*Purpose: The survival of the nonhuman primates with life-supporting pig kidney grafts has increased substantially (to >1 year). As clinical pig organ xenotransplantation draws closer, more attention is being paid to diseases that affect pigs, and which may provide a potential risk to human recipients of pig organs. Neoplasia arising from the pig organ graft is one such concern. It is well-known that there is an increased incidence of certain neoplastic diseases in organ allotransplant recipients receiving immunosuppressive therapy.

*Methods: We reviewed the literature related to malignant diseases in pigs, and data available from the US Department of Agriculture.

*Results: The incidence of neoplasia in young pigs is very low (<0.005%), but in older pigs is largely unknown (as most pigs are killed within the first 6 months of life). However, lymphosarcoma, nephroblastoma, and melanoma have been reported. These tumors should be readily identified by direct inspection and palpation before an organ is excised, and so transfer to a human recipient should be unlikely. Although (i) noninvasive modalities, e.g., ultrasonography, computed tomography, or magnetic resonance imaging, and/or (ii) invasive methods, e.g., fine needle aspiration for cytology or open biopsy for histology, could be undertaken to exclude a tumor, we suggest that, where there is any doubt, a different pig should be selected.Post-transplant lymphoproliferative disorder (PTLD) is a concern following allotransplantation, but the incidence is low following solid organ allotransplantation unless immunosuppressive therapy is particularly intensive. The organ-source pigs that will be used for clinical xenotransplantation will be bred and housed under designated pathogen-free conditions, and will be free of the gamma-herpesvirus that is a key factor in the development of PTLD in pigs. If PTLD develops in the recipient of a pig xenograft, it will therefore almost certainly be of recipient origin.

*Conclusions: We conclude that the risk of a malignant tumor developing in a transplanted organ from a young pig is very small, and that most neoplasms occurring in human recipients of a pig kidney graft are likely to arise from host (not pig) tissues. However, if the graft remains viable for many years, the incidence of malignancy in the graft may possibly increase. Long-term monitoring for de novo neoplasia will be required, as it is in recipients of allografts.

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