Introduction

For seasonal influenza A and B viruses (IAV/IBV), the major harm often does not stem from direct virus-mediated cell death, but from excessive inflammation triggered by over-activated immune responses. ELISA assays are indispensable for influenza animal-model studies and clinical sample research, widely applied for viral antigen quantification, dynamic monitoring of inflammatory cytokines, and pharmacodynamic evaluation of anti-inflammatory drugs.

However, sample matrices bring considerable challenges. Nasopharyngeal swabs, bronchoalveolar lavage fluid (BALF), and lung tissue homogenates exhibit highly complex matrices. Viral proteins are prone to degradation, proteases may interfere with detection, and high cytokine concentrations can induce hook effect. Distorted data and reviewer queries during manuscript submission are common issues. This article summarizes key ELISA targets for influenza research, assay applications in antiinflammatory studies, and frequent experimental pitfalls, providing references for virology, immunology and antiinflammatory pharmacology research.

I. Two Major Analytes Measured by ELISA in Influenza Research: Viral Antigens and ImmuneInflammatory Biomarkers

1. Viral antigens (for assessing viral load and infection level; sandwich ELISA is mostly adopted)

Three primary target proteins are frequently detected by influenzavirus ELISA: nucleoprotein (NP), hemagglutinin (HA), and neuraminidase (NA).

1. NP (Nucleoprotein): Highly conserved in sequence with distinct subtypes for influenza A and B. It is the most widely-used target in research. Suitable for cell supernatants, lung homogenates and nasopharyngeal samples to evaluate viral replication levels in infection models.
2. HA (Hemagglutinin): Surface antigen of influenza virus with substantial straintostrain variation. Mostly applied in vaccine development and neutralizing-antibody-related studies.
3. NA (Neuraminidase): Target of anti-influenza drugs such as oseltamivir, used to assess viral activity after drug intervention.

The underlying experimental logic is straightforward: measurement of viral antigens quantifies viral replication magnitude and differentiates direct viral injury from inflammatory injury. Combined with inflammatory-cytokine profiling, researchers can determine whether a candidate drug suppresses viral replication or merely mitigates cytokine storm.

2. Inflammatory / immune biomarkers (core readouts for antiinflammatory research; sandwich ELISA)

For studies on severe influenza, acute lung injury (ALI) and cytokine storm, ELISA focuses on pro-inflammatory cytokines, chemokines and anti-inflammatory mediators, which serve as direct indicators for anti-inflammatory drug pharmacodynamics.

1. Proinflammatory cytokines: TNF-α, IL-6, IL-1β; the most frequently reported biomarkers in cytokine-storm research.
2. Chemokines: CXCL1, CCL2; responsible for recruiting neutrophils and macrophages into lung tissue and amplifying local inflammation.
3. Interferon pathway: IFN-α, IFN-γ; reflect the activation magnitude of antiviral innate immunity.
4. Anti-inflammatory regulatory factor: IL-10; indicates negative-feedback immune regulation.
5. Auxiliary biomarkers: IL-8, G-CSF; evaluate neutrophil infiltration and severity of pulmonary inflammation.

Upon influenza-virus infection, alveolar macrophages and epithelial cells secrete large quantities of pro-inflammatory cytokines and recruit immune cells, which consequently induce lung tissue damage. Concurrent detection of viral NP antigen and multi-cytokine panels enables discrimination between antiviral efficacy and anti-inflammatory protective effects, a typical experimental design for current anti-influenza drug research.

II. Typical ELISA Applications in Influenza AntiInflammatory Research

1. Drug screening targeting influenzainduced cytokine storm (animal models)

Establish mouse influenza-A challenge models. Collect serum, BALF and lung-tissue homogenates at multiple time-points post-drug administration. Perform ELISA for NP (viral load), IL-6, TNF-α and IL-1β. This setup differentiates drug-mediated viral-replication inhibition versus excessive-inflammation suppression, and evaluates protective effects against acute lung injury. This workflow is widely adopted for anti-inflammatory small-molecule and JAK-inhibitor studies.

2. Immunemechanism research on viral infection

Use in-vitro infection models of alveolar epithelial cells or macrophages. Harvest supernatants and measure cytokines via ELISA to investigate how pathways such as TLR and NLRP3 inflammasome drive inflammatory amplification.

3. Retrospective clinical research on severepatient samples

Collect serum samples from influenza patients in acute and convalescent phases. Detect inflammatory cytokines and analyze correlations between cytokine levels, disease severity and prognosis. Elevation magnitudes of IL-6 and TNF-α can serve as reference indicators for severe-case early warning.

4. Vaccine immunogenicity assessment

Indirect ELISA measures anti-HA / anti-NP antibody titers to evaluate humoral-immune responses elicited by vaccines.

Important distinction: Sandwich ELISA kits are used for viralantigen and cytokine detection, whereas indirect ELISA kits are applied for serum antiviralantibody quantification. These two kit formats operate on different principles and shall not be misused.

III. Challenges and Notes for ELISA Testing on InfluenzaRelated Samples

1. Sample matrix interference (most prominent in BALF and lung homogenates)

Bronchoalveolar lavage fluid and lung homogenates contain abundant proteases, lipids, hemoglobin and mucins, which raise background signals and trigger non-specific binding and signal drift.

1. Lungtissue homogenate: Conduct all operations on ice. Protease inhibitors are mandatory. Perform thorough centrifugation to completely remove tissue debris preventing antigen encapsulation by residual fragments.
2. BALF samples: Pre-centrifuge and filter mucus-rich samples. Matrix-matched standards and dilution-linearity validation are required when obvious matrix effects occur.
3. Discard hemolyzed samples directly. Hemoglobin carries intrinsic peroxidase-like activity that induces false-positive signals, especially for analytes such as TNF-α and IL-6.

2. Protein degradation

Proinflammatory cytokines (IL-1β, IFN-α) exhibit poor stability and degrade rapidly ex-vivo.

1. Separate supernatants immediately after sample collection, aliquot and store at −80 °C. Limit freeze-thaw cycles to ≤ 2 times; repeated freeze-thaw will markedly lower measured cytokine concentrations.
2. Avoid high-temperature inactivation (e.g. heating at 60 °C). High temperature destroys cytokine epitopes and reduces detection readings. If sample inactivation is required, adopt mild detergent treatment with prior method validation.

3. Hook effect with highconcentration samples

In BALF and lung homogenates from severe influenza models, IL-6 and TNF-α concentrations often exceed kit linear ranges, leading to high-dose hook effect: higher actual analyte concentrations generate lower OD values, falsely suggesting alleviated inflammation. Perform serial dilution in preliminary experiments to identify optimal dilution factors. High-inflammation samples must be re-tested after dilution.

4. Crossreactivity and kit selection

1. Verify subtype specificity for viral-NP ELISA kits; influenza-A NP kits cannot detect influenza-B NP.
2. Ensure species matching (human / mouse / rat) for cytokine kits. Cross-species application will introduce cross-reactivity and unreliable data.
3. For antiviral-antibody detection (indirect ELISA), rheumatoid factor (RF) in serum may produce false-positive results. Heat inactivation at 56 °C for 30 min is recommended.

5. Easilyoverlooked operational details

1. Sodium azide (NaN₃) inhibits HRP activity and must not be added to samples or diluents.
2. Protect TMB chromogenic reactions from light. Strictly control incubation duration to avoid over-development.
3. Replace pipette tips frequently to prevent cross-contamination. Handle virus-containing samples following BSL-2 biosafety guidelines.

IV. Citation Suggestions for Manuscript Writing

1. Pharmacology: In influenza-challenged mice, combined ELISA measurement of viral NP and inflammatory-cytokine panels to evaluate candidate-drug efficacy in mitigating cytokine storm and relieving acute lung injury.
2. Basic immunology: In-vitro infection models of macrophages or epithelial cells, detect pro-inflammatory cytokines via ELISA to validate regulatory mechanisms of inflammasome and TLR-dependent pathways.
3. Translational clinical research: Influenza-patient cohort studies using serum ELISA inflammatory-biomarker detection to identify prognostic markers for severe-disease risk.

Summary

For influenza-related ELISA experiments, first clarify your target analyte: viral antigen, cytokine, or antiviral antibody, as these require different kit formats. Matrix interference in BALF and lung homogenates, cytokine degradation, and hook effect under high analyte concentrations represent common methodological weaknesses frequently raised by reviewers in influenza-inflammation manuscripts. Matrix-effect and dilution-linearity validation should be completed before formal experiments.