Clonal and non-clonal mast cell activation disorders

What every physician needs to know:

Mast cell activation disorders are a heterogeneous group in which patients present with signs and symptoms of disease mediated by the synthesis and release of activation products such as reformed vasoactive amines, newly generated arachidonic acid derived mediators, and induced cytokines. Mast cell activation may occur via immunoglobulin E (IgE) and/or non-IgE mediated mechanisms in the presence of clonal or non-clonally derived mast cells in tissues.

The majority of patients with symptoms due to mast cell activation have non-clonal disorders such as those mediated by specific IgE (for example, against inhalant or food allergens, Hymenoptera venom, or medications), and are cared for by allergists/immunologists. Occasionally, a patient with episodic, recurrent mast cell activation may have an unremarkable work-up for allergic causes and have no evidence of clonal mast cell disease. These patients are considered for diagnoses of idiopathic anaphylaxis or idiopathic non-clonal mast cell activation syndrome, depending on the severity of presenting symptoms.

Some patients with recurrent mast cell activation may have clonal mast cells in bone marrow as in mastocytosis (systemic or cutaneous) or a monoclonal mast cell activation (MMAS). Patients with clonal mast cell disorders generally have varying degrees of expansion of the mast cell compartment derived from a progenitor with a genetic defect that presumably reduces the cell’s threshold for activation. These patients may have elevated serum tryptase levels, carry c-kit mutations (most commonly D816V) in lesional mast cells, or have other markers of mast cell clonality such as aberrant CD25 expression.

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The “clone” in clonal mast cell disorders refers to the progeny of the hematopoietic progenitor carrying the D816V c-kit mutation. In mastocytosis, expansion of the clone is sufficient to be detected as increased tissue burden of mast cells, for example, either as skin involvement (urticaria pigmentosa), or bone marrow infiltration (systemic mastocytosis, in which the World Health Organization [WHO] diagnostic criteria are fulfilled). On the other hand, monoclonal mast cell activation syndrome defines symptoms of mast cell activation in the presence of clonal mast cells carrying the D816V c-kit mutation without fulfilling the WHO diagnostic criteria for systemic mastocytosis, and without urticaria pigmentosa skin lesions.

While one might speculate that MMAS may simply be a precursor to systemic mastocytosis, several lines of evidence suggest that this is not the case. First, follow up data available so far do not indicate progression to systemic mastocytosis in most patients with MMAS. Second, not all patients with systemic mastocytosis go through a phase with mast cell activation symptoms before developing signs or symptoms of tissue mast cell expansion. Finally, molecular studies of various hematopoietic cell lineages in patients with clonal mast cell disorders indicate that the c-kit mutation is limited to the mast cell compartment in MMAS, whereas multilineage involvement in non-mast cell lineages is common in systemic mastocytosis.

What features of the presentation will guide me toward possible causes and next treatment steps:

Disorders of systemic mast cell activation are suspected in patients presenting with recurrent anaphylactic episodes or symptoms mediated by products of mast cell activation.

Mast cell activation may be manifested in a variety of tissues and organ systems as follows:
  • Skin

– Urticaria pigmentosa is common either isolated or as part of systemic mastocytosis and characterized by infiltrated skin lesions from which a wheal and flare can be elicited. However, conventional urticaria and angioedema are generally encountered in non-clonal, IgE mediated mast cell activation and are rare in clonal mast cell disorders such as mastocytosis. Flushing is common in both groups.

  • Respiratory

– Wheezing is uncommon in clonal mast cell disorders as compared to anaphylaxis.

  • Gastrointestinal

– Nausea, vomiting, diarrhea, abdominal cramping.

  • Cardiovascular

– Tachycardia, hypotension, syncope, presyncope. Presence of these cardiovascular symptoms, particularly in the absence of urticaria and angioedema raises the index of suspicion for clonal mast cell disorders.

  • Naso-ocular

– Congestion, conjunctival injection. In patients presenting with episodic manifestations of these symptoms affecting at least two organ systems, a vigorous work-up to identify an underlying cause, including allergic (IgE-mediated) and non-allergic causes should be initiated.

Clonal mast cell disease

Clonal mast cell disease presents in the form of mastocytosis or monoclonal mast cell activation syndrome. This diagnosis should be strongly suspected in patients presenting with recurrent episodes of hypotension, syncope, or presyncope in the absence of urticaria or angioedema. Clonal mast cell disease may be found in up to 10% of patients with systemic reactions to Hymenoptera stings. Many of these patients have evidence of venom specific IgE, either by skin or serum testing.

It is therefore recommended to determine a serum tryptase level in patients presenting with anaphylactic sensitivity to Hymenoptera venom. If the level is elevated (generally greater than 12ng/ml), further work-up for clonal mast cell disease including bone marrow biopsy and aspiration to look for pathognomic markers of clonality such as D816V c-kit mutation and CD25 expression should be considered.

If work-up reveals no underlying cause in a patient with objective physical and biochemical evidence of mast cell activation, a presumptive diagnosis of mast cell activation syndrome can be entertained. Diagnostic criteria for an idiopathic systemic mast cell activation syndrome have recently been proposed. The patient would need to meet all four of the following criteria:

  • Episodic symptoms consistent with mast cell mediator release affecting at least two organ systems (see above)

  • A decrease in frequency or severity of symptoms

– With therapies targeting mast cell mediators (for example, H1 and H2 histamine receptor blockers, anti-leukotriene medications including cysLT [cysteinyl leukotriene] receptor blockers and 5-lipoxygenase inhibitors, mast-cell stabilizers).

  • Evidence of an increase in a validated urinary or serum marker of mast cell activation during symptomatic periods

– For example, serum tryptase, 24-hour urine N-methylhistamine, or PGD2 [prostaglandin D2]).

  • Rule out primary clonal (for example, mastocytosis, monoclonal mast cell activation syndrome) and non-clonal idiopathic entities (such as idiopathic anaphylaxis)

What laboratory studies should you order to help make the diagnosis and how should you interpret the results?

Serum and urine markers of mast cell burden and activation

Only a few mast cell products can be tested commercially in biological fluids. These include serum tryptase, 24-hour urine metabolites of histamine (N-methylhistamine), and PGD2 (or the metabolite, 11-beta-prostaglandin F2).


Tryptase is a tetrameric serine protease produced by mast cells in a pre-pro form, and stored in granules in the active form. Its release can be measured as a surrogate marker of mast cell burden or activation. Baseline levels of tryptase as measured by commercial assays reflect a constitutively released protryptase, and can be used as a surrogate marker of mast cell burden, while an elevation in mature (and consequently total) tryptase level is seen after a mast cell degranulation event such as anaphylaxis. Median total tryptase levels in the general population are approximately 4.5 to 5ng/ml. Most laboratories have a cutoff value of 10 to 12ng/ml as the upper limit of normal, representing two standard deviations over the mean values of general population. Baseline tryptase levels greater than 20ng/ml are suggestive of clonal mast cell disease.

Tryptase has a half-life of approximately 1.5 hours in the circulation. Therefore, tryptase levels should be checked within 4 hours after suspected episodes of mast cell activation. Mast cell activation is suspected if the levels are elevated when compared to baseline. Baseline tryptase levels may be elevated in chronic renal failure, acute or chronic myeloproliferative or dysplastic neoplasms, and chronic eosinophilic leukemia. False elevations in serum tryptase may be seen in a small group of patients due to heterophilic antibody interference in the assay. Heterophilic antibodies are human anti-animal (mouse) antibodies that bridge the capture and detection antibodies used in enzyme-linked immunosorbent assays (ELISAs) (such as those used for detection of tryptase) without presence of the analyte. They can be formed in patients who receive monoclonal antibodies raised in animals for therapeutic purposes, or as a result of occupational or domestic exposure to animals.

Urinary mast cell mediator metabolites

Other commercially available measurements of mast cell activation include urinary metabolites of histamine, and prostaglandin D2. Histamine in blood has a very short half life, can also be released by basophils, and is usually not recommended as a clinical test to confirm mast cell activation. N-methylhistamine can be measured in a 24 hour urine collection specimens as a more reliable marker of mast cell activation, however the test is neither more specific, nor sensitive than tryptase in diagnosis of mastocytosis.

To summarize, serum tryptase is the best available clinical assay in the work-up of patients with suspected mast cell disease. Elevated levels of tryptase at baseline, particularly those greater than 20ng/ml are suggestive of increased total body mast cell burden and mastocytosis, whereas transient elevations after suspected episodes of mast cell activation provide laboratory proof that mast cell degranulation or anaphylaxis has occurred.

Histopathologic studies

A bone marrow biopsy and aspiration is recommended in patients with elevated tryptase levels greater than 20ng/ml, and should be strongly considered in patients with hymenoptera induced or idiopathic anaphylaxis if their presentation includes either of the following:

  • A baseline tryptase level of greater than 12ng/ml

  • Hypotensive syncope or presyncope without urticaria or angioedema

The bone marrow biopsy should be stained immunohistochemically for tryptase to visualize mast cells. CD25 staining should be employed by immunohistochemistry or flow cytometry to look for aberrant expression of this molecule, which is a very sensitive marker of clonal mast cell disease. Mast cell morphology should be examined carefully for aberrations in morphology including spindle shapes, hypogranulation, eccentric, or multilobated nuclei which favor clonal disease. A c-kit mutational analysis for D816V mutation should be performed.

Interpretation of the bone marrow pathology: The World Health Organization recognizes one major and four minor criteria for the diagnosis of systemic mastocytosis. If one major and one minor or three minor criteria are met, the diagnosis of systemic mastocytosis is established. If only one or two minor criteria are met, the diagnosis of systemic mastocytosis is not conclusive and monoclonal mast cell activation merits consideration. If no criteria are met, a non-clonal disorder of mast cell activation should be considered in a patient with clinical signs and symptoms of mast cell activation.

What conditions can underlie mast cell activation:

Non-clonal activation

The majority of disorders resulting in mast cell activation are due to activation of non-clonal mast cells in response to IgE or non-IgE mediated stimuli.

IgE mediated mast cell activation occurs in allergic sensitization due to environmental, food, drug, or venom allergies.

Non-IgE mediated mast cell activation can be observed at least locally at tissue level in a variety of inflammatory and neoplastic disorders, and both locally and systemically in response to physical stimuli and drugs.

Clonal activation

Clonal mast cell disorders such as mastocytosis and monoclonal mast cell activation syndrome are associated with mast cell activation, and in systemic mastocytosis are due to mast cell expansion.

The incidence of symptoms and signs reminiscent of anaphylaxis including hypotensive syncope in mastocytosis is approximately 30%. In approximately 50% of this population, an IgE-mediated cause, primarily hymenoptera venom, can be demonstrated. Less severe symptoms such as flushing, lightheadedness, abdominal cramping, and diarrhea not progressing to hypotension or syncope are observed episodically in most patients with systemic mastocytosis.

In some patients with systemic reactions due to hymenoptera venom, anaphylaxis is observed in presence of allergen specific IgE and clonal mast cells carrying the D816V c-kit mutation. In these patients, multiple mechanisms contribute to anaphylaxis, including allergic sensitization with venom specific IgE, clonal expansion of mast cells carrying the c-kit mutation, and the impact of the gain in function mutation on the direct mast cell activation properties of the hymenoptera venom.

When do you need to get more aggressive tests:

A bone marrow biopsy and aspiration is recommended in patients presenting with mast cell activation symptoms in the following circumstances:

  • Baseline tryptase level less than 20ng/ml, regardless of the symptoms

  • Presence of urticaria pigmentosa or cutaneous mastocytosis in an adult

  • Recurrent hypotensive syncopal episodes in the absence of urticaria and angioedema

What imaging studies (if any) will be helpful?

Routine imaging is not recommended in mast cell activation disorders, unless there is a specific indication to do so.

In patients diagnosed with a clonal mast cell activation disorder, bone densitometry (DEXA) scans should be periodically followed due to high incidence of osteoporosis.

Other tests such as abdominal imaging to determine hepatosplenomegaly or bone scans in patients with clonal mast cell activation disorders can be considered, depending on presenting symptoms.

What therapies should you initiate immediately and under what circumstances – even if root cause is unidentified?

At least two self-injectable epinephrine devices should be prescribed to patients with a history of anaphylaxis and those with clonal mast cell disorders. The patient should be trained in its use and indications.

In addition, H1 and H2 histamine-receptor blockers should be considered depending on symptomatology. Anti-leukotriene medications such as cysteinyl leukotriene receptor 1 (cysLT) receptor antagonists or 5-lipoxygenase (5-LO) inhibitors may also be added if there is no response to antihistamines alone; oral cromolyn sodium which has poor absorption may be useful in patients with gastrointestinal symptoms.

Systemic steroids may be used in patients with recurrent anaphylactic episodes unresponsive to other anti-mediator agents. There are case reports of beneficial effects of omalizumab, an anti-IgE monoclonal antibody in some patients with mastocytosis and recurrent anaphylaxis.

What other therapies are helpful for reducing complications?

Patients with clonal mast cell disease should be advised to maintain a good calcium and vitamin D intake to reduce the complications related to osteoporosis.

Patients should be counseled about potential triggers of mast cell activation including physical stimuli, alcohol, heat, temperature changes, fever, and certain drugs including general anesthetics, opioids, contrast media nonsteroidal anti-inflammatory drugs (NSAIDs), and muscle relaxants. Those with venom allergy are at increased risk for life threatening reactions after hymenoptera stings. Those patients should be referred to an allergist and considered for venom immunotherapy. Otherwise, the incidence of IgE-mediated allergy is not higher in patients with clonal mast cell disease but co-existence of atopic disease can potentially enhance the severity of symptoms.

What should you tell the patient and the family about prognosis?

Mast cell activation disorders are heterogeneous and the prognosis for individual patients depends on the underlying etiology. Patients with mastocytosis should be classified into one of seven WHO categories and are subject to prognostic guidelines of that category.

In general, patients with cutaneous or indolent systemic mastocytosis have excellent prognosis comparable to age matched general population.

Those with aggressive mastocytosis or other associated hematologic disorders indicative of mutations elsewhere in the development pathway have poorer prognosis.

The long term natural course of monoclonal mast cell activation syndrome is not known, but a subset of patients may progress to systemic mastocytosis.

Life threatening anaphylactic reactions may be encountered in a non-clonal setting or in all patients with underlying clonal mast cell expansion.

“What if” scenarios.

What if the bone marrow biopsy is reported as normal? Can clonal mast cell activation syndrome be ruled out?

The bone marrow biopsy of patients with lower tryptase levels (less than 20ng/ml) generally does not have pathogenomic mast cell aggregates and may be reported as normal. Bone marrow biopsies and aspirates of these patients should be carefully examined by a pathologist experienced in mast cell disorders for presence of atypical spindle shaped mast cells, and serial sections should be stained by tryptase and CD25 before a clonal mast cell disease is ruled out.

Is cytoreductive therapy indicated for treatment of mast cell activation disorders?

Cytoreductive therapy is generally not indicated for patients with mast cell activation disorders due to unacceptable risk versus benefit ratio in most patients. Patients with aggressive categories of mastocytosis are candidates for cytoreductive therapies.

Imatinib inhibits wild type c-kit but not D816V mutant, and therefore is not a suitable therapy for most patients with clonal mast cell activation disorders. There have been no studies to evaluate the safety and efficacy of imatinib in non-clonal mast cell activation disorders.


Mast cell mediators

Symptoms experienced in mast cell activation syndromes result from mast cell mediators released from activated mast cells.

In IgE-mediated mast cell activation, cross-linking of high affinity IgE receptors by allergen, results in immediate exocytosis of mast cell secretory granules, followed within minutes by de novo synthesis of lipid mediators from membrane arachidonic acid, and later by synthesis of cytokines. Mast cell granules contain cationic proteases such as tryptase, chymase, and carboxypeptidase, which are found complexed with heparin and chondroitin sulfate, and vasoactive mediators such as histamine and platelet activating factor, that are capable of mediating end organ signs and symptoms of anaphylaxis.

While the exact physiologic function and substrates of proteases remain to be fully elucidated, serum or plasma tryptase levels can be used as a surrogate marker of mast cell burden and activation. Lipid mediators synthesized from nuclear membrane associated arachidonic acid liberated by the action of phospholipase A2 enter into both the cyclooxygenase and 5-lipoxygenase pathways. The former leads to prostaglandin D2 and the latter to the cysteinyl leukotrienes beginning with LTC4, from which the metabolites LTD4 and LTE4 are derived. Mast cells are also rich sources of cytokines such as TNF (tumor necrosis factor), IL(interleukin)-6, 4, 5 and 13.

Stem cell factor and Kit

Mast cell growth, differentiation, and protection from apoptosis are highly dependent on stem cell factor (SCF). SCF binds to its membrane bound receptor Kit, which has an intrinsic tyrosine kinase activity. Clonal mast cell disorders are associated with gain of function mutations in c-kit, resulting in ligand-independent constitutive activation and autophosphorylation of the receptor. The most commonly detected mutation is the somatic point mutation D816V. The likelihood of detection of c-kit mutations increases if lesional tissue is analyzed. Based on the demonstration of shared signal transduction molecules (such as NTAL), and enhancement of IgE-mediated mast cell degranulation by SCF, it has been hypothesized that constitutive activation of Kit reduces the activation threshold of mast cells via IgE and non-IgE mechanisms, thereby contributing to increased incidence of anaphylaxis and mast cell activation in clonal mast cell disorders.

Clinically, at least two patterns of mast-cell activation can be distinguished based on the underlying mechanism, including the state of the mast cells and the pathways of activation:

  • Anaphylaxis mediated by allergen binding to specific IgE

– Anaphylaxis mediated by allergen binding to specific IgE occupying the high affinity IgE receptors on the surface of the mast cells is generally seen in non-clonal mast cell disorders in the presence of urticaria and angioedema, and bronchial obstruction.

  • Clonal mast cell disorders, systemic mastocytosis and monoclonal mast cell activation syndrome

– Clonal mast cell disorders, that is, systemic mastocytosis and monoclonal mast cell activation syndrome, in which mast cell activation occurs in presence of an expanded pool of mast cells and the activating c-kit mutation are primarily associated with cardiovascular manifestations such as hypotension, tachycardia, presyncope, and syncope.

A combination of these two pathogenetic mechanisms can be seen in patients with clonal mast cell disease and IgE-mediated hymenoptera sensitivity. These patients experience predominantly cardiovascular symptoms during the sting induced anaphylactic concomitants. The underlying mechanisms for the differences in clinical symptoms with IgE-dependent mast cell activation in clonal as compared to non-clonal mast cell activation are not known, but may be related to involvement of different subsets of mast cells (constitutive versus T-cell dependent ), their anatomic locations (connective tissue and perivascular versus mucosal), the intensity of their surface IgE receptors, and the profile of elicited mediators.

What other clinical manifestations may help me to diagnose mast cell activation disorders?

Approximately 80% of patients with mastocytosis present with cutaneous involvement, most commonly urticaria pigmentosa. Therefore, a careful skin exam is necessary in all patients with suspected mast cell activation disorders. Urticaria pigmentosa is not found in patients with monoclonal mast cell activation syndrome or non-clonal mast cell activation disorders.

One should maintain a high index of suspicion for clonal mast cell disease in patients with a history of hymenoptera venom anaphylaxis and recurrent idiopathic anaphylaxis, especially if the prominent symptoms during episodes involve hemodynamic instability, instead of urticaria or angioedema.

What other additional laboratory studies may be ordered?

In certain instances, it may not be straightforward to diagnose mast cell activation due to overlapping clinical features between endocrine, neurologic, and cardiovascular disorders. Differential diagnosis of some of the symptoms encountered in mast cell activation such as flushing, pruritus, and syncope is expansive.

A tryptase level, if checked within 4 hours of a suspected mast cell activation event provides useful confirmatory information of mast cell activation if found elevated, in comparison to baseline.

In systemic mastocytosis, tryptase level is elevated at baseline and may be further elevated following an anaphylactic or hypotensive event, usually returning to that patient’s baseline within 4 hours. Normal tryptase levels however, do not necessarily rule out mast cell activation.

What’s the evidence?

Akin, C, Valent, P, Metcalfe, DD. “Mast cell activation syndrome: Proposed diagnostic criteria”. J Allergy Clin Immunol.. vol. 126. 2010. pp. 1099-1104. [This article discusses classification of clonal and non-clonal mast cell activation disorders and proposes diagnostic criteria which are later adopted by an international consensus group.]

Akin, C, Scott, LM, Kocabas, CN. “Demonstration of an aberrant mast-cell population with clonal markers in a subset of patients with "idiopathic" anaphylaxis”. Blood.. vol. 110. 2007. pp. 2331-2333. [This article is the first demonstration of clonal mast cells in recurrent anaphylaxis without meeting diagnostic criteria for systemic mastocytosis leading to definition of monoclonal mast cell activation syndrome.]

Alvarez-Twose, I, Gonzalez de Olano, D, Sanchez-Munoz, L. “Clinical, biological, and molecular characteristics of clonal mast cell disorders presenting with systemic mast cell activation symptoms”. J Allergy Clin Immunol.. vol. 125. 2010. pp. 1269-1278. [Single center experience in clinical features of clonal and non-clonal mast cell activation syndromes.]

Schwartz, LB. “Diagnostic value of tryptase in anaphylaxis and mastocytosis”. Immunol Allergy Clin North Am.. vol. 26. 2006. pp. 451-463. [Review article on the utility of tryptase levels in mast cell activation.]

Castells, M, Austen, KF. “Mastocytosis: mediator-related signs and symptoms”. Int Arch Allergy Immunol. vol. 127. 2002. pp. 147-152. [Comprehensive discussion of clinical features and pathophysiology of mast cell mediator release.]

Metcalfe, DD, Peavy, RD, Gilfillan, AM. “Mechanisms of mast cell signaling in anaphylaxis”. J Allergy Clin Immunol.. vol. 124. 2009. pp. 639-646. [Review on mast cell signaling in mast cell activation.]

Akin, C. “Anaphylaxis and mast cell disease: what is the risk?”. Curr Allergy Asthma Rep.. vol. 10. 2010. pp. 34-38. [A discussion of high incidence of anaphylaxis and its various etiologies in mastocytosis.]

Valent, P, Horny, H, Escribano, L. “Diagnostic criteria and classification of mastocytosis: a consensus proposal”. Leuk Res. vol. 25. 2001. pp. 603-625. [Consensus document adopted by WHO as current diagnostic criteria.]

Bonadonna, P, Perbellini, O, Passalacqua, G. “Clonal mast cell disorders in patients with systemic reactions to Hymenoptera stings and increased serum tryptase levels”. Journal of Allergy & Clinical Immunology.. vol. 123. 2009. pp. 680-686. [Demonstration of high rate of clonal mast cell disorders in patients with hymenoptera anaphylaxis and elevated tryptase levels.]

Valent, P, Akin, C, Arock, M. “Definitions, criteria and global classification of mast cell disorders with special reference to mast cell activation syndromes: a consensus proposal.”. Int Arch Allergy Immunol.. vol. 157. 2012. pp. 215-225. [International consensus group recommendations on diagnostic criteria for mast cell activation disorders.]