Structural Approaches to HIV-1 Immunogen Design and BNAb Analysis

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ABSTRACT There are only a handful of sites on the HIV-1 envelope protein that represent regions of vulnerability for elicitation of broadly neutralizing antibody (BNAbs). Four are associated with the gp120 subunit while one of the sites resides in the membrane proximal external region (MPER) of gp41. We previously showed that the MPER is a conserved structural element consisting of two helices separated by a hinge with tandem joints that facilitates viral hemifusion and fusion. MPER-specific BNAbs exert their anti-viral effect by disturbing gp41 conformational change required for viral entry into host cells. These BNAbs manifest extraordinary HIV-1 strain and clade neutralization breadth as exemplified by the mAb 10E8. Until recently, it was not known how to elicit anti-MPER specific Abs. The MPER is poorly immunogenic either as a component of the intact gp160 trimeric spike or in the context of an isolated gp41 subunit immunogen. Hence, anti-MPER Abs are not the most frequent specificity elicited during natural infections. This subdominance is not surprising given the fact that the MPER is lipid immersed to a significant degree. However, very recently, stealth liposomes arraying the isolated MPER and cargoing conjoint CD4 T cell epitope and TLR ligands were found to effectively foster MPER- specific bone marrow (BM) plasma cell development in the mouse, including those producing Abs with somatic hypermutation and lengthy CDRH3 as shown by microengraving and single-cell PCR analysis. Immunogenicity of this segment is dominated by residue accessibility and modulated by stereochemistry. The consequence of the latter is that unwanted specificities can be engendered during fabricated insertion of the MPER segment into a lipid membrane. Here, by appending transmembrane (TM) segments with relevant membrane tilt angles guided by computational predictions and structural/biophysical verification as well as removal of exposed misguiding chemical adducts, we shall program the desired nanovaccine immunogenicity for B cell memory and plasma cell development. In Aim I, we shall use native HIV-1 or variant TM sequences to insert the MPER into nanodiscs and liposomes. Orientation and depth of MPER residues will be assessed by NMR and EPR methods and iteratively adjusted and refined in conjunction with molecular electron microscopy and X-ray crystallographic studies using anti-MPER Abs. In Aim II, using conventional mice as well as the KyMouse with complete humanization of the mouse Ig loci, we shall establish both immunogens and vaccine schedules to engender BNAb elicitation, comparing them with those arising from natural HIV-1 infection. Existing liposome formulations, novel nanodiscs and evolving biomaterial arrays will be used for immunization and then MPER residue specificity, extraction activity, native spike binding and HIV-1 neutralizing activity of elicited antibodies will be assessed by examining bulk IgG and single BM plasma cell Abs. Optimization of several MPER sequences for induction of highly potent BNAbs against M subgroup clades will be performed in preparation for future clinical nanovaccine trials.