Abstract
Tuberculosis (TB) infection, caused by the airborne pathogen Mycobacterium tuberculosis (M.tb), resulted in almost 1.4 million deaths in 2019, and the number of deaths is predicted to increase by 20% over the next 5 years due to the COVID-19 pandemic. Upon reaching the alveolar space, M.tb comes into close contact with the lung mucosa before and after its encounter with host alveolar compartment cells. Our previous studies show that homeostatic, innate soluble components of the alveolar lining fluid (ALF) can quickly alter the cell envelope surface of M.tb upon contact, defining subsequent M.tb–host cell interactions and infection outcomes in vitro and in vivo. We also demonstrated that ALF from 60+ year old elders (E-ALF) vs. healthy 18- to 45-year-old adults (A-ALF) is dysfunctional, with loss of homeostatic capacity and impaired innate soluble responses linked to high local oxidative stress. In this study, a targeted transcriptional assay shows that M.tb exposure to human ALF alters the expression of its cell envelope genes. Specifically, our results indicate that A-ALF-exposed M.tb upregulates cell envelope genes associated with lipid, carbohydrate, and amino acid metabolism, as well as genes associated with redox homeostasis and transcriptional regulators. Conversely, M.tb exposure to E-ALF shows a lesser transcriptional response, with most of the M.tb genes unchanged or downregulated. Overall, this study indicates that M.tb responds and adapts to the lung alveolar environment upon contact, and that the host ALF status, determined by factors such as age, might play an important role in determining infection outcome.
Highlights
Mycobacterium tuberculosis (M.tb), the causative agent of tuberculosis (TB), is one of the top leading causes of mortality worldwide due to a single infectious agent, with ~1.4 million attributed deaths in 2019 [1]
Most current drugs target the M.tb cell envelope, a highly complex and dynamic structure comprised mainly of carbohydrates and lipids, which provide structural support and resistance to osmotic changes, as well as a critical immunoregulatory role during M.tb infection [7,8,9,10]. It consists of four main layers: (1) an inner plasma membrane with periplasmic space; (2) a peptidoglycan (PG) core covalently linked to arabinogalactan (AG) and mycolic acids (MAcs); (3) a peripheral layer of non-covalently linked lipids, glycolipids, and lipoglycans (e.g., phthiocerol dimycocerosates (PDIMs), trehalose dimycolate (TDM) and monomycolate (TMM), sulfolipids (SLs), phosphatidyl-myo-inositol mannosides (PIMs), lipomannan (LM), and mannose-capped lipoarabinomannan (ManLAM), among others), and; (4) the outermost layer or capsule [11,12]
We focus on early M.tb–alveolar lining fluid (ALF) interactions during the first stages of infection, and first aim to determine if short (15 min) or long (12 h) exposure to human ALF has any effects on the expression of targeted M.tb cell envelope genes, showing that gene expression is moderately altered at 15 min and that some transient expression changes happen at 12 h
Summary
Mycobacterium tuberculosis (M.tb), the causative agent of tuberculosis (TB), is one of the top leading causes of mortality worldwide due to a single infectious agent, with ~1.4 million attributed deaths in 2019 [1]. Strict lockdowns have prevented patients from having access to TB medications and clinical evaluations, and have led to decreased TB diagnosis rates, since available resources have been redirected to prevent the spread of COVID-19 [4,5] These factors are predicted to cause an increase in the number of TB cases, and to promote the development of drug-resistant TB, stressing the need for the development of new anti-TB therapies [6]. Most current drugs target the M.tb cell envelope, a highly complex and dynamic structure comprised mainly of carbohydrates and lipids, which provide structural support and resistance to osmotic changes, as well as a critical immunoregulatory role during M.tb infection [7,8,9,10]. It remains poorly understood how the M.tb cell envelope changes and adapts to the host-lung environment during the natural course of pulmonary infection, which is a critical gap in our knowledge for defining new drug targets against M.tb relevant to bacteria in the lung environment
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