The internalization and intracellular trafficking of chemokine receptors have important implications for the cellular responses elicited by chemokine receptors. The major pathway by which chemokine receptors internalize is the clathrin-mediated pathway, but some receptors may utilize lipid rafts/ caveolae-dependent internalization routes. This review discusses the current knowledge and controversies regarding these two different routes of endocytosis. The functional consequences of internalization and the regulation of chemokine receptor recycling will also be addressed. Modifications of chemokine receptors, such as palmitoylation, ubiquitination, glycosylation, and sulfation, may also impact trafficking, chemotaxis and signaling. Finally, this review will cover the internalization and trafficking of viral and decoy chemokine receptors.

The homing of proinflammatory (M1) and the "alternatively activated" anti-inflammatory (M2) macrophages plays a different role in the process of inflammation. Chemokines are the major mediators of macrophage chemotaxis, but how they differentially regulate M1 and M2 macrophages remains largely unclear. In the present study, we attempted to screen chemokines that differentially induce chemotaxis of M1 and M2 macrophages and to explore the underlying mechanism. Among the 41 chemokines that specifically bind to 20 chemokine receptors, CCL19, CCL21, CCL24, CCL25, CXCL8, CXCL10, and XCL2 specifically induced M1 macrophage chemotaxis, whereas CCL7 induced chemotaxis of both M1 and M2 macrophages. Whereas the differential effects of these chemokines on M1/M2 macrophage chemotaxis could be attributable to the predominant expression of their cognate receptors on the macrophage subsets, CCR7, the receptor for CCL19/CCL21, appeared to be an exception. Immunoblot analysis indicated an equivalent level of CCR7 in the whole cell lysate of M1 and M2 macrophages, but CCL19 and CCL21 only induced M1 macrophage chemotaxis. Both immunoblot and confocal microscopy analyses demonstrated that CCR7 was predominantly expressed on the cell surface of M1 but in the cytosol of M2 macrophages before ligand stimulation. As a result, CCL19 or CCL21 induced activation of both MEK1-ERK1/2 and PI3K-AKT cascades in M1 but not in M2 macrophages. Intriguingly, CCL19/CCL21-mediated M1 macrophage chemotaxis was blocked by specific inhibition of PI3K rather than MEK1. Together, these findings suggest that recruitment of M1 and M2 macrophages is fine tuned by different chemokines with the involvement of specific signaling pathways.

Agonist treatment of cells expressing the chemokine receptor, CXCR2, induces receptor phosphorylation and internalization through a dynamin-dependent mechanism. In the present study, we demonstrate that a carboxyl terminus-truncated mutant of CXCR2 (331T), which no longer undergoes agonist-induced phosphorylation, continues to undergo ligand-induced internalization in HEK293 cells. This mutant receptor exhibits reduced association with β-arrestin 1 but continues to exhibit association with adaptin 2 α and β subunits. Replacing Leu320-321 and/or Ile323-Leu324 with Ala (LL320,321AA, IL323,324AA, and LLIL320,321,323,324AAAA) in wild-type CXCR2 or 331T causes little change in ligand binding and signaling through Ca 2+ mobilization but greatly impairs the agonist-induced receptor sequestration and ligand-mediated chemotaxis. The LL320,321AA, IL323,324AA, and LLIL320,321,323,324AAAA mutants of CXCR2 exhibit normal binding to β-arrestin 1 but exhibit decreased binding to adaptin 2α and β. These data demonstrate a role for the LLKIL motif in the carboxyl terminus of CXCR2 in receptor internalization and cell chemotaxis and imply a role for adaptin 2 in the endocytosis of CXCR2.The chemokine receptor, CXCR2, 1 is a member of a superfamily of G protein-coupled seventransmembrane receptors (GPCRs) that transduce intracellular signals via heterotrimeric guanine nucleotide-binding proteins (G proteins). Upon stimulation by agonists, such as interleukin 8 (IL-8) or melanoma growth-stimulatory activity (MGSA)/growth-regulatory protein (GRO), CXCR2 activates a series of G protein-mediated events, including phosphatidylinositide hydrolysis, to generate inositol 1,4,5-trisphosphate and diacylglycerol, as well as mobilization of intracellular free Ca 2+ to initiate a series of cellular responses (1). In addition, CXCR2 mediates cell chemotaxis, a distinct function of chemokine receptors (2). Like many other types of GPCRs, CXCR2 undergoes a dynamic trafficking between the cell surface and the intracellular compartments (3). Such trafficking may be involved in both transmission and termination of the receptor signals and may play an important role in mediating cell chemotaxis. For CXCR2, the most remarkable trafficking process is agonistinduced receptor internalization. 1 Abbreviations: CXCR2, receptor for CXC chemokines formerly defined as interleukin 8 receptor B; IL-8, interleukin 8; MGSA/GRO, melanoma growth-stimulatory activity/growth-regulatory protein; GPCRs, G protein-coupled receptors; G proteins, guanine nucleotidebinding proteins; β2-AR, β2-adrenergic receptor; AP-2, adaptin 2; HEK293 cells, human embryonic kidney 293 cells; RBL-2H3 cells, rabbit basophilic leukemia cells; DSP, dithiobis(succinimido propionate); GRKs, G protein-coupled receptor kinases; FITC, fluorescein isothiocyanate; BSA, bovine serum albumin; DMEM, Dulbecco's modified Eagle's medium; FBS, fetal bovine serum; PBS, phosphatebuffered saline; SDS, sodium dodecyl sulfate. Agonist-induced phosphorylation of the carboxyl terminus by G protein-couple...