SIV Disrupts the Architecture of the Tuberculous Granuloma
In macaques with Mycobacterium tuberculosis infection, the lung granuloma was not simply an inflammatory mass. It behaved like a highly ordered immune structure, with distinct cellular programs separated between an inner ring and an outer ring. Myeloid cells in the inner zone were enriched for metabolic and antimicrobial pathways, while the outer ring was marked by T cell costimulatory and activation programs. That spatial arrangement matters because granulomas are meant to contain bacilli while preserving a coordinated local immune response, not merely accumulate immune cells in a disorganized cluster.
SIV co-infection altered that design at the level of transcriptional geography. The study compared granulomas from macaques infected with SIV and Mtb, macaques infected with SIV and Mtb while receiving ART, and animals infected with Mtb alone. In the co-infected granulomas, the spatially separated patterns seen in Mtb-only lesions were lost. Instead of the tidy division of labor seen in the uninfected setting, the pulmonary lesions showed broader distortion of immune signaling, with stronger enrichment of type I interferon pathways than in Mtb-only granulomas.
The biologic consequence is straightforward even if the cellular choreography is complex. When the local immune network loses organization, the granuloma becomes less able to sustain the compartmentalized functions needed for bacterial control.
Spatial Transcriptomics Reveals a Divided Granuloma in Mtb Alone
The analysis relied on spatial transcriptomics using NanoString GeoMX, paired with histology and immunofluorescence. Macaques were infected with SIV for 16 weeks and Mtb for 12 weeks, with a parallel arm receiving ART beginning 3 days after SIV infection. Lung granulomas were collected at necropsy, and individual lesions were assessed for bacterial burden, viral RNA, and tissue organization. A subset underwent spatial profiling, allowing investigators to compare local immune programs within the same lesion rather than infer function from bulk tissue averages.
In the Mtb-only granulomas, the transcriptional pattern was visibly compartmentalized. Inner-ring myeloid cells showed enrichment of metabolic and antimicrobial pathways, a pattern consistent with active containment and effector function at the site where bacilli are most likely to persist. The outer ring carried T cell activation and costimulatory signals, suggesting a coordinated interface between antigen presentation and adaptive immune recruitment. This layered organization is the kind of tissue-level response that granulomas depend on to fence in infection while preserving immune surveillance.
By contrast, SIV/Mtb lesions did not preserve those local distinctions. The loss of ring-specific specialization suggests that viral infection changes not just which cells are present, but how they communicate across space.
ART Reduced Viral Replication Without Restoring Granuloma Function
The ART-treated SIV/Mtb granulomas occupied an intermediate position. Viral replication was absent, yet the tissue was not reset to the Mtb-only state. Instead, the transcriptional pattern remained partially altered, with incomplete recovery of the spatial immune network. This finding is clinically important because it separates suppression of viral replication from full restoration of local immune architecture. ART can quiet the virus, but it does not necessarily rebuild the granuloma’s cellular map.
Even with viral replication controlled, the lung lesions retained an immune imprint of prior SIV infection. The persistence of this altered state suggests that the damage extends beyond active viremia and may involve durable shifts in tissue immunity, cell positioning, or intercellular signaling pathways. In the context of TB, that matters because the host does not fight infection in the bloodstream alone; containment depends on what happens inside the granuloma wall, where macrophages, T cells, and other immune populations must remain synchronized.
Type I Interferon Signaling Rose as Cell-Cell Communication Fell
One of the clearest molecular shifts in the co-infected granulomas was greater enrichment of type I IFN pathways. In TB, excessive or misdirected interferon signaling has long been associated with impaired antimicrobial defense, partly because it can displace protective inflammatory programs and reshape myeloid function. The study’s findings fit that framework: as the normal spatial pattern dissolved, interferon-linked signaling became more prominent.
Cell-cell communication was also reduced among the SIV/Mtb groups. That drop in signaling density gives the transcriptional findings a practical edge. Granulomas are not passive nodules; they are communication hubs. If messages between myeloid cells and lymphocytes weaken, then recruitment, activation, and local containment can all suffer. The reduction in communication may help explain why granulomas in the co-infected setting looked less organized even when ART was present.
This is not a small technical detail buried in omics data. It points to a tissue-level failure of coordination.
What the Macaque Model Adds to HIV and TB Biology
The study used simian immunodeficiency virus as a model for HIV and examined pulmonary granulomas directly, which is where TB pathology unfolds. That design allowed the investigators to focus on the local immune ecosystem rather than rely on circulating markers. In people living with HIV, the risk of TB remains elevated despite ART, and these data offer a plausible mechanistic explanation: suppression of HIV does not fully normalize the granuloma environment.
Spatially resolved analysis matters because conventional bulk measurements can blur together cell populations that are physically and functionally distinct. Here, the inner and outer rings of the granuloma were not interchangeable. Their separation in Mtb-only lesions, and their disruption in SIV/Mtb lesions, suggest that tissue architecture itself is part of host defense. Once that architecture is damaged, a lesion may retain immune cells yet still fail to behave like a functional barrier.
The work also raises a practical question for tuberculosis pathogenesis in the setting of HIV. If ART does not restore granuloma organization, then future strategies may need to address tissue-level immune repair, not only viral suppression. That could include approaches aimed at correcting interferon-skewed signaling or rebuilding the cell-cell circuits that support granuloma integrity.
Persistent Tissue Injury After Viral Control Remains the Central Problem
The central observation is stark: HIV-like SIV disrupted the spatially organized immune functions of pulmonary granulomas, and ART did not fully reverse that disruption. The lesions in co-infected macaques remained biologically distinct from Mtb-only granulomas even after viral replication was suppressed. For TB control, the unresolved issue is whether local immune architecture can be repaired once it has been distorted, or whether the granuloma carries a durable memory of viral injury that keeps bacterial containment off balance.
The study does not answer that question, but it makes the target unmistakable. In HIV and TB co-infection, the battle is fought inside the granuloma, and the fight is lost as soon as spatial order breaks down.