Mechanism of trapping of immune complexes in joint collagenous tissues

The role of acute inflammation and of pre-existing specific antibody in the retention of intra-articular antigen in joint collagenous tissues of immunized rabbits was examined. The role of the acute synovitis occurring immediately after antigen injection was investigated by the production of acute synovitis in immune and non-immune rabbits. In no case was more 125I-labelled BSA retained in the inflamed joint tissues compared to the contralateral non-inflamed joints 7 days after intrarticular antigen injection. When antigen retention was examined early after intra-articular injection, the largest amount of antigen was retained 30 min after injection, before the appearance of the acute inflammatory synovitis. These findings suggest that acute inflammation does not constitute a major factor in the long-term retention of antigen in collagenous tissues. To investigate the role of antibody in the retention of antigen, non-immune rabbits were injected intravenously with purified anti-BSA antibody 3 days prior to the intra-articular injection of BSA. Over 20 times more antigen was retained irreversibly in collagenous tissues obtained from the injected joints of passively immunized animals compared with similar tissues of control rabbits. When rabbits were injected intravenously with purified anti-BSA antibody and either killed 20 mins or 3 days later, in vitro binding of antigen by joint collagenous tissues was seen only in animals where antibody was allowed to equilibrate with the extravascular space for 3 days. These findings indicate that retention of antigen depends on the presence of extravascular antibody. Antigen retention in collagenous tissues was also present when both antibody and antigen were injected intravenously and antibody was given 3 days before the antigen. It is concluded that the trapping of immune complexes in collagenous joint tissues of immunized animals depends on: the presence of antibody in the extra-vascular space; the diffusion of antigen or soluble complexes into this space; the interaction of antigen or soluble complexes with extravascular antibody with subsequent formation of larger and more insoluble complexes; and the trapping of these complexes within the collagen fibre meshwork.