Experimental Models of Foamy Macrophages and Approaches for Dissecting the Mechanisms of Lipid Accumulation and Consumption during Dormancy and Reactivation of Tuberculosis

Abstract

Despite a slight decline since 2014, tuberculosis (TB) remains the major deadly infectious disease worldwide with about 1.5 million deaths each year and with about one-third of the population being latently infected with Mycobacterium tuberculosis, the etiologic agent of TB. During primo-infection, the recruitment of immune cells leads to the formation of highly organized granulomas. Among the different cells, one outstanding subpopulation is the foamy macrophage (FM), characterized by the abundance of triacylglycerol-rich lipid bodies (LB). M. tuberculosis can reside in FM, where it acquires, from host LB, the neutral lipids which are subsequently processed and stored by the bacilli in the form of intracytosolic lipid inclusions (ILI). Although host LB can be viewed as a reservoir of nutrients for the pathogen during latency, the molecular mechanisms whereby intraphagosomal mycobacteria interact with LB and assimilate the LB-derived lipids are only beginning to be understood. Past studies have emphasized that these physiological processes are critical to the M. tuberculosis infectious-life cycle, for propagation of the infection, establishment of the dormancy state and reactivation of the disease. In recent years, several animal and cellular models have been developed with the aim of dissecting these complex processes and of determining the nature and contribution of their key players. Herein, we review some of the in vitro and in vivo models which allowed to gain significant insight into lipid accumulation and consumption in M. tuberculosis, two important events that are directly linked to pathogenicity, granuloma formation/maintenance and survival of the tubercle bacillus under non-replicative conditions. We also discuss the advantages and limitations of each model, hoping that this will serve as a guide for future investigations dedicated to persistence and innovative therapeutic approaches against TB.

Inhibition of IL‐17A by secukinumab shows no evidence of increased Mycobacterium tuberculosis infections

“…Figures 3a and b show dual staining with Auramine‐O and Nile Red, where acid‐fast M. tuberculosis cells stain fluorescent green and lipid‐body containing M. tuberculosis cells stain fluorescent red, respectively. The loss of acid‐fastness and accumulation of lipid bodies indicate a dormant phenotype of this intracellular pathogen on day 8, 38 , 39 , 40 , 41 , 42 and showed a ratio of about 65% red and 35% green staining for the isotype control group (Figure 3b). Moreover, rifampicin tolerance is a characteristic of dormant M. tuberculosis.…”

“…To further explore any associations of secukinumab with reactivation of LTBIs, we have retrospectively re‐evaluated pooled phase 3 clinical trials in subjects with moderate to severe plaque psoriasis, who had a history of pulmonary TB or tested positive for latent TB at screening, and received secukinumab for 1 year. In addition, to more directly study the effect of secukinumab on dormant mycobacteria in comparison with an anti‐TNFα antibody treatment, we utilized a novel in vitro human M. tuberculosis H37Rv three‐dimensional microgranuloma model 38 , 39 , 40 , 41 , 42 …”

Cyclipostins and cyclophostin analogs inhibit the antigen 85C from Mycobacterium tuberculosis both in vitro and in vivo

“…In addition, under conditions where Ag85C is overexpressed, M. tuberculosis was more refractory to TAG inhibition by CyC17, further emphasizing the yet unexpected contribution of Ag85C as a player in TAG biosynthesis. Inhibition of the DGAT activity of Ag85C, and therefore TAG inhibition, by the CyC compounds is very unlikely to participate in growth inhibition of M. tuberculosis in vitro, but it may have important consequences for in vivo survival and/or for maintaining the bacilli in a nonreplicating growth phase such as in foamy macrophages in which M. tuberculosis is able to hydrolyze the host-derived TAGs from lipid bodies to fatty acids which are then reprocessed as TAGs and stored within ILIs (42,43). In these subcellular structures, TAGs represent the primary storage source of carbon and energy, allowing the bacteria to survive in a non-replicating state and to persist inside these foamy cells, which usually line the necrotic centers of tubercle granulomas and have been proposed to be the intracellular niche of M. tuberculosis during latent infection (42).…”

Galleria mellonella - a novel infection model for the Mycobacterium tuberculosis complex

“…Our data show that M. bovis BCG lux challenged larvae induce granuloma-like structures to sequester the bacteria as early as 24 h following infection, and inside these formations the bacilli lose their characteristic morphology forming amorphous acid-fast bodies by 48 h. Analysis of infected phagocytic haemocytes, extracted from G. mellonella larvae, show internalization and aggregation of M. bovis BCG lux in granuloma-like structures, with increasing accumulation of lipid bodies within M. bovis BCG lux over time. The accumulation of lipid bodies, in the form of triacylglycerides (TAGs), is evident in M. tuberculosis residing within lipid-rich foamy macrophages in human granulomas [63,64], and is proposed to serve as a source of carbon and energy representing a hallmark of persistent and non-dividing mycobacteria, labelled the drug tolerant, ‘fat and lazy’ bacilli [63,65]. It is therefore no surprise using this infection model, that mycobacteria are highly likely sourcing lipid rich material within their granuloma-like structures from their fatty G. mellonella host.…”