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Porcine endogenous retroviruses (PERV) are widely distributed in the genomes of pigs. PERV-A and PERV-B are present in all pigs. They infect human cells in vitro and therefore represent a risk for xenotransplantation when pig cells, tissues or organs are used. PERV-C infects only pig cells and is not present in the genomes of all pigs. However, PERV-A/C recombinants infecting human cells and characterized by high replication titers were found in pigs. To select PERV-C-free animals that cannot generate such recombinants, PCR-based assays were developed (Kaulitz et al., J Virol Methods, 175:60, 2011). When screening for PERV-C in German wild boars (Sus scrofa scrofa), applying these methods, a new variant of PERV-C was identified. Whereas in all 125 wild boar only the new variant of PERV-C was found, different variants were present in some landrace pigs, and most importantly, some pigs were totally free of PERV-C.

Despite enormous difficulties to induce antibodies neutralizing HIV-1, especially broadly neutralizing antibodies directed against the conserved membrane proximal external region (MPER) of the transmembrane envelope protein, such antibodies can be easily induced in the case of gammaretroviruses, among them the porcine endogenous retroviruses (PERVs). In addition to neutralizing antibodies directed against the transmembrane envelope protein p15E, neutralizing antibodies were also induced by immunization with the surface envelope protein gp70. PERVs represent a special risk for xenotransplantation using pig tissues or organs since they are integrated in the genome of all pigs and infect human cells and a vaccine may protect from transmission to the recipient. To investigate the effect of simultaneous immunization with both proteins in detail, a study was performed in hamsters. Gp70 and p15E of PERV were produced in E. coli, purified and used for immunization. All animals developed binding antibodies against the antigens used for immunization. Sera from animals immunized with p15E recognized epitopes in the MPER and the fusion peptide proximal region (FPPR) of p15E. One MPER epitope showed a sequence homology to an epitope in the MPER of gp41 of HIV-1 recognized by broadly neutralizing antibodies found in HIV infected individuals. Neutralizing antibodies were detected in all sera. Most importantly, sera from animals immunized with gp70 had a higher neutralizing activity when compared with the sera from animals immunized with p15E and sera from animals immunized with gp70 together with p15E had a higher neutralizing activity compared with sera from animals immunized with each antigen alone. These immunization studies are important for the development of vaccines against other retroviruses including the human immunodeficiency virus HIV-1.

Background: Porcine endogenous retroviruses (PERVs) represent a risk of xenotransplantation using porcine cells, tissues, or organs, as they are integrated in the porcine genome and have been shown to be able to infect human cells in vitro. To increase viral safety by RNA interference, transgenic pigs expressing a PERV-specific small hairpin (sh)RNA targeted to a highly conserved sequence in the pol gene (pol2) were generated in which expression of PERVs was reduced (Xenotransplantation, 15, 2008, 38). However, it remains to be shown how long expression of the shRNA and the RNA interference is effective in reducing PERV expression., Methods: To analyze the long-term duration of RNA interference, expression of the PERV-specific pol2 shRNA and inhibition of PERV expression was studied repeatedly in fibroblasts and peripheral blood mononuclear cells (PBMCs) of transgenic pigs over a period of 3 yr, when animals were sacrificed and expression was studied in different organs. Expression of the PERV-specific shRNA was measured using a newly developed real-time PCR, and expression of PERV was measured using a PERV-specific real-time PCR., Results: Over a period of 3 yr, PERV-specific shRNA and green fluorescent protein (GFP) as reporter of the vector system were consistently expressed in transgenic animals. PERV expression was significantly reduced during the entire period. Levels of PERV and shRNA expression were different in the various organs. PERV expression was highest in the spleen and the lungs and lowest in liver and heart. However, in all organs of the transgenic pigs, PERV expression was inhibited compared with the vector control animals., Conclusions: Transgenic pigs expressing PERV-specific shRNA maintained their specific RNA interference long term, suggesting that PERV expression in the xenotransplants will be suppressed over extended periods of time., (© 2012 John Wiley & Sons A/S.)

Porcine endogenous retroviruses (PERV) represent a risk for xenotransplantation using pig cells, tissues or organs. PERV-A and PERV-B are present in the genome of all pigs and both infect human cells in vitro. PERV-C infects only pig cells and it is integrated in the genome of most, but not all pigs. Recombinants between PERV-A and PERV-C were described that infect human cells and replicate at high titres. To avoid such recombinations, PERV-C positive animals should not be used for breeding animals suited for xenotransplantation. In order to detect PERV-C positive pigs, different methods were developed such as specific PCRs using different primers, a highly sensitive nested PCR and a real-time PCR allowing measurement of proviral copy numbers. The real-time PCR was found to be useful to discriminate between contamination and actual provirus copies. The PCRs were optimized and their sensitivity was determined. Screening can be started with PCR1, if the result is negative, PCR2 to PCR5 or the nested PCR should be used, if the result is positive, the real-time PCR should be used to exclude contaminations. All methods were used to evaluate the prevalence of PERV-C and to identify PERV-C free animals. Due to the risk of contamination with cells from other animals testing should be performed with blood cells, not with ear biopsies., (Copyright © 2011 Elsevier B.V. All rights reserved.)

Porcine endogenous retroviruses (PERVs) represent a risk for xenotransplantation using pig cells or organs since they are integrated in the genome of all pigs and infect human cells in vitro. Recombinants between PERV-A and PERV-C have been described in pigs in vivo and found de novo integrated in the genome of somatic cells, but not in the germ line. To study whether PERV-A/C can infect and have a pathogenic effect in normal pigs, German landrace pigs were inoculated with high-titre PERV-A/C. No provirus integration was found in blood cells or in various tissues, and no antibody production was observed, indicating the absence of infection.

Antibodies neutralising porcine endogenous retroviruses (PERVs) were induced in different animal species by immunisation with the transmembrane envelope protein p15E. These antibodies recognised epitopes, designated E1, in the fusion peptide proximal region (FPPR) of p15E, and E2 in the membrane proximal external region (MPER). E2 is localised in a position similar to that of an epitope in the transmembrane envelope protein gp41 of the human immunodeficiency virus-1 (HIV-1), recognised by the monoclonal antibody 4E10 that is broadly neutralising. To detect neutralising antibodies specific for PERV, a novel assay was developed, which is based on quantification of provirus integration by real-time PCR. In addition, for the first time, highly effective neutralising antibodies were obtained by immunisation with the surface envelope protein of PERV. These data indicate that neutralising antibodies can be induced by immunisation with both envelope proteins., (Copyright © 2010 Elsevier Inc. All rights reserved.)