Background/Purpose:   Rheumatoid Arthritis (RA) is characterized by extra-articular involvement including interstitial/inflammatory lung disease (ILD).  Whereas the coexistence of RA and ILD is known, mechanisms linking the two conditions are poorly understood.  The objective of this study was to combine an established organic dust extract (ODE) inhalation injury model with the collagen-induced arthritis (CIA) murine model to assess lung and bone/joint inflammatory outcomes.

Methods: Using an established intranasal inhalation protocol, DBA/1J mice were treated with saline or organic dust extract (ODE) daily for 5 weeks. CIA arthritis was induced with collagen emulsified in Freund’s complete adjuvant and injected on Days 1 and 21. Mice (9-10/group; 2 independent experiments) were assigned to 1of 4 groups: 1-Sham (saline injection/saline inhalation); 2-CIA (CIA/saline inhalation); 3-ODE (saline injection/ODE inhalation); 4-CIA+ODE (CIA/ODE inhalation).  Arthritis inflammatory scores were calculated weekly (0-4 range) based on swelling and redness of the hind paw.  Five hours following final ODE, mice were euthanized. Bones, bronchoalveolar lavage fluid (BALF), and lung tissues were collected.

Results: Table depicts experimental outcome results.  Arthritis and histologic inflammatory scores and were increased in CIA+ODE>CIA alone>ODE alone as compared to Sham. Trabecular bone micro-CT analysis showed loss of bone mineral density, volume and deterioration of bone micro-architecture and mechanical strength to be most pronounced in CIA+ODE.  Evidence of bone deterioration was also found in the CIA, but not ODE or Sham animals.  In contrast, ODE-induced airway neutrophil influx and cytokine/chemokine (TNF-α, IL-6, CXCL1, CXCL2) in BALF were greater in ODE as compared to ODE+CIA.  Similarly, lung pathology showed ODE-induced lymphoid aggregates and alveolar inflammation were increased in ODE>ODE+CIA.  By flow cytometry, ODE increased lung tissue neutrophils and activated CD11c⁺CD11b^hi macrophages to Sham. Comparable trends, but to a lesser degree, were demonstrated in CIA+ODE.  CIA increased lung activated CD11c⁺CD11b^hiairway macrophages as compared to Sham.

Conclusion: This is the first study to explore the interaction between a repetitive inhalant injury and arthritis induction with findings supporting a compartmentalized immune response. The combined exposures (CIA+ODE) resulted in the greatest degree of arthritis and diffuse bone loss.  However, data support a suppression of the lung inflammatory response in the setting of arthritis.  Activation of the lung macrophage during arthritis induction may be responsible for this paradoxical finding. Finally, this co-exposure model could be exploited further and in other murine strains to better understand the pathogenesis and response to potential treatments for RA-ILD.