Characterizing the Spectrum of Latent Mycobacterium tuberculosis in the Cynomolgus Macaque Model: Clinical, Immunologic, and Imaging Features of Evolution

Mycobacterium tuberculosis infection outcomes have been described as active tuberculosis or latent infection but a spectrum of outcomes is now recognized. We used a nonhuman primate model, which recapitulates human infection, to characterize the clinical, microbiologic, and radiographic patterns associated with developing latent M. tuberculosis infection. Four patterns were identified. "Controllers" had normal erythrocyte sedimentation rate (ESR) without M. tuberculosis growth in bronchoalveolar lavage or gastric aspirate (BAL/GA). "Early subclinicals" showed transient ESR elevation and/or M. tuberculosis growth on BAL/GA for 60 days postinfection, "mid subclinicals" were positive for 90 days, and "late subclinicals" were positive intermittently, despite the absence of clinical disease. Variability was noted regarding granuloma formation, lung/lymph node metabolic activity, lung/lymph node bacterial burden, gross pathology, and extrapulmonary disease. Like human M. tuberculosis infection, this highlights the heterogeneity associated with the establishment of latent infection, underscoring the need to understand the clinical spectrum and risk factors associated with severe disease.

SIV and Mycobacterium tuberculosis synergy within the granuloma accelerates the reactivation pattern of latent tuberculosis

Human immunodeficiency virus infection is the most common risk factor for severe forms of tuberculosis (TB), regardless of CD4 T cell count. Using a well-characterized cynomolgus macaque model of human TB, we compared radiographic, immunologic and microbiologic characteristics of early (subclinical) reactivation of latent M. tuberculosis (Mtb) infection among animals subsequently infected with simian immunodeficiency virus (SIV) or who underwent anti-CD4 depletion by a depletion antibody. CD4 depleted animals had significantly fewer CD4 T cells within granulomas compared to Mtb/SIV co-infected and Mtb-only control animals. After 2 months of treatment, subclinical reactivation occurred at similar rates among CD4 depleted (5 of 7 animals) and SIV infected animals (4 of 8 animals). However, SIV-induced reactivation was associated with more dissemination of lung granulomas that were permissive to Mtb growth resulting in greater bacterial burden within granulomas compared to CD4 depleted reactivators. Granulomas from Mtb/SIV animals displayed a more robust T cell activation profile (IFN-α, IFN-γ, TNF, IL-17, IL-2, IL-10, IL-4 and granzyme B) compared to CD4 depleted animals and controls though these effectors did not protect against reactivation or dissemination, but instead may be related to increased viral and/or Mtb antigens. SIV replication within the granuloma was associated with reactivation, greater overall Mtb growth and reduced Mtb killing resulting in greater overall Mtb burden. These data support that SIV disrupts protective immune responses against latent Mtb infection beyond the loss of CD4 T cells, and that synergy between SIV and Mtb occurs within granulomas.