Induction of M3-restricted cytotoxic T lymphocyte responses by N-formylated peptides derived from Mycobacterium tuberculosis

Major histocompatibility complex (MHC) class I-restricted CD8(+) T cells play a critical role in the protective immunity against Mycobacterium tuberculosis (Mtb). However. only a few Mtb peptides recognized by MHC class Ia-restricted CD8(+) T cells have been identified. Information on epitopes recognized by class Ib-restricted T cells is even more limited. M3 is an MHC class Ib molecule that preferentially presents N-formylated peptides to CD8+ T cells. Because bacteria initiate protein synthesis with N-formyl methionine, the unique binding specificity of M3 makes it especially suitable for presenting these particular bacterial epitopes. We have scanned the full sequence of the Mtb genome for NH2-temlinal peptides that share features with other M3-binding peptides. Synthetic peptides corresponding to these sequences were rested for their ability to bind to M3 in an immunofluorescence-based peptide-binding assay. Four of the,N-formylated Mtb peptides were able to elicit cytotoxic T lymphocytes (CTLs) from mice immunized with peptide-coated splenocytes. The Mtb peptide-specific, M3-restricted CTLs lysed the Mtb-infected macrophages effectively, suggesting that these N-fonylated Mtb peptides are presented as the naturally processed epitopes by Mtb-infected cells. Furthermore, T cells from Mtb-infected lungs, spleen, and lymph nodes responded to N-formylated Mtb peptides in an MS-restricted manner. Taken together, our data suggest that MS-restricted T cells may participate in the immune response to Mtb.