Highly Arthritis-susceptible DA Rats Express IL-1β in Articular Cartilage

There are genetically determined differences in susceptibility to arthritis among inbred rat strains. The aim of the present study was to elucidate phenotypical differences, by determining expression of TNF and IL-1β, two pivotal mediators of arthritis, in the highly arthritis-prone Dark Agouti (DA) rat compared to that of two arthritis-resistant rat strains, the major histocompatibility complex-homologous Piebald-Viral-Glaxo (PVG.1AV1) rat and the Brown Norway (BN) rat, assessed by immunohistochemistry. We demonstrate a distinct difference in articular cartilage, with chondrocytes expressing IL-1β, not TNF, in the highly arthritis-prone DA rat as opposed to the two arthritis-resistant BN or PVG.1AV1 rat strains, where no cytokine expression was documented. The results were otherwise congruent among the rat strains. We observed TNF- and IL-1β-expressing cells within the synovial lining layer in all rat strains. Other tissues studied, auricular cartilage as well as muscle, lung, thyroid gland and kidney tissue, were devoid of cytokine expression. Constitutional expression of IL-1β in chondrocytes might facilitate initiation and perpetuation of inflammation. This may offer one explanation of why erosive arthritides are so easily induced in the DA rat and also support the hypothesis that articular chondrocytes may themselves play a major role in cartilage matrix degradation.