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).