Extensive Mycobacterium bovis BCG infection of liver parenchymal cells in immunocompromised mice

A histologic study was performed on the livers of wild-type (WT), severe co mbined immunodeficient (SCID), hydrocortisone acetate (HC)-treated WT and H C-treated SCID mice infected intravenously with 10(5) CFU of Mycobacterium bovis BCG. It mas found that infection progressed faster in SCID mice than in WT mice and that HC treatment caused exacerbation of infection in both t ypes of mite. In all cases infection in the liver was confined to granuloma s that were populated predominantly by macrophages, Higher levels of infect ion in HC-treated SCID mice, bat not HC-treated WT mice, were associated wi th extensive infection and destruction of parenchymal cells at the margins of granulomas. The results indicate that in the absence of T-cell-mediated immunity and of HC-sensitive T-tell-independent defense mechanisms, macroph ages are incapable of restricting BCG growth and of confining infection to their cytoplasm. Consequently, BCG bacilli are released into the extracellu lar environment, where they are ingested by; neighboring parenchymal cells.