Non-invasive imaging demonstrates clinical features of ankylosing spondylitis in a rat adjuvant model: a case study

Ankylosing spondylitis is a common rheumatic disease involving both inflammatory erosive osteopenia and bony overgrowth. Main disease features are recapitulated in small rodents challenged with complete Freund’s adjuvant. MRI was used to follow longitudinally in vivo changes induced in the rat spine and micro-CT as terminal assessment of bone damage. Histochemistry methods were used to validate these imaging modalities in view of preclinical drug testing and translational applications of spine imaging. Animals were examined using a 3D fat-suppressed gradient-echo sequence, following the injection of gadolinium. At the end of the study, spines were excised for micro-CT and histological examination. Signals reflecting inflammation were detected at levels L5-L6 of the lumbar spine throughout the experimental period, peaking at day 27 after adjuvant. At day 14 the inflammatory response occurred along ligaments but it expanded to nearby soft tissues at later time points. From day 27 onwards inflammation was also detected within the bone, in areas where erosion occurred, and bone-like structures were formed. Micro-CT showed bone remodeling. Histology of isolated spines confirmed the inflammation and bone remodeling observed in vivo. The present study including three complementary approaches clearly demonstrates the potential of imaging for longitudinal assessments of changes in the spine in this animal model in view of preclinical pharmacological studies. The excellent correlation seen between the in vivo images and the histology underlines its fundamental role in the validation of non-invasive imaging readouts.