Characterization of humoral and cellular immunologic responses to an mRNA-based human cytomegalovirus vaccine from a phase 1 trial of healthy adults

Abstract

mRNA-1647 is an investigational mRNA-based vaccine against cytomegalovirus (CMV) that contains sequences encoding the CMV proteins glycoprotein B and pentamer. Humoral and cellular immune responses were evaluated in blood samples collected from healthy CMV-seropositive and CMV-seronegative adults who participated in a phase 1 trial of a three-dose series of mRNA-1647 (NCT03382405). Neutralizing antibody (nAb) titers against fibroblast and epithelial cell infection in sera from CMV-seronegative mRNA-1647 recipients were higher than those in sera from control CMV-seropositive samples and remained elevated up to 12 months after dose 3. nAb responses elicited by mRNA-1647 were comparable across 14 human CMV (HCMV) strains. Frequencies of antigen-specific memory B cells increased in CMV-seropositive and CMV-seronegative participants after each mRNA-1647 dose and remained elevated for up to 6 months after dose 3. mRNA-1647 elicited robust increases in frequencies and polyfunctionality of CD4⁺ T helper type 1 and effector CD8⁺ T cells in samples from CMV-seronegative and CMV-seropositive participants after stimulation with HCMV-specific peptides. The administration of three doses of mRNA-1647 to healthy adults elicited high nAb titers with wide-breadth, long-lasting memory B cells, and strong polyfunctional T-cell responses. These findings support further clinical development of the mRNA-1647 vaccine against CMV.IMPORTANCECytomegalovirus (CMV), a common virus that can infect people of all ages, may lead to serious health problems in unborn babies and those with a weakened immune system. Currently, there is no approved vaccine available to prevent CMV infection; however, the investigational messenger RNA (mRNA)-based CMV vaccine, mRNA-1647, is undergoing evaluation in clinical trials. The current analysis examined samples from a phase 1 trial of mRNA-1647 in healthy adults to better understand how the immune system reacts to vaccination. Three doses of mRNA-1647 produced a long-lasting immune response, thus supporting further investigation of the vaccine in the prevention of CMV infection.CLINICAL TRIALSRegistered at ClinicalTrials.gov (NCT03382405).

Keywords: cytomegalovirus; immune response; messenger RNA; vaccine.

Fig 1

Level and breadth of nAb activity in seronegative participants after mRNA-1647 vaccination. (A) Neutralization of HCMV in epithelial cells and fibroblasts. Black dots indicate individual values and red lines indicate the geometric mean values across the timepoints (also indicated in the blue text). (B) nAb response against 14 HCMV strains in eight seronegative participants at month 12 (6 months PD3). The assay in fibroblasts was performed in the presence of 1.56% rabbit complement. ^anAb titers against the homologous vaccine strain Merlin. ^bCMV hyperimmune globulin. CMV, cytomegalovirus; FRNT₅₀, foci-reduction neutralization test with a 50% neutralization cutoff; HCMV, human CMV; M, month; PD, post-dose.

Fig 2

IgG depletion and nAb response in epithelial cells and fibroblasts from the sera of seronegative participants who received mRNA-1647. (A) gB-, pentamer-, and mock-IgG depletion efficiency in the sera of eight seronegative participants at month 7 (1 month PD3) as measured by ELISA. Statistical significance was calculated using the Mann-Whitney test. ***P < 0.001 vs mock-depleted cells. (B) nAb response of gB-, pentamer-, and mock-IgG–depleted sera against CMV infection in epithelial cells and fibroblasts. The assay in fibroblasts was performed in the presence of 1.56% rabbit complement. Blue text indicates geometric mean values. Dotted lines indicate the limit of detection. AUC, area under the curve; CMV, cytomegalovirus; ELISA, enzyme-linked immunosorbent assay, FRNT₅₀, foci-reduction neutralization test with a 50% neutralization cutoff; gB, glycoprotein B; IgG, immunoglobulin G; nAb, neutralizing antibody; ns; not significant; OD₄₅₀, optical density at 450 nm; PD, post-dose.

Fig 3

Frequencies of anti-gB and anti-pentamer IgG-secreting memory B cells in seronegative and seropositive participants who received mRNA-1647 or placebo. (A) Memory B-cell frequencies in seronegative and seropositive participants at day 1 (baseline), month 2 (2 months PD1), month 6 (4 months PD2), and month 12 (6 months PD3). Black lines indicate frequencies in individual samples, blue lines indicate the median frequencies in the placebo cohort, and red lines indicate the median frequencies in the mRNA-647 cohort. (B) Comparison of memory B-cell frequencies between seronegative participants post mRNA-1647 at month 12 (6 months PD3, n = 10) and all seropositive participants at day 1 (baseline, n = 21). Data displayed as individual values and medians. ASC, antibody-secreting cells; CMV, cytomegalovirus; D, day; gB, glycoprotein B; IgG, immunoglobulin G; M, month; MBC, memory B cells, PD, post-dose.

Fig 4

Antigen-specific T-cell response to the stimulation with gB, gH, and gL peptides in seronegative and seropositive participants who received mRNA-1647 or placebo. Antigen-specific CD4⁺ and CD8⁺ T-cell responses following ex vivo re-stimulation with gB, gH, and gL-UL128-UL130-UL131A peptides. Samples were collected at timepoints: day 1 (baseline), day 63 (1 week PD2), and day 175 (1 week PD3). Black lines indicate frequencies in individual samples; blue or red lines indicate the arithmetic mean frequencies across all samples. The polyfunctionality graphs represent the fraction of T cells expressing one to four functional markers (expression of cytokines or ligands) for CD4⁺ T cells and one to five functional markers for CD8⁺ T cells. The shade in the figure represents 95% CI across the patients in each treatment group. CMV, cytomegalovirus; D, day; gB, glycoprotein B; gH, glycoprotein H; gL, glycoprotein L; M, month; PD, post-dose.