Further Evidence for the Immunosuppressive Activity of Transmembrane Envelope Protein p15E of Porcine Endogenous Retrovirus

Retroviruses are immunosuppressive, and there is evidence that a highly conserved immunosuppressive domain (isu domain) in their transmembrane envelope protein contributes to this activity. Studies have shown that inactivated retroviruses, their purified transmembrane envelope proteins, and synthetic peptides corresponding to the isu domain inhibit mitogen-triggered proliferation of peripheral blood mononuclear cells (PBMCs) and modulate their cytokine and gene expression. This has been demonstrated for human immunodeficiency virus type 1 (HIV-1), as well as for beta- and gammaretroviruses and for both exogenous and endogenous retroviruses, including syncytins. In the case of HIV-1, homopolymers of its isu peptide stimulated an increased release of IL-10, IL-6, and other cytokines from human PBMCs. Up-regulated genes included IL-6, IL-8, and IL-10, as well as MMP-1, TREM-1, and IL-1β. In vivo, in a mouse tumor model, tumor cells that were unable to induce tumors in immunocompetent animals gained the ability to do so when expressing the transmembrane envelope protein or the isu domain of various retroviruses on their surface. Here, we demonstrate that the transmembrane envelope protein p15E of PERV can modulate cytokine expression in human PBMCs. Human 293 cells were transfected with four constructs that express a portion of p15E, including the isu domain, and were cultured in the presence of a selection medium containing hygromycin. The p15E-expressing cells were co-cultured with human PBMCs, leading to the release of IL-6 and IL-10 protein and the modulation of multiple cytokines and other markers, including IL-6, IL-10, IFN-α, TNF-α, MMP1, and SEPP1. Similar, but more pronounced, effects were observed when PERV-producing 293 and pig cells were used in parallel; both expressed higher levels of p15E. Additionally, p15E expression reduced MHC class I expression, and preliminary data indicate that p15E expression could have a protective effect against cellular cytotoxicity. This finding underscores the need for further research to elucidate the dynamics of p15E expression and its immunosuppressive activity. It also contributes to the understanding of the immunosuppressive properties of pathogenic retroviruses. Furthermore, expressing the immunosuppressive p15E of PERV on the surface of a pig xenotransplant may reduce the need for pharmaceutical immunosuppressants.