Tuberculosis (TB) and HIV have long been known to profoundly affect the immune system and synergistically accelerate disease progression. In particular, CD4+ T cell depletion by HIV is likely to be the major cause of immunodeficiency and reactivation of latent TB. Previous studies have found that blood monocyte turnover concurrent with tissue macrophage death from viral infection better predicted AIDS onset than CD4+ T cell depletion in macaques infected with SIV. The authors examined the contribution of macrophages to the pathogenesis of Mycobacterium tuberculosis (Mtb)/SIV coinfection in a rhesus macaque model using in vivo BrdU labelling, immunostaining, flow cytometry and confocal microscopy. They found that increased monocyte and macrophage turnover and levels of SIV-infected lung macrophages all correlated with TB reactivation. All Mtb/SIV-coinfected monkeys experienced declines in their CD4+ T cells, regardless of reactivation or latency outcomes, suggesting that lower CD4+ T cell levels are not the primary cause of Mtb reactivation. Therefore SIV-related damage to macrophages may contribute to Mtb reactivation during coinfection.
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