In some embodiments, the present invention provides respiratory syncytial virus (RSV) F proteins, polypeptides and protein complexes that comprise one or more cross-links to stabilize the protein, polypeptide or protein complex in its pre-fusion conformation. In some embodiments the present invention provides RSV F proteins, polypeptides and protein complexes comprising one or more mutations to facilitate such cross-linking. In some embodiments the present invention provides compositions comprising such proteins, polypeptides or protein complexes, including vaccine compositions, and methods of making and using the same.