Recombinant TRAIL induces apoptosis and reduces white blood cell infiltrate in arthritic rabbit knees. (a) Recombinant TRAIL (rTRAIL) results in massive death of synovial cells of arthritic rabbit knees. Arthritic rabbit knee joints, induced by intra-articular injection of IL-1β producing cells, were then treated intra-articularly with 5 and 20 μg of rTRAIL per knee or saline. Sections of the synovium from the 20 μg of rTRAIL treated rabbits were analyzed at 48 h post-injection by hematoxylin and eosin ((H&E; 100×) and TUNEL (100×) staining. Examples of apoptotic cells or regions of apoptosis are indicated by the arrows. (b) Intra-articular injection of rTRAIL reduces joint inflammation. Lavage synovial fluid (LSF) from saline and rTRAIL treated arthritic rabbit knee joints were aspirated 48 h following intra-articular injection and leukocytes counted using a hemacytometer. The data were expressed as mean ± standard error of the mean ×106 per ml LSF. WBC, white blood cell.