In full social distancing crisis, far from lab work, in looking anxiously at the red dots growing every day in the graphic map of coronavirus disease 2019 pandemic [1], we wonder how were the other major pandemics and how antibodies, natural or artificial, can fight the diseases.
Several pandemics have occurred throughout history, some had more effects on human's life and/or on economics than the others. As scientists, a key step in vaccines research is to learn from the past. By that mean, what we are doing now might be cornerstone preparing us for future pandemics.
In this review, we look for the diversity and variability of coronaviruses, including SARS-CoV, MERS-CoV and SARS-CoV-2, which cause pandemics from 21 th century. The origins, impacts and molecular structure with insights of vulnerable sites of viruses who are targets of antibodies and its humoral responses will be described.
In response to pathogens including viruses, human body has evolved its immune system to protect it from invasion. Following the virus invasion, antibodies (Abs) are produced after a series of immune signaling and these Abs are able to recognize a diverse array of antigens (Ag) [2] . More specifically, the paratopes (Ag-binding sites) of Abs bind epitopes on virion-associated Ags. During an Ab response, B-cells which express Ag receptors are clonally expanded [3] . Antibodies structurally are composed of heavy (µ, α, γ, δ, ε) chains that are linked by disulfide bonds with light chains (κ, λ).
J o u r n a l P r e -p r o o f 5 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 [ 
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] .
SARS-CoV, first reported in 2002, belongs to the SARS-related coronavirus species that also includes many bat viruses.
Coronaviruses are spherical enveloped viruses with a diameter of 80 to 120 nm [17] .
The viral capsid formed by the nucleoprotein (N) and the genome is contained in the envelope and is of helical symmetry. Three structural proteins are embedded on the surface of particles, the membrane protein (M), the envelope protein (E) and the protein spike (S). They give this aspect of crown in electron microscopy that inspired the name of this viral family.
The S protein of coronaviruses (~1255 amino acids) is a highly N-glycosylated type I transmembrane protein, from 180 to 200 kDa, that plays a major role in viral entry [18] . It insures a double function in viral entry by binding the cellular receptor before conformational changes and proceeding to the fusion of the viral envelope with the membranes of the target cells. S protein has a long N-terminal domain, a short Cterminal domain and assembles into homotrimers on the surface of the viral particle [19] . S protein has a decisive role in cellular tropism and for pathogenicity [20] .
J o u r n a l P r e -p r o o f 9 of RBD but did not inhibit RBD binding. 256 is weak but specific to D480A-muted strains of 80R-escape variants. Some engineered broad nAbs, fm6 and fm39, also showed a high affinity to D480A-muted strains [33] . m396 (epitope aa 482-491) from human antibody fab library was cross-reactive [35] and used the D95 of m396 to form a salt bridge with R395 or an electrostatic interaction with D408 of SARS-CoV RBD [34] . m396 potently neutralized GD03 strain isolated from the second outbreak which resisted neutralization by 80R and S3.1. m396 also neutralized isolates from the first SARS-CoV outbreak (Urbani, Tor2) and from palm civets (SZ3, SZ16) [36] . Another By using Xenomouse in which mouse immunoglobulin genes were replaced by human immunoglobulin genes, 19 neutralizing mAbs bound S1 were found. 18 of them, 1B5 [38] , 3A7, 3C7, 3F3, 3H12, 4A10, 4E2, 4G2, 5A5, 5A7, 5D3, 5D6, 5E4, 6B1, 6B5, 6B8, 6C1 and 6C2 bound RBD (aa 318-510) to avoid virus binding to the ACE2 receptor. The last one, 4D4, bound an epitope (aa 12-261) located on the Nterminal of RBD and inhibited post-binding event but not the RBD binding. Truncation of the first 300 amino acids of S1 blocked the trimerization and the fusion of S protein [39] . Synergistic effects in some SARS-CoV strains of 4D4 with other mAbs targeting S1 or S2 proteins such as 3C7 (S1), 1F8 (HR1) or 5E9 (HR2) were also reported [40] J o u r n a l P r e -p r o o f F26 family of monoclonal Abs generated from mice (F26G9, F26G10, F26G18 and F26G19) showed neutralizing effect against SARS-CoV [42]. F26G18 binding RBD at the epitope aa 460-476 showed the most potent effect [43] . F26G19 (epitope aa 486-492 on RBD [44] ) or 80R could also bind SARS-CoV by forming salt bridge R426 (RBD)-D56 or D480 (RBD)-R162, respectively [34] .
SARS-CoV mouse antibody 240CD had a nanomolar affinity for the SARS-CoV-2 RBD but did not significantly block ACE-2 receptor binding [45] . As 240CD, CR3022 also has high affinity to SARS-CoV-2 and moreover, CR3022 had cross-neutralizing activity with this novel coronavirus [34] .
The effects of neutralizing human monoclonal antibodies, S3.1, S215.13 [46] and S230.15, from Epstein-Bar virus transformation of human B cells were observed. As m396, S230.15 had potent inhibitory activity against isolates from the first, second SARS-CoV outbreaks and from palm civets (SZ3, SZ16) [36] .
In contrast to RBD, the fusion domains are more difficult to access due to the tight folding of viral glycoproteins or the excessively transient exposure during the fusion stage. This is why few epitopes are described in these regions [6] . Interestingly, the S2 specific mAbs can neutralize pseudotyped viruses which expressing different S proteins containing RBD sequences of various clinical isolates [47] . The S2 protein is highly conserved. No mutation in HR1 was reported in an analysis of the amino acid sequences of the S protein from 94 SARS-CoV clinical isolates. Only few mutations in HR2, at amino acids K1163 or Q1183 for example, were observed in this study [47] . Some S2 epitopes inducing nAbs were reported. A peptide containing aa 1055-1192 can elicit neutralizing activity [48] . Two other proteins Trx-F3 and Trx-F9 containing linear antigenic determinants (Leu 803 to Ala 828 and Pro 1061 to Ser 1093, respectively) on the S2 domain were identified by using sera from convalescent J o u r n a l P r e -p r o o f 6H1) were reported. With these Abs, the authors showed that the combination of HmAbs targeting different regions of the S protein would likely increase the broad neutralization against different isolates [47] .
A human scFv antibody, named B1, showed a high affinity to an epitope (aa 1023-1189) on S2 protein. This antibody also showed potent neutralizing activities against SARS-CoV in vitro [50] . B1, 1F8 and 5E9 nAbs against epitopes on SARS-CoV S2 also showed effectiveness in neutralization [51] .
The protective immunity by the time in patients after SARS-CoV natural infection was observed. After 6 years, the humoral immunity continuously decreased and eventually disappeared in most infected individuals. The IgG Ab could be an indicator of neutralizing Ab for the humoral response to SARS-CoV infection [52] . [74]. SARS-CoV-2 seems to be more contagious but less pathogenic than SARS-CoV [75] . COVID-19 is a self-limiting disease in more than 80% of patients. Same as Spanish influenza viruses, SARS-CoV and SARS-CoV-2 induce a "cytokine storm" but to different degrees. The difference of some conserved interferon antagonists and of inflammasome activators explains their abilities to modulate antiviral and proinflammatory responses.
Along with the race of finding therapeutic treatment, nAbs and vaccine development are also important to control the spread in the long run. SARS-CoV-2 entries the host via the binding of its spike S protein to the ACE2 receptor -sharing receptor, but with higher affinity than SARS-CoV S [76] , suggesting a basis for the greater human-tohuman transmission of SARS-CoV-2 [51] [77] .
S protein of SARS-CoV-2 composed of 1273 amino acids [76] uses its N-terminal S1 subunit to bind ACE2 receptor with a better affinity than SARS-CoV S glycoprotein for entry [78] . Effectively, S1 subunit divides into an N-terminal domain (NTD) and a receptor-binding domain (RBD). The latter is necessary for viral binding and a potential target for nAbs. During infection, SARS-CoV-2 first binds the host cell through interaction between its S1-RBD and ACE2, triggering conformational changes in the S2 subunit that is indispensable for virus fusion and entry into the target cell [79] [80] . Some recent studies also confirmed that RBD is a conformational epitope [78] . Antibodies binding RBD may sterically hinder binding to the nearby peptide S14P5 of ACE2 receptor, thereby abolishing virus infection [34] .
J o u r n a l P r e -p r o o f SARS-CoV-2 nAbs could be detected in patients from 10-15 days after symptoms onset and the positive rate for IgG reached close to 100% around 20 days [81] [82] with the highest level during day 31-40 since onset. Some patients (5.7%) had neutralizing Abs titers under the detectable level (ID50: <40) [83] . The level of IgG antibodies was different between gender, age and clinical classification. The average IgG antibody level in female patients was higher than in male patients [84] . Patient over 40 years old developed higher levels of SARS-CoV-2 specific nAbs than the younger persons. Patients with a worse clinical classification had a higher antibody titer [83] . This remark is useful to select a research candidate and to save research time. The passive antibody therapy, such as plasma fusion containing polyclonal antibodies from COVID-19 neutralized patients has been tested. This method was tested as an option to treat other viruses such as influenza, Ebola or SARS-CoV [85] [86] [87] [88] [89] . The lack of human sera, and the possibility of contamination with other infectious agents limit this strategy. However, several groups have reported some positive results demonstrating the potential of this approach. After one dose of 200 mL of convalescent plasma (CP) derived from recently recovered donors with the neutralizing antibody titers above 1:640, the patients with SARS-CoV-2 positive revealed an improvement. Among ten patients, seven patients were virus-negative post transfusion [90] . Whereas in another study, among 5 patients received transfusion with convalescent plasma with a neutralization titer greater than 40, 3 have been discharged from the hospital (length of stay: 53, 51, and 55 days), and 2 are in stable condition at 37 days after transfusion [91] . More studies might brighter this approach but evaluation in clinical trials are also still far from a bold conclusion.
J o u r n a l P r e -p r o o f suggested that S51 may be a potential pan-coronavirus epitope. Sera from recalled SARS-CoV patients could neutralize SARS-CoV, but not the SARS-CoV-2 pseudotyped lentiviruses [78] .
In an effort to screen a set of B cell and T cell epitopes of SARS-CoV toward to the were predicted to be associated with a robust immune response to SARS-CoV-2 [30] .
Other candidate epitopes need to be confirmed [95] [108] .
Using the memory B cells from a survivor who was SARS-CoV infected in 2003, one nAb anti-RBD named S309 was found to bind to SARS-CoV-2 without interfering ACE2 binding. Besides, S309 could recognize a N343-glycan epitope that is distant from the RBM of SARS-CoV-2. Interestingly, N343-glycan of SARS-CoV-2 corresponds to SARS-CoV N330 and they are highly conserved. S309 potently neutralized both pseudotyped SARS-CoV and SARS-CoV-2 and also the authentic SARS-CoV-2 [109] .
Using machine learning approaches with the data from other virus outbreaks, some synthetic nAbs named C3, C7, C14, C17, C18, Co1, Co2 and Co4 showed a potential to against SARS-CoV-2. The authors also confirmed that the mutations of Methionine and Tyrosine could increase the affinity of antibody-target binding [110] .
19 potential immunogenicity B-cell epitopes, including 2 epitopes located within the RBD region were reported using in silico analysis. 17 of them have more than 14 amino acids. The B-cell epitopes which had highest score in this study is the 1052- A panel of five humanized single domain antibodies (sdAbs) or nanobodies, 1E2, 2F2, 3F11, 4D8 and 5F8, was recently discovered. These sdAbs bound SARS-CoV-2 tightly but not SARS-CoV, except for 5F8 could bind both viruses but with weaker affinity to SARS-CoV. They also showed neutralization activity against both pseudotyped and authentic SARS-CoV-2. 1E2, 3F11 and 4D8 completely prevented SARS-CoV-2 RBD-ACE2 binding but this effect of 2F2 and 5F8 was only partial.
Interestingly, the fusion of the human IgG1 Fc to these sdAbs improved their neutralization activity by 10-to 80-fold [112] .
Due to the lack of repairing mechanism of RNA virus replicase complex, SARS-CoV-2 mutations frequently occur during viral replication [111] . The genetic drifts of SARS-CoV-2 is a selective evolution toward less immunogenicity for host immune surveillance by T-or B-cells. The latter appearing strains are less immunogenic than earlier ones [113] . Antigenic drift is also reported in the COVID-19 pandemic. The highly prevalent 23403A>G (p.D614G) variant in the European population may result in vaccine mismatches with little protection to that group of patients [114] [115] .
Though SARS-COV-2 genome has a much lower mutation rate and genetic diversity than SARS, some of its mutations attract the special attention of scientists. Single amino acid mutation R408I in RBD can reduce the affinity of ACE2 receptor binding [115] that leads to a low or ineffective vaccine for the future epidemic. Effectively, sequence alignment showed that this 408R is strictly conserved in SARS-CoV-2, SARS-CoV. 408R located at the interface between RBD and ACE2, but positioned relatively far away from ACE2, does not have direct interaction with ACE2. 408R can form a hydrogen bond with the 90N of ACE2. This hydrogen bond is suggested to contribute to the high binding affinity of ACE2 binding [115] . [120] , but the potential duration of immunity response is not clear. Therefore, the use of singe-dose or several doses of vaccines needs to be confirmed. Moreover, once a vaccine has been approved, that does not mean that virus research and monitoring can stop.
Indeed, influenza and HIV viruses have been reported to have high mutations, making it difficult to find broadly neutralizing vaccines. In the actual pandemic of SARS-CoV-2, drift variants have been reported and that can affect COVID-19 vaccine development [114] [115] . No vaccine is available against any coronavirus [121] . Monoclonal antibodies cocktails including multiple epitopes targeting Abs could be taken into account to broaden spectrum of therapy.
Vaccine development is a long and expensive process. From identifying a virus to producing vaccines to market, it takes us a few years. If pandemic gets end before vaccines are approved, the research of vaccine candidates under development need to be continued and ready for clinical trials, in order to get emergency authorization J o u r n a l P r e -p r o o f 20 when an outbreak recurs. This statement draws on experience from Ebola pandemic in which vaccine was still under development when the Ebola outbreak ended in 2016. Ebola vaccine is recently approved [122] [123] [124] and already used in the recent outbreak in the Democratic Republic of Congo [125] .
Studying and understanding the antigen-antibody mechanism of a virus can be used as a precondition to accelerate the studying another virus during an outbreak. In the COVID-19 pandemic, some nAbs studies were based on the research of previous viruses such as SARS, MERS, Ebola and HIV. Through machine learning approaches with the data composed of HIV gp41-antibodies complexes and of 13 more different virus types, some potential nAbs against SARS-CoV-19 were found [110] . This case shows the usefulness of this review of host-antibody interactions from coronaviruses pandemics for young or mature scientists working on vaccine research.
The key findings in coronavirus antibodies investigation could reveal S protein and its subunits as major frame for antibody generation in which RBD seems to be the most efficient peptides. A lot of effort has been tried but still plenty of gaps to fill up. Other approaches and therapies are also needed to protect us from coronavirus infection. 