Identification of Amino Acid Residues of the T-Cell Epitope of Mycobacterium tuberculosis α Antigen Critical for Vβ11(+) Th1 Cells

Stimulation of Mycobacterium tuberculosis-primed lymph node cells from C57BL/6 mice with α antigen (also known as antigen 85B and MPT59) induced cell proliferation, production of interleukin 2 and gamma interferon, and expansion of Vβ11(+) CD4(+) T cells in conjunction with antigen-presenting cells in an I-A(b)-restricted manner. Using a series of 15-amino-acid peptides that overlapped each other by 5 amino acids and spanned the mature α antigen, we identified the antigenic epitope for α antigen-specific Vβ11(+) Th1 cells. That peptide (peptide-25), which corresponds to amino acid residues 240 to 254 of α antigen, contains a motif that is conserved in I-A(b) and requires processing by antigen-presenting cells. Using peptide-25-reactive Vβ11(+) T-cell clones and substituted peptide-25 mutants, we determined which amino acid residues within peptide-25 were critical for T-cell receptor (TCR) recognition. Our results showed that the amino acid residues at positions 245, 246, 248, 250, and 251 are important for recognition of TCRVβ11 and that residues at positions 244, 247, 249, and 252 are I-A(b) contact residues. We also observed that active immunization of C57BL/6 mice with peptide-25 can lead to decreased bacterial load in the lungs of M. tuberculosis H37Rv-infected mice. These results should provide us with a useful tool for delineating the regulation of Vβ11(+) Th1-cell development during M. tuberculosis infection and for developing a vaccine inducing a Th1-dominant immune response.