Longitudinal Assessment of SARS-CoV-2 Specific T Cell Cytokine-Producing Responses for 1 Year Reveals Persistence of Multi-Cytokine Proliferative Responses, with Greater Immunity Associated with Disease Severity (preprint)

Cellular-mediated immunity is critical for long-term protection against most viral infections, including coronaviruses. We studied 23 SARS-CoV-2-infected survivors over a one year post symptom onset (PSO) interval by ex vivo cytokine ELISpot assay. All subjects demonstrated SARS-CoV-2-specific IFN-{gamma}, IL-2, and Granzyme B (GzmB) T cell responses at presentation, with greater frequencies in severe disease. Cytokines, mainly produced by CD4+ T cells, targeted all structural proteins (Nucleocapsid, Membrane, Spike) except Envelope, with GzmB > IL-2 > IFN-{gamma}. Mathematical modeling predicted that: 1) cytokine responses peaked at 6 days for IFN-{gamma}, 36 days for IL-2, and 7 days for GzmB, 2) severe illness was associated with reduced IFN-{gamma} and GzmB, but increased IL-2 production rates, 3) males displayed greater production of IFN-{gamma}, whereas females produced more GzmB. Ex vivo responses declined over time with persistence of IL-2 in 86% and of IFN-{gamma} and GzmB in 70% of subjects at a median of 336 days PSO. The average half-life of SARS-CoV-2-specific cytokine-producing cells was modelled to be 139 days (~4.6 months). Potent T cell proliferative responses persisted throughout observation, were CD4 dominant, and were capable of producing all 3 cytokines. Several immunodominant CD4 and CD8 epitopes identified in this study were shared by seasonal coronaviruses or SARS-CoV-1 in the Nucleocapsid and Membrane regions. Both SARS-CoV-2-specific CD4+ and CD8+ T cell clones were able to kill target cells, though CD8 tended to be more potent.

Immunity to SARS-CoV-2 persists 9 months post-symptoms with an altered T cell phenotype compared to influenza A virus-specific memory (preprint)

SARS-CoV-2 induces T cell, B cell and antibody responses that are detected for several months in recovered individuals. Whether this response resembles a typical respiratory viral infection is a matter of debate. Here we followed T cell and antibody responses in 24 mainly non-hospitalized SARS-CoV-2 recovered subjects at two time points (median of 45- and 145-days post-symptom onset). Antibody responses were detected in 95% of subjects, with a strong correlation between plasma and salivary anti-S and anti-RBD IgG, as well as a correlation between circulating T follicular helper cells and the SARS-CoV-2-specific IgG response. Based on intracellular cytokine production or proliferation, CD4+ T cell responses to SARS-CoV-2 were detected in all subjects, decaying with a half-life of 5-6 months for S-specific IL-2-producing cells. CD4+ responses were largely of the T helper 1 phenotype, but with a lower ratio of IFN-{gamma} : IL-2 producing cells and a lower frequency of CD8+: CD4+ T cells compared to influenza A virus-(IAV)-specific memory responses within the same subjects. Analysis of secreted molecules also revealed a lower ratio of IFN-{gamma}: IL-2 and IFN-{gamma}: IL-6 and an altered cytotoxic profile for S- and N-specific compared to IAV-specific responses. These data suggest that the memory T-cell phenotype after a single infection with SARS-CoV-2 persists over time, with an altered cytokine and cytotoxic profile compared to long term memory to IAV within the same subjects.

Preclinical evaluation of a SARS-CoV-2 mRNA vaccine PTX-COVID19-B (preprint)

Safe and effective vaccines are needed to end the COVID-19 pandemic caused by SARS-CoV-2. Here we report the preclinical development of a lipid nanoparticle (LNP) formulated SARS-CoV-2 mRNA vaccine, PTX-COVID19-B. PTX-COVID19-B was chosen among three candidates after the initial mouse vaccination results showed that it elicited the strongest neutralizing antibody response against SARS-CoV-2. Further tests in mice and hamsters indicated that PTX-COVID19-B induced robust humoral and cellular immune responses and completely protected the vaccinated animals from SARS-CoV-2 infection in the lung. Studies in hamsters also showed that PTX-COVID19-B protected the upper respiratory tract from SARS-CoV-2 infection. Mouse immune sera elicited by PTX-COVID19-B vaccination were able to neutralize SARS-CoV-2 variants of concern (VOCs), including the B.1.1.7, B.1.351 and P.1 lineages. No adverse effects were induced by PTX-COVID19-B in both mice and hamsters. These preclinical results indicate that PTX-COVID19-B is safe and effective. Based on these results, PTX-COVID19-B was authorized by Health Canada to enter clinical trials in December 2020 with a phase 1 clinical trial ongoing (ClinicalTrials.gov number: NCT04765436).

A Homogeneous Split-Luciferase Assay for Rapid and Sensitive Detection of Anti-SARS CoV-2 Antibodies (preprint)

To meet the urgent demand for better diagnostic tools to combat the ongoing COVID-19 pandemic, we developed a homogeneous immunoassay to detect IgG antibodies against SARS-CoV-2. This assay is based on a tri-part Nanoluciferase (tNLuc) approach, in which the spike protein of SARS-CoV-2 and protein G, fused respectively to two different tNLuc tags, are used as antibody probes. Target engagement of the probes allows reconstitution of a functional luciferase in the presence of the third tNLuc component. The assay is performed directly in liquid phase of patient sera and enables rapid, quantitative and low-cost detection. We show that tNLuc maintains a similar sensitivity to ELISA, while its readouts are highly consistent with various neutralizing antibody assays. This proof-of-principle study suggests potential applications in diagnostics and disease and vaccination management.