5. T cell and B cell epitopes as target vaccine candidates for COVID-19
Humoral immunity by B-cell antibodies and cellular immunity by T-cells are important for effective vaccines (Rappuoli et al., 2019). On one hand, VN antibodies block the entry of viruses into human cells and on the other hand, cytotoxic CD8 T-cells and helper CD4 T-cells fully clear viruses from the infected cells (Fast and Chen, 2020). Researchers from the Hong Kong University of Science and Technology (HKUST) investigated the B-cell and T-cell S and N protein epitopes arising in response to SARS-CoV to find biomarkers that could be incorporated into vaccines to trigger an immune response against SARS-CoV-2 (Balfour, 2020). Similar to SARS-CoV, the SARS-CoV-2 S protein is likely to be immunogenic which carries several T-cell and B-cell epitopes (Fast and Chen 2020). Since 23% and 16% of known SARS-CoV T-cell and B-cell epitopes mapped identically with that of SARS-CoV-2, respectively, and no mutations have been observed in these SARS-CoV-2 epitopes, harnessing the vaccine candidate value of these epitopes may offers a significant protection against COVID-19 (Ahmed et al., 2020). Also, the high genetic similarity between SARS-CoV-2 and SARS-CoV indicates that the vaccines developed for SARS-CoV may exhibit cross-reactivity with SARS-CoV-2 (Jiang et al., 2020; Dhama et al., 2020). Profound antibody response has been generated against S protein in mouse models (Deming et al., 2006; Graham et al., 2012) and N protein in SARS-CoV infected patients (Liu et al., 2004). Though being effective, the antibody response developed in convalescent SARS patients was found to be short-lived in nature (Tang et al., 2011).
However, compared to this humoral response T-cell response against the structural proteins of SARS-CoV was found to be more dominant and provide long-term protection which makes the T-cell epitopes more interesting prospective vaccine candidate against SARS-CoV infections (Tang et al., 2011; Ng et al., 2016; Liu et al., 2017). The potential of cross-reactivity among CoVs was confirmed in a study based on similarity in the T-cell epitopes of SARS- and MERS-CoVs which expounds the possibility of broad spectrum universal CoV vaccine (Liu et al., 2017). However, Ahmed et al., (2020) argues that the prospects of T-cell epitopes in vaccine production against COVID-19 is more promising compared to B-cell epitopes. It is because of higher percentage of SARS-CoV derived T-cell epitopes that map identically with that of SARS-CoV-2, larger population expected to be covered, and long term protection conferred (Tang et al., 2011; Ng et al., 2016). Furthermore, SARS-CoV derived antibodies targeting the RBD of S1 subunit of SARS-CoV-2 S protein may not be effective due to the large genetic mismatches observed in known structural epitopes targeting this domain (Wrapp et al., 2020; Tian et al., 2020; Ahmed et al., 2020).