PolyU and M.U.S.T develop SARS-CoV-2 candidate vaccine
Researchers from the Hong Kong Polytechnic University (PolyU) and Macau University of Science and Technology (M.U.S.T.), in collaboration with other Chinese universities, have developed a recombinant vaccine targeting the Spike protein receptor-binding domain (RBD) of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which induces protective immunity in mice, rabbits and nonhuman primates.
SARS-CoV-2 gains cell entry utilizing the human angiotensin-converting enzyme 2 (ACE2) as a receptor. [Nature 2020;579:270-273; Science 2020;367:1260-1263] Based on knowledge of the viral envelope protein, the researchers hypothesized that viral RBD would be a good immunogen and could trigger a potent functional antibody response that could neutralize SARS-CoV-2 in vitro and in nonhuman primates (Macaca mulatta). [Nature 2020, doi: 10.1038/s41586-020-2599-8]
The recombinant RBD protein was prepared using insect cells and the Bac-to-Bac baculovirus expression system. “The biotechnology method used in creating the recombinant RBD vaccine is mature, generates correctly folded protein conformation, and would aid mass production of a successful vaccine,” noted Dr Johnson Lau of the Department of Applied Biology and Chemical Technology at PolyU. [Biotechnology Progress 2014;30:1-18; Journal of Invertebrate Pathology 2011;107:S31-S41; Nature 2013;500:227-231].
Mice were immunized with different doses (range, 0.1–20 μg) and regimens of the candidate vaccine. Humoral immune responses to recombinant RBD were assessed by enzyme-linked immunosorbent assay (ELISA) for RBD-specific antibodies.
“Given the urgent global need for an effective vaccine, special attention was paid to the early antibody response and ability to neutralize SARS-CoV-2,” wrote the researchers. “Sera obtained on day 7 after administration of the first candidate vaccine dose showed elevated immunoglobulin G [IgG] and IgM responses to the recombinant RBD. Furthermore, the antibody reaction was dose-dependent and could be induced with a very low dose [0.1 μg per mouse].” The dose-dependent response was also observed in rabbits.
The candidate vaccine was also given to 10 monkeys on days 0 and 7. Sera at 7 and 14 days post-immunization showed a significantly elevated IgG response to the recombinant RBD. The vaccinated monkeys also demonstrated neutralizing antibodies when challenged with live SARS-CoV-2.
“The recombinant RBD protein alone was effective at inducing the production of specific antibodies, but the addition of the Alum adjuvant induced an even higher level of specific antibodies by day 7, which was higher still by day 21,” added the researchers.
Although elevated levels of interferon-γ (INF-γ) and interleukin-4 (IL-4) were produced by lymphocytes upon RBD stimulation in vitro, no significant inflammatory cytokine increases were observed in mice, indicating that the vaccine does not induce a systemic inflammatory reaction. The vaccine’s toxicity was further evaluated in 50 nonhuman primates, which showed no pathologic changes in the liver, lung, kidney, spleen, brain, heart, or other tissues on microscopic examination. No changes in appetite, behaviour or body weight were reported; peripheral blood counts remained stable.
“It is encouraging to observe that the antibodies [to the candidate RBD vaccine] shared common binding epitopes from [SARS-CoV-2–]infected patients, that there is a strong viral neutralizing activity associated with a good humoral response, and that a simple vaccine adjuvant like Alum can further enhance the immune response. It is also encouraging to see that even one vaccine dose generates an excellent level of viral neutralizing activity, that the vaccine can protect nonhuman primates against live SARS-CoV-2 challenge, and that the toxicology studies in the nonhuman primates showed no adverse events,” commented the researchers.