Pre-existing SIV infection increases expression of T cell markers associated with activation during early Mycobacterium tuberculosis co-infection and impairs TNF responses in granulomas
Tuberculosis (TB) is the leading infectious cause of death among people living with HIV (PLHIV). PLHIV are more susceptible to contracting Mycobacterium tuberculosis (Mtb) infection and often have worsened TB disease. Understanding the immunologic defects caused by HIV and the consequences it has on Mtb co-infection is critical in combating this global health epidemic. We previously established a model of simian immunodeficiency virus (SIV) and Mtb co-infection in Mauritian cynomolgus macaques (MCM), and showed that SIV/Mtb co-infected MCM had rapidly progressive TB. We hypothesized that pre-existing SIV infection impairs early T cell responses to Mtb infection. To test our hypothesis, we infected MCM with SIVmac239 intrarectally followed by co-infection with a low dose of Mtb Erdman 6 months later. SIV-naive MCM were infected with Mtb alone as controls. Six weeks after Mtb infection, animals were necropsied and immune responses were measured by multiparameter flow cytometry. While the two groups exhibited similar TB progression at time of necropsy (Nx), longitudinal sampling of the blood (PBMC) and airways (BAL) revealed a significant reduction in circulating CD4+ T cells and an influx of CD8+ T cells in airways following Mtb co-infection of SIV+ animals. Differences in the activation markers CD69, PD-1, and TIGIT were observed. At sites of Mtb infection (i.e. granulomas), SIV/Mtb co-infected animals had a higher proportion of CD4+ and CD8+ T cells expressing PD-1 and TIGIT. In addition, there were fewer TNF-producing CD4+ and CD8+ T cells in granulomas and airways of SIV/Mtb co-infected animals. Taken together, we show that concurrent SIV infection alters T cell phenotypes in granulomas during the early stages of TB disease. As it is critical to establish control of Mtb replication soon after infection, these phenotypic changes may distinguish the immune dysfunction that arises from pre-existing SIV infection which promotes TB progression. Author SummaryPeople living with HIV are incredibly susceptible to TB and, when co-infected with Mtb, often develop serious TB disease. We do not yet understand precisely how HIV infection impairs the early stages of the adaptive immune response against Mtb bacilli. We employed a non-human primate model of HIV, using SIV as a surrogate for HIV, followed by Mtb co-infection to investigate the immunologic defects associated with pre-existing SIV infection over the first six weeks of Mtb co-infection. Our study focused on CD4+ and CD8+ T cells as these cells are known to play an important role in Mtb control. We found more CD8+ T cells in granulomas, the sites of Mtb infection, from SIV/Mtb co-infected animals, with little difference in CD4+ T cells. SIV/Mtb co-infected animals and animals infected with SIV alone had a higher proportion of both CD4+ and CD8+ T cells expressing activation markers compared to SIV-naive animals, consistent with SIV-dependent immune activation. Notably, we observed a lower proportion of TNF-producing T cells, a cytokine critical for Mtb control, in granulomas and airways of SIV/Mtb co-infected animals. Taken together, these data show that pre-existing SIV alters T cell phenotypes and reduces TNF responses early in Mtb infection.