Inhibition of leukocyte migration: immunological test and meaning

The leukocyte migration inhibition test (LMIT), commonly known in English-language literature as the leukocyte migration inhibition test, is an older laboratory method for assessing cellular immunity. Its historical purpose was to indirectly demonstrate the presence of antigen-specific T-cell sensitization: if a patient's lymphocytes "recognize" an introduced antigen, they trigger a cascade of mediators, and leukocyte migration in culture is reduced.

Essentially, RTML attempted to provide a laboratory equivalent of a delayed-type hypersensitivity reaction outside the body. This approach was particularly popular during the period when clinical immunology did not yet have modern cellular technologies and needed functional assays that would allow at least a rough assessment of the T-cell response to an antigen. [2]

Historically, the test has been used in infectious immunology, drug and contact hypersensitivity, research oncoimmunology, and certain autoimmune conditions. However, more recent reviews have shown that methodological differences between laboratories are too great, and reproducibility is too dependent on assay technique and sample quality. [3]

Today, RT-ML is rarely considered a routine laboratory test. Current reviews of T-cell-mediated hypersensitivity and in vitro drug allergy diagnostics primarily discuss patch tests, intradermal tests, lymphocyte transformation tests, and enzyme immunospot tests, while ex vivo cellular methods are generally considered tools for specialized centers rather than for general practice. [4]

Therefore, a modern article on RTML should be honest: it is an important historical and, in some places, still niche method, but it is not a "workhorse" of routine diagnostics. Understanding its principle is useful for clinical immunology, allergology, and the history of laboratory medicine, but today, the results of this test almost always require confirmation with more modern and better standardized methods.

Table 1. Where is RTML among the methods for assessing T today?

Method	What does it measure?	Current status
RTML	Inhibition of leukocyte migration after contact with an antigen	Mainly historical or highly specialized method
Application test	Delayed skin reaction to an allergen	Routine clinical method in dermatoallergology
Delayed reading intradermal test	Delayed cellular reaction in the skin	Used according to indications
Lymphocyte transformation test	Proliferation of memory T cells after stimulation	Specialized in vitro test
Enzyme immunospot test	Release of cytokines by antigen-specific cells	Specialized modern test
Interferon gamma-releasing assay	Release of interferon gamma after stimulation with tuberculosis antigens	Standardized modern test for individual tasks

Sources for the table. [6]

What is RTML and what is its immunological significance?

The immunological concept of RTML is based on the T-cell-mediated delayed-type hypersensitivity reaction. Modern reviews emphasize that such reactions are primarily associated with antigen recognition by T cells and the subsequent activation of other leukocytes via inflammatory mediators. RTML attempted to capture one of the functional outcomes of this process—a decrease in the ability of leukocytes to migrate in vitro after specific stimulation. [7]

Historically, it was assumed that sensitized lymphocytes, upon contact with an antigen, secrete soluble factors that inhibit leukocyte movement. Classically, this terminology referred to migration inhibition factors, or factors that inhibit migration, and related mediators. It was precisely this concept that underpinned the test: cell migration was compared in the presence and absence of the suspected antigen. [8]

If the antigen was recognized as significant by sensitized T cells, the migration zone became smaller, and this was interpreted as a positive reaction. If there was no noticeable inhibition, the test was considered negative. In practice, this binary approach quickly ran into a problem: leukocyte migration depends not only on antigen-specific memory but also on a multitude of technical and biological factors that are difficult to fully control.

This is why the RT-ML has always been more of a functional assay than a direct measurement of a specific immune marker. It does not identify the precise T-cell subpopulation, does not reveal the cytokine spectrum, does not provide a genetic profile, and does not allow for the reliable separation of one mechanism of T-cell response from another. In this sense, it is much cruder than modern cell-based assays. [10]

Nevertheless, in a historical context, RTML played an important role. It became one of the few early laboratory techniques that allowed for the discussion of cellular immunity not only through skin tests but also through the ex vivo response of blood cells. For its time, this was a significant step forward, although in modern practice its analytical limitations have become too obvious. [11]

Table 2. What RTML reflects and what it does not reflect

Parameter	What can I say?
The presence of cellular sensitization	May be indirectly supported by a positive reaction
The precise immune mechanism	The test doesn't reveal it completely.
Specific T cell subset	Not determined
Levels of individual cytokines	Not measured directly
The degree of clinical severity of the allergy or disease	The test does not reliably determine it
The causal role of antigen	Requires confirmation by other methods

Sources for the table. [12]

How is RTML performed and what determines the results?

The classic RTML approach involves comparing leukocyte migration in a control medium and in a medium containing the suspected antigen. Several technical variations are described in older literature: capillary methods, agarose techniques, Boyden chambers, and collagen gels. A 2000 review by Hallab explicitly lists these four main approaches and emphasizes that none has proven to be universally superior in all clinical scenarios. [13]

The result was usually expressed as a migration index or percentage inhibition relative to the control. The principle seems simple, but in reality, it involves many variables: the type of cell suspension, the quality of mononuclear cell isolation, the freshness of the sample, the cell concentration, the incubation temperature, the composition of the medium, the properties of the antigen, and even the specifics of the migration zone reading. This is why interlaboratory variability for RTML has always been high.

The pre-analytical stage also has a significant impact. The iLive page correctly notes that blood freshness, the correct anticoagulant, stable temperature, and accurate cell concentration are essential for RTML. If the sample has been stored for a long time, the cells are damaged, or the antigen is unstable, the signal becomes noisy, and the risk of false results increases.

A separate issue is the biological ambiguity of the endpoint. Inhibition of migration may depend not only on the antigen-specific T-cell response, but also on nonspecific activation, the toxicity of the test substance to cells, the serum composition, and even the mechanical parameters of the environment. Therefore, a positive test does not always indicate true, clinically significant sensitization.

For this reason, modern reviews and professional society positions emphasize standardized reading systems. Where in vitro methods are needed today, the focus is on lymphocyte proliferation tests, cytokine assays, and flow cytometry, rather than manual assessment of the migration zone. This does not mean that RTML is useless in any context, but it does mean that its analytical robustness is inferior to modern alternatives. [17]

Table 3. What most influences the RTML result?

Factor	Why is this important?
Freshness of blood	Cell aging impairs reproducibility
Cell concentration	Changes migration behavior and inhibition index
Antigen type and purity	Non-specific stimulation and false responses become more likely
Temperature and incubation environment	Affect cell viability and motility
Method format	Different systems provide different sensitivity
Method for assessing the migration zone	The human factor increases variability
Immunosuppressants and the patient's condition	May reduce cellular response and produce false negative results

Sources for the table. [18]

Where was RTML used historically and where can it still be found?

Historically, the RTML was used as a laboratory analogue of the delayed-type hypersensitivity reaction. Classic early studies evaluated it in the context of tuberculin sensitivity and other infectious antigens, attempting to correlate laboratory responses with skin tests. In 1970, Rosenberg et al. described the test specifically as an in vitro assay for delayed hypersensitivity in humans, and later studies demonstrated correlations with skin reactivity to specific antigens. [19]

In allergology and immunopathology, attempts have been made to use the RT-ML test for drug hypersensitivity, particularly in cases of delayed reactions. Older literature includes publications on drug-induced hepatitis, pneumonitis, and skin reactions, where the test was considered an ex vivo method for confirming T-cell sensitization to a suspected drug. However, current positions on drug allergy are built around other methods and emphasize the weak evidence base for many older in vitro tests. [20]

Another historical niche is contact sensitization to metals and biomaterials. Literature on orthopedic implants and metal-associated hypersensitivity mentions both the leukocyte migration inhibition test and related cellular methods. However, a modern review clearly states that this test has low sensitivity and should not be used as the sole method for detecting type IV reactions. [21]

Sometimes, RTML and related phenomena continue to be studied in research studies as inexpensive ex vivo models of cellular reactivity. However, this does not mean the test is returning to routine practice. On the contrary, modern reviews of delayed drug hypersensitivity emphasize that even more advanced ex vivo methods, such as the lymphocyte transformation test and the enzyme immunospot test, remain primarily the tools of specialized centers and research facilities. [22]

Thus, the actual modern role of RTML appears modest. It may be of historical, educational, and narrow research interest, and is sometimes used as an additional functional test in niche projects, but it is not one of the main recommended routine methods for confirming T-cell hypersensitivity in widespread clinical practice. [23]

Table 4. Historical areas of application of RTML and their current status

Direction	The historical role of RTML	Current status
Tuberculin and infectious immunology	Laboratory analogue of cellular sensitization	It has been practically replaced by the tuberculin test and interferon gamma-releasing tests.
Delayed drug hypersensitivity	In vitro drug discovery	It has been largely replaced by application tests, lymphocyte transformation tests, and enzyme immunospot tests.
Contact sensitization to metals	Additional cellular test	Not recommended as a sole method
Tumor immunology	Research marker of cellular reactivity	Historical interest
Overall T-cell score	Functional test of cellular immunity	Superseded by modern cellular and cytometric methods

Sources for the table. [24]

Why RTML disappeared from routine medicine and what replaced it

The main reason for the declining role of RT-ML is weak standardization. The ENDA and European Academy of Allergy and Clinical Immunology position paper on in vitro drug hypersensitivity testing explicitly states that for many methods, the evidence base is limited to small studies, and the results are often not confirmed later. For RT-ML, this problem is even more pronounced, as the method itself is older and more variable than modern cell-based tests. [25]

The second reason is the relatively low diagnostic reliability compared to newer methods. A review of metal hypersensitivity noted that the leukocyte migration inhibition test has poor sensitivity and is not recommended as the sole tool for diagnosing type IV reactions. This is an important formulation: the method is not simply "old," but rather insufficiently reliable as a standalone basis for clinical decision-making. [26]

The third reason is the emergence of tests that provide more meaningful biological information. The lymphocyte transformation test measures the proliferation of antigen-specific memory T cells after exposure to a suspected substance, and its modern modifications use various read-out parameters, including proliferation and cytokine response. This allows for deeper interpretation and, in some scenarios, greater reproducibility. [27]

The fourth reason is the development of specialized tests for specific diseases. For tuberculosis infection, for example, the CDC recommends interferon gamma-releasing assays as standardized blood tests measuring the immune response to Mycobacterium tuberculosis antigens. This is a good example of how a historically general functional assay has been replaced by a more specific and better-validated technology. [28]

The fifth reason is the expanding role of in vivo testing and integrated algorithms. In modern diagnostics of delayed drug hypersensitivity, patch tests and intradermal tests with delayed readings have become the leading "clinical" approaches, while ex vivo tools, including the lymphocyte transformation test and enzyme immunospot test, are used in specialized centers and are not considered routine. Against this backdrop, RTML appears even more peripheral. [29]

Table 5. Why RTML gave way to other tests

Cause	How did this affect the fate of the method?
Weak standardization	Results are difficult to compare between laboratories
Low and inconsistent sensitivity	The risk of missing sensitization increases
Limited specificity	A positive result is not always clinically significant.
Lack of cellular detail	It is not possible to distinguish subpopulations and cytokine profiles
The emergence of new technologies	Lymphocyte transformation test, enzyme immunospot test, and flow cytometry have become more informative.
Development of specialized algorithms	More validated methods have emerged for specific tasks.

Sources for the table. [30]

How to interpret the RTML result today if the test is still performed

If the RTML test is positive, it can only be considered an indirect indication that the patient's cells responded to a specific antigen in vitro by inhibiting migration. This result does not automatically prove a clinically significant allergy, does not determine the severity of a future reaction, and does not replace a full clinical diagnosis. For this reason, even in areas where the test is still under discussion, its use as the sole criterion is not recommended. [31]

A negative result doesn't resolve the issue. Modern reviews of in vitro delayed hypersensitivity tests emphasize that even more advanced methods have limited sensitivity and negative predictive value (NPV) is far from 100%. For RTML, this limitation is especially relevant due to its outdated methodology and high dependence on sample quality and laboratory equipment. [32]

RTM assessment should be performed only in the context of the clinical situation. If drug hypersensitivity is suspected, the algorithm should take into account the medical history, the timing of the reaction's onset, the morphology of the rash, patch test data, and, if necessary, the lymphocyte transformation test and other available methods. When it comes to metals, current literature prioritizes patch tests and, if indicated, lymphocyte methods, but not RTM assessment as a standalone solution. [33]

It is especially dangerous to draw clinically harsh conclusions from the RTML without confirmation. For example, excluding an important drug, changing the surgical plan, or blaming a specific metal solely on this reaction should not be done. Modern allergology and clinical immunology base decisions on a combination of data, not on a single functional test of the old type. [34]

Therefore, if RTML is performed in an uncommon laboratory or is mentioned in old records, the optimal modern approach is to view it as additional historically interesting information that can support a hypothesis, but it should almost never be the final diagnostic point. Decisions on further management should be based on more validated methods and the clinical context.

Table 6. How possible RTML results are usually interpreted

Result	What can we assume?	What to do next
Positive	There is indirect ex vivo reactivity to the antigen.	Confirm with more modern methods and clinical evidence
Negative	There was no obvious reaction in this production.	Do not consider this a definitive exception to sensitization
Questionable or borderline	The role of technical and biological factors is high	Re-evaluation in a specialized laboratory is of little use without compelling reasons.
Contradiction with the clinic	A false positive or false negative result is possible.	Rely on clinical evidence and validated tests
Positive without symptoms	It does not in itself prove illness.	Do not make isolated decisions on treatment or prohibitions

Sources for the table. [36]

What has replaced RTML in modern practice and what is usually used instead?

When diagnosing contact allergies and other delayed-type skin reactions, patch testing remains the primary modern tool. The European Contact Dermatitis Society considers patch testing a basic diagnostic method for suspected contact allergies and related delayed-type hypersensitivity conditions of the skin and mucous membranes. [37]

When it comes to delayed-type drug hypersensitivity, current reviews discuss a combination of clinical history, patch testing, delayed-read intradermal tests, and, in specialized centers, in vitro methods, primarily the lymphocyte transformation test and enzyme immunospot test. Frontiers 2021 explicitly states that these ex vivo tools remain largely the domain of specialized laboratories rather than routine practice. [38]

When it comes to tuberculosis infection, modern clinics use the tuberculin skin test and interferon gamma-releasing assays. The CDC describes the latter as standardized blood tests that measure the release of interferon gamma in response to tuberculosis antigens. Against this background, the historical use of the RTML for tuberculosis has remained essentially a training exercise. [39]

When assessing more subtle cellular mechanisms, flow cytometry, cell proliferation analysis, subpopulation phenotyping, cytokine panels, and molecular methods are now becoming increasingly important. These approaches provide insight into which cells are involved, the strength of the response, which mediators predominate, and whether there is a profile associated with a specific disease. RTM does not provide such depth. [40]

Therefore, in modern practice, the choice of test depends not on the laboratory's habits, but on the specific clinical question. For one task, skin is sufficient, for another, blood, and for a third, a cellular test from a specialized center. In this context, RTML remains primarily a historical precursor to modern functional methods, rather than a fully-fledged alternative. [41]

FAQ

1. What does RTML actually show?
It attempts to indirectly demonstrate whether a patient has antigen-specific T-cell reactivity capable of inhibiting leukocyte migration in vitro. It is a functional test of cellular immunity, not a direct marker of a specific disease.

2. Is RTML a modern, routine test?
No. Even the iLive homepage explicitly calls it an old functional test, while modern reviews of delayed hypersensitivity and in vitro diagnostics emphasize other methods.

3. Why has the RT-ML been largely abandoned?
Due to poor standardization, high interlaboratory variability, limited sensitivity, and the advent of more reproducible tests, including the lymphocyte transformation test, enzyme immunospot test, and flow cytometry. [44]

4. Can a positive RTML test alone prove a drug or metal allergy?
No. The literature on metals clearly states that the test is not recommended for use alone to detect type IV reactions, and in modern drug hypersensitivity algorithms, it is not considered a core routine test at all. [45]

5. Can a negative RTML rule out the problem?
No, it doesn't. A negative result doesn't rule out sensitization, especially if the sample was imperfect, the antigen was poorly selected, or the immune response was suppressed by medications or other factors.

6. In what areas has RTML been used most frequently?
Historically - in infectious immunology, drug hypersensitivity, contact sensitization to metals, and in individual research studies on tumor immunity. [47]

7. How does the RTML differ from the lymphocyte transformation test?
The RTML evaluates the inhibition of leukocyte migration as the final functional effect, while the lymphocyte transformation test measures the proliferation of antigen-specific memory T cells after stimulation in vitro. The latter method is considered much more modern today. [48]

8. Which method is now more commonly used instead of RT-MS for contact allergies?
Usually, patch tests. Patch testing is considered the primary diagnostic method for suspected contact allergies and other delayed-type hypersensitivity skin conditions. [49]

9. Is there a modern analogue of the RTML for tuberculosis infection?
Interferon gamma-releasing assays and the tuberculin test are currently used to assess the cellular immune response to tuberculosis antigens, rather than the RTML. [50]

10. Does the average patient need this test now?
In most situations, no. If contact allergy, drug hypersensitivity, or another T-cell reaction is suspected, more modern and better-validated methods are usually chosen. [51]

Key findings

The RTML is an old functional test of cellular immunity, historically used as a laboratory equivalent of the delayed-type hypersensitivity reaction. Its immunological concept is clear and important for the history of clinical immunology, but the analytical limitations of the method are too significant for widespread modern routine use.

The main problems with this method are poor standardization, high variability of results, limited sensitivity, and the inability to use it as an independent basis for serious clinical decisions. This is particularly evident in the current literature on metal-associated hypersensitivity and delayed drug hypersensitivity. [53]

Today, patch tests, delayed-read intradermal tests, lymphocyte transformation tests, enzyme immunospot tests, and specialized modern cell-based analysis methods are commonly used for similar purposes. Therefore, the RTML should be viewed primarily as a historically significant and, in some cases, auxiliary test, rather than as a modern routine standard. [54]