Understanding the mechanisms and impact of booster vaccinations are essential in the design and delivery of vaccination programs. Here we show that a three dose regimen of a synthetic peptide vaccine elicits an accruing CD8+ T cell response against one SARS-CoV-2 Spike epitope. We see protection against lethal SARS-CoV-2 infection in the K18-hACE2 transgenic mouse model in the absence of neutralizing antibodies, but two dose approaches are insufficient to confer protection. The third vaccine dose of the single T cell epitope peptide results in superior generation of effector-memory T cells and tissue-resident memory T cells, and these tertiary vaccine-specific CD8+ T cells are characterized by enhanced polyfunctional cytokine production. Moreover, fate mapping shows that a substantial fraction of the tertiary CD8+ effector-memory T cells develop from re-migrated tissue-resident memory T cells. Thus, repeated booster vaccinations quantitatively and qualitatively improve the CD8+ T cell response leading to protection against otherwise lethal SARS-CoV-2 infection.

Understanding the mechanisms and impact of booster vaccinations can facilitate decisions on vaccination programmes. This study shows that three doses of the same synthetic peptide vaccine eliciting an exclusive CD8+ T cell response against one SARS-CoV-2 Spike epitope protected all mice against lethal SARS-CoV-2 infection in the K18-hACE2 transgenic mouse model in the absence of neutralizing antibodies, while only a second vaccination with this T cell vaccine was insufficient to provide protection. The third vaccine dose of the single T cell epitope peptide resulted in superior generation of effector-memory T cells in the circulation and tissue-resident memory T (TRM) cells, and these tertiary vaccine-specific CD8+ T cells were characterized by enhanced polyfunctional cytokine production. Moreover, fate mapping showed that a substantial fraction of the tertiary effector-memory CD8+ T cells developed from remigrated TRM cells. Thus, repeated booster vaccinations quantitatively and qualitatively improve the CD8+ T cell response leading to protection against otherwise lethal SARS-CoV-2 infection.SummaryA third dose with a single T cell epitope-vaccine promotes a strong increase in tissue-resident memory CD8+ T cells and fully protects against SARS-CoV-2 infection, while single B cell epitope-eliciting vaccines are unable to provide protection.

Human cytomegalovirus (HCMV) is an ubiquitous herpesvirus that can cause serious morbidity and mortality in immunocompromised or immune-immature individuals. A vaccine that induces immunity to CMV in these target populations is therefore highly needed. Previous attempts to generate efficacious CMV vaccines primarily focused on the induction of humoral immunity by eliciting neutralizing antibodies. Current insights encourage that a protective immune response to HCMV might benefit from the induction of virus-specific T cells. Whether addition of antiviral T cell responses enhances the protection by antibody-eliciting vaccines is however unclear. Here, we assessed this query in mouse CMV (MCMV) infection models by developing synthetic vaccines with humoral immunity potential, and deliberately adding antiviral CD8+ T cells. To induce antibodies against MCMV, we developed a DNA vaccine encoding either full-length, membrane bound glycoprotein B (gB) or a secreted variant lacking the transmembrane and intracellular domain (secreted (s)gB). Intradermal immunization with an increasing dose schedule of sgB and booster immunization provided robust viral-specific IgG responses and viral control. Combined vaccination of the sgB DNA vaccine with synthetic long peptides (SLP)-vaccines encoding MHC class I-restricted CMV epitopes, which elicit exclusively CD8+ T cell responses, significantly enhanced antiviral immunity. Thus, the combination of antibody and CD8+ T cell-eliciting vaccines provides a collaborative improvement of humoral and cellular immunity enabling enhanced protection against CMV.