How IL‑39 ELISA Kits May Support Rheumatoid Arthritis Research

Editorially reviewed by: A to Zen Therapies Editorial Team in accordance with our Editorial Policy.

Rheumatoid arthritis (RA) is a chronic autoimmune disease in which immune activity targets the synovium—the tissue lining the joints. This can cause persistent inflammation, pain, warmth, swelling and stiffness. Without effective treatment, inflammation may damage cartilage, bone, tendons and ligaments.

Unlike osteoarthritis, which is primarily associated with structural changes within joints, RA is a systemic inflammatory condition. It often affects similar joints on both sides of the body and may also involve the eyes, lungs, heart, blood vessels and skin.

Cytokines are signalling proteins that help coordinate immune responses. Established RA research has identified important roles for tumour necrosis factor (TNF), interleukin‑6 (IL‑6), interleukin‑1 and several other inflammatory pathways. IL‑39 is a much less established candidate.

Commercial IL‑39 ELISA assays may give researchers a way to investigate this proposed cytokine. However, their findings must be considered exploratory until the existence, secretion and biological activity of native human IL‑39 have been more firmly demonstrated.

ELISA can quantify proteins in biological samples, but novel cytokine assays require careful validation before their results can be interpreted.

Quick Answer

IL‑39 ELISA kits may help researchers explore whether a proposed IL‑23p19/EBI3 protein complex can be detected in experimental samples. They can support preliminary comparisons between groups, but IL‑39 is not an established human RA biomarker. Assay specificity, cross-reactivity, matrix effects and independent confirmation are therefore essential.

What is IL‑39?

IL‑39 is the name proposed for a heterodimer containing the IL‑23p19 and EBI3 protein subunits. Mouse studies have suggested inflammatory activity, particularly in experimental lupus. Human studies have not yet consistently demonstrated that immune cells naturally produce a functional, secreted IL‑39 cytokine.

Key Takeaways

• IL‑39 is proposed to consist of IL‑23p19 and EBI3.

• Its expression and biological function have been demonstrated more convincingly in certain mouse models than in humans.

• IL‑39 is not used clinically to diagnose or monitor rheumatoid arthritis.

• ELISA may support exploratory measurement, provided that the antibodies recognise the intact complex.

• Cross-reactivity with IL‑23, IL‑27, IL‑35 or free subunits must be investigated.

• Results should ideally be confirmed using a second method, such as immunoprecipitation or mass spectrometry.

• Researchers should assess detection range, matrix effects, precision, recovery and dilutional parallelism before analysing study samples.

Table of Contents

1. Understanding Rheumatoid Arthritis

2. Why Cytokines Matter in RA Research

3. What the Evidence Says About IL‑39

4. How an IL‑39 ELISA Works

5. Potential Applications in RA Research

6. Important Limitations of IL‑39 ELISA Research

7. Selecting and Validating an IL‑39 ELISA Kit

8. Interpreting Experimental Results Responsibly

9. Frequently Asked Questions

Understanding Rheumatoid Arthritis

RA develops through a complicated interaction between genetic susceptibility, immune dysregulation and environmental exposures. Smoking is an established modifiable risk factor, while sex, age and certain genetic variants may also influence susceptibility.

Common symptoms include:

• Painful, swollen or warm joints

• Morning stiffness lasting longer than ordinary stiffness

• Fatigue

• Reduced appetite

• Low-grade fever

• Difficulty completing everyday activities

• Reduced joint movement or strength

Hands, wrists and feet are frequently affected, although RA can involve many joints.

According to the National Institute of Arthritis and Musculoskeletal and Skin Diseases, diagnosis draws on symptoms, physical examination, blood tests and imaging. Common laboratory investigations include rheumatoid factor, anti-CCP antibodies, C-reactive protein and erythrocyte sedimentation rate.

No single blood result can confirm or exclude every case. IL‑39 is not part of current routine diagnostic testing.

Early assessment matters because joint damage can begin during the first years of disease. The NHS guidance on rheumatoid arthritis explains that early treatment can help reduce progression and the risk of permanent joint damage.

Readers seeking a broader clinical overview can also explore common bone and joint problems treated by orthopaedic specialists.

Why Cytokines Matter in RA Research

Cytokines allow immune and tissue cells to communicate. In RA, signalling networks can recruit immune cells into the synovium, promote inflammation and contribute to tissue destruction.

Some of the better-established mediators include:

• TNF

• IL‑6

• IL‑1

• GM-CSF

• IL‑17-related pathways

• Chemokines that attract immune cells

The clinical success of treatments targeting TNF and IL‑6 demonstrates why cytokine research matters. However, it does not mean every newly proposed cytokine will become a useful biomarker or treatment target.

Novel candidates must move through several stages:

1. Demonstrating that the protein exists naturally

2. Confirming which cells produce it

3. Identifying its receptor and signalling pathway

4. Showing reproducible biological effects

5. Establishing an association with disease

6. Demonstrating that measurement changes diagnosis or treatment decisions

IL‑39 research has not yet completed this pathway in humans.

What the Evidence Says About IL‑39

IL‑39 was proposed as a member of the IL‑12 cytokine family because it combines two subunits used by related cytokines:

• IL‑23p19, which is also part of IL‑23

• EBI3, which is incorporated into IL‑27 and IL‑35

Initial experiments identified the p19/EBI3 combination in activated B cells from lupus-prone mice. The proposed cytokine was associated with neutrophil expansion and inflammatory activity in those experimental models.

These findings do not establish the same function in human RA.

A human-cell investigation published in PLOS One found that p19 and EBI3 could form a complex when deliberately overexpressed in laboratory cells. However, researchers could not reliably detect native IL‑39 protein from stimulated human B cells, macrophages or keratinocytes. The proposed cytokine also failed to activate human immune cells or STAT3 signalling under the tested conditions. The researchers concluded that evidence for IL‑39 expression and function in humans remained insufficient.

A more recent review of IL‑35 and IL‑39 in rheumatoid arthritis likewise describes IL‑39 as a theoretical cytokine in humans.

Therefore, it is premature to state that IL‑39:

• Causes human rheumatoid arthritis

• Provides early diagnosis

• Predicts disease severity

• Monitors treatment response

• Represents a validated drug target

These remain research questions rather than established clinical applications.

📊 Evidence Snapshot

Claim	Current evidence	Confidence
IL‑23p19 and EBI3 can form a complex	Demonstrated under engineered laboratory conditions	Moderate
Native IL‑39 is secreted by human immune cells	Not consistently demonstrated	Low
IL‑39 has inflammatory activity in mice	Reported mainly in lupus-prone models	Preliminary
IL‑39 contributes to human RA	Direct evidence remains limited	Very low
IL‑39 can diagnose or monitor RA	No validated clinical use	Unsupported
ELISA can explore proposed IL‑39 signals	Possible with appropriate validation	Method-dependent

How an IL‑39 ELISA Works

ELISA stands for enzyme-linked immunosorbent assay. It uses antibody binding and an enzyme-generated signal to detect or quantify an analyte in a sample.

A sandwich ELISA generally involves the following steps:

1. A capture antibody is attached to the microplate.

2. The sample is added, allowing the target to bind.

3. Unbound material is removed through washing.

4. A detection antibody binds to another site on the target.

5. An enzyme-linked detection system is introduced.

6. A substrate produces a measurable colour signal.

7. Sample readings are compared with a standard curve.

For a proposed heterodimer such as IL‑39, an ideal design would use one antibody recognising IL‑23p19 and another recognising EBI3. This configuration may help indicate that both subunits are present in the captured complex.

Even this does not automatically prove that the signal represents native, biologically active IL‑39. The assay could potentially detect:

• Non-functional p19/EBI3 complexes

• Aggregated proteins

• Associated but non-covalently paired subunits

• Cross-reacting cytokines

• Interference from rheumatoid factor or other antibodies

• Recombinant standards that behave differently from natural proteins

Researchers comparing commercial products may encounter IL39 ELISA kits promoted for use with human serum, plasma, cell-culture supernatants or tissue samples. Before purchasing, the full technical manual and lot-specific validation data should be reviewed. Inclusion of a supplier does not independently validate or endorse the assay.

Potential Applications in RA Research

Comparing Experimental Groups

Researchers may compare assay signals across:

• Healthy control samples

• Samples from people with established RA

• Different experimental disease models

• Treated and untreated laboratory groups

• Samples collected at multiple study time points

Such comparisons can identify patterns worth investigating, but they cannot establish causation.

Exploring Cytokine Networks

An exploratory study could measure proposed IL‑39 alongside better-characterised mediators such as TNF, IL‑6, IL‑1β or IL‑17.

Correlations should be interpreted carefully. If two measurements rise together, this does not prove that one controls the other. Both may reflect a broader inflammatory response.

Investigating Cellular Sources

Cell-culture studies may examine whether activated B cells, macrophages, fibroblast-like synoviocytes or other cells produce a detectable p19/EBI3 complex.

Appropriate controls should distinguish:

• Gene expression from protein production

• Intracellular association from secretion

• Free subunits from an intact heterodimer

• Protein detection from biological activity

Evaluating Experimental Interventions

Researchers may test whether an experimental intervention changes the ELISA signal. This can generate mechanistic hypotheses, but it does not demonstrate that the intervention works through IL‑39.

Because p19 and EBI3 participate in other cytokines, reduced assay signals could reflect wider changes in cytokine expression rather than selective effects on IL‑39.

Supporting Early Biomarker Discovery

ELISA could contribute to the earliest stages of biomarker exploration. A candidate biomarker would then require independent replication, assay standardisation and comparison with established clinical measures.

For perspective on how validated clinical markers are interpreted, see our guide to blood biomarkers, inflammation and health monitoring.

Important Limitations of IL‑39 ELISA Research

Shared Subunits Create a Specificity Problem

IL‑23p19 is shared with IL‑23, while EBI3 contributes to IL‑27 and IL‑35. Antibodies must

therefore be tested against:

• Recombinant IL‑23

• Recombinant IL‑27

• Recombinant IL‑35

• Free IL‑23p19

• Free EBI3

• Other related proteins likely to occur in the sample

Without these experiments, an ELISA signal cannot confidently be assigned to intact IL‑39.

Recombinant Standards May Not Match Native Protein

Commercial assays commonly use recombinant material to create the calibration curve. The recombinant standard may differ from naturally produced protein in folding, glycosylation, stability or subunit arrangement.

An assay can therefore detect its standard successfully while performing poorly with biological samples.

Rheumatoid Factor May Interfere

RA samples can contain rheumatoid factor and other heterophilic antibodies capable of interacting with immunoassay antibodies. This may generate misleadingly high or low

signals.

Blocking reagents, alternative antibody formats and orthogonal confirmation can help investigate this interference.

Results Below the Detection Range Are Uncertain

A numerical result should not be reported simply because software extrapolates it from a curve. Values below the lower limit of quantification may not be sufficiently accurate or precise.

Researchers should distinguish between:

• Limit of detection

• Lower limit of quantification

• Working measurement range

• Undetectable samples

• Results requiring dilution

ELISA Does Not Demonstrate Biological Activity

Detecting a p19/EBI3 complex does not show that it binds a receptor, activates a signalling pathway or influences RA pathology. Functional cellular experiments are needed to test those questions.

Selecting and Validating an IL‑39 ELISA Kit

Before using a commercial kit, review:

• Intended species

• Validated sample types

• Capture- and detection-antibody targets

• Measurement range

• Analytical sensitivity

• Cross-reactivity data

• Intra-assay precision

• Inter-assay precision

• Spike-and-recovery results

• Dilutional linearity or parallelism

• Sample-volume requirements

• Storage and stability

• Lot-to-lot consistency

• Full standard-curve data

• Evidence involving native biological samples

The practical guide to immunoassay validation explains that recovery, precision, parallelism and specificity are central to evaluating whether an immunoassay is suitable for its intended purpose.

💡 Expert Tip: Do not validate an exploratory IL‑39 assay using only the supplied recombinant standard. Test pooled RA serum, synovial fluid or the intended experimental matrix for spike recovery and dilutional parallelism. Include related IL‑12-family cytokines to examine cross-reactivity.

Essential Experimental Controls

A robust study may require:

• Blank wells

• Zero-standard controls

• Known positive and negative controls

• Technical duplicates or triplicates

• Serially diluted samples

• Spike-and-recovery experiments

• Matrix-matched standards

• Cross-reactivity controls

• Heterophilic-antibody blocking controls

• Inter- and intra-assay precision testing

• Multiple reagent lots

• Randomised plate placement

• Blinded sample analysis

The importance of spike recovery and dilutional linearity is explained in this ELISA validation overview. Poor recovery or non-parallel dilution curves may reveal interference from the sample matrix.

Reliable ELISA research requires controls for cross-reactivity, sample interference, recovery, dilutional linearity and plate-to-plate variation.

Interpreting Experimental Results Responsibly

A detected ELISA signal should initially be described as IL‑39-reactive material or a

p19/EBI3-associated signal, unless assay specificity has been convincingly demonstrated.

Stronger evidence would combine ELISA with an independent method, such as:

• Immunoprecipitation followed by western blotting

• Size-exclusion chromatography

• Targeted mass spectrometry

• Proximity-based protein assays

• Cell-signalling experiments

• Receptor-binding studies

Researchers should report negative and inconclusive findings as clearly as positive ones.

Failure to detect IL‑39 may reflect true absence, low abundance, unsuitable sample handling or inadequate assay sensitivity.

Similarly, a positive signal does not prove that IL‑39 is biologically active or clinically relevant.

Frequently Asked Questions

Are IL‑39 ELISA kits used to diagnose rheumatoid arthritis?

No. IL‑39 ELISA kits are research tools. RA diagnosis currently relies on clinical assessment, established blood tests and imaging where appropriate.

Is IL‑39 a confirmed human cytokine?

IL‑23p19 and EBI3 can form a complex under engineered conditions, but natural secretion and biological activity in human immune cells have not been consistently established.

Is IL‑39 an established RA biomarker?

No. Evidence is insufficient to use IL‑39 for predicting RA risk, measuring disease activity or monitoring treatment.

Can an IL‑39 ELISA distinguish IL‑39 from IL‑23?

Only if its antibody pair and validation data demonstrate appropriate specificity. Because the proposed cytokines share the p19 subunit, cross-reactivity must be tested.

Which samples could be studied?

Depending on the validated kit, researchers may examine serum, plasma, cell-culture supernatants, tissue lysates or synovial fluid. Each matrix requires separate validation.

What is the difference between detecting IL‑39 and proving its function?

ELISA indicates antibody-reactive material. Functional evidence requires experiments showing receptor binding, signalling or a reproducible effect on cells or disease processes.

Final Thoughts

IL‑39 ELISA kits may provide a practical starting point for exploratory cytokine research, but their use in rheumatoid arthritis requires unusually careful interpretation. IL‑39 remains a proposed human cytokine, and its relevance to RA has not been clinically established.

The strongest studies will verify that an assay detects the intact p19/EBI3 complex, evaluate cross-reactivity with related cytokines and confirm findings through an independent analytical method.

IL‑39 ELISA results should not be presented as evidence of early diagnosis, disease severity or treatment response until larger human studies and independently validated assays support those applications.

For further evidence-informed information about inflammatory joint conditions and long-term discomfort, continue exploring our Pain Management Hub.

References

• Ling et al. Divergent roles of IL‑35 and IL‑39 in rheumatoid arthritis

• Ecoeur et al. Lack of evidence for expression and function of IL‑39 in human immune cells

• Wang et al. A novel IL‑23p19/EBI3 cytokine mediates inflammation in lupus-like mice

• NIAMS: Rheumatoid arthritis diagnosis and treatment

• Andreasson et al. A practical guide to immunoassay method validation