The complement anaphylatoxin C5a is a proinflammatory component of host defense that functions through two identified receptors, C5a receptor (C5aR) and C5L2. C5aR is a classical G protein-coupled receptor, whereas C5L2 is structurally homologous but deficient in G protein coupling. In human neutrophils, we show C5L2 is predominantly intracellular, whereas C5aR is expressed on the plasma membrane. Confocal analysis shows internalized C5aR following ligand binding is co-localized with both C5L2 and ␤-arrestin. Antibody blockade of C5L2 results in a dramatic increase in C5a-mediated chemotaxis and ERK1/2 phosphorylation but does not alter C5a-mediated calcium mobilization, supporting its role in modulation of the ␤-arrestin pathway. Association of C5L2 with ␤-arrestin is confirmed by cellular co-immunoprecipitation assays. C5L2 blockade also has no effect on ligand uptake or C5aR endocytosis in human polymorphonuclear leukocytes, distinguishing its role from that of a rapid recycling or scavenging receptor in this cell type. This is thus the first example of a naturally occurring seven-transmembrane segment receptor that is both obligately uncoupled from G proteins and a negative modulator of signal transduction through the ␤-arrestin pathway. Physiologically, these properties provide the possibility for additional fine-tuning of host defense.The complement anaphylatoxin C5a is one of the most potent inflammatory mediators of the innate immune system, with the ability to activate all classes of myeloid cells as well as cells of many other lineages. Multiple studies using experimental animals deficient in C5 or mice with targeted deletion of the C5a receptor (C5aR) 4 have demonstrated the critical role for this anaphylatoxin in host defense. Generation of C5a has also been associated with disease conditions, including asthma, contact sensitivity reactions, autoimmune arthritis, and sepsis (reviewed in Refs. 1-3). C5a manifests its activities by interaction with two known receptors, C5aR and C5L2. Cellular stimulation of the C5aR through G␣ i2 , G␣ i3 , or G␣ 16 results in intracellular calcium mobilization and activation of signaling pathways including phosphatidylinositol 3-kinase, diacylglycerol, MAPK, ERK, and others (4, 5), (reviewed in Ref. 6).Like the C5aR, C5L2 is a putative seven-transmembrane segment protein that was identified by Ohno et al. (7) as a cDNA with homology to the C5aR. It has similar sequence homology with the formyl peptide receptor (FPR) and the chemokine receptor chemR23. C5L2 is expressed on neutrophils, macrophages, and immature dendritic cells in coordination with the C5aR, although its mRNA is present at significantly reduced levels (7,8). Expression has additionally been reported in adrenal gland, spinal cord, thyroid, liver, lung, spleen, brain, and heart (9, 10).Studies of the distinct properties of C5L2-utilizing transfection systems demonstrate its ability to bind C5a with affinity similar to that of the C5aR. It binds the metabolite des-Arg C5a with higher avidity (11,12). In con...

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