Morphological and immunohistochemical characterization of one case of PTLD observed in a pig-to-primate xenotransplantation programme with cynomolgus monkeys

Objectives: Human post-tranplant lymphoproliferative disorder (PTLD) has been shown to be strongly associated with Epstein-Barr virus (EBV) infection. Sporadic cases of PTLD associated with the presence of EBV with a B cell phenotype have also been reported in transplanted non-human primates. In our centre, only 1 primate in 50 renal xenotransplant recipients, developed PTLD. In the present study we evaluated the morphological characteristics, immunophenotype and the possible association with EBV of the PTLD observed in this transplanted primate. Material and Methods: Nephrectomized primates were transplanted with hDAF transgenic porcine kidney and received different regimens of immunosuppression. At euthanasia, samples from all tissues and organs were collected and prepared for H&E staining and immunohistochemical analyses for T cells, B cells and EBV nuclear antigen 2 (EBNA2). Results: Macroscopic evaluation of the primate specimens showed a moderate diffuse lymphoadenomegaly. The microscopical analysis demonstrated the presence of a monomorphous atypical lymphoid population, densely arranged in sheets, which altered the lymph node architecture and infiltrated lungs, stomach and liver. Dense cellular lymphoid aggregates were also frequently observed in small arteries and lymphatic vessels. The cells showed a high nuclear/cytoplasmic ratio and scant to moderate homogeneous eosinophilic cytoplasm with indistinct cell boundaries. The nucleus (10-14 microns in diameter) was central, round, with hyperchromatic clumped chromatin and one prominent eosinophilic central nucleolus. There was mild anisocytosis, moderate anisokaryosis and scattered apoptotic lymphoid cells. Between 2 to 6 lymphoid mitotic cells were present per high power field. The immunohistochemical analysis of neoplastic cells showed diffuse and intense specific reactivity with the anti-CD20 monoclonal antibody (mAb). Scattered reactive lymphocytes and occasional pseudo-follicular aggregates were moderately positive to anti-CD3, CD5 and CD79αcy mAbs. There was a diffuse specific nuclear reaction to the anti-EBNA2 mAb. Conclusion: In our series of transplanted primates, only one case of multiorgan monomorphic B-cell PTLD, associated with an EBV-related herpesvirus could be observed. This study demonstrated that PTLD is an uncommon event following the transplantation of pig organs into immunosuppressed cynomolgus monkeys.