Table of Contents - 6 1 Introduction - 8 2 Env and HIV transmission - 34 2.1 Differential transmission of HIV-1 by distinct subsets of effector dendritic cells - 34 3 Env antigenicity - 52 3.1 The mannose-dependent epitope for neutralizing antibody 2G12 on HIV-1 glycoprotein gp120 - 52 4 Env folding - 76 4.1 Folding and functionality of HIV-1 envelope glycoprotein disulfide bond mutants: discrepancies between ER quality control verdicts and viral fitness requirements - 76 4.2 A stable ß-sheet fold can substitute for a disulfide bond in HIV-1 gp120 - 98 4.3 Local and distal compensatory changes upon evolution of the HIV-1 envelope glycoproteins lacking the N-terminal disulfide bond in gp120 - 118 5 Env modifications for vaccines - 128 5.1 A recombinant HIV-1 envelope glycoprotein complex stabilized by an intermolecular disulfide bond between the gp120 and gp41 subunits is an antigenic mimic of the trimeric virion-associated structure - 128 5.2 Variable loop-deleted variants of the HIV-1 envelope glycoprotein can be stabilized by the introduction of an intermolecular disulfide bridge between the gp120 and gp41 subunits - 162 5.3 Biophysical and antigenic properties of a proteolytically mature, disulfide stabilized HIV-1 gp140 envelope glycoprotein - 182 5.4 Evolution of the HIV-1 envelope glycoproteins with a disulfide bond between gp120 and gp41 - 212 5.5 Enhancing the proteolytic maturation of HIV-1 envelope glycoproteins - 226 5.6 Stabilization of the soluble, trimeric form of the envelope glycoprotein complex of HIV-1 - 248 5.7 Evolutionary repair of HIV-1 gp41 with a kink in the N-terminal helix leads to restoration of the six-helix bundle structure - 276 6 Concluding remarks - 290 Appendix Mutational analyses and natural variability of the gp41 ectodomain - 296 Summary - 322 Samenvatting (Dutch) - 328 Dankwoord (Dutch) - 334 Publications - 336