Lower expression of Th1‐related cytokines and inducible nitric oxide synthase in mice with streptozotocin‐induced diabetes mellitus infected with Mycobacterium tuberculosis
Diabetes mellitus is an important predisposing factor for tuberculosis. The aim of this study was to investigate the mechanism underlying this association using a murine model. Mice with streptozotocin‐induced diabetes mellitus were prone to Mycobacterium tuberculosis infection, as indicated by increased numbers of live bacteria in lung, liver and spleen. In diabetic mice, the levels of IL‐12 and IFN‐γ in the lung, liver and spleen were lower than those in control animals on day 14 postinfection, while the opposite was true for IL‐4 levels in the lung and liver. The expression pattern of inducible nitric oxide synthase (iNOS), in the two mice types was as for IL‐12 and IFN‐γ. In addition, peritoneal exudate cells obtained from diabetic mice produced lower amounts of IL‐12 and NO than those from control mice, when stimulated in vitro with M. bovis BCG. Spleen cells from diabetic mice infected with M. tuberculosis produced a significantly lower amount of IFN‐γ upon restimulation with purified protein derivatives (PPD) than those from infected nondiabetic mice. Interestingly, addition of high glucose levels (33 mM) to the cultures of PPD‐restimulated spleen cells reduced the synthesis of IFN‐γ only in diabetic mice, and not in nondiabetic mice. Finally, control of blood glucose levels by insulin therapy resulted in improvement of the impaired host protection and Th1‐related cytokine synthesis. Our results suggest that the reduced production of Th1‐related cytokines and NO account for the hampered host defense against M. tuberculosis infection under diabetic conditions.