Technology testing of SARS-CoV-2 antibodies

In the COVID-19 pandemic, tests for viral ribonucleic acid (RNA) or viral particles are used to detect and isolate infected people. The proportion of the population carrying antibodies following infection or vaccination determines herd immunity. How long protective antibodies persist after infection or vaccination remains to be determined. Large-scale population screenings will provide this valuable information and facilitate surveillance during the pandemic.

Many platforms for SARS-CoV-2 antibody testing have been launched [1], typically requiring specialized liquid handling and reading devices for result assessment. We have recently developed C19-kodecyte reactive red blood cells suitable for routine manual and automated assays with antiglobulin techniques available in most blood banks and hospital laboratories [2, 3].

C19-kodecyte reactive red blood cells can be prepared in any laboratory in 2 h by inserting Kode Technology constructs into the membranes of blood group O red blood cells. The C19-kodecytes are thus coated with 15 amino acid peptides from long derivatives of the SARS-CoV-2 spike protein (SP) attached to the red blood cell membrane by a spacer and a lipid. The resulting reactive red blood cells are then tested against undiluted serum or plasma samples in any indirect antiglobulin platform.

As many immunohematology laboratories around the world have automated blood group analyzers, they are able to perform large-scale testing and are uniquely positioned to continuously survey their presumptively healthy blood donor populations for immunity to COVID-19. Here, we evaluated the C19-kodecyte assay in 130 convalescent plasma donors.

The results were compared with an established enzyme immunoassay (ELISA) and plaque reduction neutralization assay [1]. Additionally, we transferred the C19-kodecyte test to an automated blood group analyzer and evaluated 231 samples from a vaccination surveillance study.

2.1 Convalescent COVID-19 donor and control samples Serum samples were from blood donors who had recovered from mild to moderate PCR-confirmed COVID-19 disease and being evaluated as convalescent plasma donors for a prospective randomized trial for the treatment of patients with COVID-19. 19 severe (CAPSID; EudraCT #2020-001310-38; identifier NCT04433910).

All 130 samples were tested with the Euroimmun ELISA for antibodies against SP and for antibodies against nucleocapsid protein (NCP). Additionally, 88 of these samples had been tested with the SARS-CoV-2 plaque reduction neutralization assay (PRNT) [1, 4] which detects the reduction of wild-type virus-induced cell culture plaques. PRNT results are reported as the sample titer at which a 50% (PRNT50) or 90% (PRNT90) plaque reduction is observed. For the present study, we used the results of the PRNT50.

For negative controls, 38 serum samples were obtained from healthcare workers and their dependents (not known to have had COVID-19 or been vaccinated). Eleven of these control samples were included in a recently published study [1].

2.2 Plasma samples from the SARS-CoV-2 vaccination screening program
Informed consent was obtained and individuals were tested for SARS-CoV-2 antibodies before and after vaccination. This study was approved by the ethics committee of the University of Ulm (no. 488/20).

2.3 C19-kodocytes C19-kodecyte reactive red blood cells were prepared as previously described [2]. Briefly, Kode constructs FSL-1147 and FSL-1255 were both dispersed in red blood cell stabilizer solution (ID-Cellstab 005650; Bio-Rad, München, Germany) at concentrations of 1.5 μmol/L and 2.5 μmol/L, respectively. FSL-1147 + 1255 construct mix was incubated with washed group O red blood cells for 2 h at 37°C and then adjusted to 1% using red blood cell stabilization solution.

2.4 Assay of C19-kodocytes Serum samples from convalescent COVID-19 donors and controls were tested manually using Grifols DG antiglobulin and saline cards (#210342 and 210343, respectively; Grifols S.A., Barcelona, ​​Spain). The cards were used according to the manufacturer's recommendations. Briefly, 25 μl of serum was incubated with 50 μl of 1% C19-kodecytes in