# Development of a Novel Homogeneous Liposome-Based One-Step Assay for SARS-CoV‑2 Antibody Detection in Human Serum Based on Fluorescent Liposomes and Complement Activity

## TL;DR

A new one-step test using fluorescent liposomes detects SARS-CoV-2 antibodies in blood with high accuracy and no need for washing steps.

## Contribution

A novel homogeneous liposome-based assay for SARS-CoV-2 antibody detection with high sensitivity and specificity.

## Key findings

- The assay achieved 95% specificity and 93% sensitivity in detecting SARS-CoV-2 antibodies.
- It showed strong correlation with ELISA (R² = 0.82) and pseudovirus neutralization tests (R² = 0.72).
- The method is wash-free and adaptable to other viral targets by changing the surface antigen.

## Abstract

Monitoring antibodies
in patient serum enables the diagnosis
of
infectious and chronic diseases, the assessment of an individual’s
immune status, and possible protection against infection and could
thus be a ubiquitous tool for pandemic preparedness and personalized
medicine. Advancing from the traditional enzyme-linked immunosorbent
assay (ELISA), a homogeneous assay format was developed that simplifies
assay procedures and enables a wash-free one-step performance. SARS-CoV-2
was chosen as the model virus, and its Spike protein-derived receptor
binding domain (RBD) was covalently coupled to fluorescent liposomes.
The binding of patient antibodies triggered the complement system,
led to liposome lysis, and allowed quantitative fluorescent detection.
The liposome assay was optimized with respect to liposome lipid composition,
RBD coverage and surface chemistry, incubation conditions, and pretreatment
of a standardized complement source. A proof-of-principle was demonstrated
through artificially supplemented anti-RBD antibodies and full titrations
with known positive sera. Testing of 37 SARS-CoV-2 negative sera and
28 sera from individuals with SARS-CoV-2 (breakthrough) infections
resulted in a specificity of 95%, sensitivity of 93% and an excellent
correlation (R
2 = 0.82, Spearman r = 0.90) with antibody titers determined in an ELISA approved
for diagnostic use. Finally, the liposome assay showed a good correlation
to a pseudovirus neutralization test (pVNT) (R
2 = 0.72, Spearman r = 0.84), similar to the
diagnostic ELISA. As the new liposome assay does not require any wash
steps and can be easily adapted to other viral targets by changing
the surface antigen, it provides a new avenue for high-throughput
immunodiagnostics.