Abstract
Mycobacterium tuberculosis survival in cells requires mycobactin siderophores. Recently, the search for lipid antigens presented by the CD1a antigen-presenting protein led to the discovery of a mycobactin-like compound, dideoxymycobactin (DDM). Here we synthesize DDMs using solution phase and solid phase peptide synthesis chemistry. Comparison of synthetic standards to natural mycobacterial mycobactins by nuclear magnetic resonance and mass spectrometry allowed identification of an unexpected alpha-methyl serine unit in natural DDM. This finding further distinguishes these pre-siderophores as foreign compounds distinct from conventional peptides, and we provide evidence that this chemical variation influences the T cell response. One synthetic DDM recapitulated natural structures and potently stimulated T cells, making it suitable for patient studies of CD1a in infectious disease. DDM analogs differing in the stereochemistry of their butyrate or oxazoline moieties were not recognized by human T cells. Therefore, we conclude that T cells show precise specificity for both arms of the peptide, which are predicted to lie at the CD1a-T cell receptor interface.
Highlights
Pathogens are detected by the host when antigenic molecules directly contact immune receptors during the early stages of infection
Comparison of synthetic standards to natural mycobacterial mycobactins by nuclear magnetic resonance and mass spectrometry allowed identification of an unexpected ␣-methyl serine unit in natural DDM. This finding further distinguishes these pre-siderophores as foreign compounds distinct from conventional peptides, and we provide evidence that this chemical variation influences the T cell response
To intracellular pathogens because they can respond to antigens that are generated inside cells and transported to the surface of infected cells after binding to antigen-presenting molecules
Summary
NMR studies of limiting amounts of natural material derived from the pathogenic organisms, so that not all elements of its chemical structure could be formally determined. Highlighting the physiological importance of the mycobactin pathway, deletion of mycobactin synthase B limits M. tuberculosis survival in cells [13, 14]. The small available yields of natural material highlighted the need for a straightforward method to synthesize DDM for studies of its role in mycobacterial iron acquisition and testing T cell responses in human populations, as well as to provide authentic standards to investigate unknown aspects of natural DDM stereochemistry. Comparison of synthetic and natural DDMs gives unexpected insight into the stereochemical structures of the methylserine, oxazoline, and butyrate moieties of DDM and provides direct evidence that the T cell response is highly specific for a unique aspect of DDM structure that protrudes from the surface of the CD1aDDM complexes
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