Peripheral mononuclear blood cells (PBMCs) are the most widely used study materials for immunomonitoring and antigen-specific T-cell identification. However, limited patient PBMCs and low-frequency antigen-specific T cells remain as significant technical challenges. To address these limitations, we established a novel platform comprised of optimized HLA-matched immortalized B cells transfected with mRNA of a prototype viral or tumor antigen conjugated to MHC class-I trafficking domain protein (MITD) to increase the efficiency of epitope expression in antigen-presenting cells (APCs) essential to expanding antigen-specific T cells. When applied to CMV as a model, the IBMAM platform could successfully expand CMV-specific T cells from low-frequency CMV PBMCs from seropositive donors. Additionally, this platform can be applied to the validation of antigen specific TCRs. Together, compared to using APCs with synthesized peptides, this platform is an unlimited, highly efficient, and cost-effective resource in detecting and expanding antigen-specific T cells and validating antigen-specific TCRs.

Neutralizing antibodies are established correlates of protection against SARS-CoV-2, yet T-cell responses are also thought to contribute substantially to limiting disease severity and enhancing durability of protection. To examine whether cellular immunity alone can confer protection, we engineered DNA vaccines encoding modified Spike proteins, including C-terminal truncation (SARS-CoV-2-ΔC, ΔC) and cleavage-site–deleted, linker-inserted (SARS-CoV-2-Linker-ΔT, Linker-ΔT) variants, with or without genetic fusion to MIP3α, which has been shown to enhance targeting of antigen-presenting cells (APC) and preferentially induce T-cell responses. In BALB/c mice, ΔC constructs induced non-neutralizing Spike- and RBD-binding antibodies across variants, as well as robust CD4 + and CD8 + T cell responses, whereas Linker-ΔT elicited strong Th1-skewed cellular immunity in the absence of humoral responses. In K18-hACE2 mice antibody neutralizing activity was not detected by any of the vaccines, and none conferred protection following lethal virus challenge, despite robust specific T-cell cytokine responses. These findings support chemokine-fusion strategies to enhance T cell priming, while indicating that the cellular immunity elicited by this vaccine alone does not confer protection against SARS-CoV-2. Integrating such APC-targeting strategies with structural modifications that preserve pre-fusion neutralizing epitopes may be worthwhile.

Neutralizing antibodies are established correlates of protection against SARS-CoV-2, yet T-cell responses are also thought to contribute substantially to limiting disease severity and enhancing durability of protection. To examine whether cellular immunity alone can confer protection, we engineered DNA vaccines encoding modified Spike proteins, including C-terminal truncation (SARS-CoV-2-ΔC, ΔC) and cleavage-site–deleted, linker-inserted (SARS-CoV-2-Linker-ΔT, Linker-ΔT) variants, with or without genetic fusion to MIP3α, which has been shown to enhance targeting of antigen-presenting cells (APC) and preferentially induce T-cell responses. In BALB/c mice, ΔC constructs induced non-neutralizing Spike- and RBD-binding antibodies across variants, as well as robust CD4 + and CD8 + T cell responses, whereas Linker-ΔT elicited strong Th1-skewed cellular immunity in the absence of humoral responses. In K18-hACE2 mice antibody neutralizing activity was not detected by any of the vaccines, and none conferred protection following lethal virus challenge, despite robust specific T-cell cytokine responses. These results support vaccine designs incorporating chemokine fusion to enhance T-cell priming, but cellular responses alone are insufficient for SARS-CoV-2 protection. Integrating such APC-targeting strategies with structural modifications that preserve pre-fusion neutralizing epitopes may be worthwhile.