Deconstructing the Antiviral Neutralizing-Antibody Response: Implications for Vaccine Development and Immunity

Abstract

The antibody response plays a key role in protection against viral infections. While antiviral antibodies may reduce the viral burden via several mechanisms, the ability to directly inhibit (neutralize) infection of cells has been extensively studied. Eliciting a neutralizing-antibody response is a goal of many vaccine development programs and commonly correlates with protection from disease. Considerable insights into the mechanisms of neutralization have been gained from studies of monoclonal antibodies, yet the individual contributions and dynamics of the repertoire of circulating antibody specificities elicited by infection and vaccination are poorly understood on the functional and molecular levels. Neutralizing antibodies with the most protective functionalities may be a rare component of a polyclonal, pathogen-specific antibody response, further complicating efforts to identify the elements of a protective immune response. This review discusses advances in deconstructing polyclonal antibody responses to flavivirus infection or vaccination. Our discussions draw comparisons to HIV-1, a virus with a distinct structure and replication cycle for which the antibody response has been extensively investigated. Progress toward deconstructing and understanding the components of polyclonal antibody responses identifies new targets and challenges for vaccination strategies.

Copyright © 2016, American Society for Microbiology. All Rights Reserved.

Figures

FIG 1

Structure and diversity of the surface glycoproteins of flaviviruses. (A) Dendrogram depicting the relatedness of selected flavivirus E proteins (the bar represents 0.1 amino acid substitutions per site). JEV, Japanese encephalitis virus; MVEV, Murray Valley encephalitis virus; WNV, West Nile virus; SLEV, Saint Louis encephalitis virus; TBEV, tick-borne encephalitis virus; POWV, Powassan virus; YFV, yellow fever virus; DENV, dengue virus. (B) Structure of the ectodomain of the flavivirus E protein dimer (PDB accession number 1OAN) from a side view (top) and top view (bottom). Domains I, II, and III are shown in red, yellow, and blue, respectively. The fusion loop in domain II is shown in green. (C) Structure of a mature flavivirus virion (PDB accession number 4CCT). The E protein is arranged as antiparallel homodimers that densely coat the virion surface. (D) The two possible glycans on the E protein are highlighted in red in the E protein dimer (PDB accession number 1OAN) from a side view (top) and top view (bottom).

FIG 2

Structure and diversity of the surface glycoproteins of HIV-1. (A) Dendrogram depicting the relatedness of Env from HIV-1 groups M, N, O, and P and SIVcpz (CPZ) (the bar represents 0.1 amino acid substitutions per site). (B) Structure of the ectodomain of the HIV-1 Env trimeric spike (PDB accession number 4TVP), consisting of trimers of gp120/gp41 heterodimers, from a side view (left) and top view (right). gp120 and gp41 are shown in gray and cyan, respectively. On gp120, variable loops are shown in green, while the CD4-binding site is in purple. (C) Schematic of the HIV-1 virion. Env exists as trimeric spikes that sparingly populate the virion surface (shown in red and gray). A variety of cellular proteins are incorporated into the virion (shown in blue) (91). (D) The HIV glycan shield is highlighted in red on the Env trimer (PDB accession number 4TVP) from a side view (left) and top view (right). The model was generated by using GlyProt (298).

FIG 3

Sources of antibodies. Upon naive B cell recognition of an antigen and activation by a cognate T cell, activated B cells are characterized by extensive proliferation. Activated B cells can then follow one of several paths: (i) they may terminally differentiate into short-lived plasma cells (PCs), which have low surface Ig levels and high Ig secretion rates; (ii) they may differentiate into memory B cells (MBCs), which retain BCR expression but do not constitutively secrete antibody; and (iii) they can participate in the formation of germinal centers (GCs), along with follicular helper T (TFH) cells and follicular dendritic cells (FDC). In GCs, B cells undergo rapid proliferation, further diversification of their antibody gene through somatic hypermutation, and class switch recombination (CSR), during which the Fc region of the Ig gene may be exchanged for another to determine antibody effector function. Selected GC B cells receive signals to differentiate into PCs or MBCs; other GC B cells undergo apoptosis. PCs may be short-lived and remain in the lymphoid organs or become long-lived plasma cells (LLPCs) and migrate to the bone marrow, where they continue to secrete antibody independent of the presence of the antigen. LLPCs are most likely responsible for the long-lived, pathogen-specific antibody titers in serum that can last years or decades following infection or vaccination. Distinct from LLPCs, MBCs are long-lived cells that remain in circulation and peripheral lymphoid tissue. Through expression of their BCR, they can be reactivated by an antigen. Upon restimulation, they may set up germinal centers, undergo further somatic hypermutation and class switching, and differentiate into antibody-secreting plasmablasts and PCs.

FIG 4

Flavivirus entry and mechanisms of antibody-mediated neutralization. (A) Flavivirus entry occurs following virus interaction with attachment factors such as the C-type lectins DC-SIGN and DC-SIGNR, mannose receptor, glycosaminoglycans (GAGS), and phosphatidylserine receptors of the TIM and TAM protein families. (B) Following virus attachment, flaviviruses undergo clathrin-mediated endocytosis. (C) Flaviviruses can then enter cells by pH-dependent fusion, typically in the late endosome for dengue viruses (67). Antibody-mediated neutralization of flaviviruses may be achieved by inhibiting virus infectivity at a number of virus entry steps such as (i) preventing virus attachment to the cell surface, (ii) promoting virus detachment from cells, and (iii) inhibiting virus fusion with endosomal membranes. (Inset) Neutralization occurs when antibodies bind flaviviruses with a stoichiometry that exceeds a particular threshold (160). Antibody-dependent enhancement of infection (ADE) can occur if the number of antibodies bound to the virion does not reach the stoichiometric threshold for neutralization. The number of antibodies bound per virion is modulated by antibody affinity as well as by epitope accessibility. Therefore, antibodies that bind cryptic epitopes that are poorly accessible for antibody recognition may not be able to achieve a stoichiometry sufficient to exceed the threshold requirements for neutralization despite high affinity for the epitope. In contrast, antibodies that bind highly accessible epitopes can exceed the stoichiometric threshold for neutralization at low occupancy.

FIG 5

Epitopes targeted by antiflavivirus neutralizing antibodies in polyclonal sera. (A) The two residues highlighted in the E protein dimer, E126 in green (DII) and E157 in cyan (DI), were found to be major targets of the neutralizing-antibody response to DENV1 vaccination (228). A DENV1 variant with mutations E126K and E157K was not sensitive to neutralization by type-specific antibodies in sera from recipients of a DENV1 vaccine candidate. (B) The E126 and E157 variants were selected from a large panel of DENV1 variants, with mutations at each of the residues highlighted in purple. (C) The 40 residues highlighted in green on the E protein dimer were found to be the main targets of neutralizing antibodies in DENV2 primary sera (250). Transplantation of this DIII epitope from DENV2 into DENV4 resulted in a chimeric DENV4/2 virus that was neutralized by DENV2 primary sera with a potency similar to that measured with DENV2 (250). (D) The epitope of MAb 2D22 (172) is highlighted in purple. This potent human MAb was used to guide the construction of the DENV4/2 chimeric virus in panel C.

FIG 6

Examples of epitopes targeted by antiflavivirus monoclonal antibodies. Epitopes of selected MAbs are shown in green on the E protein dimer (PDB accession number 1OAN) (top) or in red on the mature virion (PDB accession number 4CCT) (bottom). (A) Monoclonal antibody E16 targets the DIII lateral ridge, a highly accessible epitope on the virion surface (180). (B) Monoclonal antibody E111 recognizes a cryptic epitope on DIII that is poorly accessible on the mature virion (207). This MAb may rely on virus breathing for epitope accessibility. (C) Monoclonal antibody EDE2-B7 targets a quaternary epitope that spans adjacent E protein monomers at the dimer interface (169).