Simplifying glycan monitoring of complex antigens such as the SARS-CoV-2 spike to accelerate vaccine development

Modifying the glycosylation profile of SARS-CoV-2 spike-based subunit vaccines alters focusing of the humoral immune response in a mouse model

BACKGROUND

Protein subunit vaccines have a strong track record of efficacy and safety and have been widely applied for prevention of a variety of infectious diseases. The impacts of post-translational modifications of vaccine antigens are often overlooked, despite the fact that they can vary significantly depending on the expression hosts (e.g., bacteria, yeast, plant, insect or mammalian cells) and the culture conditions used for their manufacturing.

METHODS

Using SARS-CoV-2 spike trimers as model antigens, we sought to evaluate the immunological impact of modulating their state of glycosylation. Spike proteins rich in complex-type (CT), high-mannose (HM) or paucimannose (PM) N-linked glycans were produced using Chinese Hamster Ovary (CHO) cells (cultured with or without the mannosidase inhibitor kifunensine) or insect cells.

RESULTS

Here we show that when these antigens are adjuvanted with liposomes composed of sulfated lactosyl archaeol (SLA), all glycoforms are highly immunogenic and induce abundant spike-specific serum IgG and IFN-γ producing T-cells within female C57BL/6 mice. The spike antigen with CT glycans induces a significantly more potent neutralizing immune response, which directly correlates to more abundant receptor binding domain (RBD)-specific IgG when comparing to the antigen with HM glycans. This observation remains true whether the spike is resistin- or T4 foldon-trimerized, indicating that the glycosylation effect is not trimerization domain-specific. Spike with PM glycans induces remarkably low titers of neutralizing antibodies and RBD-specific IgG.

CONCLUSIONS

The results highlight the significant impacts of a vaccine's antigen glycosylation profile in directing the immune response, which should be an important consideration for designing efficient protein-based vaccines.

Identification of glycosylated nucleosides in small synthetic glyco-RNAs

Recently, the post-transcriptional modification of RNA with N-glycans was reported, changing the paradigm that RNAs are not commonly N-glycosylated. Moreover, glycan modifications of RNA are investigated for therapeutic targeting purposes. But the glyco-RNA field is in its infancy with many challenges to overcome. One question is how to accurately characterize glycosylated RNA constructs. Thus, we generated glycosylated forms of Y5 RNA mimics, a short non-coding RNA. The simple glycans lactose and sialyllactose were attached to the RNA backbone using azide-alkyne cycloadditions. Using nuclease digestion followed by LC-MS, we confirmed the presence of the glycosylated nucleosides, and characterized the chemical linkage. Next, we probed if glycosylation would affect the cellular response to Y5 RNA. We treated human foreskin fibroblasts in culture with the generated compounds. Key transcripts in the innate immune response were quantified by RT-qPCR. We found that under our experimental conditions, exposure of cells to the Y5 RNA did not trigger an interferon response, and glycosylation of this RNA did not have an impact. Thus, we have identified a successful approach to chemically characterize synthetic glyco-RNAs, which will be critical for further studies to elucidate how the presence of complex glycans on RNA affects the cellular response.