Anti-citrullinated protein antibodies bind surface-expressed citrullinated Grp78 on monocyte/macrophages and stimulate tumor necrosis factor alpha production.
OBJECTIVE
Anti-citrullinated protein antibodies (ACPAs), which are the most specific autoantibody marker in patients with rheumatoid arthritis (RA), correlate with disease activity; however, the role of ACPAs in RA pathogenesis has not been elucidated. We hypothesized that ACPAs may directly stimulate mononuclear cells to produce inflammatory cytokines. Thus, we identified cognate antigens of ACPAs on monocyte/macrophages and examined their immunopathologic roles in the pathogenesis of RA.


METHODS
ACPAs were purified from pooled ACPA-positive RA sera by cyclic citrullinated peptide-conjugated affinity column. After coculture of U937 cells with ACPAs, the tumor necrosis factor alpha (TNFalpha) production and NF-kappaB DNA binding activity of the cells were measured by enzyme-linked immunosorbent assay. The cognate antigens of ACPAs on the U937 cell surface were probed by ACPAs, and the reactive bands were examined via proteomic analysis.


RESULTS
ACPAs specifically enhanced TNFalpha production and increased the DNA-binding activity of NF-kappaB in U937 cells. Proteomic analysis revealed that Grp78 protein (72 kd) was one of the cognate antigens of ACPAs. The truncated form of cell surface-expressed Grp78 (55 kd) on U937 cells contained citrulline capable of binding with ACPAs. After citrullination, glutathione S-transferase-tagged recombinant Grp78 (97.52 kd) became a 72-kd fragment and bound with ACPAs. ACPAs also bound to human monocytes and lymphocytes to promote TNFalpha production.


CONCLUSIONS
We clearly demonstrated that ACPAs enhance NF-kappaB activity and TNFalpha production in monocyte/macrophages via binding to surface-expressed citrullinated Grp78.
Lu
Ming-Chi
MC
Buddhist Dalin Tzu-Chi General Hospital, Chia-Yi, Taiwan, National Taiwan University College of Medicine, Taipei, Taiwan, and School of Medicine, Tzu-Chi University, Hualien, Taiwan.
Lai
Ning-Sheng
NS
Yu
Hui-Chun
HC
Huang
Hsien-Bin
HB
Hsieh
Song-Chou
SC
Yu
Chia-Li
CL
D000954:Antigens, Surface / Q000276:immunology / Q000378:metabolism; D001172:Arthritis, Rheumatoid / Q000276:immunology* / Q000378:metabolism; D001323:Autoantibodies / Q000097:blood / Q000276:immunology / Q000494:pharmacology; D015153:Blotting, Western; D000091342:Endoplasmic Reticulum Chaperone BiP; D006360:Heat-Shock Proteins / Q000276:immunology*; D006801:Humans; D047468:Immunoprecipitation; D008214:Lymphocytes / Q000276:immunology / Q000378:metabolism; D008264:Macrophages / Q000276:immunology* / Q000378:metabolism; D009000:Monocytes / Q000276:immunology* / Q000378:metabolism; D016328:NF-kappa B / Q000378:metabolism; D010456:Peptides, Cyclic / Q000276:immunology*; D040901:Proteomics; D014409:Tumor Necrosis Factor-alpha / Q000276:immunology / Q000378:metabolism*; D020298:U937 Cells
D000954:Antigens, Surface; D001323:Autoantibodies; D000091342:Endoplasmic Reticulum Chaperone BiP; C000717247:HSPA5 protein, human; D006360:Heat-Shock Proteins; D016328:NF-kappa B; D010456:Peptides, Cyclic; D014409:Tumor Necrosis Factor-alpha; C487763:cyclic citrullinated peptide