1. Introduction
The successful containment of SARS-CoV in 2003 led to the establishment of a global collaboration system for preventing and controlling infectious diseases (Ksiazek et al., 2003, Rota et al., 2003). During this period, extensive research was carried out, covering various aspects such as epidemiology, pathophysiology, immunology, vaccine development, and structural studies of SARS-CoV. These studies significantly contributed to our understanding of SARS-CoV and the broader coronavirus family, while also advancing methodologies for virus research.
Almost a decade after the emergence of SARS, a new coronavirus known as Middle East respiratory syndrome coronavirus (MERS-CoV) was discovered in the Middle East (Zaki et al., 2012). This virus caused an outbreak of acute respiratory illness in a public hospital in Zarqa, Jordan, in April 2012 (Hijawi et al., 2013). Imported cases have emerged in various countries such as France, Germany, Italy, Tunisia, the United States, and the United Kingdom (Su et al., 2016, Wong et al., 2015). In 2015, a MERS-CoV outbreak occurred in South Korea, resulting in 36 deaths out of 186 confirmed cases (http://www.who.int/emergencies/mers-cov/en/). One patient who traveled from South Korea to China marked the first case of MERS-CoV in China (Su et al., 2015, Wang et al., 2015).
The clinical features of patients infected by MERS-CoV are very similar to SARS, i.e., severe pneumonia (Feng and Gao, 2007, Zaki et al., 2012). SARS-CoV-like viruses have been discovered in insectivorous Rhinolophid bats, and viruses genetically related to MERS-CoV have also been detected in Neoromicia capensis bats from Africa (Corman et al., 2014, Lau et al., 2013, Li et al., 2005b, To et al., 2013). Recently, the bat origins of MERS-CoV were further supported by evidence that bat coronavirus HKU4 also uses the human MERS-CoV receptor CD26 for virus entry (Wang et al., 2014). Additionally, MERS-CoV-like viruses are also widespread in dromedary camels, with sero-epidemiological studies indicating 90% seroprevalence in adult animals (Reusken et al., 2014).
A pivotal role for virus-specific memory T-cells in broad and long-term protection against SARS-CoV infection has been elucidated (Channappanavar et al., 2014, Zhao et al., 2010a). Indeed, the crucial protective role of T-cell immune responses in coronavirus infections has been clearly documented in several animal models, e.g., feline infectious peritonitis virus (FIPV), mouse hepatitis virus (MHV), and avian infectious bronchitis virus (IBV) (Li et al., 2016, Takano et al., 2014, Trujillo et al., 2014). These animal models provide useful references for the study of human infection by SARS-CoV or MERS-CoV. Infection of mice with MHV, a member of the same betacoronavirus group as SARS-CoV and MERS-CoV, was used as an early experimental model to elucidate the role of T-cells in viral clearance and cytotoxicity (Le Prevost et al., 1975) and in T-cell oriented vaccine development (MacNamara et al., 2008, Zust et al., 2007). A primary role for cytotoxic T lymphocytes (CTL) has been demonstrated in protection from MHV and virus clearance (Stohlman et al., 1995).
In this review, we focus on lessons learned from T-cell immunological studies of SARS-CoV, including the immunogenicity of SARS-CoV structural proteins, T-cell epitopes identified, T-cell-oriented vaccine development, and the structural immunology of SARS-CoV based on human leucocyte antigen class I (HLA I)/peptide structures (Hilgenfeld and Peiris, 2013). Based on the knowledge of the T-cell immunity of SARS and recent studies on MERS, the immune correlation and potential T-cell cross-reactivity between SARS-CoV and MERS-CoV are evaluated, which may have implications for vaccine development against human coronavirus infections.
