Natural T Cell Epitope Containing Methyl Lysines on Mycobacterial Heparin-Binding Hemagglutinin

Area of Science:

• Immunology
• Microbiology
• Biochemistry

Background:

• T cell epitopes are typically nonmodified peptides, with known posttranslationally modified examples originating from viral or self-proteins.
• The mycobacterial heparin-binding hemagglutinin (HBHA) is an antigen recognized by T cells in humans with latent Mycobacterium tuberculosis infection and features a complex C-terminal methylation pattern.

Purpose of the Study:

• To investigate the role of posttranslational modifications, specifically methylation, in bacterial T cell epitopes.
• To identify and characterize a novel bacterial methylated T cell epitope.

Main Methods:

• Comparison of native HBHA with recombinant HBHA (rHBHA-Ms) produced in Mycobacterium smegmatis to identify antigenic differences linked to methylation profiles.
• Peptide scan analysis to pinpoint specific epitopes.
• Monoclonal antibody (mAb) binding assays to assess epitope recognition.
• Interferon-gamma (IFN-γ) release assays using T cells from latently infected humans to confirm T cell epitope function.

Main Results:

• A peptide containing methyl lysines was identified, which was recognized by a mAb with significantly higher affinity for native HBHA compared to rHBHA-Ms.
• This methylated peptide also elicited IFN-γ release from T cells of latently infected humans, indicating T cell recognition.
• The nonmethylated version of the peptide did not induce IFN-γ release, supporting the role of methylation in epitope function.

Conclusions:

• The study provides evidence for a bacterial methylated T cell peptide epitope.
• Methylation of lysine residues within the HBHA antigen is critical for T cell epitope recognition in Mycobacterium tuberculosis infection.
• This finding expands the understanding of T cell epitope diversity beyond nonmodified peptides.