ART fails to stop TB spreading beyond lungs in macaque study

People who are infected with both HIV and Mycobacterium tuberculosis (Mtb) often develop more severe, disseminated forms of tuberculosis and experience faster progression of HIV. Philana Ling Lin and colleagues set out to test a simple but important question: if antiretroviral treatment (ART) suppresses the virus, does that stop Mtb from spreading beyond the lungs? To study this, the team used a macaque model of simian immunodeficiency virus (SIV) and Mtb co-infection. Macaques were assigned to one of three groups: infected with SIV B670, infected with SIV B670 and given ART, or given saline as a control. After these initial steps, all animals received a low-dose Mtb inoculation. The researchers followed the animals with serial clinical checks, microbiological tests, PET CT imaging, and immunologic assessments to monitor disease course. At the end of the study they performed necropsies to examine gross pathology, measure viremia and bacterial burden, and compare a range of immunologic parameters. The design allowed a direct comparison of how ART-modulated SIV infection influenced subsequent tuberculosis disease in this controlled animal model.

The study used matched groups of macaques to compare outcomes after low-dose Mtb challenge. Animals in the SIV-TB group (infected with SIV B670 and then Mtb) showed greater gross pathology and higher total bacterial burden than animals that received Mtb alone (TB only) and animals that were infected with SIV B670 but treated with ART (SIV/ART/TB). However, a surprising result emerged: despite having normal blood CD4 counts and undetectable SIV RNA, the SIV/ART/TB macaques showed clinical measures and extrapulmonary involvement similar to the untreated SIV-TB animals. The team also analyzed barcoded-Mtb to track dissemination and found evidence suggesting that control of SIV replication by ART did not prevent Mtb from spreading beyond the lungs. These findings came from combining clinical observation, microbiology, PET CT imaging, and postmortem measures of bacterial burden and pathology to capture both the systemic and localized aspects of disease.

Taken strictly from these results, the study indicates that suppressing SIV replication with ART does not necessarily stop Mtb from disseminating to extrapulmonary sites in this macaque model. That means people living with HIV who are on effective ART might still be at high risk of bacterial spread and extrapulmonary tuberculosis, even when blood measures such as CD4 counts and viral load look reassuring. The authors highlight a practical problem: extrapulmonary disease can be missed when methods to identify it are inconsistent, so relying solely on blood markers may give a false sense of security. These data underscore the need to better understand the mechanisms that allow Mtb to spread in the context of HIV/ART, and to develop more reliable clinical approaches to detect and manage extrapulmonary TB in people with treated HIV infection.