The Transmembrane Envelope Protein of Porcine Endogenous Retroviruses Modulates Cytokine Release and Gene Expression in Human Immune Cells

Porcine endogenous retroviruses (PERVs), their transmembrane envelope protein p15E and peptides corresponding to a domain in p15E that is highly conserved among retroviruses, the immunosuppressive (isu) domain, demonstrated immunosuppressive properties. They inhibited in vitro immune reactions, and induced release of IL-6 and IL-10 in human peripheral blood mononuclear cells (PBMCs). We recently showed that p15E of PERV expressed on 293T cells reduced MHC expression, induced cytokine release in co-incubated human PBMCs, and inhibited cytotoxic cells. The cell-surface expression of p15E was chosen to simulate an artificially introduced expression on a transplant. Here, a new construct with a higher expression of p15E and consequently higher effects on cytokine release was designed and used. An intracellular cytokine assay was newly developed and applied, whereas a commercial cytokine array was used to analyze the release of 105 cytokines into the supernatant. The differential gene expression was analyzed by sequencing the RNA of PBMCs incubated with p15E-expressing and wild-type 293T cells. PERV p15E induced an elevated expression of IL-10, IL-6 and 24 other cytokines and modulated the expression of hundreds of genes. Furthermore, coating porcine L23 cells with the synthetic isu peptide of the PERV p15E protein and subsequently co-incubating them with human PBMCs induced IL-10 production, whereas direct addition of the peptide to PBMCs alone did not induce cytokine production. These results demonstrate that the PERV p15E protein can modulate cytokine release by human immune cells and suppress their cytotoxic activity. This immunomodulatory property may have potential applications in protecting transplanted tissues from immune-mediated rejection.