FRNT
Overview
The Focus Reduction Neutralization Test (FRNT) is a cell-based assay for quantifying virus-neutralizing antibodies. Serial dilutions of patient serum are incubated with virus, and the mixture is applied to susceptible cell monolayers. Infected foci are visualised by immunostaining and counted; the serum dilution achieving 50% focus reduction (FRNT₅₀) is reported as the neutralizing titer. FRNT is the gold standard for measuring DENV neutralizing antibodies and is preferred over plaque reduction (PRNT) for dengue due to its higher throughput and compatibility with all four DENV serotypes.
Key Points from Literature
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Neutralizing titers do NOT differ between mild and severe dengue: FRNT₅₀ titers against the infecting DENV serotype were not significantly different between dengue with/without warning signs and severe dengue — despite higher anti-NS1 and anti-prM/M/E IgG binding antibody titers in severe disease. This replicates the neutralizing antibody paradox previously observed in COVID-19 by Woodruff2020: EF-associated antibody quantity does not predict neutralization quality (see Ansari2025 - Peripheral T Helper Subset Drives B Cell Response in Dengue, n=170 cohort).
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FRNT performed against all 4 DENV serotypes: Cross-neutralization assessed against DENV-1 through DENV-4, enabling serotype-specific and cross-reactive neutralization profiling (see Ansari2025 - Peripheral T Helper Subset Drives B Cell Response in Dengue).
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FRNT₅₀ used to demonstrate OAS in secondary dengue mAbs: Priyamvada2016 measured FRNT₅₀ of 53 plasmablast-derived mAbs against DENV1–4 using a focus-forming assay (FFA). 46/53 mAbs neutralised ≥1 serotype; 34/46 were cross-neutralising. In 2/4 patients, mAbs preferentially neutralised DENV1 (FRNT₅₀ 0.16 µg/ml) over the infecting DENV2 (FRNT₅₀ 1.2 µg/ml) — functional evidence of original antigenic sin. 15 mAbs neutralised virus without binding recombinant E, indicating virion-dependent conformational epitopes not detectable by standard ELISA (see Priyamvada2016 - Cross-Reactive Memory Plasmablasts in Secondary Dengue, n=4 secondary DHF, FFA-based FRNT₅₀).
Contradictions & Debates
None documented in current wiki sources.
Related Pages
PRNT, IgG, Extrafollicular Response, Plasmablast