Mycobacterium tuberculosis has established infection in one third of the world population. Toll‐like receptor 2 is important for control of mycobacterial infection,
and is important for numerous immunological processes. Toll‐like receptor 2 has an
unusually diverse ligand repertoire, including lipoproteins, glycolipids, proteins, and
glycans. The purpose of the studies described in this thesis was to better
characterize the mechanisms of TLR2 activation by mycobacterial lipoproteins. In
the first part, we examined co‐receptor and accessory receptor contribution to
TLR2‐mediated detection of the mycobacterial lipoproteins LpqH, LprA, LprG, and
PhoS1. We found that while most of the lipoproteins required TLR2, TLR1, and
CD14, some did not require CD14, and LprA did not require TLR1. These results
suggest that the protein component of bacterial lipoproteins contributes
significantly to their ability to activate TLR2. Furthermore, in an effort to determine
whether different TLR2 co‐ and accessory receptor requirements might impact
mycobacterial infection, we analyzed murine lung antigen presenting cell subsets
and found that expression of TLR2, CD14, and CD36 varied amongst them. This
impacted their ability to respond to the mycobacterial lipoprotein LpqH. Lung
macrophages expressed less TLR2 than alveolar macrophages or dendritic cells, and
were significantly less responsive to LpqH than alveolar macrophages.
A second focus of this thesis was to describe the structural motifs responsible for
the ability of LprG to signal independent of its covalently‐attached acyl chains.
Toward this end, we found that LprG carried glycolipids in a hydrophobic pocket
and delivered them to TLR2 for recognition. A point mutation in the hydrophobic
pocket abrogated glycolipid binding and TLR2 activation by the non‐acylated LprG.
We found that while LprA could carry diacylated glycolipids, only LprG carried
triacylated glycolipids, and that these glycolipids signaled in a TLR2 and TLR1
dependent manner. Furthermore, we found that LprG increased the potency of
glycolipids for TLR2 activation by several orders of magnitude, and that the
glycolipids carried in the pocket of acylated LprG explain the difference between
LprA and LprG. We propose that LprG functions to carry mycobacterial glycolipids
for the purpose of envelope construction. Together, the studies described in this
thesis demonstrate that TLR2 detects its agonists via multiple mechanisms, and
include the description of a novel recognition mechanism by which TLR2 co‐opts a
bacterial glycolipid carrier as a ligand delivery system.