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The covalent modification of bacterial (lipo)polysaccharides with discrete substituents may impact their biosynthesis, export and/or biological activity. Whether mycobacteria use a similar strategy to control the biogenesis of its cell envelope polysaccharides and modulate their interaction with the host during infection is unknown despite the report of a number of tailoring substituents modifying the structure of these glycans. Here, we show that discrete succinyl substituents strategically positioned on Mycobacterium tuberculosis (Mtb) lipoarabinomannan govern the mannose-capping of this lipoglycan and, thus, much of the biological activity of the entire molecule. We further show that the absence of succinyl substituents on the two main cell envelope glycans of Mtb, arabinogalactan and lipoarabinomannan, leads to a significant increase of pro-inflammatory cytokines and chemokines in infected murine and human macrophages. Collectively, our results validate polysaccharide succinylation as a critical mechanism by which Mtb controls inflammation., Competing Interests: The authors have declared that no competing interests exist., (Copyright: © 2023 Palčeková et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.)

Although leprosy is curable with drug treatment, the identification of biomarkers of infection, disease progression and treatment efficacy would greatly help to reduce the overall prevalence of the disease. Reliable biomarkers would also reduce the incidence of grade-2 disability by ensuring that those who are most at risk are diagnosed and treated early or offered repeated treatments in the case of relapse. In this study, we examined the reactivity of sera from lepromatous and tuberculoid leprosy patients (LPs) against a panel of 12 recombinant Mycobacterium leprae proteins and found that six proteins were strongly recognised by multibacillary (MB) patients, while only three were consistently recognised by paucibacillary patients. To better understand the dynamics of patient antibody responses during and after drug therapy, we measured antibody titres to four recombinant proteins, phenolic glycolipid-I and lipoarabinomannan at baseline and up to two years after diagnosis to investigate the temporal changes in the antibody titres. Reactivity patterns to individual antigens and decreases in antibody titres were patient-specific. Antibody titres to proteins declined more rapidly vs. those to carbohydrate and glycolipid antigens. Compared to baseline values, increases in antibody titres were observed during reactional episodes in one individual. Additionally, antibody responses against a subset of antigens that provided a good prognostic indicator of disease progression were analysed in 51 household contacts of MB index cases for up to two years. Although the majority of these contacts showed no change or exhibited decreases in antibody titres, seven individuals developed higher titres towards one or more of these antigens and one individual with progressively higher titres was diagnosed with borderline lepromatous leprosy 19 months after enrolment. The results of this study indicate that antibody titres to specific M. leprae antigens can be used to monitor treatment efficacy in LPs and assess disease progression in those most at risk for developing this disease.

A simple serodiagnostic test based on the Mycobacterium leprae-specific phenolic glycolipid I(PGL-I), for individuals with leprosy is nearly universally positive in leprosy patients with high bacillary loads but cannot be used as a stand-alone diagnostic test for the entire spectrum of the disease process. For patients with early infection with no detectable acid-fast bacilli in lesions or with low or no antibody titer to PGL-I, as in those at the tuberculoid end of the disease spectrum, this diagnostic approach has limited usefulness. To identify additional M. leprae antigens that might enhance the serological detection of these individuals, we have examined the reactivity patterns of patient sera to PGL-I, lipoarabinomannan (LAM), and six recombinant M. leprae proteins (ML1877, ML0841, ML2028, ML2038, ML0380, and ML0050) by Western blot analysis and enzyme-linked immunosorbent assay (ELISA). Overall, the responses to ML2028 (Ag85B) and ML2038 (bacterioferritin) were consistently high in both multibacillary and paucibacillary groups and weak or absent in endemic controls, while responses to other antigens showed considerable variability, from strongly positive to completely negative. This analysis has given a clearer understanding of some of the differences in the antibody responses between individuals at opposite ends of the disease spectrum, as well as illustrating the heterogeneity of antibody responses toward protein, carbohydrate, and glycolipid antigens within a clinical group. Correlating these response patterns with a particular disease state could allow for a more critical assessment of the form of disease within the leprosy spectrum and could lead to better patient management.