Mycolic Acid Modification by the mmaA4 Gene of M. tuberculosis Modulates IL-12 Production

Dee N Dao,Dan Roshevsky,Kari Sweeney,Gurdyal S Besra,Steven A Porcelli,John Chan,William R Jacobs,Tsungda Hsu,Ivan P Nascimento,Sudagar S Gurcha,Lalita Ramakrishnan

doi:10.1371/journal.ppat.1000081

Dee N Dao, Dan Roshevsky + Show 9 more

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https://doi.org/10.1371/journal.ppat.1000081

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Abstract

Mycobacterium tuberculosis has evolved many strategies to evade elimination by the host immune system, including the selective repression of macrophage IL-12p40 production. To identify the M. tuberculosis genes responsible for this aspect of immune evasion, we used a macrophage cell line expressing a reporter for IL-12p40 transcription to screen a transposon library of M. tuberculosis for mutants that lacked this function. This approach led to the identification of the mmaA4 gene, which encodes a methyl transferase required for introducing the distal oxygen-containing modifications of mycolic acids, as a key locus involved in the repression of IL-12p40. Mutants in which mmaA4 (hma) was inactivated stimulated macrophages to produce significantly more IL-12p40 and TNF-α than wild-type M. tuberculosis and were attenuated for virulence. This attenuation was not seen in IL-12p40-deficient mice, consistent with a direct linkage between enhanced stimulation of IL-12p40 by the mutant and its reduced virulence. Treatment of macrophages with trehalose dimycolate (TDM) purified from the ΔmmaA4 mutant stimulated increased IL-12p40, similar to the increase observed from ΔmmaA4 mutant-infected macrophages. In contrast, purified TDM isolated from wild-type M. tuberculosis inhibited production of IL-12p40 by macrophages. These findings strongly suggest that M. tuberculosis has evolved mmaA4-derived mycolic acids, including those incorporated into TDM to manipulate IL-12-mediated immunity and virulence.

Highlights

Tuberculosis (TB) is the second leading cause of death from an infectious disease worldwide [1,2]
We discovered that mutation of the mycobacterial mmaA4 gene, which controls the chemical modification of complex lipids of M. tuberculosis called mycolic acids, renders the bacterium unable to block IL-12 production
Mycolic acids incorporated into a secreted bacterial molecule called trehalose dimycolate (TDM) from M. tuberculosis had the ability on their own to suppress the production of IL-12 by activated macrophages; we showed that TDM from the mmaA4 mutant of M. tuberculosis is attenuated for suppression

Summary

Introduction

Tuberculosis (TB) is the second leading cause of death from an infectious disease worldwide [1,2]. Mycobacterium tuberculosis is well adapted to the human host, and possesses a variety of mechanisms that promote immune evasion and thereby permit latent infection in the presence of host innate and adaptive immune responses [3,4]. This latent reservoir of M. tuberculosis can eventually develop into active disease when the host immune system is compromised by any of a variety of factors, the most common of which are aging, malnutrition, and concurrent infection by HIV [5,6,7]. Work on developing new and more immunogenic vaccine candidates is crucial and requires advances in our understanding of the host-pathogen interaction

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