Composition of allogeneic equine orthobiologics and their repeated intra-articular administrations

Background: Orthobiologics such as platelet-rich plasma (PRP) and alpha-2-macroglobulin (A2MG) are increasingly used for equine musculoskeletal injuries. However, their composition and safety with repeated allogeneic intra-articular administration remain poorly investigated. Objectives: To characterise orthobiological preparations with or without 1 h incubation at 37°C (PRP, PRP + Inc, A2MG, A2MG + Inc) and evaluate immunological safety following repeated intra-articular administration (autologous and allogeneic) of PRP combined with A2MG + Inc in healthy horses. Study Design: In vivo experiments. Methods: This was a two-phase study with biochemical characterisation followed by repeated intra-articular injections in four healthy horses. Blood was processed to produce PRP or A2MG, with or without incubation, and characterised using haematology, biochemistry and enzyme-linked immunosorbent assay. For immunogenic assessment, healthy horses received three intra-articular injections of combined PRP and A2MG + Inc at 2-week intervals (Days 0, 14 and 28) (allogeneic n = 3, autologous n = 1). Blood and synovial fluid were collected on Days 0, 14, 28 and 42. Blood was analysed for haematology, biochemistry, CD4/CD8 ratio and SAA, while synovial fluid was analysed for leukocyte counts and immunoglobulins. Results: Biochemical characterisation demonstrated effective removal of blood cells from all orthobiologics. PRP-based formulations were enriched in platelets and growth factors, whereas A2MG preparations had higher alpha-2-macroglobulin and lower growth factor levels. Repeated intra-articular administration of PRP and A2MG + Inc caused no clinically relevant changes in systemic or synovial immune cells, lymphocyte subsets, or acute-phase markers, indicating no immunological activation. Main Limitations: Small sample sizes, the limited scope of characterised compositions and the use of combined PRP and A2MG + Inc for which only individual components were characterised. Conclusions: PRP may promote repair via growth factors, and A2MG is proposed to modulate immunity and catabolism. Repeated intra-articular injections of PRP with A2MG + Inc caused no systemic or synovial immune response, with no differences between autologous and allogeneic use.