Recent studies have identified the CD1 family of cell surface glycoproteins as novel antigen presenting molecules encoded by genes located outside of the major histocompatibility complex. Identification of naturally occurring antigens presented by CD1 has revealed the surprising finding that these prominently include a range of foreign lipids and glycolipids, such as several that are known to exist in the cell walls and membranes of pathogenic mycobacteria. Among the currently known CD1-presented antigens, the best structurally characterized are the mycolic acids and their monoglucosylated analogue, glucose monomycolate (GMM). These compounds have a lipid structure that is unlike any found in mammalian tissues, and occur abundantly as a major outer cell membrane component in a variety of pathogenic bacteria, including Mycobacterium tuberculosis. Recognition of these lipids appears to be a frequent feature of CD1b-restricted M. tuberculosis-specific T cells in humans, suggesting that T cell responses to mycolic acids and GMM may play a significant role in the protective host response to this important pathogen. Studies in the current proposal will assess whether mycolic acids or GMM are among the immunodominant T cell antigens of M. tuberculosis in humans infected with this bacterium, and will seek to clarify the molecular basis for the presentation and recognition of these novel T cell antigens. A combination of preparative and synthetic chemistry will be used to generate a range of structural analogues of mycolic acids and GMM. These will be studied for their ability to be recognized by CD1b-restricted T cells, and for their ability to bind directly to CD1b proteins. In addition, the structural basis for the binding of these antigens to the CD1b protein will be analyzed by carrying out site directed mutagenesis of residues forming the predicted ligand binding groove of CD1b. These studies will lead to a detailed understanding of the fundamental rules governing the interaction of a newly recognized class of T cell antigens with their antigen presenting molecule. The identification and detailed analysis of this newly recognized pathway for T cell antigen recognition is likely to be important for understanding the human immune response to M. tuberculosis and related pathogens, and has potential implications for future vaccine development efforts.
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