Serum IFI16 and anti-IFI16 antibodies in psoriatic arthritis

Abstract

Nuclear interferon-inducible protein 16 (IFI16) and anti-IFI16 antibodies have been detected in subjects with several rheumatic diseases, often correlating with disease severity, and in this study we investigated their prevalence and clinical associations in psoriatic arthritis (PsA) compared to psoriasis (Pso). We tested sera and synovial fluids of patients with PsA for IFI16 protein levels by capture enzyme-linked immunosorbent assay (ELISA) and for anti-IFI16 immunoglobulin (Ig)G and IgA by ELISA, protein radio-immunoprecipitation and immunoprecipitation-Western blot of IgG. Sera from patients with Pso and healthy subjects were used as controls, and in a subgroup of patients with PsA we also studied sera after treatment with etanercept. IFI16 was detectable in the sera of 66% of patients with Pso, 46% with PsA and 19% of controls. Among PsA cases, 51% of IFI16-positive cases had elevated levels of C-reactive protein (CRP) compared to 31% of patients with undetectable IFI16. Anti-IFI16 of both IgG and IgA isoforms were detected with significantly higher frequency in PsA and Pso compared to healthy controls, with higher IgG titres in patients with elevated C-reactive protein (CRP) (P = 0·015). Immunoprecipitation confirmed the presence of anti-IFI16 IgG antibodies and these preferentially recognized epitopes outside the N-terminus of the protein. Lastly, IFI16 was detected in one of seven and anti-IFI16 in three of seven synovial fluids from patients with PsA. Therefore, IFI16 and anti-IFI16 are detectable in serum and synovial fluid of PsA patients, especially in cases of elevated CRP.

Keywords: IFI16; anti-IFI16 antibodies; psoriatic arthritis.

Figure 1

Serum interferon‐inducible protein 16 (IFI16) and anti‐IFI16 immunoglobulin (Ig)G and IgA levels in patients with psoriatic arthritis (PsA), psoriasis and healthy controls (HCs). (a) IFI16 protein levels in patients’ and controls’ sera were determined using an in‐house capture enzyme‐linked immunosorbent assay (ELISA). Each dot represents the concentration of IFI16 protein (expressed in ng/ml on a linear scale) in each individual subject. The horizontal bar represents the median values. Values over the dotted line indicate the percentage of subjects with IFI16 protein levels above the cut‐off value (27 ng/ml). (b) Serum IgG and (c) IgA specific for IFI16 were quantified by ELISA in HC and patients suffering from psoriatic arthritis and psoriasis. Each dot represents the autoantibody level for each subject sample expressed in arbitrary units on a linear scale. The horizontal bars in each group represent the median values. Values over the dotted line indicate the percentage of subjects with antibody titres above the cut‐off value (113 U/ml for anti‐IFI16 IgG and 9·6 U/mL for anti‐IFI16 IgA, calculated as the 95th percentile of the control population). Statistical significance: ***P < 0·0001 (Mann–Whitney U‐tests).

Figure 2

Analysis of protein component of autoantigen interferon‐inducible protein 16 (IFI16) [8% sodium dodecyl sulphate‐polyacrylamide (SDS‐PAGE)] by radio‐immunoprecipitation (IP) (a), IP‐Western blotting (WB) (b) and indirect immunofluorescence (c). The IP pattern of psoriatic arthritis (PsA) sera recognizing a 90 kDa protein (black arrows, a). IP‐WB confirming the identity of the IP bands corresponding to IFI16 (b). The nuclear pattern for a representative sample positive for ani‐IFI16 IgG in IIF (larger image, c) compared to an anti‐IFI16‐negative patient (small image in the upper right side, c) and to a normal subject (small image in the bottom right side, c).

Figure 3

Epitope mapping for anti‐interferon‐inducible protein 16 (IFI16) immunoglobulin (Ig)G. To determine the target of the anti‐IFI16 IgG antibodies found in the sera of psoriatic arthritis (PsA) patients, the DAPIN (spanning from aa 1 to 88), HINA (from aa 131 to 337) and HINB (from aa 506 to 705) domains of IFI16 were purified as recombinant peptides and 2 μg/ml each used to perform enzyme‐linked immunosorbent assay (ELISA). Each dot represents the value of the absorbance at 450 nm for 158 sera from study patients (PsA) and 101 healthy controls (CTRL) tested against the three domains. The horizontal bars in each scatter represent the median values. Values over the dotted line indicate the percentage of subjects with antibody titres above the cut‐off value calculated as the 95th percentile of the control population (0·395, 0·900 and 0·776 for DAPIN, HINA and HINB, respectively). Statistical significance: ***P < 0·0001, **P < 0·001, *P < 0·05 (Mann–Whitney U‐tests).