Infection barriers to successful xenotransplantation focusing on porcine endogenous retroviruses

Abstract

Xenotransplantation may be a solution to overcome the shortage of organs for the treatment of patients with organ failure, but it may be associated with the transmission of porcine microorganisms and the development of xenozoonoses. Whereas most microorganisms may be eliminated by pathogen-free breeding of the donor animals, porcine endogenous retroviruses (PERVs) cannot be eliminated, since these are integrated into the genomes of all pigs. Human-tropic PERV-A and -B are present in all pigs and are able to infect human cells. Infection of ecotropic PERV-C is limited to pig cells. PERVs may adapt to host cells by varying the number of LTR-binding transcription factor binding sites. Like all retroviruses, they may induce tumors and/or immunodeficiencies. To date, all experimental, preclinical, and clinical xenotransplantations using pig cells, tissues, and organs have not shown transmission of PERV. Highly sensitive and specific methods have been developed to analyze the PERV status of donor pigs and to monitor recipients for PERV infection. Strategies have been developed to prevent PERV transmission, including selection of PERV-C-negative, low-producer pigs, generation of an effective vaccine, selection of effective antiretrovirals, and generation of animals transgenic for a PERV-specific short hairpin RNA inhibiting PERV expression by RNA interference.

Fig 1

Structure of proviral PERV. (A) Genes and open reading frames are shown as open boxes. cap, transcriptional start site; PBS, primer binding site; SD, splice donor; SA, splice acceptor; SU/TM, surface/transmembrane envelope protein cleavage site in Env; PPT, polypurine tract, poly(A) addition site; LTR, long terminal repeat; gag, group-specific antigen gene; ppro/pol, protease/polymerase gene; env, envelope protein gene. (B) Schematic presentation of the subtypes of PERV and the recombination events and increase in the length of the LTR during passaging on human cells. Boxes in the LTR indicate sequence repeats.

Fig 2

PERVs produced by infected human cells as shown by transmission (A) and scanning (B) electron microscopy. (Courtesy of Klaus Boller, Paul Ehrlich Institute, Langen, Germany.)

Fig 3

Schematic presentation of the life cycle of PERVs. For further details, see the text and Table 4.

Fig 4

Immunohistochemical evidence for the expression of PERV in the white pulp of the spleen of a Yucatan miniature swine. The tissue was stained with an antibody specific for the transmembrane envelope protein of PERV. The immunohistochemical reaction was visualized using the avidin-biotin complex (ABC) method, using aminoethylcarbazole (AEC) as the chromogen. (Courtesy of Joachim Denner and Iris Bittmann, Institute of Pathology, Rotenburg, Germany.)

Fig 5

Interference of different cellular restriction factors with replication of PERVs. For further details, see the text and Table 4.