Potent Therapeutic Strategies for COVID-19 with Single-Domain Antibody Immunoliposomes Neutralizing SARS-CoV-2 and Lip/cGAMP Enhancing Protective Immunity.

The worldwide spread of COVID-19 continues to impact our lives and has led to unprecedented damage to global health and the economy. This highlights the need for an efficient approach to rapidly develop therapeutics and prophylactics against SARS-CoV-2. We modified a single-domain antibody, SARS-CoV-2 VHH, to the surface of the liposomes. These immunoliposomes demonstrated a good neutralizing ability, but could also carry therapeutic compounds. Furthermore, we used the 2019-nCoV RBD-SD1 protein as an antigen with Lip/cGAMP as the adjuvant to immunize mice. Lip/cGAMP enhanced the immunity well. It was demonstrated that the combination of RBD-SD1 and Lip/cGAMP was an effective preventive vaccine. This work presented potent therapeutic anti-SARS-CoV-2 drugs and an effective vaccine to prevent the spread of COVID-19.

A two-adjuvant multiantigen candidate vaccine induces superior protective immune responses against SARS-CoV-2 challenge.

An ideal vaccine against SARS-CoV-2 is expected to elicit broad immunity to prevent viral infection and disease, with efficient viral clearance in the upper respiratory tract (URT). Here, the N protein and prefusion-full S protein (SFLmut) are combined with flagellin (KF) and cyclic GMP-AMP (cGAMP) to generate a candidate vaccine, and this vaccine elicits stronger systemic and mucosal humoral immunity than vaccines containing other forms of the S protein. Furthermore, the candidate vaccine administered via intranasal route can enhance local immune responses in the respiratory tract. Importantly, human ACE2 transgenic mice given the candidate vaccine are protected against lethal SARS-CoV-2 challenge, with superior protection in the URT compared with that in mice immunized with an inactivated vaccine. In summary, the developed vaccine can elicit a multifaceted immune response and induce robust viral clearance in the URT, which makes it a potential vaccine for preventing disease and infection of SARS-CoV-2.

Delivery of STING agonists for adjuvanting subunit vaccines.

Adjuvant is an essential component in subunit vaccines. Many agonists of pathogen recognition receptors have been developed as potent adjuvants to optimize the immunogenicity and efficacy of vaccines. Recently discovered cyclic GMP-AMP synthase-stimulator of interferon genes (cGAS-STING) pathway has attracted much attention as it is a key mediator for modulating immune responses. Vaccines adjuvanted with STING agonists are found to mediate a robust immune defense against infections and cancer. In this review, we first discuss the mechanisms of STING agonists in the context of vaccination. Next, we present recent progress in novel STING agonist discovery and the delivery strategies. We next highlight recent work in optimizing the efficacy while minimizing toxicity of STING agonist-assisted subunit vaccines for protection against infectious diseases or treatment of cancer. Finally, we share our perspectives of current issues and future directions in further developing STING agonists for adjuvanting subunit vaccines.

Inclusion of cGAMP within virus-like particle vaccines enhances their immunogenicity.

Cyclic GMP-AMP (cGAMP) is an immunostimulatory molecule produced by cGAS that activates STING. cGAMP is an adjuvant when administered alongside antigens. cGAMP is also incorporated into enveloped virus particles during budding. Here, we investigate whether inclusion of cGAMP within viral vaccine vectors enhances their immunogenicity. We immunise mice with virus-like particles (VLPs) containing HIV-1 Gag and the vesicular stomatitis virus envelope glycoprotein G (VSV-G). cGAMP loading of VLPs augments CD4 and CD8 T-cell responses. It also increases VLP- and VSV-G-specific antibody titres in a STING-dependent manner and enhances virus neutralisation, accompanied by increased numbers of T follicular helper cells. Vaccination with cGAMP-loaded VLPs containing haemagglutinin induces high titres of influenza A virus neutralising antibodies and confers protection upon virus challenge. This requires cGAMP inclusion within VLPs and is achieved at markedly reduced cGAMP doses. Similarly, cGAMP loading of VLPs containing the SARS-CoV-2 Spike protein enhances Spike-specific antibody titres. cGAMP-loaded VLPs are thus an attractive platform for vaccination.

Lymphocyte Changes in Severe COVID-19: Delayed Over-Activation of STING?

Upon recognition of microbial DNA or self-DNA, the cyclic-GMP-AMP synthase (cGAS) of the host catalyzes the production of the cyclic dinucleotide cGAMP. cGAMP is the main activator of STING, stimulator of interferon genes, leading to interferon synthesis through the STING-TBK1-IRF3 pathway. STING is also a hub for activation of NF-κB and autophagy. The present review details the striking similarities between T and B cell responses in severe coronavirus disease 2019 (COVID-19) and both animal or human models of STING gain of function (SAVI syndromes: STING-associated vasculopathy with onset in infancy). Those similarities may be further clues for a delayed activation of STING in severe COVID-19 patients, due to DNA damages following severe acute respiratory syndrome coronaviruses (SARS-CoV-2) infection and unusual role of STING in SARS-CoV-2 control. In early stages, Th2 differentiation are noticed in both severe COVID-19 and SAVI syndromes; then, CD4+ and CD8+ T cells functional exhaustion/senescent patterns due to TCR hyper-responsiveness are observed. T cell delayed over-responses can contribute to pneumonitis and delayed cytokine secretion with over-production of IL-6. Last, STING over-activation induces progressive CD4+ and CD8+ T lymphopenia in SAVI syndromes, which parallels what is observed in severe COVID-19. ACE2, the main receptor of SARS-CoV-2, is rarely expressed in immune cells, and it has not been yet proven that some human lymphocytes could be infected by SARS-CoV-2 through CD147 or CD26. However, STING, expressed in humans T cells, might be triggered following excessive transfer of cGAMP from infected antigen presenting cells into activated CD4+ and CD8+ T cells lymphocytes. Indeed, those lymphocytes highly express the cGAMP importer SLC19A1. Whereas STING is not expressed in human B cells, B cells counts are much less affected, either in COVID-19 or SAVI syndromes. The recognition of delayed STING over-activation in severe COVID-19 patients could prompt to target STING with specific small molecules inhibitors already designed and/or aspirin, which inhibits cGAS.