To reassess the role of autoantibodies to type II collagen in the pathogenesis of diseases, we studied antibodies from patients with rheumatoid arthritis (RA) and from patients with relapsing polychondritis for species specificity and collagen type specificity, using an improved enzyme‐linked immunosorbent assay. Antibodies were found in the sera of 15% of the RA patients and 50% of the relapsing polychondritis patients, as well as in the cartilage of 69% of the RA patients examined. Reaction with both homologous and heterologous type II collagens was common. Analysis of 19 selected RA sera revealed that autoantibodies were generally associated with specific antibodies to some species of heterologous type II collagen. In contrast, antibodies found in 4% of the non‐RA controls were specific for either bovine or chick type II collagen. These findings indicate that autoantibody formation in RA and relapsing polychondritis may occur as a result of an immune response to heterologous type II collagen. However, since RA and relapsing polychondritis patient sera differed in their reactivity with the cyanogen bromide‐digested peptides, it is possible that the clinical manifestation of collagen autoimmunity might be influenced by the epitope specificity of the antibodies.

The physiological role of mechanoreceptors in the anterior cruciate ligament (ACL) was studied in unanaesthetised decerebrate-spinalised cats and dogs. Tonic activity in the quadriceps and the hamstring increased in response to physiological loading of the ACL. Evoked potentials in the posterior articular nerve (PAN) were elicited by electrical stimulation of the surface of the ligament. ACL loading also induced significant discharges from the PAN. The results suggest that ACL loading has an excitatory effect on the thigh muscles through a multimotor neurone output, and that the PAN is one of the afferent routes from the mechanoreceptors of the ACL. The ACL-muscle reflex may therefore play a physiological role in maintaining knee kinematics.

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