正式論文：
https://www.nature.com/articles/s41467-021-27611-y
https://imgur.com/kiGiKgr
大意與原理： 研究人員表示，他們發現鯊魚特有的一種蛋白質能夠中和 COVID-19
病毒及其變種。“這些微型的抗體蛋白可以進入人類抗體無法進入的角落和縫隙。
這種稱為 VNAR 的蛋白質是由鯊魚免疫系統自然產生的。它們的大小大約是人類抗體的十
分之一，這使得它們足夠小和靈活，可以邊緣化地與傳染性病原體產生的蛋白質結合併停
止它們的功能。
在數十億個 VNAR 中，他們發現了三個抗體蛋白對阻止病毒感染人類細胞特別有效。
The COVID-19 pandemic caused by the severe acute respiratory syndrome
coronavirus 2 (SARS-CoV-2) has resulted in a devastating global health
crisis. Though vaccines are the centrepiece for controlling the pandemic, the
benefits of vaccines depend upon complex population vaccination strategies
that remain vulnerable to manufacturing or deployment delays. The widely
implemented two-dose requirement to achieve efficacy, leaves the possibility
of non-compliance for the second dose, a situation that may be exacerbated
further by the decision in certain areas to extend the time interval between
dosing. The rapid evolution of SARS-CoV-2 into highly infectious variants
across the globe also has the potential to impact vaccine efficacy.
Researchers have reported that the new SARS-CoV-2 variants can result in
reduced sensitivity to antibody therapies, convalescent plasma, and vaccine
sera1,2,3,4. It has been documented that people with compromised immune
systems respond poorly to COVID-19 vaccines, thus necessitating the
development of additional antiviral therapeutics5,6. As we enter the next key
stage in our global escape plan from this pandemic, it is vital to develop
alternative therapeutic approaches and, concurrently, expand our knowledge of
this virus.
Neutralizing antibody (NAb) therapeutics that block virus entry into the host
cell have demonstrated efficacy at treating COVID-19 infection. Two NAb
therapeutics (LY3819253 and REGN-COV2) received emergency use authorization
status from the Food and Drug Administration for use in the clinic7.
SARS-CoV-2 NAbs target the trimeric spike (S) protein on the viral surface
that mediates cell entry. The S protein has two distinct functional subunits
that facilitate cell attachment (S1) and fusion of the viral and host cell
membranes (S2). The receptor-binding domain (RBD) on the S1 subunit is
responsible for engaging angiotensin-converting enzyme 2 (ACE2)—the cognate
receptor required for membrane fusion. The RBD exists in two different
conformations; the closed “down” conformation and the open “up”
conformation which is highly accessible to ACE2. Studies with NAbs that
target the RBD have revealed mechanisms of viral neutralization based on
changes in the “up” and “down” conformations. In general, NAbs act by
blocking the ACE2 binding interface or by trapping the RBD in the unstable “
up” conformation. Complicating the development of effective NAbs is the
emergence of new SARS-CoV-2 variants with highly mutated S proteins. Studies
have shown that mutational changes in the RBD observed in the variants
correspond to surface-exposed residues within or proximal to the ACE2 binding
interface. These mutations can result in the modification of NAb epitopes
leading to attenuated or abrogated neutralization of the virus by antibodies.
Thus, there is a need for NAbs that can recognize cryptic epitopes
inaccessible to human antibodies that are impervious to mutational drift.
Variable New Antigen Receptors (VNARs) represent an unexplored technology for
the development of next-generation NAbs for SARS-CoV-2. VNARs are the
smallest (~11Da) naturally occurring independent heavy chain-only binding
domains in the vertebrate kingdom. Part of the adaptive immune system of
sharks, VNARs are evolutionarily distinct from immunoglobulins despite
sharing some structural similarity with mammalian heavy and light variable
chains. VNARs further differentiate themselves from classical antibodies and
single-domain camelid antibodies by lacking a CDR2, but possess the benefits
of two additional hypervariable loops (HV2 and HV4), yielding a total of four
loops of diversity into their small and simple domain architecture8. With
characteristic protruding paratopes, the VNARs are pre-disposed to access and
bind epitopes not normally available to the planar binding sites of classical
human antibodies. This feature allows for the identification of highly potent
binders that reach deep into pockets and grooves within the target
antigen9,10,11. Furthermore, their amenability to reformatting,
cost-effective expression at scale in non-mammalian systems, and exceptional
stability in diverse formulation conditions, makes VNARs ideal for clinical
translation12,13.
Here, we describe the identification and characterization of three monomeric
antiviral VNARs (3B4, 2C02, 4C10) that were found to be potent neutralizers
of pseudotype and authentic SARS-CoV-2 virus. These VNARs were uniquely
potent as monovalent constructs, with efficacies rivalling multimeric
variable-heavy-heavy (VHH) camelid antibodies and Fc-bound bivalent
constructs, including conventional full-length immunoglobulins. The crystal
structures of VNARs 3B4 and 2C02 showed markedly different mechanisms of
neutralization and underscored the likely resilience of VNARs to SARS-CoV-2
variants. Additionally, the VNARs showed neutralization capabilities against
other beta coronaviruses supporting the potential broad therapeutic
application of these VNARs and the VNAR platform against both known diseases
and future emergent disease. Together the three VNARs described here
reinforce the need for the continued expansion of the single-domain
heavy-chain only antibody-like drug class.