DNA Sensing via TLR-9 Constitutes a Major Innate Immunity Pathway Activated during Erythema Nodosum Leprosum.

The chronic course of lepromatous leprosy may be interrupted by acute inflammatory episodes known as erythema nodosum leprosum (ENL). Despite its being a major cause of peripheral nerve damage in leprosy patients, the immunopathogenesis of ENL remains ill-defined. Recognized by distinct families of germline-encoded pattern recognition receptors, endogenous and pathogen-derived nucleic acids are highly immunostimulatory molecules that play a major role in the host defense against infections, autoimmunity, and autoinflammation. The aim of this work was to investigate whether DNA sensing via TLR-9 constitutes a major inflammatory pathway during ENL. Flow cytometry and immunohistochemistry analysis showed significantly higher TLR-9 expression in ENL when compared with nonreactional lepromatous patients, both locally in the skin lesions and in circulating mononuclear cells. The levels of endogenous and pathogen-derived TLR-9 ligands in the circulation of ENL patients were also higher. Furthermore, PBMCs isolated from the ENL patients secreted higher levels of TNF, IL-6, and IL-1ß in response to a TLR-9 agonist than those of the nonreactional patients and healthy individuals. Finally, E6446, a TLR-9 synthetic antagonist, was able to significantly inhibit the secretion of proinflammatory cytokines by ENL PBMCs in response to Mycobacterium leprae lysate. Our data strongly indicate that DNA sensing via TLR-9 constitutes a major innate immunity pathway involved in the pathogenesis and evolution of ENL. Thus, the use of TLR-9 antagonists emerges as a potential alternative to more effectively treat ENL aiming to prevent the development of nerve injuries and deformities in leprosy.

Modulation of lipid droplets by Mycobacterium leprae in Schwann cells: a putative mechanism for host lipid acquisition and bacterial survival in phagosomes.

The predilection of Mycobacterium leprae (ML) for Schwann cells (SCs) leads to peripheral neuropathy, a major concern in leprosy. Highly infected SCs in lepromatous leprosy nerves show a foamy, lipid-laden appearance; but the origin and nature of these lipids, as well as their role in leprosy, have remained unclear. The data presented show that ML has a pronounced effect on host-cell lipid homeostasis through regulation of lipid droplet (lipid bodies, LD) biogenesis and intracellular distribution. Electron microscopy and immunohistochemical analysis of lepromatous leprosy nerves for adipose differentiation-related protein expression, a classical LD marker, revealed accumulating LDs in close association to ML in infected SCs. The capacity of ML to induce LD formation was confirmed in in vitro studies with human SCs. Moreover, via confocal and live-cell analysis, it was found that LDs are promptly recruited to bacterial phagosomes and that this process depends on cytoskeletal reorganization and PI3K signalling. ML-induced LD biogenesis and recruitment were found to be independent of TLR2 bacterial sensing. Notably, LD recruitment impairment by cytoskeleton drugs decreased intracellular bacterial survival. Altogether, our data revealed SC lipid accumulation in ML-containing phagosomes, which may represent a fundamental aspect of bacterial pathogenesis in the nerve.

Mycobacterium leprae downregulates the expression of PHEX in Schwann cells and osteoblasts.

Neuropathy and bone deformities, lifelong sequelae of leprosy that persist after treatment, result in significant impairment to patients and compromise their social rehabilitation. Phosphate-regulating gene with homologies to endopeptidase on the X chromosome (PHEX) is a Zn-metalloendopeptidase, which is abundantly expressed in osteoblasts and many other cell types, such as Schwann cells, and has been implicated in phosphate metabolism and X-linked rickets. Here, we demonstrate that Mycobacterium leprae stimulation downregulates PHEX transcription and protein expression in a human schwannoma cell line (ST88-14) and human osteoblast lineage. Modulation of PHEX expression was observed to a lesser extent in cells stimulated with other species of mycobacteria, but was not observed in cultures treated with latex beads or with the facultative intracellular bacterium Salmonella typhimurium. Direct downregulation of PHEX by M. leprae could be involved in the bone resorption observed in leprosy patients. This is the first report to describe PHEX modulation by an infectious agent.

Mycobacterium leprae downregulates the expression of PHEX in Schwann cells and osteoblasts

Neuropathy and bone deformities, lifelong sequelae of leprosy that persist after treatment, result in significant impairment to patients and compromise their social rehabilitation. Phosphate-regulating gene with homologies to endopeptidase on the X chromosome (PHEX) is a Zn-metalloendopeptidase, which is abundantly expressed in osteoblasts and many other cell types, such as Schwann cells, and has been implicated in phosphate metabolism and X-linked rickets. Here, we demonstrate that Mycobacterium leprae stimulation downregulates PHEX transcription and protein expression in a human schwannoma cell line (ST88-14) and human osteoblast lineage. Modulation of PHEX expression was observed to a lesser extent in cells stimulated with other species of mycobacteria, but was not observed in cultures treated with latex beads or with the facultative intracellular bacterium Salmonella typhimurium. Direct downregulation of PHEX by M. leprae could be involved in the bone resorption observed in leprosy patients. This is the first report to describe PHEX modulation by an infectious agent.