Granuloma formation in severe combined immunodeficient (SCID) mice in response to progressive BCG infection. Tendency not to form granulomas in the lung is associated with faster bacterial growth in this organ.

Intravenous inoculation of Mycobacterium bovis, strain bacillus Calmette-Guerin caused infection in the lungs, livers, and spleens of severe combined immunodeficient (SCID) mice and in the same organs in immunocompetent co-isogenic C.B17 mice. However, whereas infection in the latter mice was stabilized and partly resolved, it was progressive in SCID mice and eventually lethal, with the most rapid bacterial growth occurring in the lungs. Histological examination of infected organs showed that well-developed, compact epithelioid granulomas formed at sites of bacterial multiplication in livers, spleens, and lungs of C.B17 mice. Granulomas also formed in the livers and spleens of SCID mice, despite their inability to generate immunity. However, in the lungs of SCID mice, bacillus Calmette-Guerin was regionally distributed mostly in isolated alveolar macrophages and in aggregates of macrophages resembling small granulomas. The possibility that this tendency not to form granulomas in the lung is the reason for the more rapid growth of bacillus Calmette-Guerin in this organ is discussed.