Strain-dependent CNS dissemination in guinea pigs after Mycobacterium tuberculosis aerosol challenge

Abstract

Clinical reports suggest an association of distinct Mycobacterium tuberculosis strains with CNS disease. We therefore examined CNS dissemination by different laboratory strains (two M. tuberculosis H37Rv, one CDC1551) in a guinea pig aerosol infection model. Although all strains grew exponentially in lungs, with similar bacterial burdens at the time of extrapulmonary dissemination, M. tuberculosis CDC1551 disseminated to the CNS significantly more than the H37Rv strains. No CNS lesions were observed throughout the study, with only a modest cytokine response. These data suggest that M. tuberculosis may have virulence factors that promote CNS dissemination, distinct from those required for pulmonary TB.

Figure 1. Guinea pig brain and spleen CFU following aerosol infection

A. Extrapulmonary dissemination was first observed 14 days after infection for all 3 strains. M. tuberculosis H37Rv JHU demonstrated a lower bacterial burden at day 14, although there was no significant difference between CDC1551 and H37Rv TAMU at this time point or between any strains after day 28. B. M. tuberculosis CDC1551 were detected at significantly higher levels in the brain at all time points, compared to the two H37Rv strains (P < 0.04) (Fig. 1B). No bacteria were recovered from brains of guinea pigs infected with M. tuberculosis H37Rv JHU at any of the time points examined, while bacteria were detected only in low numbers from brains of guinea pigs infected with M. tuberculosis H37Rv TAMU at 14 and 56 days after infection. C. Blood CFU counts shown are extrapolated to the whole blood volume (25 ml) of the guinea pig. Bacteria were detected in the whole blood starting at day 14 after infection. Bacillemia was most consistent for strain CDC1551, but remained low in each animal (< 2.5 log₁₀ CFU) at each time point. At least 3 animals were used for data collection at each time point.

Figure 2. Immune response in guinea pig brain tissue following aerosol infection with M. tuberculosis CDC1551

The cytokines IL-1β and TNF-α were measured at days 1, 14, 28 and 56 after infection. Both of these proinflammatory mediators were found to be modestly but significantly elevated (relative to day 1) at days 14 and 56 (P < 0.0001) following aerosol infection, although at much lower levels than is typically observed in the lungs. At day 28, TNF-α levels were only slightly elevated (P = 0.003), and no change was observed for IL-1β (P = 0.6). Three animals were used for cytokine analysis at each time point.