Date Published: October 26, 2018
Publisher: Public Library of Science
Author(s): Brian A. Norris, Joel D. Ernst, Marcel A. Behr.
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.
Mononuclear phagocytes (MNP) harbor Mycobacterium tuberculosis in tissues of humans  and experimental animals [2–4]; and MNP are essential elements of granulomas, the characteristic tissue lesions in tuberculosis [5, 6]. Although macrophages have been characterized as prominent cellular hosts for M. tuberculosis in vivo, recent studies have revealed the roles of distinct populations of MNP, including tissue-resident (i.e., lung alveolar) macrophages, monocyte-derived recruited macrophages, and dendritic cells, as host cells for the bacteria in experimental animals  and humans . Although cells in these subsets exhibit functional differences during M. tuberculosis infection, including the ability to transport bacteria from the lungs to the local lymph nodes [8–10] and their ability to present antigens for activation of CD4 T cells , there is little known regarding the population dynamics of MNP in tuberculosis or any other chronic infection.
In this work, we advanced the understanding of host cell and bacterial dynamics during the chronic stage of M. tuberculosis infection, after the development of adaptive immunity. Consistent with reports in sterile inflammation models, we found that Ly6Chi blood monocytes differentiate into multiple subsets of dendritic cells and macrophages in the lungs and lymph nodes of M. tuberculosis-infected mice. Using a recently-described method of distinguishing cells that reside in the vascular space from those in the tissue parenchyma, we determined that Ly6Chi monocytes differentiate into Ly6Clo monocytes that remain in the lung vascular space in M. tuberculosis-infected mice. Using the same method, we determined that differentiation of Ly6Chi cells into other subsets happens after egress from the vascular compartment, without transitioning through a Ly6Clo state. In particular, we determined that Ly6Chi monocytes rapidly differentiate into Ly6Chi CD11c+ MHCII+ population of DC, which is a transient intermediate state followed by differentiation into CD11b+ DC in the lungs. Although our evidence is indirect, we found kinetic evidence that both Ly6Chi and CD11b+ DC migrate and transport bacteria from the lungs to the lung-draining mediastinal lymph node, consistent with previous reports [8–10].