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Screening Assays for PERV during the First Clinical Trials

Ralf R. Tönjes

Paul-Ehrlich-Institute, Federal Institute for Vaccines and Biomedicines, Division of Medical Biotechnology, Langen, Germany

Xenotransplantation of pig cells, tissues or organs bears microbiological as well as virological risks, possibly inducing diseases by zoonotic or xenotic pathogens in human recipients. Transmission of most if not all microorganisms and viruses with the exception of porcine endogenous retroviruses (PERV) may be prevented by designated or qualified pathogen-free breeding and screening of donor pigs. PERV cannot be easily eliminated as they are present in the genome of all pigs. Furthermore, PERV have the capacity to infect human cells in vitro.

Until now, no PERV transmission was observed in vivo, neither in preclinical and clinical xenotransplantations nor in infection experiments. A number of case studies suggest that PERV infection in human recipients of xenotransplants either does not occur or happens at very low levels. However, most of the human recipients were exposed to porcine tissue for relatively brief periods of time. Nevertheless, any clinical trial using xenotransplants has to be addressed adequately regarding the safety concerns for infectious risk.

In the patient monitoring program materials from the pig donor and the human recipient need to be sampled and archived as outlined in guidelines and consensus statements [U.S. DHHS/FDA/CBER2003,Guidance for Industry: Source Animal, Product, Preclinical, and Clinical Issues Concerning the Use of Xenotransplantation Products in Humans; European Medicines Agency EMEA/CHMP/CPWP/83508/2009, Guideline on Xenogeneic Cell-based Medicinal Products; Denner et al. 2009, The International Xenotransplantation Association consensus statement on conditions for undertaking clinical trials of porcine islet products in type 1 diabetes - Chapter 5: Strategies to prevent transmission of porcine endogenous retroviruses, Xenotransplantation 16(4)].

Donor animal tissues including cryopreserved samples of peripheral blood mononuclear cells (PBMC) allow retrospective testing. Recipients’ serum and PBMC samples taken at regular intervals before and after xenotransplantation allow screening for antibodies and virus in the blood and in PBMC. Xenotransplant recipients can be screened for PERV transmission using i) direct methods such as PCR to measure the presence of virus in the recipient’s blood as well as integration and expression in infected cells and ii) indirect methods to evaluate antibody responses by the recipient. Specifically, PERV monitoring will include screening for viral RNA in the blood, testing of PBMC for proviral PERV DNA, detection of antibodies using ELISA and Western blot analyses or immunofluorescence assays, co-cultivation experiments using PBMCs from the patient and highly susceptible human cells as well as screening for microchimerism.