Antibodies to Citrullinated Proteins (ACPA) Associate with Markers of Osteoclast Activation and Bone Destruction in the Bone Marrow of Patients with Rheumatoid Arthritis

Abstract

Normalizing bone metabolism is a challenge in rheumatoid arthritis (RA). Studies in mice suggest that anti-citrullinated protein antibodies (ACPAs) can trigger osteoclast activation and bone resorption in the bone marrow. However, data on the presence and role of ACPAs in human bone marrow are scarce. We investigated whether ACPAs can contribute to osteoclast activation and bone erosion in RA bone marrow. Anti-cyclic citrullinated peptide antibodies (anti-CCP Abs), osteoclast activation indicators-the tartrate-resistant acid phosphatase 5b (TRAP5b) and cathepsin K, and bone degradation marker-C-terminal telopeptide of type I collagen (CTX-I) were measured in the bone marrow and peripheral blood of RA patients using ELISAs. We found that ACPAs present in RA bone marrow was associated with increased amounts of TRAP5b, cathepsin K and CTX-I in this location. Levels of IL-8, the key mediator of anti-citrullinated protein antibody (ACPA)-induced bone resorption, were also elevated in bone marrow containing anti-CCP Abs and positively correlated with TRAP5b and cathepsin K concentrations. Higher levels of TRAP5b, cathepsin K, CTX-I and IL-8 in bone marrow compared to peripheral blood indicate local generation of these molecules. Our results complement data from animal studies and highlight the relevance of ACPAs and bone marrow in bone resorption in RA.

Keywords: ACPAs; anti-CCP antibodies; bone destruction; bone marrow; osteoimmunology; rheumatoid arthritis.

Figure 1

Concentrations of TRAP5b, cathepsin K and CTX-I in the bone marrow of RA patients. (A–C) Anti-cyclic citrullinated peptides antibodies-positive (anti-CCP Abs-pos)/anti-cyclic citrullinated peptides antibodies-negative (anti-CCP Abs-neg) bone marrow samples: n = 20/18 (A), n = 19/17 (B) and n = 22/20 (C). Comparisons were performed using two-tailed Student’s t-test (A and C) or two-tailed Mann–Whitney U test (B). TRAP5b, tartrate-resistant acid phosphatase 5b; CTX-I, C-terminal telopeptide of type I collagen.

Figure 2

Concentrations of IL-8 in the bone marrow of RA patients. (A) Concentrations of IL-8 in anti-cyclic citrullinated peptides antibodies-positive (anti-CCP Abs-pos), n = 20, and anti-cyclic citrullinated peptides antibodies-negative (anti-CCP Abs-neg), n = 17, bone marrow samples. Comparison was performed using two-tailed Mann–Whitney U test. (B) Correlation between IL-8 and anti-CCP Abs concentrations, n = 37. The relationship between variables was measured using the Spearman’s rank correlation coefficient.

Figure 3

Concentrations of TRAP5b, cathepsin K and CTX-I in bone marrow containing high or low levels of IL-8. (A–C) Levels of TRAP5b, cathepsin K and CTX-I in IL-8-low and IL-8-high bone marrow samples (description in the text), n = 23/11; Comparisons were performed using two-tailed Mann– Whitney U test; ns, not significant; (D–E) Correlation between IL-8 and TRAP5b (D), and cathepsin K (E) concentrations in anti-CCP Abs-positive bone marrow, n = 19; the relationships between variables were measured using the Spearman’s rank correlation coefficient. TRAP5b, tartrate-resistant acid phosphatase 5b; CTX-I, C-terminal telopeptide of type I collagen.

Figure 4

Concentrations of IL-8, TRAP5b, cathepsin K and CTX-I in paired bone marrow and peripheral blood samples of ACPA-positive RA patients. (A–D) Bone marrow/peripheral blood samples: n = 16 (A), n = 16 (B), n = 14 (C), n = 15 (D). Comparisons were performed using two-tailed Mann–Whitney U test (A,C) or two-tailed Student’s t-test (B,D). TRAP5b, tartrate-resistant acid phosphatase 5b; CTX-I, C-terminal telopeptide of type I collagen; ACPA, anti-citrullinated protein antibodies.