Involvement of Envelope-Glycoprotein Glycans in HIV-1 Biology and Infection

The immunogenicity of p24 protein from HIV-1 virus is strongly supported and modulated by coupling with liposomes and mannan

Anti-viral and anti-tumor vaccines aim to induce cytotoxic CD8+ T cells (CTL) and antibodies. Conserved protein antigens, such as p24 from human immunodeficiency virus, represent promising component for elicitation CTLs, nevertheless with suboptimal immunogenicity, if formulated as recombinant protein. To enhance immunogenicity and CTL response, recombinant proteins may be targeted to dendritic cells (DC) for cross presentation on MHCI, where mannose receptor and/or other lectin receptors could play an important role. Here, we constructed liposomal carrier-based vaccine composed of recombinant p24 antigen bound by metallochelating linkage onto surface of nanoliposomes with surface mannans coupled by aminooxy ligation. Generated mannosylated proteonanoliposomes were analyzed by dynamic light scattering, isothermal titration, and electron microscopy. Using murine DC line MutuDC and murine bone marrow derived DC (BMDC) we evaluated their immunogenicity and immunomodulatory activity. We show that p24 mannosylated proteonanoliposomes activate DC for enhanced MHCI, MHCII and CD40, CD80, and CD86 surface expression both on MutuDC and BMDC. p24 mannosylated liposomes were internalized by MutuDC with p24 intracellular localization within 1 to 3 h. The combination of metallochelating and aminooxy ligation could be used simultaneously to generate nanoliposomal adjuvanted recombinant protein-based vaccines versatile for combination of recombinant antigens relevant for antibody and CTL elicitation.

Engineering immunogens that select for specific mutations in HIV broadly neutralizing antibodies

Vaccine development targeting rapidly evolving pathogens such as HIV-1 requires induction of broadly neutralizing antibodies (bnAbs) with conserved paratopes and mutations, and, in some cases, the same Ig-heavy chains. The current trial-and-error search for immunogen modifications that improve selection for specific bnAb mutations is imprecise. To precisely engineer bnAb boosting immunogens, we used molecular dynamics simulations to examine encounter states that form when antibodies collide with the HIV-1 Envelope (Env). By mapping how bnAbs use encounter states to find their bound states, we identified Env mutations that were predicted to select for specific antibody mutations in two HIV-1 bnAb B cell lineages. The Env mutations encoded antibody affinity gains and selected for desired antibody mutations in vivo. These results demonstrate proof-of-concept that Env immunogens can be designed to directly select for specific antibody mutations at residue-level precision by vaccination, thus demonstrating the feasibility of sequential bnAb-inducing HIV-1 vaccine design.

Shear-reversible clusters of HIV-1 in solution: stabilized by antibodies, dispersed by mucin

IMPORTANCE

The phenomenon of reversible clustering is expected to further nuance HIV immune stealth because virus surfaces can escape interaction with antibodies (Abs) by hiding temporarily within clusters. It is well known that mucin reduces HIV virulence, and the current perspective is that mucin aggregates HIV-1 to reduce infections. Our findings, however, suggest that mucin is dispersing HIV clusters. The study proposes a new paradigm for how HIV-1 may broadly evade Ab recognition with reversible clustering and why mucin effectively neutralizes HIV-1.

The endoplasmic reticulum proteostasis network profoundly shapes the protein sequence space accessible to HIV envelope

The sequence space accessible to evolving proteins can be enhanced by cellular chaperones that assist biophysically defective clients in navigating complex folding landscapes. It is also possible, at least in theory, for proteostasis mechanisms that promote strict quality control to greatly constrain accessible protein sequence space. Unfortunately, most efforts to understand how proteostasis mechanisms influence evolution rely on artificial inhibition or genetic knockdown of specific chaperones. The few experiments that perturb quality control pathways also generally modulate the levels of only individual quality control factors. Here, we use chemical genetic strategies to tune proteostasis networks via natural stress response pathways that regulate the levels of entire suites of chaperones and quality control mechanisms. Specifically, we upregulate the unfolded protein response (UPR) to test the hypothesis that the host endoplasmic reticulum (ER) proteostasis network shapes the sequence space accessible to human immunodeficiency virus-1 (HIV-1) envelope (Env) protein. Elucidating factors that enhance or constrain Env sequence space is critical because Env evolves extremely rapidly, yielding HIV strains with antibody- and drug-escape mutations. We find that UPR-mediated upregulation of ER proteostasis factors, particularly those controlled by the IRE1-XBP1s UPR arm, globally reduces Env mutational tolerance. Conserved, functionally important Env regions exhibit the largest decreases in mutational tolerance upon XBP1s induction. Our data indicate that this phenomenon likely reflects strict quality control endowed by XBP1s-mediated remodeling of the ER proteostasis environment. Intriguingly, and in contrast, specific regions of Env, including regions targeted by broadly neutralizing antibodies, display enhanced mutational tolerance when XBP1s is induced, hinting at a role for host proteostasis network hijacking in potentiating antibody escape. These observations reveal a key function for proteostasis networks in decreasing instead of expanding the sequence space accessible to client proteins, while also demonstrating that the host ER proteostasis network profoundly shapes the mutational tolerance of Env in ways that could have important consequences for HIV adaptation.

The host cell’s endoplasmic reticulum proteostasis network has a profound, constraining impact on the protein sequence space accessible to HIV’s envelope protein, which is a major target of the host’s adaptive immune system; in particular, upregulation of stringent quality control pathways appears to restrict the viability of destabilizing envelope variants.

Glycosylation of viral surface proteins probed by mass spectrometry

Glycosylation is a common and biologically significant post-translational modification that is found on numerous virus surface proteins (VSPs). Many of these glycans affect virulence through modulating virus receptor binding, masking antigenic sites, or by stimulating the host immune response. Mass spectrometry (MS) has arisen as a pivotal technique for the characterization of VSP glycosylation. This review will cover how MS-based analyses, such as released glycan profiles, glycan site localization, site-occupancy, and site-specific heterogeneity, are being utilized to map VSP glycosylation. Furthermore, this review will provide information on how MS glycoprofiling data are being used in conjunction with molecular and structural experiments to provide a better understanding of the role of specific glycans in VSP function.

Functional insights into pathogen biology from 3D electron microscopy

In recent years, novel imaging approaches revolutionised our understanding of the cellular and molecular biology of microorganisms. These include advances in fluorescent probes, dynamic live cell imaging, superresolution light and electron microscopy. Currently, a major transition in the experimental approach shifts electron microscopy studies from a complementary technique to a method of choice for structural and functional analysis. Here we review functional insights into the molecular architecture of viruses, bacteria and parasites as well as interactions with their respective host cells gained from studies using cryogenic electron tomography and related methodologies.

Blood Groups in Infection and Host Susceptibility

Blood group antigens represent polymorphic traits inherited among individuals and populations. At present, there are 34 recognized human blood groups and hundreds of individual blood group antigens and alleles. Differences in blood group antigen expression can increase or decrease host susceptibility to many infections. Blood groups can play a direct role in infection by serving as receptors and/or coreceptors for microorganisms, parasites, and viruses. In addition, many blood group antigens facilitate intracellular uptake, signal transduction, or adhesion through the organization of membrane microdomains. Several blood groups can modify the innate immune response to infection. Several distinct phenotypes associated with increased host resistance to malaria are overrepresented in populations living in areas where malaria is endemic, as a result of evolutionary pressures. Microorganisms can also stimulate antibodies against blood group antigens, including ABO, T, and Kell. Finally, there is a symbiotic relationship between blood group expression and maturation of the gastrointestinal microbiome.

The cytoplasmic tail of retroviral envelope glycoproteins

Retroviruses comprise a large, diverse group that infects a broad range of host organisms. Pathogenicity varies widely; the human immunodeficiency virus is the causative agent of acquired immunodeficiency syndrome, one of the world's leading infectious causes of death, while many nonhuman retroviruses cause cancer in the host. Retroviruses have been studied intensively, and great strides have been made in understanding aspects of retroviral biology. While the principal functions of the viral structural proteins are well understood, there remain many incompletely characterized domains. One of these is the cytoplasmic tail (CT) of the envelope glycoprotein. Several functions of the CT are highly conserved, whereas other properties are unique to a specific retrovirus. For example, the lentiviruses encode envelope glycoproteins with particularly large cytoplasmic domains. The functions of the long lentiviral envelope CT are still being deciphered. The reported functions of retroviral envelope CTs are discussed in this chapter.

Differential glycosylation of envelope gp120 is associated with differential recognition of HIV-1 by virus-specific antibodies and cell infection

BACKGROUND

HIV-1 entry into host cells is mediated by interactions between the virus envelope glycoprotein (gp120/gp41) and host-cell receptors. N-glycans represent approximately 50% of the molecular mass of gp120 and serve as potential antigenic determinants and/or as a shield against immune recognition. We previously reported that N-glycosylation of recombinant gp120 varied, depending on the producer cells, and the glycosylation variability affected gp120 recognition by serum antibodies from persons infected with HIV-1 subtype B. However, the impact of gp120 differential glycosylation on recognition by broadly neutralizing monoclonal antibodies or by polyclonal antibodies of individuals infected with other HIV-1 subtypes is unknown.

METHODS

Recombinant multimerizing gp120 antigens were expressed in different cells, HEK 293T, T-cell, rhabdomyosarcoma, hepatocellular carcinoma, and Chinese hamster ovary cell lines. Binding of broadly neutralizing monoclonal antibodies and polyclonal antibodies from sera of subtype A/C HIV-1-infected subjects with individual gp120 glycoforms was assessed by ELISA. In addition, immunodetection was performed using Western and dot blot assays. Recombinant gp120 glycoforms were tested for inhibition of infection of reporter cells by SF162 and YU.2 Env-pseudotyped R5 viruses.

RESULTS

We demonstrated, using ELISA, that gp120 glycans sterically adjacent to the V3 loop only moderately contribute to differential recognition of a short apex motif GPGRA and GPGR by monoclonal antibodies F425 B4e8 and 447-52D, respectively. The binding of antibodies recognizing longer peptide motifs overlapping with GPGR epitope (268 D4, 257 D4, 19b) was significantly altered. Recognition of gp120 glycoforms by monoclonal antibodies specific for other than V3-loop epitopes was significantly affected by cell types used for gp120 expression. These epitopes included CD4-binding site (VRC03, VRC01, b12), discontinuous epitope involving V1/V2 loop with the associated glycans (PG9, PG16), and an epitope including V3-base-, N332 oligomannose-, and surrounding glycans-containing epitope (PGT 121). Moreover, the different gp120 glycoforms variably inhibited HIV-1 infection of reporter cells.

CONCLUSION

Our data support the hypothesis that the glycosylation machinery of different cells shapes gp120 glycosylation and, consequently, impacts envelope recognition by specific antibodies as well as the interaction of HIV-1 gp120 with cellular receptors. These findings underscore the importance of selection of appropriately glycosylated HIV-1 envelope as a vaccine antigen.

Humoral immune responses to HIV in the mucosal secretions and sera of HIV-infected women

Although sera and all external secretions contain antibodies to human immunodeficiency virus (HIV), their levels, specificity, isotypes, and relevant effector functions display a great degree of variability. Antibodies that bind HIV antigens and neutralize the virus are predominantly associated with the IgG isotype in sera and in all external secretions, even where total levels of IgG are much lower than those of IgA. Rectal fluid that contains high IgA, but low IgG levels, displayed low neutralizing activity independent of antibodies. Therefore, external secretions should be evaluated before and after selective depletion of Ig. At the systemic level, HIV-specific IgA may interfere with the effector functions of IgG, as suggested by recent studies of individuals systemically immunized with an experimental HIV vaccine. Although HIV-specific IgG and IgA antibodies may exhibit their protective activities at mucosal surfaces through interference with viral entry and local neutralization at the systemic level, such antibodies may display discordant effector functions.

HIV-1 envelope glycoprotein biosynthesis, trafficking, and incorporation

The HIV-1 envelope (Env) glycoproteins play an essential role in the virus replication cycle by mediating the fusion between viral and cellular membranes during the entry process. The Env glycoproteins are synthesized as a polyprotein precursor (gp160) that is cleaved by cellular proteases to the mature surface glycoprotein gp120 and the transmembrane glycoprotein gp41. During virus assembly, the gp120/gp41 complex is incorporated as heterotrimeric spikes into the lipid bilayer of nascent virions. These gp120/gp41 complexes then initiate the infection process by binding receptor and coreceptor on the surface of target cells. Much is currently known about the HIV-1 Env glycoprotein trafficking pathway and the structure of gp120 and the extracellular domain of gp41. However, the mechanism by which the Env glycoprotein complex is incorporated into virus particles remains incompletely understood. Genetic data support a major role for the cytoplasmic tail of gp41 and the matrix domain of Gag in Env glycoprotein incorporation. Still to be defined are the identities of host cell factors that may promote Env incorporation and the role of specific membrane microdomains in this process. Here, we review our current understanding of HIV-1 Env glycoprotein trafficking and incorporation into virions.