Journal: The Journal of Experimental Medicine
Article Title: FcRn is a CD32a coreceptor that determines susceptibility to IgG immune complex–driven autoimmunity
doi: 10.1084/jem.20200359
Figure Lengend Snippet: CD32a H exhibits increased bridging with IgG IC and FcRn under acidic conditions. (a) Schematic representation of ELISA of hIgG1 ICs binding to CD32a variants. Neutravidin-immobilized C-terminal biotinylated CD32a R or CD32a H variants were exposed to titrated concentrations of hIgG1 ICs at pH 5.6. hIgG1 WT and all derived mutants were monoclonal mouse/human chimeric IgG1, composed of hIgG1 heavy chain associated with murine λ light chain, both with NIP specificity. Bound ICs were detected with anti-Fc HRP-conjugated F(ab′) 2 fragments. (b) Log of hIgG1 WT IC concentration (1.67–214.3 nM) versus OD mean values ± SEM of triplicate technical replicates fitted with nonlinear regression curves are shown (solid line; R 2 = 0.99; EC 50 compared by extra sum-of-squares F test; P < 0.0001), representative of three independent experiments. (c) FcRn–hIgG1 IC–CD32a ternary complex structural model based on the superposition of the FcRn–hIgG1 Fc (PDB 4N0U ) and CD32a R– hIgG1 Fc (PDB 3RY6 ) crystal structures with root mean square deviation of 1.378 Å. The binding sites on IgG Fc (green) for FcRn (red) and CD32a (orange) are between ∼39 and 53 Å apart on the ipsilateral and contralateral Fc heavy chain, respectively. The hIgG1 Fc residues critical for binding to FcRn (IHH; green spheres) and CD32a (N297; gray spheres) are indicated by black arrowheads. (d) Schematic representation of ELISA setup used to detect hIgG1 IC bridging between FcRn and CD32a. CD32a R or CD32a H variants were immobilized and incubated with hIgG1 WT ICs as described in panel a, followed by incubation with soluble hFcRn. hFcRn was detected with anti-hFcRn-ALP–conjugated nanobody. (e) Log of hIgG1 IC concentration (1.67–214.3 nM) versus OD mean values ± SEM of triplicate technical replicates fitted with nonlinear regression curves are shown (solid line; R 2 = 0.99; EC 50 compared by extra sum-of-squares F test; P < 0.0001), representative of three independent experiments. (f) Confocal microscopic images of PLA performed between CD32a and FcRn, using PLA probes targeting the cytoplasmic tails of CD32a and mFcRn, respectively, in CD32a R (R)–, CD32a H (H)–, or vector control (Vector) plasmid–transfected RAW 264.7 cells treated with fluorescent hIgG1 WT ICs. Amplification of adequately proximate PLA probe oligonucleotides that enabled hybridization of fluorescent complementary oligos were visualized at 63× magnification under glycerol immersion and are indicated by white arrowheads. Representative images are shown of nuclei (blue), hIgG1 ICs (green), and PLA signals (red) and merged images (red and yellow). Scale bars = 3 µm. (g) Representative multiplex immunoblot showing coimmunoprecipitation of CD32a R or CD32a H variants (red) with hFcRn (green) after treatment with hIgG1 WT or hIgG1 IHH ICs. Data are representative of two (f and g) or three (b and e) independent experiments. H, CD32a H ; MW, molecular weight; R, CD32a R ; Vector, control vector; IP, immunoprecipitation; WB, Western blot.
Article Snippet: For primary cell studies, CD11c + MHCII + DCs were purified rom spleen by magnetically activated cell sorting in two steps, first using negative selection (CD19 MicroBeads; Miltenyi; #130-052-201) followed by positive selection (CD11c MicroBeads UltraPure; Miltenyi; #130-108-338), from CD32a H-Tg , CD32a R-Tg , or FcγR KO mice that had been inoculated subcutaneously with 5 × 10 6 GM-CSF–secreting B16-F10 melanoma cells 2 wk before spleen harvest, as described previously ( ; ).
Techniques: Enzyme-linked Immunosorbent Assay, Binding Assay, Derivative Assay, Concentration Assay, Incubation, Plasmid Preparation, Control, Transfection, Amplification, Hybridization, Multiplex Assay, Western Blot, Molecular Weight, Immunoprecipitation