CXCL17, a recently discovered chemokine, is constitutively expressed in mucosal tissues such as lung, stomach, and the small intestines. These organs are continuously exposed to a wide variety of pathogens via food intake and breathing, and so must be on high alert to mount a rapid and efficient immune response. CXCL17 could play an important role in this process, as it selectively recruits immature dendritic cells (DCs). These cells play a key role as immunological sentinels, and are professional antigen-presenting cells, and key regulators of T cell functions. Currently nothing is known about how CXCL17 mediates its in vivo functions. Characteristic features of chemokines are that they adopt the same three-dimensional structure, have four conserved cysteines that form two disulfide bonds, and bind glycosaminoglycans (GAGs). Both the disulfides are essential for receptor activation, and GAG binding is critical for directed migration of leukocytes. Most remarkably, sequence analysis reveals that CXCL17, has only three of the four conserved cysteines, and so may not adopt the typical chemokine structure;it also has a distinctly different distribution of positively charged residues, suggesting a novel GAG-binding mechanism. Another unusual feature of CXCL17 is that it is active only at high concentrations, and shows optimal activity at 100-1000x higher concentrations than is normally observed for most chemokines (?M vs. nM). As structure dictates function, we hypothesize that CXCL17-mediated recruitment of immature DCs from the vasculature to the tissues under steady-state conditions plays an important and non-redundant role in mucosal immunosurveillance.
Our Specific Aims for this R21 grant are to:
Aim 1. Characterize CXCL17-mediated recruitment of DC subtype(s) in vivo in a mouse lung, and recruitment of human DC subtype(s) in vitro using chemotaxis assays.
Aim 2. Determine the CXCL17 solution structure and characterize its binding with GAG using NMR spectroscopy;
and Aim 3. Determine whether CXCL17 can be converted to a more potent agonist by systematically deleting its N-terminal residues.
Organs such as the lung and stomach are continuously exposed to a wide variety of pathogens via food intake and breathing, and so must be on high alert for an efficient and quick immune response, a process during which specialized immune cells called dendritic cells (DCs) function as sentinels. We propose that CXCL17, a newly discovered chemokine, plays an important role in regulating DC function, and we further propose that CXCL17 is structurally unique, and that its structural properties are intimately linked to its ability to regulate DC function. Our planned studies on the structural and functional characterization of CXCL17 should provide critical insights into the immunosurveillance process in humans.
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|Joseph, Prem Raj B; Sawant, Kirti V; Isley, Angela et al. (2013) Dynamic conformational switching in the chemokine ligand is essential for G-protein-coupled receptor activation. Biochem J 456:241-51|