ESTABLISHMENT of AN ANTI-RBD THRESHOLD THAT PREDICTS ROBUST NEUTRALIZING ACTIVITY

ABSTRACT

Background: Although presence of SARS-CoV-2 neutralising antibodies can provide protection against development of COVID-19, how reflective circulating anti-SARS-CoV-2 antibody levels are of underlying neutralising capacity, and whether a threshold exists to predict sufficient neutralising capacity remains unclear. Methods: In plasma from individuals with PCR-confirmed COVID-19 recruited to the All Ireland Infectious Diseases Cohort Study, we measured IgG concentrations against RBD, Spike protein sub-unit 1 and 2 (S1, S2) and Nucleocapsid (NC) using multiplex electrochemiluminescence (normalised to World Health Organisation reference serum as IU/mL). Neutralising capacity was measured against live SARS-CoV-2 virus (clinical isolate 2019-nCoV/Italy-INMI1) by determining the maximum plasma dilution required to maintain 50% inhibition of infection of Vero E6 cells (50% Neutralisation Titre (NT50)), by flow cytometry-based micro-neutralisation assay. Given that the Beta SARS-CoV-2 variant of concern (VOC) reduces neutralising activity up to six fold, we estimated a NT50 of 11000 against wild type SARS-CoV-2 would maintain neutralising activity against VOC. We used Spearman correlation and linear regression to model relationships between NT50 and IgG concentrations. Data are presented as median (IQR) unless specified. Results: In 190 individuals (age 50 (40-64) years, 55% female, time from symptom onset 98 (35-179) days), NT50 most highly correlated with anti-RBD IgG (Rho 0.81 p<0.001, Fig 1a) compared with other IgG classes (S1;Rho 0.8, S2;0.73, NC;0.72, all p<0.001). Median RBD titre was 246 (71-662) but trended lower over time, with a median of 319 (61-1012) IU/ml at 0-90 days, 244 (86-523) IU/ml at 90-180 days and 157 (80-364) IU/ml at >180 days post symptom onset respectively (p=0.08, Fig 1b). RBD IgG titres of 476 IU/ml corresponded to a NT50 of 11000. Overall, RBD ≥476 IU/ml predicted NT50 ≥11000 with a sensitivity of 77 (95% CI 65-87)% and specificity 89 (95% CI 82-93)%. This improved in an analysis restricted to convalescent samples (>30 days post symptom onset, n=148), with a sensitivity 88% (95% CI 74-96%) and specificity 90% (95%CI 82-95%) respectively. Conclusion: In convalescent plasma, RBD IgG titres ≥476IU/mL is sensitive and specific for predicting robust underlying neutralising capacity. Further research is required to validate these findings in other cohorts and confirm these thresholds in post-vaccinated individuals.