Strong CD4+ T-Cell Responses to Ancestral and Variant Spike Proteins Are Established by NVX-CoV2373 Severe Acute Respiratory Syndrome Coronavirus 2 Primary Vaccination

Abstract

Background: NVX-CoV2373 is an efficacious coronavirus disease 2019 (COVID-19) vaccine comprising full-length recombinant severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spike (rS) glycoprotein and Matrix-M adjuvant. Phase 2 of a randomized, placebo-controlled, phase 1/2 trial in healthy adults (18-84 years of age) previously reported good safety/tolerability and robust humoral immunogenicity.

Methods: Participants were randomized to placebo or 1 or 2 doses of 5-µg or 25-µg rS with 50 µg Matrix-M adjuvant 21 days apart. CD4+ T-cell responses to SARS-CoV-2 intact S or pooled peptide stimulation (with ancestral or variant S sequences) were measured via enzyme-linked immunosorbent spot assay and intracellular cytokine staining.

Results: A clearly discernable spike antigen-specific CD4+ T-cell response was induced after 1 dose, but markedly enhanced after 2 doses. Counts and fold increases in cells producing Th1 cytokines exceeded those secreting Th2 cytokines, although both phenotypes were clearly present. Interferon-γ responses to rS were detected in 93.5% of 2-dose 5-µg recipients. A polyfunctional CD4+ T-cell response was cross-reactive and of equivalent magnitude to all tested variants, including Omicron BA.1/BA.5.

Conclusions: NVX-CoV2373 elicits a moderately Th1-biased CD4+ T-cell response that is cross-reactive with ancestral and variant S proteins after 2 doses.

Clinical trials registration: NCT04368988.

Keywords: CD4+ T-cell response; COVID-19 vaccine; Matrix-M adjuvant; polyfunctional; variant cross-reactivity.

Figure 1.

Cytokine-secreting cells after recombinant severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spike glycoprotein (rS) stimulation, assessed by enzyme-linked immunosorbent spot assay (ELISpot). Cells secreting interferon gamma (IFN-γ; A), tumor necrosis factor alpha (TNF-α; B), IFN-γ and TNF-α (C), or interleukin 5 (IL-5; D) after SARS-CoV-2 rS stimulation were detected by ELISpot. Geometric mean count (GMC) of cells per million peripheral blood mononuclear cells (PBMCs) secreting the requisite cytokine is plotted with 95% confidence intervals (CIs). Geometric mean fold rise (GMFR) in cytokine-secreting cells relative to day 0 (preimmunization) is indicated above the bars along with 95% CIs. Group A: placebo (day 0/7, n = 31; day 28, n = 29). Group B: 2-dose 5 µg rS + 50 µg Matrix-M adjuvant (n = 32, n = 31). Group C: 1-dose 5 µg rS + 50 µg Matrix-M adjuvant (n = 28, n = 25). Group D: 2-dose 25 µg rS + 50 µg Matrix-M adjuvant (n = 33, n = 30). Group E: 1-dose 25 µg rS + 50 µg Matrix-M adjuvant (n = 31, n = 28).

Figure 2.

Response rate of participants producing cytokines after recombinant severe acute respiratory syndrome coronavirus 2 spike glycoprotein (rS) stimulation, assessed by enzyme-linked immunosorbent spot assay (ELISpot). The response rate, defined as the proportion of participants in each treatment group at that time point with a count that exceeds the 95th percentile of pooled day 0 counts, was determined for each group, assessed by ELISpot. Response rate is plotted, shown as a percentage, for interferon gamma (IFN-γ; A), tumor necrosis factor alpha (TNF-α; B), IFN-γ and TNF-α (C), or interleukin 5 (IL-5; D). Group A: placebo (day 0/7, n = 31; day 28, n = 29). Group B: 2-dose 5 µg rS + 50 µg Matrix-M adjuvant (n = 32, n = 31). Group C: 1-dose 5 µg rS + 50 µg Matrix-M adjuvant (n = 28, n = 25). Group D: 2-dose 25 µg rS + 50 µg Matrix-M adjuvant (n = 33, n = 30). Group E: 1-dose 25 µg rS + 50 µg Matrix-M adjuvant (n = 31, n = 28).

Figure 3.

Stimulation of polyfunctional type 1 T-helper (Th1) CD4⁺ T-cell responses by variant spike proteins in peripheral blood mononuclear cells (PBMCs) of individuals treated with 2 doses of placebo or NVX-CoV2373. PBMCs from a convenience sample of 6 placebo 2-dose and 18 NVX-CoV2373 2-dose recipients at day 28 were stimulated for 6 hours with intact recombinant spike proteins of ancestral, Delta, Beta, and Omicron BA.1, BA.2, and BA.5 sequences. CD4⁺ T cells producing all 3 Th1 cytokines (interferon gamma [IFN-γ], tumor necrosis factor alpha [TNF-α], and interleukin 2 [IL-2]) were quantified via intracellular cytokine staining assay. Geometric mean counts of triple Th1 cytokine-positive CD4⁺ T cells per million CD4⁺ T cells (with 95% confidence intervals [CIs]) are shown. Two different experiments were run, with the first testing against ancestral, Delta, Omicron BA.1, and Omicron BA.2 strains, and the second testing against ancestral, Beta, and Omicron BA.5 strains.