Increased cellular infiltrate in inflammatory synovia of osteoarthritic knees

To determine the nature and origin of synovial inflammation in knees with osteoarthritis (OA).

Synovial samples were obtained from 21 medial compartmental knee OAs from 19 patients. First, using 11 medial knee synovial samples from 9 patients, a quantitative estimation of synovitis was made with both ordinary and immunohistochemical staining. Second, from the other 10 knees, tissue samples were taken from both the medial and the lateral compartments to quantify cells that infiltrated into the synovium. Medial synovium was immunostained using antibodies to anti-type II collagen, CD68, CD2, CD4, CD8, CD15, CD19, CD25, HLA-DR, CD1a and LN5. The lateral synovium was immunostained with anti-type II collagen, CD68, HLA-DR and CD4 antibody as a control.

Denatured cartilaginous detritus was found captured by synovial lining cells with a strong immunoreactivity to CD68 antibody, and whose phagocytic potential was activated. The number of anti-type II collagen-positive fragments in the medial compartment of the knee was larger than that found in the lateral compartment. Moreover, the population of CD68-positive cells in synovial tissue and HLA-DR-positive cells in the lining layer was larger in the medial compartment than in the lateral compartment. The number of CD4-positive cells (defined as helper/inducer T lymphocytes) was greater in medial synovium than in lateral synovium.

Overall, this study strongly supports the concept that the synovitis observed in patients with knee OA might be induced by an immunological mechanism involving, to some extent, a macrophage/helper T cell interaction.