Treatment of Recurrent Refractory Pediatric Pre-B Acute Lymphoblastic Leukemia Using Inotuzumab Ozogamicin Monotherapy Resulting in CD22 Antigen Expression Loss as a Mechanism of Therapy Resistance.

BACKGROUND: Inotuzumab ozogamicin is a novel antibody-drug conjugate that targets CD22, a common antigen on pre-B acute lymphoblastic leukemia cells. OBSERVATIONS: A 7-year-old boy with pre-B acute lymphoblastic leukemia in his second relapse was given 2 cycles of inotuzumab ozogamicin. He responded morphologically with a negative bone marrow evaluation. However, he relapsed in cycle 3 of therapy with a loss of CD22 expression on his lymphoblast population. CONCLUSION: To our knowledge, this is the first published case of CD22 expression loss as a mechanism of therapy resistance for inotuzumab ozogamicin.

Ectopic expression of a novel CD22 splice-variant regulates survival and proliferation in malignant T cells from cutaneous T cell lymphoma (CTCL) patients.

CD22 is a member of the Sialic acid-binding Ig-like lectin (Siglec) family of lectins described to be exclusively present in B lymphocytes and B cell-derived neoplasms. Here, we describe a novel splice form of CD22 (designated CD22∆N), which lacks the N-terminal domain as demonstrated by exon-specific RT-PCR and differential recognition by anti-CD22 antibodies. Importantly, CD22∆N mRNA is expressed in skin lesions from 39 out of 60 patients with cutaneous T cell lymphoma (CTCL), whereas few patients (6 out of 60) expresses full-length, wild type CD22 (CD22wt). In addition, IHC staining of tumor biopsies confirmed the expression of CD22 in CD4+ T cells. Moreover, four out of four malignant T cell lines express CD22: Two cell lines express CD22∆N (MyLa2059 and PB2B) and two express CD22wt (MAC-1 and MAC-2A). siRNA-mediated silencing of CD22 impairs proliferation and survival of malignant T cells, demonstrating a functional role for both CD22∆N and CD22wt in these cells.In conclusion, we provide the first evidence for an ectopic expression of CD22 and a novel splice variant regulating malignant proliferation and survival in CTCL. Analysis of expression and function of CD22 in cutaneous lymphomas may form the basis for development of novel targeted therapies for our patients.

Inotuzumab ozogamicin in B-cell acute lymphoblastic leukemias and non-Hodgkin's lymphomas.

INTRODUCTION: The expression profile of the CD22 antigen and its role in B-cell function make it an important target in B-cell leukemias and lymphomas. Inotuzumab ozogamicin (IO), a humanized monoclonal antibody targeting CD22, is one of the most promising monoclonal antibodies for the treatment of relapsed/refractory B-cell acute lymphoblastic leukemia (ALL). AREAS COVERED: This article reviews the current literature of IO in adult leukemias and lymphomas. EXPERT OPINION: Single-agent IO has demonstrated activity in patients with relapsed B-cell ALL and non-Hodgkin lymphoma (NHL). It has also demonstrated favorable early results when combined with chemotherapy in older patients with ALL. There is potential for IO to be combined with other targeted therapies under development for these diseases; data are still early and further studies of IO are warranted. While the pivotal randomized study of IO for relapsed NHL versus physician's choice did not show a statistically significant advantage in response rate, the results of the pivotal study in ALL are not yet available.

Genetic regulation of gene expression in the lung identifies CST3 and CD22 as potential causal genes for airflow obstruction.

BACKGROUND: COPD is a complex chronic disease with poorly understood pathogenesis. Integrative genomic approaches have the potential to elucidate the biological networks underlying COPD and lung function. We recently combined genome-wide genotyping and gene expression in 1111 human lung specimens to map expression quantitative trait loci (eQTL). OBJECTIVE: To determine causal associations between COPD and lung function-associated single nucleotide polymorphisms (SNPs) and lung tissue gene expression changes in our lung eQTL dataset. METHODS: We evaluated causality between SNPs and gene expression for three COPD phenotypes: FEV(1)% predicted, FEV(1)/FVC and COPD as a categorical variable. Different models were assessed in the three cohorts independently and in a meta-analysis. SNPs associated with a COPD phenotype and gene expression were subjected to causal pathway modelling and manual curation. In silico analyses evaluated functional enrichment of biological pathways among newly identified causal genes. Biologically relevant causal genes were validated in two separate gene expression datasets of lung tissues and bronchial airway brushings. RESULTS: High reliability causal relations were found in SNP-mRNA-phenotype triplets for FEV(1)% predicted (n=169) and FEV(1)/FVC (n=80). Several genes of potential biological relevance for COPD were revealed. eQTL-SNPs upregulating cystatin C (CST3) and CD22 were associated with worse lung function. Signalling pathways enriched with causal genes included xenobiotic metabolism, apoptosis, protease-antiprotease and oxidant-antioxidant balance. CONCLUSIONS: By using integrative genomics and analysing the relationships of COPD phenotypes with SNPs and gene expression in lung tissue, we identified CST3 and CD22 as potential causal genes for airflow obstruction. This study also augmented the understanding of previously described COPD pathways.

A reevaluation of CD22 expression in human lung cancer.

CD22 is a transmembrane glycoprotein expressed by mature B cells. It inhibits signal transduction by the B-cell receptor and its coreceptor CD19. Recent reports indicate that most human lung cancer cells and cell lines express CD22, making it an important new therapeutic target for lung cancer. The objective of our studies was to independently validate these results with the goal of testing the efficacy of our CD22 immunotoxins on lung cancer cell lines. As determined by quantitative real-time PCR analysis, we found that levels of CD22 mRNA in a panel of human lung cancer cell lines were 200 to 60,000-fold lower than those observed in the human CD22(+) Burkitt lymphoma cells, Daudi. Using flow cytometry with a panel of CD22 monoclonal antibodies and Western blot analyses, we could not detect surface or intracellular expression of CD22 protein in a panel of lung cancer cell lines. In addition, the in vitro proliferation of the lung tumor cell lines was not affected by either CD22 antibodies or our highly potent anti-CD22 immunotoxin. In contrast, CD22(+) Daudi cells expressed high levels of CD22 mRNA and protein, and were sensitive to our CD22 immunotoxin. Importantly, primary non-small cell lung cancers from more than 250 patient specimens did not express detectable levels of CD22 protein as assessed by immunohistochemistry. We conclude that CD22 is not expressed at measurable levels on the surface of lung cancer cells, and that these cells cannot be killed by anti-CD22 immunotoxins.

Functional evaluation of activation-dependent alterations in the sialoglycan composition of T cells.

Sialic acids (Sias) are often conjugated to the termini of cellular glycans and are key mediators of cellular recognition. Sias are nine-carbon acidic sugars, and, in vertebrates, the major species are N-acetylneuraminic acid (Neu5Ac) and N-glycolylneuraminic acid (Neu5Gc), differing in structure at the C5 position. Previously, we described a positive feedback loop involving regulation of Neu5Gc expression in mouse B cells. In this context, Neu5Gc negatively regulated B-cell proliferation, and Neu5Gc expression was suppressed upon activation. Similarly, resting mouse T cells expressed principally Neu5Gc, and Neu5Ac was induced upon activation. In the present work, we used various probes to examine sialoglycan expression by activated T cells in terms of the Sia species expressed and the linkages of Sias to glycans. Upon T-cell activation, sialoglycan expression shifted from Neu5Gc to Neu5Ac, and the linkage shifted from α2,6 to α2,3. These changes altered the expression levels of sialic acid-binding immunoglobulin-like lectin (siglec) ligands. Expression of sialoadhesin and Siglec-F ligands increased, and that of CD22 ligands decreased. Neu5Gc exerted a negative effect on T-cell activation, both in terms of the proliferative response and in the context of activation marker expression. Suppression of Neu5Gc expression in mouse T and B cells prevented the development of nonspecific CD22-mediated T cell-B cell interactions. Our results suggest that an activation-dependent shift from Neu5Gc to Neu5Ac and replacement of α2,6 by α2,3 linkages may regulate immune cell interactions at several levels.

Results of inotuzumab ozogamicin, a CD22 monoclonal antibody, in refractory and relapsed acute lymphocytic leukemia.

BACKGROUND: CD22 expression occurs in >90% of patients with acute lymphocytic leukemia (ALL). Inotuzumab ozogamicin, a CD22 monoclonal antibody bound to calicheamicin, is active in ALL. METHODS: Patients with refractory-relapsed ALL received treatment with inotuzumab. The first 49 patients received single-dose, intravenous inotuzumab at doses of 1.3 to 1.8 mg/m2 every 3 to 4 weeks. In the next 41 patients, the schedule was modified to inotuzumab weekly at a dose of 0.8 mg/m2 on day 1 and at a dose of 0.5 mg/m2 on days 8 and 15, every 3 to 4 weeks, based on higher in vitro efficacy with more frequent exposure. RESULTS: Ninety patients were treated; 68% were in salvage 2 or beyond. Overall, 17 patients (19%) achieved a complete response (CR), 27 (30%) had a CR with no platelet recovery (CRp), and 8 (9%) had a bone marrow CR (no recovery of counts), for an overall response rate of 58%. Response rates were similar for single-dose and weekly dose inotuzumab (57% vs 59%, respectively). The median survival was 6.2 months overall, 5.0 months with the single-dose schedule, and 7.3 months with the weekly dose schedule. The median survival was 9.2 months for patients in salvage 1 (37% at 1 year), 4.3 months for patients in salvage 2, and 6.6 months for patients in salvage 3 or later. The median remission duration was 7 months. Reversible bilirubin elevation, fever, and hypotension were observed less frequently on the weekly dose. In total, 36 of 90 patients (40%) underwent allogeneic stem cell transplantation. Veno-occlusive disease was noted in 6 of 36 patients after stem cell transplantation (17%), was less frequent after the weekly schedule (7%), and with less alkylators in the preparative regimen. CONCLUSIONS: Inotuzumab single-agent therapy was highly active, safe, and convenient in patients with refractory-relapsed ALL. A weekly dose schedule appeared to be equally effective and less toxic than a single-dose schedule.

Immunoconjugates and new molecular targets in hairy cell leukemia.

Hairy cell leukemia (HCL) is a B-cell malignancy that in its classic form is exquisitely sensitive to single-agent purine analog therapy, but that is associated in many patients with late relapse and eventual purine analog resistance. Minimal residual disease, which is present in most patients achieving complete remission with purine analogs, retains Ags that are ideal for targeted therapy. Rituximab, which targets CD20, is active as a single agent, particularly if combined with purine analogs. Recombinant immunotoxins targeting either CD25 or CD22 and containing truncated Pseudomonas exotoxin have achieved major responses in relapsed/refractory HCL. Moxetumomab pasudotox in phase 1 testing achieved responses in 86% of such patients (complete in 46%) without dose limiting toxicity and often without MRD. Soluble CD22 has been used for improved detection and monitoring of HCL, particularly the poor-prognosis variant that lacks CD25. Ig rearrangements unique for each HCL patient have been cloned, sequenced, and followed by real-time quantitative PCR using sequence-specific reagents. Analysis of these rearrangements has identified an unmutated IGVH4-34-expressing poor-prognosis variant with immunophenotypic characteristics of either classic or variant HCL. The BRAF V600E mutation, reported in 50% of melanomas, is present in > 85% of HCL cases that are both classic and express rearrangements other than IGVH4-34, making HCL a potential target for specific inhibitors of BRAF V600E. Additional targets are being defined in both classic and variant HCL, which should improve both detection and therapy.

CD22 antigen is broadly expressed on lung cancer cells and is a target for antibody-based therapy.

Most patients with lung cancer still die from their disease, necessitating additional options to improve treatment. Here, we provide evidence for targeting CD22, a cell adhesion protein known to influence B-cell survival that we found is also widely expressed in lung cancer cells. In characterizing the antitumor activity of an established anti-CD22 monoclonal antibody (mAb), HB22.7, we showed CD22 expression by multiple approaches in various lung cancer subtypes, including 7 of 8 cell lines and a panel of primary patient specimens. HB22.7 displayed in vitro and in vivo cytotoxicity against CD22-positive human lung cancer cells and tumor xenografts. In a model of metastatic lung cancer, HB22.7 inhibited the development of pulmonary metastasis and extended overall survival. The finding that CD22 is expressed on lung cancer cells is significant in revealing a heretofore unknown mechanism of tumorigenesis and metastasis. Our work suggests that anti-CD22 mAbs may be useful for targeted therapy of lung cancer, a malignancy that has few tumor-specific targets.

Early murine T-lymphocyte activation is accompanied by a switch from N-Glycolyl- to N-acetyl-neuraminic acid and generation of ligands for siglec-E.

It is well established that murine T-lymphocyte activation is accompanied by major changes in cell-surface sialylation, potentially influencing interactions with sialic acid-binding immunoglobulin-like lectins (siglecs). In the present study, we analyzed early activation of murine CD4+ and CD8+ T-lymphocytes at 24 h. We observed a striking and selective up-regulation in the binding of a recombinant soluble form of siglec-E, an inhibitory siglec, which is expressed on several myeloid cell types including antigen-presenting dendritic cells. In contrast, much lower levels of T cell binding were observed with other siglecs, including sialoadhesin, CD22, and siglec-F and the plant lectins Maackia amurensis leukoagglutinin and Sambucus nigra agglutinin. By mass spectrometry, the sialic acid content of 24-h-activated CD4+ and CD8+ T-lymphocytes exhibited an increased proportion of N-acetyl-neuraminic acid (NeuAc) to N-glycolyl-neuraminic acid (NeuGc) in N-glycans. Reduced levels of NeuGc on the surface of activated T cells were demonstrated using an antibody specific for NeuGc and the expression levels of the gene encoding NeuAc- to NeuGc-converting enzyme, CMP-NeuAc hydroxylase, were also reduced. Siglec-E bound a wide range of sialylated structures in glycan arrays, had a preference for NeuAc versus NeuGc-terminated sequences and could recognize a set of sialoglycoproteins that included CD45, in lysates from activated T-lymphocytes. Collectively, these results show that early in T cell activation, glycan remodelling involves a switch from NeuGc- to NeuAc-terminating oligosaccharides on cell surface glycoproteins. This is associated with a strong up-regulation of siglec-E ligands, which may be important in promoting cellular interactions between early activated T-lymphocytes and myeloid cells expressing this inhibitory receptor.

Variables affecting the quantitation of CD22 in neoplastic B cells.

BACKGROUND: Quantitative flow cytometry (QFCM) is being applied in the clinical flow cytometry laboratory for diagnosis, prognosis, and assessment of patients receiving antibody-based therapy. ABC values and the effect of technical variables on CD22 quantitation in acute lymphoblastic leukemia (ALL), chronic lymphocytic leukemia (CLL), mantle cell lymphoma (MCL), follicular lymphoma (FCL), hairy cell leukemia (HCL) and normal B cells were studied. METHODS: The QuantiBrite System® was used to determine the level of CD22 expression (mean antibody bound per cell, ABC) by malignant and normal B cells. The intra-assay variability, number of cells required for precision, effect of delayed processing as well as shipment of peripheral blood specimens (delayed processing and exposure to noncontrolled environments), and the effect of paraformaldehyde fixation on assay results were studied. RESULTS: The QuantiBRITE method of measuring CD22 ABC is precise (median CV 1.6%, 95% confidence interval, 1.2-2.3%) but a threshold of 250 malignant cells is required for reliable CD22 ABC values. Delayed processing and overnight shipment of specimens resulted in significantly different ABC values whereas fixation for up to 12 h had no significant effect. ABC measurements determined that CD22 expression is lower than normal in ALL, CLL, FCL, and MCL but higher than normal in HCL. CONCLUSIONS: CD22 expression was atypical in the hematolymphoid malignancies studied and may have diagnostic utility. Technical variables such as cell number analyzed and delayed processing or overnight shipment of specimens impact significantly on the measurement of antigen expression by QFCM in the clinical laboratory.

A novel method for making human monoclonal antibodies.

We have developed a B cell immortalization method for low B cell numbers per well using simultaneous B cell stimulation by CpG2006 and B cell infection by Epstein-Barr virus (EBV), followed by an additional CpG2006 and interleukin-2 (IL-2) stimulus. Using this method, immunoglobulin G (IgG)-producing immortalized B cell lines were generated from peripheral blood IgG(+)CD22(+) B cells with an efficiency of up to 83%. Antibody can already be obtained from the culture supernatant after 3-4 weeks. Moreover, clonality analysis demonstrated monoclonality in 87% of the resulting immortalized B cell lines. Given the high immortalization efficiency and monoclonality rate, evidence is provided that no further subcloning is necessary. An important application of this B cell immortalization method is the characterization of (autoreactive) antibodies from patients with autoimmune disease. This could eventually lead to the identification of new autoantigens, disease markers or targets for therapy.

CD22 expression mediates the regulatory functions of peritoneal B-1a cells during the remission phase of contact hypersensitivity reactions.

Although contact hypersensitivity (CHS) has been considered a prototype of T cell-mediated immune reactions, recently a significant contribution of regulatory B cell subsets in the suppression of CHS has been demonstrated. CD22, one of the sialic acid-binding immunoglobulin-like lectins, is a B cell-specific molecule that negatively regulates BCR signaling. To clarify the roles of B cells in CHS, CHS in CD22(-/-) mice was investigated. CD22(-/-) mice showed delayed recovery from CHS reactions compared with that of wild-type mice. Transfer of wild-type peritoneal B-1a cells reversed the prolonged CHS reaction seen in CD22(-/-) mice, and this was blocked by the simultaneous injection with IL-10 receptor Ab. Although CD22(-/-) peritoneal B-1a cells were capable of producing IL-10 at wild-type levels, i.p. injection of differentially labeled wild-type/CD22(-/-) B cells demonstrated that a smaller number of CD22(-/-) B cells resided in lymphoid organs 5 d after CHS elicitation, suggesting a defect in survival or retention in activated CD22(-/-) peritoneal B-1 cells. Thus, our study reveals a regulatory role for peritoneal B-1a cells in CHS. Two distinct regulatory B cell subsets cooperatively inhibit CHS responses. Although splenic CD1d(hi)CD5(+) B cells have a crucial role in suppressing the acute exacerbating phase of CHS, peritoneal B-1a cells are likely to suppress the late remission phase as "regulatory B cells." CD22 deficiency results in disturbed CHS remission by impaired retention or survival of peritoneal B-1a cells that migrate into lymphoid organs.

Apoptosis of marginal zone B-cells in unimmunized mice.

Although evidence suggests that peripheral B cell death plays a role in maintenance of B cell homeostasis including elimination of self-reactive B cells, the site of B cell death in unimmunized peripheral lymphoid organs has not yet been studied. Here we demonstrate that cells exhibiting reduced mitochondrial membrane potential and those expressing active caspase 3, both of which are characteristic for apoptosis, are specifically accumulated in the marginal zone (MZ) B cell compartment compared to the other B cell subsets in normal unimmunized mouse spleen. These apoptotic MZ B cells exhibit increased expression of MHC class II and CD22, whose expression is shown to be increased upon B cell activation, but not other activation markers such as CD69 and CD86, suggesting that apoptotic MZ B cells are activated with partial expression of B-cell-activation-associated antigens. These results indicate that MZ is the major site of peripheral B cell apoptosis in unimmunized mice. Apoptosis of MHC class II(hi) CD22(hi) MZ B cells may contribute to B cell homeostasis.

Effects of recombinant erythropoietin on breast cancer-initiating cells.

BACKGROUND: Cancer anemia causes fatigue and correlates with poor treatment outcome. Erythropoietin has been introduced in an attempt to correct these defects. However, five recent clinical trials reported a negative impact of erythropoietin on survival and/or tumor control, indicating that experimental evaluation of a possible direct effect of erythropoietin on cancer cells is required. Cancer recurrence is thought to rely on the proliferation of cancer initiating cells (CICs). In breast cancer, CICs can be identified by phenotypic markers and their fate is controlled by the Notch pathway. METHODS: In this study, we investigated the effect of erythropoietin on CICs in breast cancer cell lines. Levels of erythropoietin receptor (EpoR), CD24, CD44, Jagged-1 expression, and activation of Notch-1 were assessed by flow cytometry. Self-renewing capacity of CICs was investigated in sphere formation assays. RESULTS: EpoR expression was found on the surface of CICs. Recombinant human Epo (rhEpo) increased the numbers of CICs and self-renewing capacity in a Notch-dependent fashion by induction of Jagged-1. Inhibitors of the Notch pathway and PI3-kinase blocked both effects. CONCLUSIONS: Erythropoietin functionally affects CICs directly. Our observation may explain the negative impact of recombinant Epo on local control and survival of cancer patients with EpoR-positive tumors.

Expression of CD22 on peripheral B cells in patients with rheumatoid arthritis: relation to CD5-positive B cells.

B cells in patients with rheumatoid arthritis (RA) are hyperactivated. Although B cell receptor signal transduction may be affected by various response regulators, CD22 plays an important role as a response regulator of B cells. Therefore, we investigated and examined CD22 expression on peripheral blood B cells of patients with RA. Thirty-two RA patients and 16 controls were enrolled in this study, and CD22 expressions on B cells were analyzed by flow cytometry. In patients with RA, CD22(+) B cells significantly decreased in comparison to the controls (ratio: P < 0.05). However, there was no correlation between this decrease and the clinical data. Interestingly, CD5(+) CD22(-) B cells significantly increased in RA patients. The decrease in CD22(+) B cells and increased in CD5(+)CD22(-) B cells play critical roles in the pathogenesis of RA mediated by the activation of B cells.

Comparative biodistributions of pretargeted radioimmunoconjugates targeting CD20, CD22, and DR molecules on human B-cell lymphomas.

Pretargeted radioimmunotherapy (PRIT) using streptavidin (SA)-conjugated antibodies (Abs), followed by clearing agent and radiolabeled biotin is a promising method that can increase the effectiveness of RIT, while decreasing the toxicities associated with directly labeled Abs. Although CD20 has been the traditional target antigen for RIT of non-Hodgkin lymphoma (NHL), studies targeting HLA DR and CD22 have yielded promising results. Targeting all 3 antigens at once may further augment the effect of PRIT. This study compares the targeting of Ramos, Raji, and FL-18 lymphoma xenografts with either anti-CD20 Ab/SA (1F5/SA), anti-HLA DR Ab/SA (Lym-1/SA), anti-CD22 Ab/SA (HD39/SA), or all 3 conjugates in combination, followed 24 hours later by a biotin-N-acetyl-galactosamine clearing agent, and 3 hours after that by (111)In-DOTA-biotin. The Ab/SA conjugate yielding the best tumor uptake and tumor-to-normal organ ratios of radioactivity varied depending on the target antigen expression on the cell line used, with 1F5/SA and Lym-1/SA yielding the most promising results overall. Also, the best tumor-to-normal organ ratios of absorbed radioactivity were obtained using single conjugates optimized for target tumor antigen expression rather than the combination therapy. This study highlights the importance of screening the antigenic expression on lymphomas to select the optimal reagent for PRIT.

Decreased expression levels of CD22 and L-selectin on peripheral blood B lymphocytes from patients with bullous pemphigoid.

Bullous pemphigoid (BP), an autoimmune subepidermal-blistering disease of the elderly, is caused by antibodies against BP antigens at the epidermal basement membrane zone (BMZ). CD22 is a B lymphocyte specific response regulator, which is down-regulated after B-cell activation. Old CD22-deficient mice produce class-switched autoantibodies. To assess the role of CD22 in the pathogenesis of BP, we examined CD22 expression on B cells from BP patients and correlated its expression with clinical parameters. B cell expression of CD22 was 20% lower in BP patients when compared to healthy control subjects. In addition, B cells from BP patients showed decreased expression of L-selectin, which is an indicator of leukocyte activation, and CD22 expression levels were correlated with L-selectin expression. These results suggest that the decreased CD22 expression may be associated with the activation of B cells in BP. CD22 expression levels in BP patients did not correlate with the levels of anti-epidermal BMZ antibodies, and old CD22-deficient mice did not develop the anti-epidermal BMZ antibody. These results suggest that a decrease in CD22 expression may not be associated with BP-specific antibody production.

Urticaria pigmentosa and mastocytosis: the role of immunophenotyping in diagnosis and determining response to treatment.

Recent advances in our understanding of mast cell biology and disease resulted in identification of important differences in expression of mast cell surface antigens in normal and neoplastic mast cells. Most notably, detection of aberrant expression of CD25 and CD2 on the surface of neoplastic mast cells but not on their normal counterparts leads to the inclusion of this immunophenotypic abnormality in the World Health Organization's diagnostic criteria for systemic mastocytosis. Aberrant mast cell surface marker expression can be detected in the bone marrow aspirate by flow cytometry, even in patients with limited disease that lacks histopathologically detectable aggregates of mast cells in bone marrow biopsy sections. Flow cytometric analysis of bone marrow mast cells is therefore a sensitive method of diagnosis of mast cell disease and is expected to find increasing use in determining response to emerging mast cell cytoreductive therapies.