We discuss the possible applications of these antibodies in the global fight against COVID-19. The highly contagious novel coronavirus, SARS-CoV-2, continues to infect tens of millions worldwide, with the estimated number of deaths from COVID-19 fast approaching two million. is studded with ~40 copies of a trimeric club-shaped glycoprotein called Spike [2,3]. Spikes RBD promotes virus attachment to host cells via angiotensin-converting enzyme 2 (ACE2) [4,5]. From what is known about other coronaviruses, the SARS-CoV-2 Spike undergoes a conformation change following ACE2 binding that results in membrane fusion and delivery of the virus and its genomic contents into the cytoplasm of host cells, where replication ensues [6]. SARS-CoV-2 infection of the airways triggers an inflammatory cytokine storm that can progress to life-threatening acute respiratory distress syndrome (ARDS) [7,8]. The devastating impact of the SARS-CoV-2 pandemic has mobilized efforts around the globe to develop drugs, vaccines, and other intervention strategies to arrest the novel coronavirus in its tracks and stop the spread of COVID-19. Of particular interest is a series of recent reports describing camelid-derived and synthetic sdAbs with potent (nanomolar) and even ultrapotent (picomolar) SARS-CoV-2-neutralizing activities [9., 10., 11., 12., 13.]. X-ray crystallography and cryogenic electron microscopy (cryo-EM) has revealed that the most potent sdAbs target a common region on the RBD that blocks ACE2 interactions, thereby preventing SARS-CoV-2 attachment to host cells. DL-Adrenaline Here, we summarize the strategies used to isolate these unique sdAbs and speculate on how to best use them in the fight against SARS-CoV-2. The Versatile World of sdAbs While antibodies come in all shapes and sizes, the conventional arrangement in most mammals is typified by human IgG in which a heavy chain (H) and light chain (L) pair homodimerize to form Rabbit Polyclonal to mGluR7 a Y-shaped molecule (150 kDa) (Figure 1 ). The variable domains of the H (VH) and L (VL) chains of each arm of the ‘Y’ are in close spatial proximity to each other and form a single interface involved in target (antigen) recognition. The VH and VL elements each contain three hypervariable complementarity determining regions (CDRs) that, over the course of an immune response, may undergo multiple rounds of affinity maturation to achieve a ‘best fit’ with a pathogen target [14]. While it is not uncommon that VH elements dominate an antibodyCantigen interaction, rarely does a VH or VL element on its own have sufficient affinity to engage with an antigen. Open in a separate window Figure 1 Structures of Conventional, Heavy-Chain-Only (HCAb), and Single-Domain Antibodies (sdAbs). Conventional IgG antibodies consist of H and L chain pairs that form Y-shaped structures. Camelids (e.g., alpacas, llamas) make a heavy-chain-only (HCAb) class of antibody. The variable elements (VHH) of HCAbs, when expressed as autonomous units, are known as single-domain antibodies (sdAbs) or nanobodies. The crystal structure of a representative sdAb (PDB ID 6OBC) is shown with the complementarity determining regions (CDRs) 1, 2, and 3 colored blue, yellow, and red, respectively. Image created in BioRender. Heavy-chain-only antibodies (HCAbs) are different. First discovered in Arabian camels (using a combination of bioinformatics and structural biology information available in GenBank and the PDB [20]. In either case, once constructed, the power of phage- and yeast-display technologies enables the isolation of rare sdAbs against targets of interest. However, synthetic and na?ve libraries generally yield sdAbs of lower affinities than immune libraries, thereby necessitating affinity maturation and further rounds of screening to obtain sdAbs with low DL-Adrenaline or subnanomolar activities [11,13,21]. Discovery of sdAbs That Target the SARS-CoV-2 Spike Protein Leveraging the strategies outlined in the previous text, there have been a flurry of studies in the past several months describing the discovery, structural analysis, and engineering of ultrapotent sdAbs targeting the SARS-CoV-2 Spike protein [9., 10., 11., 12., 13.]. In fact, according to the coronavirus antibody database (CoV-AbDab), there are now more than 300 SARS-CoV-2-specific sdAbs [22]. Here we summarize DL-Adrenaline strategies used to discover some of the most potent RBD-specific sdAbs and speculate as to their potential to contribute to the fight against COVID-19. The first sdAbs with SARS-CoV-2 neutralizing activity were reported within months of the appearance of the novel coronavirus onto the worlds stage [12]. By chance, Wrapp and colleagues had generated a VHH-phage display library from a llama immunized with the Spike proteins of two related human beta coronaviruses, MERS and SARS-CoV-1 [23]. DL-Adrenaline The Spike proteins of SARS-CoV-1 and SARS-CoV-2 are ~75% similar to each other [24]. One sdAb, called VHH-72, was identified as targeting the RBD of SARS-CoV-1 Spike protein, but reacted with an epitope shared with SARS-CoV-2 RBD. The structure of VHH-72 bound to SARS-CoV-1 RBD revealed its binding site as being outside the ACE2 receptor binding motif (Figure.