Mononuclear cell dynamics in M. tuberculosis infection provide opportunities for therapeutic intervention

Abstract

Mycobacterium tuberculosis causes chronic infection of mononuclear phagocytes, especially resident (alveolar) macrophages, recruited macrophages, and dendritic cells. Despite the importance of these cells in tuberculosis (TB) pathogenesis and immunity, little is known about the population dynamics of these cells at the sites of infection. We used a combination of congenic monocyte adoptive transfer, and pulse-chase labeling of DNA, to determine the kinetics and characteristics of trafficking, differentiation, and infection of mononuclear phagocytes during the chronic, adaptive immune phase of M. tuberculosis infection in mice. We found that Ly6Chi monocytes traffic rapidly to the lungs, where a subpopulation become Ly6Clo and remain in the lung vascular space, while the remainder migrate into the lung parenchyma and differentiate into Ly6Chi dendritic cells, CD11b+ dendritic cells, and recruited macrophages. As in humans with TB, M. tuberculosis-infected mice have increased numbers of blood monocytes; this is due to increased egress from the bone marrow, and not delayed egress from the blood. Pulse-chase labeling of dividing cells and flow cytometry analysis revealed a T1/2 of [~]15 hrs for Ly6Chi monocytes, indicating that they differentiate rapidly upon entry to the parenchyma of infected lungs; in contrast, cells that differentiate from Ly6Chi monocytes turn over more slowly, but diminish in frequency in less than one week. New cells (identified by pulse-chase labeling) acquire bacteria within 1-3 days of appearance in the lungs, indicating that bacteria regularly encounter new cellular niches, even during the chronic stage of infection. Our findings that mononuclear phagocyte populations at the site of M. tuberculosis infection are highly dynamic provide support for specific approaches for host-directed therapies directed at monocytes, including trained immunity, as potential interventions in TB, by replacing cells with limited antimycobacterial capabilities with newly-recruited cells better able to restrict and kill M. tuberculosis.\n\nAuthor summaryDuring certain chronic infections such as tuberculosis, inflammatory cells, including macrophages and dendritic cells, are recruited to infected tissues where they aggregate to form tissue lesions known as granulomas. Although granulomas can persist long term, the dynamics of the cell populations that comprise granulomas are not well understood. We used a combination of methods to discover that, during chronic infection of mice with Mycobacterium tuberculosis, the monocyte, macrophage, and dendritic cell populations are highly dynamic: recently-proliferated cells traffic rapidly to infected lung tissues, yet they persist with a half-life of less than one week. We also found that recently-proliferated cells become infected with M. tuberculosis as soon as one day after their arrival in the lungs, indicating that the bacteria are regularly moving to new cellular niches, even during the chronic stage of infection. The dynamic nature of the cell populations that encounter M. tuberculosis suggests that interventions such as trained immunity have potential therapeutic roles, by replacing cells that have poor antimycobacterial activity with cells with enhanced antimycobacterial activity. These interventions could improve the outcomes of treatment of drug resistant tuberculosis.