Lung Mucosa Lining Fluid Modification of Mycobacterium tuberculosis to Reprogram Human Neutrophil Killing Mechanisms

Tuberculosis is caused by the airborne pathogen Mycobacterium tuberculosis, with 8.7 million new tuberculosis cases annually worldwide [1]. Although many studies focus on delineating interaction of M. tuberculosis with host cells, there have been limited studies investigating how the alveolar mucosa, or alveolar lining fluid (ALF), participates in this interaction. The M. tuberculosis bacilli are exposed to ALF in the alveolar space when an individual is first infected with M. tuberculosis, on release from infected host cells, when M. tuberculosis is extracellular within lung cavities, and potentially during transmission in active tuberculosis. We have demonstrated that human ALF contains an array of hydrolases that are capable of altering the M. tuberculosis cell envelope with 2 outcomes: modifications to the M. tuberculosis cell envelope surface and release of M. tuberculosis-derived cell envelope fragments [2]. The M. tuberculosis cell envelope modifications were evident after exposure to human ALF for as little as 15 minutes, which has previously been shown to improve the control of M. tuberculosis infection by human macrophages [2]. Neutrophils are a key first line of defense against microbes [3]. They possess extracellular and intracellular killing mechanisms to control microbial infections [4]. Neutrophil extracellular killing mechanisms include (1) secretion of azurophilic/primary, specific/secondary, and secretory/tertiary granules containing hydrolytic enzymes and antimicrobial peptides [5]; (2) production of reactive oxygen intermediates [6]; and (3) release of neutrophil extracellular traps (NETs), composed of DNA fibers with embedded granules capable of extracellular binding and killing of microbes [7]. Second to phagocytosis, the major neutrophil intracellular killing mechanism is phagosome-lysosome fusion [4]. In humans, neutrophils represent the most abundant cell population harboring M. tuberculosis in bronchoalveolar lavage and sputum samples from patients with active tuberculosis [8, 9], yet the impact of the lung environment and particularly the role of ALF on neutrophil innate responses during microbial infection remain unknown. In the current study, we determined how alterations of the M. tuberculosis cell surface from exposure to human ALF affect the interaction of M. tuberculosis with human neutrophils. In contrast to human macrophages, neutrophils had enhanced recognition of ALF-exposed M. tuberculosis. Despite this, ALF-exposed M. tuberculosis did not trigger degranulation, release of NETs, apoptosis, or the oxidative response of infected neutrophils. Instead, we observed enhance intracellular clearance of ALF-exposed M. tuberculosis, primarily due to increased phagosome-lysosome fusion. ALF-exposed M. tuberculosis also triggered a significant increase in the production of the chemoattractant interleukin 8 (IL-8). Interestingly, ALF-exposed M. tuberculosis–infected neutrophils did not trigger the release of IL-12 by resting macrophages. Thus, our results demonstrate that human ALF-derived modifications of the M. tuberculosis cell envelope contribute to M. tuberculosis intracellular clearance by neutrophils but are not accompanied by some of the characteristic tissue damaging acute inflammatory responses associated with neutrophils.