Fast isolation of sub-nanomolar affinity alpaca nanobody against the Spike RBD of SARS-CoV-2 by combining bacterial display and a simple single-step density gradient selection

Despite the worldwide efforts to avoid disease progression of COVID-19 into a severe acute respiratory syndrome and avoid its severe impact on health systems; the situation remains critical. Effective diagnosis, treatment, and prophylactic measures are required to meet the worldwide demand: recombinant antibodies such as alpaca Nanobodies fulfill these requirements. Here, we develop a fast track for nanobody isolation against the receptor-binding-domain (RBD) SARS-CoV-2 Spike protein following an optimized immunization, efficient construction of the VHH library for E. coli surface display, and single-step selection of high-affinity nanobodies using a simple density gradient centrifugation of the bacterial library. Following this procedure, we isolate and characterize an alpaca Nanobody against Spike RBD of SARS-CoV-2 in the sub-nanomolar range.