TLR-9 Plays a Role in Mycobacterium leprae-Induced Innate Immune Activation of A549 Alveolar Epithelial Cells

André Alves Dias,Natasha Ribeiro Cardoso Linhares,Aislan De Carvalho Vivarini,Carlos Adriano De Matos E Silva,Ulisses Gazos Lopes,Patrícia Sammarco Rosa,Maria Cristina Vidal Pessolani,João Pedro Sousa Santos,Camila Oliveira Da Silva,Maria Ângela De Mello Marques,Márcia Berrêdo-Pinho

doi:10.3389/fimmu.2021.657449

Abstract

The respiratory tract is considered the main port of entry of Mycobacterium leprae, the causative agent of leprosy. However, the great majority of individuals exposed to the leprosy bacillus will never manifest the disease due to their capacity to develop protective immunity. Besides acting as a physical barrier, airway epithelium cells are recognized as key players by initiating a local innate immune response that orchestrates subsequent adaptive immunity to control airborne infections. However, to date, studies exploring the interaction of M. leprae with the respiratory epithelium have been scarce. In this work, the capacity of M. leprae to immune activate human alveolar epithelial cells was investigated, demonstrating that M. leprae-infected A549 cells secrete significantly increased IL-8 that is dependent on NF-κB activation. M. leprae was also able to induce IL-8 production in human primary nasal epithelial cells. M. leprae-treated A549 cells also showed higher expression levels of human β-defensin-2 (hβD-2), MCP-1, MHC-II and the co-stimulatory molecule CD80. Furthermore, the TLR-9 antagonist inhibited both the secretion of IL-8 and NF-κB activation in response to M. leprae, indicating that bacterial DNA sensing by this Toll-like receptor constitutes an important innate immune pathway activated by the pathogen. Finally, evidence is presented suggesting that extracellular DNA molecules anchored to Hlp, a histone-like protein present on the M. leprae surface, constitute major TLR-9 ligands triggering this pathway. The ability of M. leprae to immune activate respiratory epithelial cells herein demonstrated may represent a very early event during infection that could possibly be essential to the generation of a protective response.

Highlights

Leprosy is a chronic infectious disease caused by the obligate intracellular pathogen Mycobacterium leprae
An investigation was launched into whether M. leprae was able to induce the secretion of the chemokines MCP-1 (CCL2) and IL-8 (CXCL8) in alveolar type II pneumocytic epithelial A549 cells, known to play an important role in mediating immune cell migration to the infection site [26]
For the quantification of MCP-1 and IL-8, the A549 cells were incubated or not with live or irradiated M. leprae for 24 and 48 hours and the chemokines were measured by ELISA in culture supernatants

Summary

Introduction

Leprosy is a chronic infectious disease caused by the obligate intracellular pathogen Mycobacterium leprae The ability of this mycobacterium to infect Schwann cells may result in peripheral nerve damage with severe physical deformities, a hallmark of the disease [1]. The disease is transmitted from person to person and the respiratory tract has been considered the most probable route of bacterial entry [3]. It is well established, for example, that, in urban areas, the nasal discharge from leprosy patients with a high bacillary load (multibacillary [MB] patients) is the main source of infection [4]. Most M. leprae-infected individuals will generate an effective T helper 1 (Th1)-based immune response upon producing IFN-g and will never develop any clinical sign of the disease [5, 6]

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Results

Conclusion