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. 2007 Dec 18;104(51):20534-9.
doi: 10.1073/pnas.0707873105. Epub 2007 Dec 12.

Placental syncytins: Genetic disjunction between the fusogenic and immunosuppressive activity of retroviral envelope proteins

Marianne Mangeney  1 , Martial Renard, Géraldine Schlecht-Louf, Isabelle Bouallaga, Odile Heidmann, Claire Letzelter, Aurélien Richaud, Bertrand Ducos, Thierry Heidmann
Affiliations

Placental syncytins: Genetic disjunction between the fusogenic and immunosuppressive activity of retroviral envelope proteins

Marianne Mangeney et al. Proc Natl Acad Sci U S A. .

Abstract

We have previously demonstrated that the envelope proteins of a murine and primate retrovirus are immunosuppressive in vivo. This property was manifested by the ability of the proteins, when expressed by allogeneic tumor cells normally rejected by engrafted mice, to have the env-expressing cells escape (at least transiently) immune rejection. Here, we analyzed the immunosuppressive activity of the human and murine syncytins. These are envelope genes from endogenous retroviruses independently coopted by ancestral hosts, conserved in evolution, specifically expressed in the placenta, and with a cell-cell fusogenic activity likely contributing to placenta morphogenesis. We show that in both humans and mice, one of the two syncytins (human syncytin-2 and mouse syncytin-B) is immunosuppressive and, rather unexpectedly, the other (human syncytin-1 and mouse syncytin-A) is not (albeit able to induce cell-cell fusion). Delineation of the immunosuppressive domain by deletion analysis, combined with a comparison between immunosuppressive and nonimmunosuppressive sequences, allowed us to derive a mutation rule targeted to specific amino acids, resulting in selective switch from immunosuppressive to nonimmunosuppressive envelope proteins and vice versa. These results unravel a critical function of retroviral envelopes, not necessarily "individually" selected for in the retrovirus endogenization process, albeit "tandemly" conserved in evolution for the syncytin pairs in primates and Muridae. Selective inactivation of immunosuppression, under conditions not affecting fusogenicity, should be important for understanding the role of this function in placental physiology and maternofetal tolerance.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
Analysis of the immunosuppressive activity of syncytins and of retroviral envelope subdomains. (A) Schematic representation of a retroviral envelope protein with its surface (SU) and transmembrane (TM) subunits and ISD. (B) Rationale of the in vivo immunosuppression assay. MCA205 tumor cells transduced with the env-expressing pDFG vector were selected in hygromycin-containing medium. The Env-expressing cells were then injected s.c. into mice, and tumor growth was monitored. (C) Immunosuppressive activity of syncytin-1 and -2. MCA205 cells transduced with the pDFG vector encoding the syncytin-1 and -2 env or an empty vector (none) were injected into allogeneic BALB/c mice (106 cells per mice, 8 mice per group), and tumor size was measured. Shown are the percentages of animals with tumor (gray bars) and mean tumor areas when >1 mm2 (black bars). (D) Control growth of the MCA205 cells in C (black circles, syncytin-1; gray circles, syncytin-2; open circles, none) injected into syngeneic C57BL/6 mice; mean tumor areas ± SD are indicated (106 cells per mice, 3–5 mice per group). (E) Immunosuppressive indexes (see text) of syncytin-1 (open bar) and syncytin-2 (gray bar) (same experimental conditions as in C; means ± SD, n = 3). (F) Functional delineation of the ISD of MoMLV Env. The IS activity of full-length and truncated/deleted envelopes (structures at Left) was tested by using the MCA205 tumor-rejection assay as in B–D (immunosuppression indexes at Right; means ± SD, n = 3).
Fig. 2.
Fig. 2.
Identification of the mutations selectively affecting the IS activity of retroviral Env proteins. (A) Aligned sequences of the ISD of syncytin-1 and MPMV Env, with the four divergent residues grayed. The bracket links the two key residues reciprocally substituted in the double-mutant (dm) constructs. (B) Model structures of syncytin-1 (Left) and MPMV Env (Right) ectodomains calculated from the molecular structure of the syncytin-2 ectodomain previously determined by x-ray crystallography (27). Side chains are represented only for the four divergent residues. (C) Cell–cell fusion activity of syncytin-1 (wild-type, wt) and its mutant derivatives in HeLa cells. Fusion indexes were determined as indicated in Materials and Methods (means ± SD, n = 3). (D) Infectivity of MPMV Env and its mutant derivatives as expressed on the surface of MLV viral pseudotypes, using HeLa cells as a target. Titers (LacZ-positive focus forming units) were measured as in ref. (means ± SD, n = 3). (E) In vivo IS activity of syncytin-1 and MPMV Env and their mutant derivatives. Same experimental conditions as in Fig. 1E.
Fig. 3.
Fig. 3.
Immunosuppressive properties of human placental Env proteins. (A) Alignment of the TM subunit ectodomain sequences from syncytin-1, syncytin-2, and ERV-3 Env. The ISD is framed, and the bracket indicates the positions mutated to generate the double-mutant (dm) derivatives. (B) In vivo IS activity of the ectodomain of the wild-type (wt) syncytin-1, syncytin-2, and ERV-3 Env, and their mutant (dm) derivatives. Immunosuppression indexes for the ectodomains were measured as in Fig. 1 (means ± SD, n = 3). (C) Immunosuppressive activity of syncytins as revealed by the differential antibody response to the wild-type and mutant recombinant ectodomains. Swiss mice (five animals per experiment) were immunized twice with either wild-type or double-mutant recombinant ectodomains (100 μg per i.v. injection). Serum levels of IgG antibodies directed against the ectodomain of each protein were assayed on ELISA plates coated with either the wild-type (dark gray) or mutated (light gray) corresponding ectodomains (mean values ± SD, n = 3).
Fig. 4.
Fig. 4.
Sequence alignment of rodent syncytin-A and -B, and IS properties of the murine syncytins. (A) Alignment of the TM subunit ectodomain sequences from murine syncytin-A and -B with those of the orthologous proteins from several rodents (GenBank entries AY849973 through AY849981). Only the residues differing from the murine sequences are indicated, with the ISD residues in boldface and the key 14th residue boxed. (B) Cell–cell fusion activity of the murine syncytin-A and -B and their mutant derivatives, in 293T and A23 cells, respectively (mean values ± SD). (C) In vivo IS activity of the murine syncytin-A and -B ectodomains and their mutant derivatives. Same experimental conditions as in Fig. 3B (mean values ± SD).
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References

    1. Pötgens AG, Schmitz U, Bose P, Versmold A, Kaufmann P, Franck HG. Placenta. 2002;23:S107–S113. - PubMed
    1. Moffett A, Loke C. Nat Rev Immunol. 2006;6:584–594. - PubMed
    1. Leiser R, Kaufmann P. Exp Clin Endocrinol. 1994;102:122–134. - PubMed
    1. Blaise S, de Parseval N, Bénit L, Heidmann T. Proc Natl Acad Sci USA. 2003;100:13013–13018. - PMC - PubMed
    1. Blond JL, Lavillette D, Cheynet V, Bouton O, Oriol G, Chapel-Fernandes S, Mandrand B, Mallet F, Cosset FL. J Virol. 2000;74:3321–3329. - PMC - PubMed

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