Expression of thrombospondin-1 (TSP-1) and signal-regulatory protein-alpha (SIRPα) by human articular chondrocytes and molecular associations of CD47/IAP. (a) Protein extracts from primary cultures of human articular chondrocytes were separated by SDS-PAGE and immunoblotted with antibodies against TSP-1 and SIRPα. For TSP-1, a 6% gel under reducing conditions was used, whereas for SIRPα, an 8% gel under non-reducing conditions was used. (b) Protein extracts from primary cultures of human articular chondrocytes were immunoprecipitated with antibodies against CD47/IAP and immunoblotted for TSP-1, SIRPα, or α5 integrin. TSP-1, SIRPα, and α5 integrin are identified in non-immunoprecipitated extracts, but only TSP-1 and α5 integrin co-immunoprecipitate with CD47/IAP. IP, immunoprecipitated protein extracts; MW, molecular weight; N, normal human articular chondrocytes; Non IP, non-immunoprecipitated protein extracts; OA, osteoarthritic human articular chondrocytes.