One result of virus infection in animals is the production of antibodies by plasma B-cells. Antibodies are immune system proteins responsible for neutralising viruses by binding to them and preventing infection of further cells. Most mammals produce only one form of antibody that is made from a combination of heavy and light protein chains. These form a 'Y'-shaped structure with the heavy chain proteins forming the backbone of the entire 'Y' and the light-chain proteins lining the arms. It is the top of the 'Y'-shape that contains a region which tightly and specifically binds to an area on the antigen. Variations in this region allow for specific binding to a vast range of different antigen structures.
Llamas and alpacas differ, as in addition to the antibody structure described, they also possess a smaller variant (around 40% of the total) known as a single domain antibody . This antibody form still has the ‘Y’ structure though the arms are shorter due to the absence of light-chain proteins - the variable regions are still located along the arms however. Separation and isolation of the arms from the stem yields the nanobody.
The Coronavirus family of viruses causes a wide range of illnesses in animals. These range from the common cold to Middle East Respiratory Syndrome (MERS) and Severe Acute Respiratory Syndrome (SARS) in humans but can also cause diarrhoea, hepatitis, encephalomyelitis and respiratory illnesses in other species. A coronavirus infection causing diarrhoea in young camelid crias has also been identified [28 and 29]. These viruses can be transmitted between animals and people, a process known as zoonosis, so it is significant that wildlife can act as reservoirs of coronaviruses [31 and 32]. SARS-CoV-2, more widely known as COVID-19, is also a member of the Coronavirus group and likely originated from bats, possibly transmitting to humans through contact with pangolins, although this has yet to be proven.
A critical element of the COVID-19 infection process is binding of the viral ‘spike’ glycoprotein to the cell receptor protein (angiotensin-converting enzyme 2 (ACE2)), a process that allows the virus particle to be taken into the cell. A nanobody, raised against the COVID-19 spike protein, strongly binds to it  thus preventing binding to the receptor and blocking cell entry. A variety of physical techniques (eg. Synchrotrons) are being used to determine the structure of nanobodies bound to the SARS-CoV-2 spike protein and determine the exact target area of the antigen.
The nanobodies are obtained by immunising an alpaca or llama with specific fragments of the virus' spike protein. After a number of days, blood is taken from the alpaca and the antibodies extracted. Their small size, solubility and stability make them attractive as candidate therapeutic agents. These properties give them the potential to be used similarly to a convalescent serum, effectively stopping progression of the disease and also developed into an inhaled treatment  since the nanobodies would be deposited directly into the lungs, the primary location of COVID-19 infection.
Very recently, a paper has appeared  demonstrating that a further receptor, Neuropilin-1, which is abundant on the cells of the nasal cavity, significantly potentiates SARS-CoV-2 infectivity. The researchers showed that the effect is blocked by a monoclonal antibody against Neuropilin-1. Speculatively, this may be another suitable target for nanobodies.
Given the severity of the COVID-19 pandemic and the urgent need for innovative therapeutic solutions, there are several international scientific groups now researching this area using both alpacas and llamas as the source of nanobodies. A notable result (23/2/2021) is highlighted in "The Guardian", reporting the development of a electrochemical coronavirus test for Covid-19 using camelid nanobodies. This test is three times faster than lateral flow antigen tests and almost as accurate as PCR assays.
Since the discovery of nanobodies, a large number of review papers have been published describing a wide variety of potential applications in the bio-medical fields, for examples [36, 37, 44, 45 and 48 ]. For full appreciation of the breadth of research, readers are recommended to consult these papers.Back to the top
Most of the literature below can be accessed by clicking on the highlighted link. Some links will access the appropriate web page from which the article can be downloaded but others will immediately start downloading the full reference.
27. Hamers-Casterman, C., Atarhouch, T., Muyldermans, S., Robinson, G., Hamers, C.,Songa, E.B., Bendahman, N., Hamers, R. (1993). Naturally occurring antibodies devoid of light chains. Nature, 363: 446–448.
28. Cebra, C.K., Mattson, D.E., Baker, R.J., Sonn, R.J., Dearing, P.L. (2003). Potential pathogens in feces from unweaned llamas and alpacas with diarrhea. J. Am. Vet. Med. Assoc. 223(12): 1806–1808.
29. Jin, L, Cebra, C.K., Baker, R.J., Mattson, D.E., Cohen, S.A., Alvarado, D.E. and Rohrmann, G.F. (2007). Analysis of the genome sequence of an alpaca coronavirus. Virol., 365(1): 198-203.
30. Vanlandschoot, P., Stortelers, C., Beirnaert, E., Ibañez, L.I., Schepens, B., Depla, E., Saelens, X. (2011). Nanobodies: new ammunition to battle viruses. Antivir. Res., 92: 389-407.
31. Poon, L.L.M., Chu, D.K.W., Chan, K.H., Wong, O.K., Ellis, T.M., Leung, Y.H.C., Lau, S.K.P., Woo, P.C.Y., Suen, K.Y., Yuen, K.Y., Guan, Y. and Peiris, J.S.M. (2005). Identification of a Novel Coronavirus in Bats. J. Virol., 79(4): 2001–2009.
32. Lam, T.T., Jia, N., Zhang, Y. et al. (2020). Identifying SARS-CoV-2-related coronaviruses in Malayan pangolins. Nature, 583: 282–285.
33. Hanke, L., Perez, L.V., Sheward, D.J., Das, H., Schulte, T., Moliner-Morro, A., Corcoran, M., Achour, A., Karlsson Hedestam, G.B. Hällberg, B.M., Murrell, B. and McInerney, G.M. (2020). An alpaca nanobody neutralizes SARS-CoV-2 by blocking receptor interaction. Posted 8th July 2020. doi: https://doi.org/10.1101/2020.06.02.130161.
35. Gai, J., Ma, L., Li, G., Zhu, M., Qiao, P., Li, X., Zhang, H., Zhang, Y., Chen, Y., Gong, R. and Wan, Y. (2020). A potent neutralizing nanobody against SARS-CoV-2 with inhaled delivery potential. BioRxiv Preprint server.
36. Jovčevska, I. and Muyldermans, S. (2020). The Therapeutic Potential of Nanobodies. BioDrugs, 34: 11-26.
37. Steeland, S., Vandenbroucke, R.E. and Libert, C. (2016). Nanobodies as therapeutics: big opportunities for small antibodies. Drug Disc. Today, 21(7): 1076-1113.
38. Lin, J., Gu, Y., Xu, Y., Yu, J., Tang, J., Wu, L., Zhou, Z., Chen, C., Liu, M., Chun, X., Liu, H., Nian, R., Song, H. and Zhang, J. (2020). Characterization and applications of nanobodies against Pseudomonas aeruginosa Exotoxin A selected from single alpaca B cells. Biotech. Biotechnol. Equip., 34(1): 1028–1037.
39. Chames, P. and Rothbauer, U. (2020). Special Issue: Nanobody. Antibodies 2020, 9(1), 6; https://doi.org/10.3390/antib9010006
40. Ji, L., Dong, C., Fan, R. and Qi, S. (2020). A high affinity nanobody against endothelin receptor type B: a new approach to the treatment of melanoma. Mol. Biol. Rep., 47, 2137-2147.
41. Xia, L., Teng, Q., Chen, Q. and Zhang, F. (2020). Preparation and Characterization of Anti-GPC3 Nanobody Against Hepatocellular Carcinoma. Int. J. Nanomed., 15: 2197-2205.
42. Dumoulin, M., Last, A.M., Desmyter, A., Decanniere, K., Canet, D., Larsson, G., Spencer, A., Archer, D.B., Sasse, J., Muyldermans, S., Wyns, L., Redfield, C., Matagne, A., Robinson, C.V. and Dobson, C.M. (2003). A camelid antibody fragment inhibits the formation of amyloid fibrils byhuman lysozyme. Nature, 424: 783-788.
43. He, Y., Ren, Y., Guo, B., Yang, Y., Ji, Y., Zhang, D., Wang, J., Wang, Y. and Wang, H. (2020). Development of a specific nanobody and its application in rapid and selective determination of Salmonella enteritidis in milk. Food Chem., 310, 25 April 2020, 125942.
44. Chames, P. and Rothbauer, U. (2020). Special Issue: Nanobody. Antibodies, 9(1): 6-9.
45. Schumacher, D., Helma, J., Schneider, A.F.L., Leonhardt, H. and Hackenberger, C.P.R. (2018). Nanobodies: Chemical Functionalization Strategies and Intracellular Applications. Angew. Chem. Int. Ed., 57: 2314-2333.
46. Cantuti-Castelvetri, L., Ojha, R., Pedro, L. D., Djannatian, M., Franz, J., Kuivanen, S., van der Meer, F., Kallio, K., Kaya, T., Anastasina, M., Smura, T., Levanov, L., Szirovicza, L., Tobi, A., Kallio-Kokko, H., Österlund, P., Joensuu, M., Meunier, F. A., Butcher, S. J., Winkler, M.S., Mollenhauer, B., Helenius, A., Gokce, O., Teesalu, T., Hepojoki, J., Vapalahti, O., Stadelmann, C., Balistreri, G. and Simons, M. (2020). Neuropilin-1 facilitates SARS-CoV-2 cell entry and infectivity. Sci., 20 Oct.
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