Mechanistic insights into concurrent cART + 3HP therapy in TB/HIV co-infection

The high stakes of TB/HIV co-infection

Tuberculosis (TB) and HIV continue to be two of the most significant infectious threats worldwide. When these diseases intersect, the challenges become even greater. People living with HIV are at substantially increased risk of reactivating latent TB infection (LTBI), making prevention and treatment strategies vital for global health. The World Health Organization (WHO) now recommends combining antiretroviral therapy (cART) with the short-course TB preventive regimen known as 3HP, a once-weekly 12-week treatment composed of high-dose isoniazid and rifapentine. Clinical trials have shown that this combination is safe, effective, and compatible with dolutegravir-based cART. However, the biological mechanisms underlying the benefits and limitations of this concurrent therapy have not been fully defined.

To address this knowledge gap, we used rhesus macaques co-infected with Mycobacterium tuberculosis (Mtb) and simian immunodeficiency virus (SIV) to explore how concurrent cART + 3HP affects immune function, bacterial burden, and lung pathology. Our findings offer critical insights into why TB/HIV co-infection remains difficult to manage, even under optimal therapeutic conditions.

Modeling the unmodelable: How NHPs reveal TB/HIV mechanisms

Nonhuman primates (NHPs), particularly rhesus macaques, offer an unparalleled model for TB/HIV co-infection. Their immune systems closely resemble those of humans, and they develop TB infection states; latent, chronic, and reactivated that mirror human disease progression. Importantly, rhesus macaques can be safely examined at the tissue level, allowing scientists to analyze lung immune responses that would be inaccessible in human clinical studies[1, 2]. While human studies provide clinical data, they cannot capture the mechanistic details of how treatments alter immune activity in specific tissues. Therefore, modeling concurrent cART and 3HP in NHPs offers a powerful tool for understanding how these therapies influence bacterial control, viral suppression, and immune recovery.

A deep dive into how combined therapy alters immune activation, T cell responses

The central aim of our study was to determine whether early initiation of concurrent cART and 3HP would better control immune activation, promote immune reconstitution, and prevent LTBI reactivation compared to cART alone. To accomplish this, we compared animals receiving cART[3] or cART + 3HP to LTBI-only macaques and cART-naïve Mtb/SIV co-infected macaques. We evaluated multiple parameters, including pulmonary and systemic immune activation, the frequency and functionality of Mtb-specific T cell responses, bacterial burden, and lung inflammation measured through both pathology and PET/CT imaging. Transcriptomic and single-cell RNA sequencing further assessed how treatments influenced gene expression patterns within lung tissue and immune cell populations.

Bacterial control and pathology: strong benefits of cART + 3HP

One of the most striking findings was the marked reduction in bacterial burden in animals receiving cART + 3HP. Five of six macaques showed complete sterilization of Mtb in lung tissue, and all demonstrated prevention of extrapulmonary bacterial spread. Lung pathology supported these results. Pathological examination revealed that the remaining granulomas were small, with a mix of non-necrotizing and caseous structures. PET/CT imaging further demonstrated a decrease in the number of lesions in the cART + 3HP group, although the lesions that remained showed high 18F-FDG uptake, indicating ongoing inflammation.

These findings highlight a key point: while combined therapy effectively reduces bacterial burden and disease severity, it does not completely eliminate inflammatory lesions or fully resolve lung immune disturbances.

Incomplete immune reconstitution despite pathogen suppression

Although cART + 3HP achieved strong bacterial and viral suppression, we found that immune reconstitution in the lung remained significantly impaired. This aligns with earlier findings that cART alone does not fully restore lung-resident immunity, even when viral loads are suppressed[3, 4].

One major concern is that chronic immune activation persisted in both the pulmonary compartment and the periphery. Markers of activation such as HLA-DR and CD69 on CD4+ T cells remained elevated, revealing that early SIV-driven immune dysregulation was not resolved by treatment. Elevated CXCR3+CCR6+CD4+ T cell populations in the blood also indicated persistent systemic activation.

Functional skewing of Mtb-specific T cell responses

We observed qualitative differences in Mtb-specific immune responses following treatment. After 12 weeks of cART + 3HP, macaques showed increased frequencies of IFN-g and IL-17-producing CD4⁺ T cells. These T_H1 and T_H17responses are important for anti-mycobacterial activity. However, the frequency of TNF-a-producing Mtb-specific CD4⁺ T cells was reduced compared to cART-only animals. Because TNF-a is essential for granuloma formation and maintenance, this skewed cytokine response may negatively affect long-term protection.

Transcriptomic insights: persistent inflammation and cell death

Bulk and Single-cell RNA sequencing of lung tissue showed increased expression of type I interferon–related genes and apoptosis-associated pathways in cART + 3HP animals compared to cART-only macaques. Type I interferons, while essential in antiviral defense, have been repeatedly implicated in exacerbating TB pathology and promoting macrophage cell death, which can facilitate bacterial spread. Together, these data indicate that even under successful pathogen control, the lung environment remains inflamed, activated, and prone to dysfunctional immune signaling.

Conclusion: The need for host-directed interventions

Ultimately, the study demonstrates that while concurrent cART + 3HP therapy effectively suppresses HIV and Mtb,immune reconstitution in the lungs remains incomplete and inflammation persists. Protective CD4⁺ T_EM cells fail to recover, Mtb-specific responses are functionally skewed, and transcriptomic signatures show continued immune dysregulation. These findings underscore the need for host-directed therapies; such as IL-21-based biologics or IDO-1 inhibitors to be combined with standard treatments. By targeting the host immune response during early HIV co-infection, such interventions may enhance long-term immune recovery, reduce chronic inflammation, and more effectively prevent TB reactivation.

The study not only expands our understanding of TB/HIV co-infection but also sets the stage for next-generation therapeutic strategies aimed at improving outcomes for millions living with both infections worldwide.

1. Sharan, R., et al., Isoniazid and rifapentine treatment effectively reduces persistent M. tuberculosis infection in macaque lungs. J Clin Invest, 2022. 132(18).
2. Sharan, R., et al., Characterizing Early T Cell Responses in Nonhuman Primate Model of Tuberculosis. Front Immunol, 2021. 12: p. 706723.
3. Sharan, R., et al., Antiretroviral therapy timing impacts latent tuberculosis infection reactivation in a Mycobacterium tuberculosis/SIV coinfection model. J Clin Invest, 2022. 132(3).
4. Ganatra, S.R., et al., Antiretroviral therapy does not reduce tuberculosis reactivation in a tuberculosis-HIV coinfection model. J Clin Invest, 2020. 130(10): p. 5171-5179.