Experimental mycobacterial granulomas in guinea pig lymph nodes: ultrastructural observations

A systematic ultrastructural study has been performed of the mononuclear phagocytes in granulomas induced by different types of mycobacteria, e.g. live BCG (Pasteur), irradiated M. leprae and irradiated BCG (Pasteur) in guinea pig lymph nodes. Live BCG (Pasteur) induces a granuloma which peaks at 2-3 weeks and by light microscopy, a large number of the infiltrating cells have the appearance of epithelioid cells. Ultrastructurally a large proportion of these cells have a distinct appearance. They are characterised by the presence of very large nuclei, and prominent nucleoli, paucity of cytoplasmic organelles and swollen rough endoplasmic reticulum. Cells from 2 week granulomas induced by live M. kansasii and BCG (Glaxo), has a similar ultrastructural appearance. BCG (Pasteur) granulomas slowly resolved and by light and electron microscopy fibrosis and collagen deposition was seen by 7 weeks and was very extensive in some nodes at 10 weeks. The nodes from M leprae injected animals showed peak infiltration at 5 weeks and by light microscopy the infiltrating cell population was more mixed, with most of the cells having a macrophage appearance. Electron-microscopy showed these infiltrating cells to be mainly activated macrophages, containing phagocytosed organisms. Nodes from irradiated BCG (Pasteur) injected animals had peak infiltration at 1 week. The light and electron microscopic appearance showed areas consisting mainly of fibroblasts and mononuclear phagocytes. Thus, the granulomas induced by M. leprae are very different from those induced by BCG in the guinea pig. It is suggested that lymph nodes draining areas of local injection of BCG and irradiated M. leprae could respectively form a good model in the guinea pig for cellular and biochemical studies of the granulomas of tuberculoid and lepromatous leprosy.