Serum alpha-mannosidase as an additional barrier to eliciting oligomannose-specific HIV-1-neutralizing antibodies

Abstract

Oligomannose-type glycans on HIV-1 gp120 form a patch that is targeted by several broadly neutralizing antibodies (bnAbs) and that therefore is of interest to vaccine design. However, attempts to elicit similar oligomannose-specific bnAbs by immunizing with oligomannosidic glycoconjugates have only been modestly successful so far. A common assumption is that eliciting oligomannose-specific bnAbs is hindered by B cell tolerance, resulting from the presented oligomannosides being sensed as self molecules. Here, we present data, along with existing scientific evidence, supporting an additional, or perhaps alternate, explanation: serum mannosidase trimming of the presented oligomannosides in vivo. Mannosidase trimming lessens the likelihood of eliciting antibodies with capacity to bind fullsized oligomannose, which typifies the binding mode of existing bnAbs to the oligomannose patch. The rapidity of the observed trimming suggests the need for immunization strategies and/or synthetic glycosides that readily avoid or resist mannosidase trimming upon immunization and can overcome possible tolerance restrictions.

A glycoside analog of mammalian oligomannose formulated with a TLR4-stimulating adjuvant elicits HIV-1 cross-reactive antibodies

“…All mice immunized with the GLA-SE formulation mounted a rapid IgG response to the CRM 197 carrier protein by day 10 after priming (Supplementary Fig. 3 ), consistent with previous reports 20 and the expected high immunogenicity of the carrier. The magnitude of the antibody response was somewhat slower in mice immunized with the Alhydrogel and AddaVax formulations.…”

“…2 A). As reported recently 20 , these antibodies bind NIT211 at least as good as the BSA conjugate loaded at similar density.

Figure 1 Schematic of neoglycoconjugate NIT211.

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“…One possibility is that the glycoside configuration on the BSA conjugate, used to assess antibody responses to the oligomannose mimetic without interference from carrier-specific antibodies, allows for more avid IgG3 binding compared to IgG2. As reported elsewhere 20 , several of the oligomannose-specific PGT antibodies (PGT125, 126 and 130) bind the CRM 197 conjugate better than the BSA conjugate, suggesting differences in glycoside configuration. The BSA conjugate may thus allow for mouse IgG3 binding but not IgG2.…”

“…However, a thesis put forth recently by Nguyen et al 44 suggests that trimming of oligomannose-based conjugates upon immunization, perhaps rather than immune tolerance, may be impeding the elicitation of oligomannose-specific bnAbs. As published recently, our own investigations revealed that after prime and two boosters, sera of mice immunized with NIT211 + GLA-SE produced an IgG response that bound better to serum mannosidase-treated (i.e., trimmed) conjugate compared to the buffer-treated conjugate 20 . This might explain our inability to measure much of a response to the target glycoside until after the second booster (day 49; Fig.…”

“…The BSA conjugate of the ligand, termed NIT82b, with an average loading density of 4 glycosides per BSA molecule, was also prepared as previously reported 5 . The CRM 197 conjugates, collectively dubbed NIT211, at densities ranging from 2–6 glycosides per CRM 197 molecule, were synthesized as described elsewhere recently 20 . Biotinylated NIT211 was generated with a sulfo-NHS biotinylation kit (ThermoFisher) in the same manner as reported previously for the BSA conjugate 5 .…”