cultures and the supernatants were harvested 72 hr later; means of triplicate cultures. Increased IFN-γ production by anti-IL-10 treatment was significant in both strains at P < 0·05. Other differences P> 0·05. Representative of two experiments with mice that were not infected with T.cruzi.

Figure 3

Interferon-γ production by Con A- and parasite-antigen-(T-Ag) stimulated spleen cell cultures from BALB/c mice coming from chronically MHV-infected or from MHV⁻ colonies and infected with different numbers of T. cruzi blood forms derived from MHV⁻ donors. Data from day 5 of infection, 72 hr supernatants. Means of duplicates; representative of three experiments; differences significant at P < 0·01 (*) or at P < 0·05 (**).

Figure 4

Interferon-γ production by Con A- and parasite-antigen-(T-Ag) stimulated spleen cell cultures from C57BL/6 (B6) mice coming from chronically MHV-infected or from MHV⁻ colonies and infected with different numbers of T. cruzi blood forms derived from MHV⁻ donors. Data from day 5 of infection, 72 hr supernatants. Means of duplicates; representative of three experiments; differences significant at P < 0·01 (*).

Figure 5

Interleukin-10 (units/ml) production by anti-CD3 stimulated and spleen cell cultures from BALB/c mice coming from chronically MHV-infected or from MHV⁻ colonies. The cultures were performed in the presence (or not) of the neutralizing anti-IFN-γ mAb XMG1.2; 72 hr supernatants. The mice were tested before T. cruzi infection (normal) and on days 6, 11 and 22 after infection with 500 T. cruzi blood forms. Means of duplicates; representative of two experiments; MHV⁺ and anti-IFN-γ mAb-treated groups significantly different from untreated MHV⁺ groups and from MHV⁻ groups at P < 0·01 (*).