Summary Pentraxins are a family of ancient innate immune mediators conserved throughout evolution. The classical pentraxins include serum amyloid P component (SAP) and C-reactive protein (CRP), that are part of acute phase proteins synthesized in response to infection1, 2. Both recognize microbial pathogens and activate the classical complement pathway through C1q3,4. More recently, members of the pentraxin family were found to interact with cell surface Fcγ receptors (FcγR) and activate leukocyte-mediated phagocytosis5-8. We now describe the structural mechanism for pentraxin binding to FcγR and its functional activation of FcγR-mediated phagocytosis and cytokine secretion. The complex structure between human SAP and FcγRIIa reveals a diagonally bound receptor on each SAP pentamer with both D1 and D2 domains of the receptor contacting the ridge helices from two SAP subunits. The 1:1 stoichiometry between SAP and FcγRIIa infers the requirement for multivalent pathogen binding for receptor aggregation. Mutational and binding studies show that pentraxins are diverse in their binding specificity to FcγR isoforms but conserved in their recognition structure. The shared binding site for SAP and IgG results in competition for FcγR binding and the inhibition of immune complex-mediated phagocytosis by soluble pentraxins. These results establish the antibody-like functions for pentraxins in the FcγR pathway, suggest an evolutionary overlap between the innate and adaptive immune systems, and have novel therapeutic implications for autoimmune diseases.

Significance Serum amyloid A (SAA) is a major serum acute-phase protein and a cause of secondary amyloidosis, which impacts ∼1% of patients with chronic inflammation such as rheumatoid arthritis and neoplastic diseases. The lack of structural information has hampered our understanding of SAA-mediated amyloidosis and the development of effective therapies. Here we report a crystal structure of human SAA1.1 as a prototypic member of the family. SAA1.1 exists as a hexamer with subunits displaying a unique four-helix bundle fold. We further defined binding sites for heparin and high-density lipoprotein, identified major amyloidogenic epitopes, and visualized SAA-mediated protofibril formation using electron microscopy. These studies provide mechanistic insights into amyloidogenic conformational transition of SAA.

Fc gamma receptor I (FcγRI) contributes to protective immunity against bacterial infections, but exacerbates certain autoimmune diseases. The sole high-affinity IgG receptor, FcγRI plays a significant role in immunotherapy. To elucidate the molecular mechanism of its high-affinity IgG binding, we determined the crystal structure of the extracellular domains of human FcγRI in complex with the Fc domain of human IgG 1 . FcγRI binds to the Fc in a similar mode as the low-affinity FcγRII and FcγRIII receptors. In addition to many conserved contacts, FcγRI forms additional hydrogen bonds and salt bridges with the lower hinge region of Fc. Unique to the high-affinity receptor-Fc complex, however, is the conformation of the receptor D2 domain FG loop, which enables a charged KHR motif to interact with proximal carbohydrate units of the Fc glycans. Both the length and the charge of the FcγRI FG loop are well conserved among mammalian species. Ala and Glu mutations of the FG loop KHR residues showed significant contributions of His-174 and Arg-175 to antibody binding, and the loss of the FG loopglycan interaction resulted in an ∼20-to 30-fold decrease in FcγRI affinity to all three subclasses of IgGs. Furthermore, deglycosylation of IgG 1 resulted in a 40-fold loss in FcγRI binding, demonstrating involvement of the receptor FG loop in glycan recognition. These results highlight a unique glycan recognition in FcγRI function and open potential therapeutic avenues based on antibody glycan engineering or small molecular glycan mimics to target FcγRI for certain autoimmune diseases.CD64 | FcgRI | IgG recognition | crystal structure | glycan recognition I gGs and pentraxins are circulating immune components that directly recognize pathogens. On formation of immune complexes or opsonization, they activate cellular response through Fc receptors (FcRs) (1, 2). The FcRs for IgGs include FcγRI (CD64); FcγRII (CD32) with A, B, and C isoforms; and FcγRIII (CD16) with two isoforms (3). Most of these are activating receptors either containing an intracellular immunoreceptor tyrosine-based activation motif or associated with an FcR common γ chain (4). FcγRIIB is an inhibitory receptor that contains an intracellular immunoreceptor tyrosine-based inhibitory motif. FcγRIIIB does not have a cytosolic domain and is anchored to the plasma membrane through glycosylphosphatidylinositol linkage. The binding affinity to IgG ranges from 10 −8 M for FcγRI to 10 −5-10 −7 M for FcγRII and III (3).FcγRI plays an important role in the protection against bacterial infections, but also exacerbates certain autoimmune diseases (5). Owing to its high-affinity antibody binding, FcγRI is important in antibody therapy as well (6, 7). To date, the structure of the ligand-bound high-affinity receptor has not been determined, however. Consequently, the mechanism of its highaffinity antibody recognition remains to be elucidated. The role of glycan in antibody function has been a subject of intense study. Differential glycosylation of Fc, notably fucosylated Fc, is...