Our goal is to determine the biochemical and structural basis for binding of Mycobacterium tuberculosis (MTB) lipoproteins to Toll-like receptor (TLR)-2 and resulting agonist activity. TLR2 recognition of MTB lipoproteins initiates innate immunity and influences adaptive immunity to MTB. Despite this critical role for TLR2 in tuberculosis, the structural basis for TLR2 recognition of MTB lipoproteins remains poorly understood. In addition, TLR2 functions in recognition of other pathogenic species, yet the structural determinants of TLR2 agonist activity are largely unexplored. It is known that acyl structures of lipoproteins influence their recognition by TLR2, but the influence of protein structures on TLR2 binding is essentially unknown. We have characterized three distinct MTB lipoproteins that signal through TLR2: LpqH (19-kDa lipoprotein), LprG and LprA. These lipoproteins are all TLR2 agonists but differ in potency and apparent structural determinants of their activity. Our data indicate that both lipid and protein components of MTB lipoproteins can influence TLR2 agonist activity. We are constructing recombinant tagged lipoproteins and soluble TLR2 fusion proteins to dissect structure-function relationships in TLR2-ligand interactions relevant to these pathophysiologically important TLR2 agonists from MTB.
Aim 1 will use cellular cytokine secretion readouts to study the activity of His-tagged recombinant MTB lipoproteins and their receptor dependence (use of TLR1 or TLR6 as co-receptors in heterodimers with TLR2, as well as use of accessory receptors, CD14 and CD36).
Aim 2 will determine structural features of MTB lipoproteins that affect interations with TLR2, TLR1, TLR6 and accessory receptors (CD14 and CD36) by use of macrophages and dendritic cells from mice that are genetically deficient in there receptors and analyses of MTB lipoprotein variantswithout acylation and/or with deletions, truncations or mutations in the protein sequence (or use of minimal active constructs expressed as recombinant proteins or made as synthetic peptides).
Aim 3 will use direct biochemical binding assays to study binding of tagged recombinant soluble TLR and lipoprotein molecules. We will measure the affinities of different MTB lipoproteins and structural variants thereof for TLR2 to further understand the structural determinants of agonist binding to TLR2. Overall we will determine the structural basis for binding of MTB lipoproteins to TLR2, including contributions of lipid and protein components.
These studies will provide unique and novel insights into the mechanisms by which TLR2 recognizes MTB. TLR2 is a key immune system receptor involved in recognition of MTB. Greater understanding of its function will help reveal important mechanisms in immunity that lead to host resistance and/or evasion of immunity during chronic infection by MTB. This may help develop better treatments for tuberculosis. It may also aid in design of better immune adjuvants for a wide array of therapeutic uses.
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