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Chapter 18 : Effects of Whole-Body X-Irradiation on Tuberculosis
In an experiment to study the effects of sublethal wholebody irradiation, commercial rabbits were irradiated with 400 rads of whole-body X-irradiation—a sublethal dose. At 2 or 10 days thereafter, they were injected intradermally with BCG. Between 2 and 4 weeks after irradiation, the BCG lesions and 48-h tuberculin reactions in the irradiated group were smaller than those of the nonirradiated controls. The BCG lesions in the irradiated group also contained more bacilli. Pulmonary alveolar macrophages (AM) recovered by bronchoalveolar lavage (BAL) from irradiated rabbits contained higher levels of hydrolytic enzymes than did AM from nonirradiated controls. The AM from the irradiated group were apparently an older (more activated) cell population, because they had ingested inhaled particles for a longer period of time. The irradiation evidently had reduced the young macrophage population that replenishes the AM population. Rabbits were infected by aerosol with virulent human-type tubercle bacilli (H37Rv) at 12 or 30 days after irradiation. In each case, 5 weeks after infection, the number of primary pulmonary tubercles in their lungs was the same in both the irradiated and the nonirradiated groups. The number of viable bacilli in these tubercles was the same. Therefore, this sublethal dose of irradiation had no appreciable effect on the development and progress of primary pulmonary tubercles in rabbits. In brief, X-irradiation reduces the bone marrow’s capacity to provide defense cells to protect the host against infection. When the host is challenged by inhaled virulent human-type tubercle bacilli, an adequate supply of defense cells is available.