TLR6-Driven Lipid Droplets in Mycobacterium leprae-Infected Schwann Cells: Immunoinflammatory Platforms Associated with Bacterial Persistence

Abstract

The mechanisms responsible for nerve injury in leprosy need further elucidation. We recently demonstrated that the foamy phenotype of Mycobacterium leprae-infected Schwann cells (SCs) observed in nerves of multibacillary patients results from the capacity of M. leprae to induce and recruit lipid droplets (LDs; also known as lipid bodies) to bacterial-containing phagosomes. In this study, we analyzed the parameters that govern LD biogenesis by M. leprae in SCs and how this contributes to the innate immune response elicited by M. leprae. Our observations indicated that LD formation requires the uptake of live bacteria and depends on host cell cytoskeleton rearrangement and vesicular trafficking. TLR6 deletion, but not TLR2, completely abolished the induction of LDs by M. leprae, as well as inhibited the bacterial uptake in SCs. M. leprae-induced LD biogenesis correlated with increased PGE2 and IL-10 secretion, as well as reduced IL-12 and NO production in M. leprae-infected SCs. Analysis of nerves from lepromatous leprosy patients showed colocalization of M. leprae, LDs, and cyclooxygenase-2 in SCs, indicating that LDs are sites for PGE2 synthesis in vivo. LD biogenesis Inhibition by the fatty acid synthase inhibitor C-75 abolished the effect of M. leprae on SC production of immunoinflammatory mediators and enhanced the mycobacterial-killing ability of SCs. Altogether, our data indicated a critical role for TLR6-dependent signaling in M. leprae–SC interactions, favoring phagocytosis and subsequent signaling for induction of LD biogenesis in infected cells. Moreover, our observations reinforced the role of LDs favoring mycobacterial survival and persistence in the nerve. These findings give further support to a critical role for LDs in M. leprae pathogenesis in the nerve.

Deciphering the contribution of lipid droplets in leprosy: multifunctional organelles with roles in Mycobacterium leprae pathogenesis

“…These mechanisms exhibit similarities to M. bovis BCG infection during which lipid droplet biogenesis is induced (i) in a time and dose-dependent manner, (ii) by both live and dead mycobacteria, (iii) by mycobacterial cell wall components and (iv) in bacterium-associated Almeida et al 2009, Mattos et al 2010). Interestingly, we have shown that the mechanisms of lipid droplet induction by M. leprae in Schwann cells share similarities with those observed in macrophages, but also exhibit some differences (Table) (Mattos et al 2010(Mattos et al , 2011b. Lipid droplet biogenesis is induced in a time and dose-dependent manner and requires bacterial phagocytosis.…”

“…The stimulation of TLR signalling by bacteria and bacterial molecules has been shown to promote the accumulation of lipids in macrophages in the form of lipid droplet organelles. Experiments performed using macrophages from TLR-deficient mice have revealed that lipid droplet formation in response to bacteria is dependent on TLR signalling, particularly via TLR2 (Pacheco et al 2002, D'Avila et al 2006, Cao et al 2007, Almeida et al 2009, Mattos et al 2010, 2011b. Moreover, it has been shown that killed bacteria and purified molecules derived from bacteria are also capable of promoting lipid accumulation in treated macrophages (Pacheco et al 2002, Kazemi et al 2005, D'Avila et al In leprosy, the mechanisms of LD formation by ML in Schwann cells share similarities, but also exhibit some differences compared with those observed in macrophages.…”

“…First, in contrast to macrophages, live M. leprae, but not dead bacteria or isolated mycobacterial components, are able to induce LD in in vitro-cultured Schwann cells, suggesting that the bacteria may play an active role in lipid droplet formation in this cell type (Mattos et al 2011b). Second, in M. leprae-infected cultures, M. leprae-induced lipid droplet formation is observed in cells associated with bacteria, but not in cells without internalised bacteria (Mattos et al 2011b).…”

“…First, in contrast to macrophages, live M. leprae, but not dead bacteria or isolated mycobacterial components, are able to induce LD in in vitro-cultured Schwann cells, suggesting that the bacteria may play an active role in lipid droplet formation in this cell type (Mattos et al 2011b). Second, in M. leprae-infected cultures, M. leprae-induced lipid droplet formation is observed in cells associated with bacteria, but not in cells without internalised bacteria (Mattos et al 2011b). In deep contrast to the observations in macrophages, a culture medium conditioned by M. lepraestimulated Schwann cells is unable to mimic the ability of the bacteria to induce LD, confirming the absence of any soluble factors secreted by infected Schwann cells that are able to promote lipid droplet biogenesis in uninfected neighbouring cells (Mattos et al 2011a, b) (see below for further discussion).…”

Statins Increase Rifampin Mycobactericidal Effect

“…Our group recently described the mechanism by which M. leprae induces lipid droplet formation in macrophages and Schwann cells (5,7,28), showing that host-derived cholesterol represents the major lipid component inside these organelles. The importance of lipids in M. leprae energetic metabolism is further supported by the high prevalence of genes involved in lipid anabolism and catabolism, despite the huge reductive evolution that occurred in its genome, producing essentially a minimal gene set for survival (14).…”