Background/Purpose:

Some but not all antibodies against citrullinated modified proteins (ACPA) promote osteoclastogenesis and bone destruction in vitro and in vivo. We aimed to investigate the ACPA specificity pattern that is related to this effect and if this effect is limited to ACPA or encompasses also other RA-associated antibodies.

Methods:

Polyclonal ACPA IgG and IgGs others than ACPA were obtained from the peripheral blood of RA patients by purification on a G column followed by an anti-CCP2 column. Monoclonal ACPA, anti-MDA and rheumatoid factor (RF) IgGs were generated from either single plasma cells isolated from the synovial fluid or tetramer-positive sorted single B-cell isolated from the plasma of RA patients. Osteoclasts were generated from CD14+ monocytes of healthy individuals or bone marrow cells of Fc gamma III or Fc gamma chain knockout mice, in the presence or absence of polyclonal ACPA, monoclonal antibodies (ACPA, and anti-MDA antibodies and RF) and IgG controls. TRAP positive multinucleated cells were counted and bone erosion assay was done in parallel.

Results:

Polyclonal ACPA increased osteoclastogenesis, by a fold of 1.6±0.03One out of 4 tested plasma cell derived monoclonals ACPAs and one out of the five tested tetramer positive B-cell derived monoclonals have similar effects (with a fold increase of 1.63 ±0.15 for the plasma cell derived antibody and 1.4± 0.16 for the tetramer positive B-cell derived) have similar OC effects. Two additional tetramer positive B-cell derived monoclonals inhibited osteocalstogenesis while the remaining had no significant effect. All monoclonal ACPA were relatively highly cross-reactive to several citrullinated epitopes but not to native arginine peptides. Anti-MDA monoclonals antibodies displaying somatic hypermutations and low reactivity had significant in vitro functional properties and enhanced osteoclastogenesis (fold increase of for one antibody 4.0±0.76 and fold increase of for the second one 2.3±0.2), while the natural antibody related high-reactivity anti-MDA antibody did not. Anti MDA antibodies had no cross reactivity to other antigen modifications such as citrullination or carbamylation. Monoclonal RF had no direct effect on ostecoalstogenesis but were able to significantly increase ACPA-mediated osteoclastogenesis (fold increase of 1.68 ±0.03 for ACPA alone and for the combination of ACPA and RF 3.15±0.24). Dimeric Fab fragments of polyclonal ACPA increased OC numbers by a fold of 1.78, suggesting that epitope recognition is involved in the observed osteocalstogenetic effect of ACPA. Interestingly however while ACPA increased osteoclastogenesis from bone marrow precursors of wild type mice, it had no effect on the bone marrow precursors of the Fc gamma chain knockout and Fc gamma III mice bone marrow samples, implying a more complex mechanism than epitope recognition alone that involves Fc receptors.

Conclusion:

We demonstrate that RA-associated antibodies targeting different post translational modifications have the capacity to increase osteoclastogenesis while others have not. The mechanism is mediated through both Fc dependent and independent mechanisms.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/ra-associated-antibodies-targeting-post-translational-modification-have-different-osteoclastogenetic-potential/