The outbreak of the COVID-19 pandemic, caused by the highly virulent severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), triggered the development of multiple vaccines, which effectively protected billions from severe illness by prompting humoral immunity, through antibody production. However, immunocompromised patients who cannot generate a humoral immune response to natural infection or vaccination, face an increased risk of severe COVID-19 outcomes even after the end of the pandemic. This vulnerability particularly affects those with congenital or acquired hypogammaglobulinemia, the later comprising cancer patients undergoing B-cell depletion due to lymphoprolypherative disorders.

To address the lack of humoral protection against SARS-CoV-2 in these individuals, we aimed at harnessing SARS-CoV-2-specific T cells, based on their central role to combat COVID-19 and providing a long-lasting immunity against viral disease. Therefore, we have developed the peptide-based T-cell activator CoVac-1. After successful evaluation in a first-in-human Phase I study in healthy volunteers (Heitmann et al., Nature 2022), we here evaluated CoVac-1 within a Phase II trial in patients with acquired or congenital immunodeficiency.

CoVac-1 comprises six human leukocyte antigen (HLA)-DR-restricted SARS-CoV-2 T-cells epitopes, selected from diverse viral proteins (including spike, nucleocapsid, membrane, envelope, and ORF8). CoVac-1 peptides are adjuvanted with the novel synthetic lipopeptide XS15, which functions as a TLR1/2 ligand emulsified in MontanideTM ISA51 VG. One dose of the T cell activator was administered subcutaneously. The trial was conducted between July 2021 and January 2022, at three referral German institutions: the Clinical Collaboration Unit (CCU) Translational Immunology, University Hospital Tübingen; the Institute of Clinical Cancer Research, Krankenhaus Nordwest, University Cancer Center, Frankfurt; and the Department of Hematology, Oncology, and Cancer Immunology, Campus Benjamin Franklin, Charité-Universitätsmedizin Berlin. The study recruited 54 adult patients (median age 61.8 years, range 37-90), with congenital or acquired B-cell deficiencies and lacking SARS-CoV-2 antibody response. 93% of the study patients had cancer-related acquired B-cell deficiencies, with chronic lymphocytic leukemia (CLL, 30%), mantle cell lymphoma (MCL, 24%), and follicular lymphoma (FL, 20%) being the most prevalent diagnoses. Furthermore, 83% of patients reported receiving an approved COVID-19 vaccine prior to study inclusion, with a median of two vaccinations per patient. Patients showed low CD4⁺ T-cell counts (range 123-2,501/µl, (median 458/µl)). All patients received a single CoVac-1 dose on day 1 and were monitored for safety up to day 56, with 49 patients eligible for immunogenicity analysis up to day 28.

Safety assessment was performed for all patients from diary cards (for 28 days) and safety visits (until day 56). Local reactogenicity in terms of solicited adverse events (AEs) occurred in all trial patients, being mostly of CTCAE (Common Terminology Criteria for Adverse Events)-grade 0 to 2 (87% of patients). As expected and intended, 94% of subjects developed a localized granuloma formation, which enables continuous local stimulation of SARS-CoV-2-specific T cells required for induction of long-lasting T cell responses without systemic inflammation. Thus, systemic reactogenicity was largely mild or absent. No CoVac-1-related SAE or grade 4 AE were reported.

The immunogenicity endpoint was reached, as shown through the evaluation of T-cell response to the six SARS-CoV-2 HLA-DR CoVac-1 T-cell epitopes with ELISPOT assays performed at baseline (day 1), on day 7, day 14, and day 28 after CoVac-1 application. Overall, 86% of patients exhibited induction of SARS-CoV-2-specific T-cell responses targeting multiple CoVac-1 peptides, with a 32‑fold increase from baseline. CoVac-1-induced CD4⁺ T cells expressed IFN-g, tumor necrosis factor (TNF), interleukin-2 (IL-2), and CD107a, thus displaying a T-helper 1 (Th1) phenotype.

Noteworthy observations related to seroconversion and antibody production emerged. Low-level induction of SARS-CoV-2 anti-spike IgG antibodies was detected in a subset of patients (n = 8) on day 28.

Of note, the magnitude of CoVac-1-induced T-cell responses exceeded that of spike-specific T-cell responses following mRNA vaccine in B-cell deficient patients. Furthermore, pre-existing spike-specific T-cell responses after mRNA vaccination were boosted by CoVac-1 and extended to further SARS-CoV-2 peptides.

 These responses remained effective against prevailing Omicron variants that had no impact on the selected peptides, emphasizing the robustness of CoVac-1.

Comparisons between CoVac-1-induced T-cell responses in B-cell deficient patients and T-cell responses in immunocompetent SARS-CoV-2 convalescent individuals with asymptomatic or mild disease yielded compelling insights, with the intensity of CoVac-1-induced IFN-γ T-cell responses at day 28 resembling or even exceeding the one observed in healthy controls after infection. This pattern persisted when considering immunocompetent individuals who did not develop a humoral anti-spike IgG response after infection. These findings suggest that CoVac-1 induced T-cell responses could prevent B-cell deficient patients from developing severe COVID-19 manifestations.

In summary, this trial underscores the remarkable ability of the T-cell activator to elicit potent T-cell responses against SARS-CoV-2 even in highly immunocompromised individuals. This work provides proof-of-concept of the peptide-based T cell activator technology which is currently developed for many other infectious diseases that threaten immunocompromised tumor patients. In addition, this study is an important contribution to the development of therapeutic peptide-based vaccines for cancer patients, which we are currently evaluating in patients with various solid tumors and blood cancers.

Figure 1. Schematic overview of trial concept and design, safety, and immunogenicity outcomes. The peptide-based T-cell activator CoVac-1 was designed to address the lack of protection against SARS-CoV-2 in immunocompromised patients, who cannot develop a humoral immunity after vaccination or infection, due to congenital or acquired B-cell deficiency. These patients (n = 54) were included in the Phase I/II trial and received one single dose of CoVac-1 subcutaneously in the lower abdomen (top left panel). Safety and efficacy were assessed until day +56 and +28, respectively. After the administration of CoVac-1, 94% of patients developed the expected and intended granuloma at vaccination site. This served as a depot for the continuous priming of T cell, based on the persistence of peptides and the local infiltration of immune cells (bottom left panel). Systemic reactogenicity was only documented in a minority of study subjects. The immunogenicity endpoint was reached and a robust induction of SARS-CoV-2 specific T cells directed to multiple CoVac-1 peptides was observed in 86% of patients (bottom right panel).