3  Coronaviruses
Coronaviruses are a group of related viruses in the family Coronaviridae and subfamily Coronavirinae, order Nidovirales. In the subfamily Coronavirinae, coronaviruses include 4 genres: alphacoronavirus, betacoronavirus, gammacoronavirus and deltacoronavirus (Fig. 1 ). Coronaviruses infect a wide variety of hosts including many species of birds, mammals and humans [7]. Alphacoronaviruses and betacoronaviruses circulate among mammals, gammacoronaviruses and deltacoronaviruses infect birds and mammals. Within betacoronaviruses, there are 4 lineages: lineage A contains human coronaviruses HKU1 and OC43, lineage B to which SARS-CoV (Severe Acute Respiratory Syndrome Coronavirus) belongs, lineage C belongs to MERS-CoV (Middle East Respiratory Coronavirus Syndrome) and the lineage D has the bat coronaviruses HKU4 and HKU5 who are close to MERS-CoV.
Fig. 1 Isolates of coronaviruses discussed in this review and their receptors, host and reservoirs. SARS-CoV and SARS-CoV-2 from the lineage B use ACE2 as receptor. MERS-CoV from the lineage C enters into host cells by binding DPP4. SARS-CoV has the masked palm civets as an intermediate host in which the virus has adapted from the Chinese Horseshoe bat reservoir to ultimately infect humans [13]. SARS-CoV-2 has bats and pangolins as natural reservoir and can infect ferrets or domestics animals, with a high susceptibility in cats [14]. MERS-CoV has the origin from bats [15] but maybe this virus had an adaptation through camels before its emergence into human [16].
The first human coronaviruses 229E (HCoV-229E) and OC43 (HCoV-OC43) were isolated in the 1960s and are now classified respectively as alphacoronaviruses and betacoronaviruses. As these viruses were not very pathogenic and often associated with colds [8], this family of viruses attracted little interest from scientists until the 2000s. In November 2002, SARS-CoV, first reported in Guangdong province, China, became the first highly pathogenic coronavirus that emerged in the human population. This virus was responsible for an epidemic of severe acute respiratory syndromes that started in China before spreading rapidly over the world with around 8000 infected people and with a mortality rate of around 10%, depending on patients' age [9]. However, this coronavirus from animal origin was initially unable to use the human angiotensin 2 converting enzyme (ACE2) as receptor [[10], [11], [12]]. It has been suggested that the masked palm civet (Paguma larvata) may be an intermediate host in which the viruses have adapted to ultimately infect humans. A recent study suggests that Chinese Horseshoe bats in the family Rhinolophidae may be the natural reservoir for SARS-CoV. One of these two viral isolates in this study, WIV1, was able to recognize the human ACE2 receptor and to replicate in certain human cell lines, suggesting that this virus can directly infect humans without adaptation [13].