LprG-mediated surface expression of lipoarabinomannan is essential for virulence of Mycobacterium tuberculosis.

Rajiv L Gaur,Sabine Ehrt,Richard N Zare,Kangning Ren,Sara Gibbs,Niaz Banaei,Joel D Ernst,Antje Blumenthal,Mary Jackson,Suresh Bhamidi,Marcel A Behr

doi:10.1371/journal.ppat.1004376

Abstract

Mycobacterium tuberculosis employs various virulence strategies to subvert host immune responses in order to persist and cause disease. Interaction of M. tuberculosis with mannose receptor on macrophages via surface-exposed lipoarabinomannan (LAM) is believed to be critical for cell entry, inhibition of phagosome-lysosome fusion, and intracellular survival, but in vivo evidence is lacking. LprG, a cell envelope lipoprotein that is essential for virulence of M. tuberculosis, has been shown to bind to the acyl groups of lipoglycans but the role of LprG in LAM biosynthesis and localization remains unknown. Using an M. tuberculosis lprG mutant, we show that LprG is essential for normal surface expression of LAM and virulence of M. tuberculosis attributed to LAM. The lprG mutant had a normal quantity of LAM in the cell envelope, but its surface was altered and showed reduced expression of surface-exposed LAM. Functionally, the lprG mutant was defective for macrophage entry and inhibition of phagosome-lysosome fusion, was attenuated in macrophages, and was killed in the mouse lung with the onset of adaptive immunity. This study identifies the role of LprG in surface-exposed LAM expression and provides in vivo evidence for the essential role surface LAM plays in M. tuberculosis virulence. Findings have translational implications for therapy and vaccine development.

Highlights

Mycobacterium tuberculosis is a human pathogen that has infected one-third of world’s population and causes 8 million new cases and over one million deaths each year [1]
Fitness of lprG mutant is unaltered in broth culture To investigate the function of LprG in M. tuberculosis, we constructed a deletion mutant of lprG (DlprG) in H37Rv and complemented it with lprG-Rv1410c (::lprG) (Figure S1)
We showed that the lprG mutant has normal abundance of lipids, lipoglycans, and glycans in the cell wall and the capsule, but it has an altered surface with reduced expression of surface-exposed LAM

Summary

Introduction

Mycobacterium tuberculosis is a human pathogen that has infected one-third of world’s population and causes 8 million new cases and over one million deaths each year [1]. Many of the virulence determinants of M. tuberculosis are constituents of the cell envelope [3]. The mycobacterial cell envelope is an impermeable barrier composed of an inner plasma membrane; a cell wall core; an outer mycomembrane; and a surface capsule composed of polysaccharides, lipids, and proteins [4,5,6]. The constituents of the capsule can be concealed within the ,35 nm capsular layer or they can be exposed on the surface with the ability to interact with the host [6]. Mannose-capped lipoarabinomannan (LAM), an abundant surface-exposed lipoglycan anchored to the inner and outer membranes via a mannosyl phosphate inositol [7], is considered one of the key virulence determinants of Author Summary

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