Histological analysis of human meniscal allografts - A preliminary report

Background: Little is known about the biology of meniscal allograft transpl antation in humans. In particular, little information is available about th e phenotype of the cells that repopulate the allograft, whether an immune r esponse is elicited against the graft, and whether the! repopulating tells synthesize normal extracellular matrix components. Methods: A small biopsy specimen of the meniscal allograft (twenty-eight me nisci in twenty-five patients) and the adjacent synovial membrane (sixteen patients) was harvested during follow-up arthroscopy in patients who had un dergone meniscal allograft transplantation at a mean of sixteen months earl ier, Seventeen patients had undergone concomitant reconstruction of the ant erior cruciate ligament with an allograft, Normal menisci (implanted allogr afts) and synovial specimens from age-matched controls were examined as wel l. All twenty-weight meniscal allografts were examined histologically, Immu nohistochemical analysis was carried out on ten menisci and nine synovial s pecimens with use of monoclonal antibodies to class-I and class-II major hi stocompatibility complex antigens, CD-8, CD-11b, and CD-19 epitopes, as wel l as other epitopes, to demonstrate immunogenic macromolecules, cytotoxic T -lymphocytes, activated macrophages, and B-lymphocytes. Results: Most of the specimens demonstrated incomplete repopulation with vi able cells. The repopulating cells stained positively with phenotype marker s for both synovial cells and fibroblasts. Polarized light microscopy demon strated evidence of active remodeling of the matrix. The cells in frozen, u nimplanted menisci stained positively for class-I and class-II human leukoc yte antigens, indicating immunogenicity at the time of transplantation, Ove rall, nine of twelve specimens contained immunoreactive cells (B-lymphocyte s or cytotoxic T-cells) in the meniscus or synovial tissue. However, only a small number of these cells was present. There was no evidence of frank im munological rejection. The clinical outcome (success or failure of the tran splant) was not related to the overall histological score or to the presenc e of an immune response in the meniscal or synovial biopsy specimen, Conclusions: Human meniscal allograft transplants are repopulated with cell s that appear to be derived from the synovial membrane; these cells appear to actively remodel the matrix. Although there is histological evidence of an immune response directed against the transplant, this response does not appear to affect the clinical outcome, The presence of histocompatibility a ntigens on the meniscal surface at the time of transplantation (even after freezing) indicates the potential for an immune response against the transp lant. Clinical Relevance: Despite the absence of frank immunological rejection, a subtle immune reaction may affect the healing, incorporation, and revascul arization of the graft. It is possible that the structural remodeling assoc iated with cellular repopulation may render the meniscus more susceptible t o injury.