Genetically based resistance to the antiinflammatory effects of methotrexate in the air-pouch model of acute inflammation

Objective. Low-dose methotrexate (MTX), a mainstay in the treatment of rheumatoid arthritis, is effective in only 60-70% of patients, a finding mirrored by poor antiinflammatory efficacy in some animal models, most notably collagen-induced arthritis. To determine whether genetic factors or the model itself is responsible for the poor response to MTX, we directly compared the responses of 4 inbred mouse strains to MTX in the air-pouch model of acute inflammation.. Methods. The exudate leukocyte count and adenosine concentration were determined in inbred mice treated with MTX (0.75 mg/kg intraperitoneally every week for 4 weeks) or vehicle 4 hours after injection of carrageenan into the air pouch using previously described methods. Quantitative trait locus mapping was performed using an in silico, or computer-based, method to identify loci potentially associated with each phenotype. Results. MTX significantly reduced the exudate leukocyte count in C57BL/6J and BALB/cJ mice, but not DBA/1J (the strain used in the collagen-induced arthritis model) or DBA/2J mice. In a parallel manner, MTX increased adenosine concentration in inflammatory exudates of C57BL/6J and BALB/cJ mice, but not DBA/1J or DBA/2J mice. Antiinflammatory and adenosine responses to MTX in DBA/1J X C57BL/6J F-1 and F-2 offspring were most consistent with single genetic loci being responsible for each phenotype. In silico mapping identified partially overlapping loci containing candidate genes involved in both responses. Conclusion. Genetic factors contribute to the antiinflammatory efficacy of MTX, and a single locus involved in MTX-induced adenosine up-regulation is likely responsible for the observed resistance to MTX in DBA/1J mice.