Role of common cytokine receptor γ chain (γc)– and Jak3-dependent signaling in the proliferation and survival of murine mast cells

CD56-positive Angioimmunoblastic T-cell Lymphoma Complicated by Chylothorax

A 77-year-old woman presented with systemic lymphadenopathy and bilateral pleural effusion. Angioimmunoblastic T-cell lymphoma (AITL) was diagnosed based on the results of a lymph node biopsy. AITL cells expressed the aberrant antigen of CD56. The bilateral pleural effusion was attributed to chylothorax, not the infiltration of lymphoma cells into the pleura, as determined by the pleural fluid analysis. We therefore diagnosed her with CD56-positive AITL complicated by chylothorax. She achieved complete remission by multidrug chemotherapy. AITL is commonly complicated by pleural effusion, but rarely by chylothorax. This is the first case of CD56-positive AITL complicated by chylothorax.

Cytokine Storm Syndrome in SARS-CoV-2 Infections: A Functional Role of Mast Cells

Cytokine storm syndrome is a cascade of escalated immune responses disposing the immune system to exhaustion, which might ultimately result in organ failure and fatal respiratory distress. Infection with severe acute respiratory syndrome-coronavirus-2 can result in uncontrolled production of cytokines and eventually the development of cytokine storm syndrome. Mast cells may react to viruses in collaboration with other cells and lung autopsy findings from patients that died from the coronavirus disease that emerged in 2019 (COVID-19) showed accumulation of mast cells in the lungs that was thought to be the cause of pulmonary edema, inflammation, and thrombosis. In this review, we present evidence that a cytokine response by mast cells may initiate inappropriate antiviral immune responses and cause the development of cytokine storm syndrome. We also explore the potential of mast cell activators as adjuvants for COVID-19 vaccines and discuss the medications that target the functions of mast cells and could be of value in the treatment of COVID-19. Recognition of the cytokine storm is crucial for proper treatment of patients and preventing the release of mast cell mediators, as impeding the impacts imposed by these mediators could reduce the severity of COVID-19.

Progression-free survival at 24 months as a predictor of survival outcomes after CHOP treatment in patients with peripheral T-cell lymphoma: a single-center validation study in a Japanese population

Peripheral T-cell lymphoma (PTCL) is a group of aggressive lymphomas commonly treated with cyclophosphamide, doxorubicin, vincristine, and prednisone (CHOP). Progression-free survival at 24 months (PFS24) constitutes a survival predictor for some lymphomas but has not been examined in Asian populations. We retrospectively investigated whether PFS24 was predictive of survival outcomes after CHOP treatment in 73 Japanese patients with PTCL. Patients without PFS24 had shorter median subsequent overall survival (OS) (20.2 vs. 121.0 months, p < 0.001) and shorter median subsequent progression-free survival (5.0 vs. 17.1 months, p = 0.03). Patients without PFS24 had worse overall (62.5% vs. 100%) and complete response rates (45.8% vs. 96.0%) (both p < 0.001). PFS24 absence (hazard ratio: 3.34, p = 0.004) and poor performance status (hazard ratio: 3.17, p = 0.04) were independently predictive of shorter OS. These findings suggest that PFS24 is predictive of survival after CHOP treatment in Japanese patients with PTCL.

The clinicopathological relevance of uniform CD56 expression in anaplastic large cell lymphoma: a retrospective analysis of 18 cases

Background

Anaplastic large cell lymphoma (ALCL) with uniform CD56 expression is a rare condition, that has been described in limited literature, and its clinicopathological features have not yet been well illustrated. The aim of our study was to fully investigate the clinical, histological, immunohistochemical and molecular features of CD56+ ALCL.

Methods

The clinical and histological characteristics of CD56+ ALCL cases were retrospectively evaluated. The immunohistochemical phenotype, status of Epstein-Barr virus (EBV) and T-cell receptor (TCR) gene rearrangement were examined. Overall survival was also analyzed.

Results

Eighteen (5.8%) cases with diffuse CD56 expression were identified out of 313 archived ALCL cases with CD56 test. CD56 expression was significantly higher in ALK+ systemic ALCLs (sALCLs) (13/64, 20.3%) than in ALK- sALCLs (3/101, 3.0%) (p < 0.001) as well as primary cutaneous ALCLs (2/148, 1.4%) (p < 0.001). Regarding the CD56+ ALK+ sALCLs, the median age was 20 years (range, 8–60 years) with a male-to-female ratio of 2.3:1, and these cases more frequently affected extranodal sites (11/38, 28.9%) rather than lymph nodes (2/26, 7.7%) (p = 0.038). Eleven (84.6%) cases presented with stage I-II diseases, which was significantly more than their CD56- ALK+ counterparts (45.5%) (p = 0.015). Histologically, 2 ALK+ cases were of small cell variant and all the others displayed characteristic morphology of classic ALCL. Regarding the immunophenotype, both CD30 and CD56 were diffusely positive in all cases. CD3, CD43, anaplastic lymphoma kinase-1 (ALK1), TIA-1, EMA expression was observed in 30.8% (4/13), 90.9% (10/11), 100% (13/13), 100% (9/9), and 80.0% (8/10) cases, respectively. EBV infection was consistently absent. Monoclonal TCR gene rearrangement was found in 100% (5/5) of investigated ALK+ cases. Chemotherapy with a CHOP regimen was most frequently employed. The 3-year overall survival (OS) rate for CD56+ ALK+ patients was 92.0%, compared with 73.0% for their CD56- counterparts, but there was no significant difference in OS between the two groups (p = 0.264).

Conclusions

Uniform CD56 expression is an unexpected condition in ALCL. Of ALK+ ALCLs, CD56 expression correlated with a high frequency of early stage and an extranodal predominance. It is of great importance to raise awareness of this condition and familiarity with its characteristic features to avoid diagnostic and therapeutic pitfalls. Further investigations are warranted for a better understanding of this unusual phenotype and the significance of CD56 expression in ALCL.

IgE and mast cells: The endogenous adjuvant

Mast cells and IgE are most familiar as the effectors of type I hypersensitivity reactions including anaphylaxis. It is becoming clear however that this pair has important immunomodulatory effects on innate and adaptive cells of the immune system. In this purview, they act as endogenous adjuvants to ignite evolving immune responses, promote the transition of allergic disease into chronic illness and disrupt the development of active mechanisms of tolerance to ingested foods. Suppression of IgE-mediated mast cell activation can be exerted by molecules targeting IgE, FcɛRI or signaling kinases including Syk, or by IgG antibodies acting via inhibitory Fcγ receptors. In 2015 we reviewed the evidence for the adjuvant functions of mast cells. This update includes the original text, incorporates some important developments in the field over the past five years and discusses how interventions targeting these pathways might have promise in the development of strategies to treat allergic disease.

Tofacitinib suppresses mast cell degranulation and attenuates experimental allergic conjunctivitis

BACKGROUND

Allergic conjunctivitis (AC), a common eye inflammation that affects patients' health and quality of life, is still a therapeutic challenge for ophthalmologists. Tofacitinib, a new Janus kinase (JAK) inhibitor, has been successfully used in the treatment of several disorders. Nonetheless, its effect in AC and the potential anti-allergic mechanisms are still unclear. The objective of the current study was to explore the roles of tofacitinib in preventing AC and elucidate the potential underlying mechanisms.

METHODS

Tofacitinib was used topically in BALB/c mice with experimental allergic conjunctivitis (EAC). Ocular allergic symptoms and biological modifications were examined. To assess the anti-allergic mechanisms of tofacitinib, RBL-2H3 cells and HUVECs were cultured in vitro. The inhibitory effects and mechanisms of tofacitinib were studied and measured by real-time quantitative PCR, ELISA, western blot analysis, and flow cytometry.

RESULTS

Topical administration of tofacitinib reduced the clinical symptoms of OVA-induced EAC, with a substantial mitigation in inflammatory cell infiltration, histamine release, and TNF-α mRNA as well as IL-4 mRNA expression. In vitro, tofacitinib repressed the degranulation and cytokine production in RBL-2H3 cells and reduced histamine-induced vascular hyperpermeability. The underlying mechanism might involve the downregulation of phosphorylation of JAK3/STATs signaling molecules in RBL-2H3 cells and HUVECs.

CONCLUSIONS

Our findings provide evidence that tofacitinib prevented EAC by targeting the JAK3/STATs pathway. We recommend the use of tofacitinib as an innovative approach for the treatment of AC.

Decreased MYC-associated factor X (MAX) expression is a new potential biomarker for adverse prognosis in anaplastic large cell lymphoma

MYC-associated factor X (MAX) is a protein in the basic helix-loop-helix leucine zipper family, which is ubiquitously and constitutively expressed in various normal tissues and tumors. MAX protein mediates various cellular functions such as proliferation, differentiation, and apoptosis through the MYC-MAX protein complex. Recently, it has been reported that MYC regulates the proliferation of anaplastic large cell lymphoma. However, the expression and function of MAX in anaplastic large cell lymphoma remain to be elucidated. We herein investigated MAX expression in anaplastic large cell lymphoma (ALCL) and peripheral T-cell lymphoma, not otherwise specified (PTCL-NOS) and found 11 of 37 patients (30%) with ALCL lacked MAX expression, whereas 15 of 15 patients (100%) with PTCL-NOS expressed MAX protein. ALCL patients lacking MAX expression had a significantly inferior prognosis compared with patients having MAX expression. Moreover, patients without MAX expression significantly had histological non-common variants, which were mainly detected in aggressive ALCL cases. Immunohistochemical analysis showed that MAX expression was related to the expression of MYC and cytotoxic molecules. These findings demonstrate that lack of MAX expression is a potential poor prognostic biomarker in ALCL and a candidate marker for differential diagnosis of ALCL and PTCL-NOS.

PD-L1 expression is associated with ALK positivity and STAT3 activation, but not outcome in patients with systemic anaplastic large cell lymphoma

The programmed cell death 1 (PD-1) pathway is a recently recognized mechanism of tumor immune evasion. In this study, programmed cell death ligand 1 (PD-L1) expression was evaluated in 95 patients with systemic anaplastic large cell lymphoma: 45 ALK+ and 50 ALK-. ALK+ anaplastic large cell lymphoma was more often positive for PD-L1 than ALK- anaplastic large cell lymphoma (76% vs 42%, p = 0.002). ALK- anaplastic large cell lymphoma showed a strong correlation between PD-L1 expression and STAT3 activation (measured by pSTAT3^Tyr705) (r = 0.8, p < 0.0001). In contrast, the PD-L1/pSTAT3 correlation was weaker in ALK+ anaplastic large cell lymphoma (r = 0.4, p = 0.08). In ALK- anaplastic large cell lymphoma, the PD-L1+ subgroup was more often EMA positive (69% vs 20%, p = 0.02) and tended to be less often CD2+ (50% vs 83%, p = 0.059). In ALK+ anaplastic large cell lymphoma, PD-L1 was not associated with pathologic features (all p > 0.05). Negative ALK status and high IPI score (≥3) were associated with shorter overall survival (p = 0.009 and p = 0.0005, respectively). Overall survival was not different between patients with PD-L1+ vs PD-L1- anaplastic large cell lymphoma (p = 0.44), regardless of ALK status and International Prognostic Index (IPI) score. We conclude that PD-L1 expression is more common in ALK+ anaplastic large cell lymphoma than ALK- anaplastic large cell lymphoma. In ALK- anaplastic large cell lymphoma, PD-L1 is strongly correlated with STAT3 activation and is associated with more frequent EMA and less frequent CD2 expression. PD-L1 has no prognostic significance in predicting the outcome of patients with systemic anaplastic large cell lymphoma, regardless of ALK status. PD-L1 expression on the anaplastic large cell lymphoma cells suggests these patients as potential candidates for PD-1 blockade immunotherapy.

Cutaneous permeability barrier function in signal transducer and activator of transcription 6-deficient mice is superior to that in wild-type mice

BACKGROUND

Th2 cytokines exhibit a variety of inhibitory effects on permeability barrier function via signal transducer and activator of transcription 6 (STAT6). However, the role of STAT6 signaling on the construction and/or homeostasis of permeability barrier function in the physiological state has not been fully assessed.

OBJECTIVE

We compared permeability barrier function between Stat6-deficient and wild-type C57BL/6 mice at steady state.

METHODS AND RESULTS

Measurement of transepidermal water loss and quantitative penetration assay revealed that permeability barrier function was superior in Stat6-deficient mice. Accordingly, expressions of loricrin, acidic sphingomyelinase (aSMase) and β-glucocerebrosidase (β-GlcCer'ase) in epidermis and ceramide levels in stratum corneum were elevated in STAT6-deficient mice. On the other hands, up-regulations of loricrin, aSMase and β-GlcCer'ase were not observed in 3-dimensionally cultured human keratinocytes transfected with siRNA for STAT6. Meanwhile, number of mast cells in the dermis was decreased in Stat6-deficient mice.

CONCLUSIONS

These results suggest that STAT6 signaling negatively affects permeability barrier function in vivo, even in the physiological state. However, the superior permeability barrier function in Stat6-deficient mice may be a secondary effect exerted via cells other than keratinocytes, such as mast cells, since mast cells are known to influence permeability barrier function in vivo. Blockade of STAT6 signaling might be a strategy to augment the permeability barrier function.

ALK expression plays different roles in anaplastic large-cell lymphomas and outcome of crizotinib use in relapsed/refractory ALK+ patients in a Chinese population

The prognostic value of anaplastic lymphoma kinase (ALK) expression in patients with anaplastic large-cell lymphoma (ALCL) remains controversial. Data on the clinical features of ALCL in a Chinese population are limited. We retrospectively reviewed 1293 patients with pathologically diagnosed lymphoma at Guangdong General Hospital from June 2007 through August 2016. We evaluated the incidence of ALCL, clinical characteristics, survival status, and outcome of crizotinib use in four relapsed/refractory ALK-positive patients. Among the 1293 patients, 1193 (92.3%) were non-Hodgkin's lymphoma, and 53 (4.4%) of whom were ALCL. Of the 50 ALCL patients, with a median age of 34 years, were evaluated. Among them, 33 (66.0%) were ALK-positive and 17 (34.0%) were ALK-negative. Significantly, more patients younger than 40 years old were ALK-positive than ALK-negative (66.7 vs. 23.5%; P = 0.003). The 5-year progression-free survival (PFS) for ALK-positive and ALK-negative patients were 61 and 11%, and the 5-year overall survival (OS) were 70 and 22%, respectively. Median PFS and OS were significantly better for patients with ALK-positive than ALK-negative (60.1 vs. 9.4 months, P = 0. 017; not reached vs. 32.7 months, P = 0.021). Multivariate analyses identified ALK expression, stage, and bone marrow involvement as independent prognostic factors for PFS and OS. Four relapsed ALK-positive patients were treated with crizotinib and two died. Our results suggest that ALK expression has different prognostic significance in patients with ALCL. Mechanisms underlying early relapse after chemotherapy and resistance to crizotinib need further investigation.

Allergen-specific IgG antibody signaling through FcγRIIb promotes food tolerance

BACKGROUND

Patients with food allergy produce high-titer IgE antibodies that bind to mast cells through FcεRI and trigger immediate hypersensitivity reactions on antigen encounter. Food-specific IgG antibodies arise in the setting of naturally resolving food allergy and accompany the acquisition of food allergen unresponsiveness in oral immunotherapy.

OBJECTIVE

In this study we sought to delineate the effects of IgG and its inhibitory Fc receptor, FcγRIIb, on both de novo allergen sensitization in naive animals and on established immune responses in the setting of pre-existing food allergy.

METHODS

Allergen-specific IgG was administered to mice undergoing sensitization and desensitization to the model food allergen ovalbumin. Cellular and molecular mechanisms were interrogated by using mast cell- and FcγRIIb-deficient mice. The requirement for FcγRII in IgG-mediated inhibition of human mast cells was investigated by using a neutralizing antibody.

RESULTS

Administration of specific IgG to food allergy-prone IL4raF709 mice during initial food exposure prevented the development of IgE antibodies, T_H2 responses, and anaphylactic responses on challenge. When given as an adjunct to oral desensitization in mice with established IgE-mediated hypersensitivity, IgG facilitated tolerance restoration, favoring expansion of forkhead box protein 3-positive regulatory T cells along with suppression of existing T_H2 and IgE responses. IgG and FcγRIIb suppress adaptive allergic responses through effects on mast cell function.

CONCLUSION

These findings suggest that allergen-specific IgG antibodies can act to induce and sustain immunologic tolerance to foods.

Disequilibrium in the CD8+CD28+/CD8+CD28- T Lymphocyte Balance Is Related to Prognosis in Rats with Trinitrobenzenesulfonic Acid-Induced Colitis

PURPOSE

The CD8⁺CD28⁺/CD8⁺CD28^- T lymphocyte balance is vital for human ulcerative colitis (UC) but has not been defined in experimental colitis. This investigation will try to identify the changes that occur in the CD8⁺CD28⁺/CD8⁺CD28^- T lymphocyte balance during the progression of trinitrobenzenesulfonic acid (TNBS)-induced colitis in rats.

METHODS

The frequencies of blood CD8⁺CD28⁺ and CD8⁺CD28^- T lymphocytes were detected in the rats belonging to the normal, model, and treated groups on five days using flow cytometry. The treated rats were administered with mesalazine and were euthanized after a 14-day treatment, as were the normal and model rats. The sensitivity and specificity of the CD8⁺CD28⁺/CD8⁺CD28^- T lymphocyte balance in diagnosing early colitis were analyzed by receiver operating characteristics (ROC) curves. The frequencies of CD8⁺CD28⁺ and CD8⁺CD28^- T lymphocytes in the colon tissue were tested via immunofluorescence. ELISA was used to measure the levels of the cytokines. Immunohistochemistry and Western blotting were used to detect the colonic expression of JAK3, STAT6, NFATc2, and GATA3.

RESULTS

We found that the ratio of CD8⁺CD28⁺/CD8⁺CD28^- T lymphocytes decreased, as did the level of interleukin-7, but not IL-12p40, IL-13, or IL-15, in the blood; however, the ratio increased along with JAK3, STAT6, NFATc2, and GATA3 in the colon of the rats with colitis. The changes were effectively reversed through the administration of mesalazine for 13 days. Surprisingly, the balance in the blood could sensitively distinguish rats with early colitis from normal rats.

CONCLUSION

These data show that increase in CD8⁺CD28⁺ T cells in blood and decrease in CD8⁺CD28^- T cells in colon are associated with experimental colitis.

A humanized mouse model of anaphylactic peanut allergy

BACKGROUND

Food allergy is a growing health problem with very limited treatment options. Investigation of the immunologic pathways underlying allergic sensitization to foods in humans has been greatly constrained by the limited availability of intestinal tissue and gut-resident immune cells. Although mouse models have offered insights into pathways of food sensitization, differences between rodent and human immune physiology limit the extension of these findings to our understanding of human disease.

OBJECTIVE

We sought to develop a strategy for the generation of mice with humanized adaptive immune systems, complete with tissue engraftment by human mast cells that are competent to mount specific IgE-mediated responses and drive systemic anaphylaxis on ingestion challenge.

METHODS

Nonobese diabetic severe combined immunodeficient mice lacking the cytokine receptor common gamma chain (γc-/-) and carrying a human stem cell factor transgene were engrafted with human hematopoietic stem cells. The impact of peanut (PN) feeding and IgE neutralization on the development of immune responses, mast cell homeostasis, and anaphylactic food allergy was assessed in these animals.

RESULTS

Humanized nonobese diabetic severe combined immunodeficient common gamma chain-deficient stem cell factor (huNSG) mice exhibited robust engraftment with functional human T and B lymphocytes and human mast cells were found in significant numbers in their tissues, including the intestinal mucosa. Following gavage feeding with PN, they mounted specific antibody responses, including PN-specific IgE. When enterally challenged with PN, they exhibited mast-cell-mediated systemic anaphylaxis, as indicated by hypothermia and increases in plasma tryptase levels. Anti-IgE (omalizumab) treatment ablated this anaphylactic response.

CONCLUSIONS

huNSG mice provide a novel tool for studying food allergy and IgE-mediated anaphylaxis.

TSLP induces mast cell development and aggravates allergic reactions through the activation of MDM2 and STAT6

Thymic stromal lymphopoietin (TSLP) is known to promote T helper type 2 cell-associated inflammation. Mast cells are major effector cells in allergic inflammatory responses. We noted that the population and maturation of mast cells were reduced in TSLP-deficient mice (TSLP-/-). Thus, we hypothesized that TSLP might affect mast cell development. We found that TSLP induced the proliferation and differentiation of mast cells from bone marrow progenitors. TSLP-induced mast cell proliferation was abolished by depletion of mouse double minute 2 (MDM2) and signal transducers and activators of transcription 6 (STAT6), as an upstream activator of MDM2. TSLP-/-, in particular, had a considerable deficit in the expression of MDM2 and STAT6. Also, the TSLP deficiency attenuated mast cell-mediated allergic reactions through the downregulation of STAT6 and MDM2. In an antibody microarray chip analysis, MDM2 expression was increased in atopic dermatitis patients. These observations indicate that TSLP is a factor for mast cell development, and that it aggravates mast cell-mediated immune responses.

CD56-positive adult T-cell leukemia/lymphoma: a case report and a review of the literature

A 67-year-old woman presented with a swelling on both sides of the neck. Biopsy of an enlarged cervical lymph node on the left side and flow cytometric analysis revealed CD56-positive CD4(+)CD8(+) abnormal NK/T cells. A Southern blot analysis of the cervical lymph node biopsy specimen showed a human T-cell leukemia virus type 1 provirus DNA monoclonal band. Based on these findings, the patient was diagnosed with CD56-positive adult T-cell leukemia/lymphoma. After five cycles of cyclophosphamide, doxorubicin, vincristine, and prednisone therapy, the general condition of the patient gradually declined, indicating resistance to treatment, and approximately 9 months after the onset of symptoms, the patient died. CD56 is recognized as an unfavorable prognostic marker in cases of acute myeloid leukemia with t(8;21), acute promyelocytic leukemia, and anaplastic large cell lymphoma. Only eight cases of CD56-positive adult T-cell leukemia/lymphoma have been reported so far in the literature. Most of these cases were in the advanced stage at diagnosis and had poor outcomes. It appears that the correlation between CD56 expression and outcomes in patients with adult T-cell leukemia/lymphoma should be clarified by investigating a larger number of cases in the future.

Hematopoietic cell transplantation for systemic mature T-cell non-Hodgkin lymphoma

PURPOSE

To analyze outcomes of hematopoietic cell transplantation (HCT) in T-cell non-Hodgkin lymphoma.

PATIENTS AND METHODS

Outcomes of 241 patients (112 anaplastic large-cell lymphoma, 102 peripheral T-cell lymphoma not otherwise specified, 27 angioimmunoblastic T-cell lymphoma) undergoing autologous HCT (autoHCT; n = 115; median age, 43 years) or allogeneic HCT (alloHCT; n = 126; median age, 38 years) were analyzed. Primary outcomes were nonrelapse mortality (NRM), relapse/progression, progression-free survival (PFS), and overall survival (OS). Patient, disease, and HCT-related variables were analyzed in multivariate Cox proportional hazard models to determine association with outcomes.

RESULTS

AutoHCT recipients were more likely in first complete remission (CR1; 35% v 14%; P = .001) and with chemotherapy-sensitive disease (86% v 60%; P < .001), anaplastic large-cell histology (53% v 40%; P = .04), and two or fewer lines of prior therapy (65% v 44%; P < .001) compared with alloHCT recipients. Three-year PFS and OS of autoHCT recipients beyond CR1 were 42% and 53%, respectively. Among alloHCT recipients who received transplantations beyond CR1, 31% remained progression-free at 3 years, despite being more heavily pretreated and with more refractory disease. NRM was 3.5-fold higher (95% CI, 1.80 to 6.99; P < .001) for alloHCT. In multivariate analysis, chemotherapy sensitivity (hazard ratio [HR], 1.8; 95% CI, 1.16 to 2.87) and two or fewer lines of pretransplantation therapy (HR, 5.02; 95% CI, 2.15 to 11.72) were prognostic of survival.

CONCLUSION

These data describe the roles of autoHCT and alloHCT in T-cell non-Hodgkin lymphoma and suggest greater effectiveness earlier in the disease course, and limited utility in multiply relapsed disease. Notably, autoHCT at relapse may be a potential option for select patients, particularly those with anaplastic large-cell lymphoma histology.

Direct effects of IL-4 on mast cells drive their intestinal expansion and increase susceptibility to anaphylaxis in a murine model of food allergy

Interleukin (IL)-4 has critical roles in allergic disorders, including food hypersensitivity. The direct effects of the cytokine on the survival and function of mast cells, the key effectors of food anaphylaxis, have not been established. In this study, we demonstrate that IL-4 induces a marked intestinal mastocytosis in mice. This phenotype is reproduced in animals expressing Il4rαF709, an activating variant of the IL-4 receptor α-chain (IL-4Rα). Il4rαF709 mice exhibit enhanced anaphylactic reactions but unaltered physiological responses to vasoactive mediators. IL-4 induces Bcl-2 and Bcl-X(L) and enhances survival and stimulates proliferation in cultured bone marrow-derived mast cells (BMMC). These effects are STAT6 (signal transducer and activator of transcription factor 6)-dependent and are amplified in Il4rαF709 BMMC. In competitive bone marrow chimeras, Il4rαF709 mast cells display a substantial competitive advantage over wild-type mast cells, which, in turn, prevail over IL-4Rα⁻/⁻ mast cells in populating the intestine, establishing a cell-intrinsic effect of IL-4 in intestinal mast cell homeostasis. Our results demonstrate that IL-4-signaling is a key determinant of mast cell expansion in food allergy.

Long-term outcome of adults with systemic anaplastic large-cell lymphoma treated within the Groupe d'Etude des Lymphomes de l'Adulte trials

PURPOSE

Systemic anaplastic large-cell lymphoma (ALCL) is a T-cell lymphoma, whose anaplastic lymphoma kinase (ALK) expression varies according to age. Long-term outcomes of chemotherapy-treated adults are not definitively established and should be evaluated.

PATIENTS AND METHODS

Patients treated in three Groupe d'Étude des Lymphomes de l'Adulte prospective clinical trials with confirmed systemic ALCL after immunohistopathologic review and defined ALK expression status were analyzed.

RESULTS

Among the 138 adult patients with ALCL, 64 (46%) were ALK positive, and 74 (54%) were ALK negative. Median follow-up was 8 years. At diagnosis, significantly more patients younger than 40 years old were ALK positive than ALK negative (66% v 23%, respectively; P < .001). Comparing patients with ALK-positive and ALK-negative ALCL, β(2)-microglobulin was ≥ 3 mg/L in 12% and 33% (P = .017); International Prognostic Index was high (score, 3 to 5) in 23% and 48% (P = .03); complete response rates to first-line treatment were 86% and 68% (P = .01); and 8-year overall survival (OS) rates were 82% (95% CI, 69% to 89%) and 49% (95% CI, 37% to 61%), respectively (P < .001). The survival difference mostly affected patients age ≥ 40 years. Multivariate analysis identified β(2)-microglobulin ≥ 3 mg/L (P < .001) and age ≥ 40 years (P = .029), but not ALK status, as prognostic for OS. These two variables distinguished four survival risk groups, with 8-year OS ranging from 84% to 22%. CONCLUSION Results of this long-term study enabled refinement of the prognosis of adult systemic ALCL, with ALK prognostic value dependent on age, and could provide guidance for eventual treatment adjustment.

Population PK and IgE pharmacodynamic analysis of a fully human monoclonal antibody against IL4 receptor

PURPOSE

For AMG 317, a fully human monoclonal antibody to interleukin receptor IL-4Rα, we developed a population pharmacokinetic (PK) model by fitting data from four early phase clinical trials of intravenous and subcutaneous (SC) routes simultaneously, investigated important PK covariates, and explored the relationship between exposure and IgE response.

METHODS

Data for 294 subjects and 2183 AMG 317 plasma concentrations from three Phase 1 and 1 Phase 2 studies were analyzed by nonlinear mixed effects modeling using first-order conditional estimation with interaction. The relationship of IgE response with post hoc estimates of exposure generated from the final PK model was explored based on data from asthmatic patients.

RESULTS

The best structural model was a two-compartment quasi-steady-state target-mediated drug disposition model with linear and non-linear clearances. For a typical 80-kg, 40-year subject, linear clearance was 35.0 mL/hr, central and peripheral volumes of distribution were 1.78 and 5.03 L, respectively, and SC bioavailability was 24.3%. Body weight was an important covariate on linear clearance and central volume; age influenced absorption rate. A significant treatment effect was observable between the cumulative AUC and IgE response measured.

CONCLUSION

The population PK model adequately described AMG 317 PK from IV and SC routes over a 60-fold range of doses with two dosing strengths across multiple studies covering healthy volunteers and patients with mild to severe asthma. IgE response across a range of doses and over the sampling time points was found to be related to cumulative AMG 317 exposure.

An overview of IL-7 biology and its use in immunotherapy

Interleukin (IL)-7 is required for T-cell development as well as for the survival and homeostasis of mature T-cells. In the thymus, the double negative (DN) CD4(-) CD8(-) thymocyte progenitor transition into double positive CD4+ CD8+ cells requires Notch and IL-7 signaling. Importantly, IL-7 seems to have a dose effect on T-cell development and, at high doses, DN progression is blocked. Naïve T-cells in the thymus, and after their exit to the periphery, are dependent on IL-7 and TCR signaling for survival. Upon antigen exposure, they proliferate and differentiate into memory T-cells. Because IL-7 intervenes at all stages of T-cell development and maintenance, it has been introduced recently into clinical trials as an immunotherapeutic agent for cancer patients (of particular note, those who had undergone T-cell depleting therapy) in an attempt to increase their population sizes of CD4+ and CD8+ cells overall, and specifically of CD8+ (CD45RA+)CCR7+ and/or CD27+), CD4+ (CD45RA+CD31+), and CD4+ central memory T-cells (CD45RA(-)CCR7+). Interestingly, IL-7 in humans induced a preferential expansion of naïve T-cells, resulting in a broader T-cell repertoire than before the treatment; this effect was independent of age. This suggests that IL-7 therapy could enhance immune responses in patients with limited naïve T-cell numbers as in aged patients or after disease-induced or iatrogenic T-cell depletion. This overview highlights the role of IL-7 on T-cells in mice and humans.

Janus kinase-3 dependent inflammatory responses in allergic asthma

Allergic asthma is a chronic inflammatory condition of the lung characterized by reversible airway obstruction, high serum immunoglobulin (Ig) E levels, and chronic airway inflammation. A number of cells including mast cells, T cells, macrophages and dendritic cells play a role in the pathogenesis of the disease. Janus kinase (JAK)-3, a non-receptor protein tyrosine kinase, traditionally known to mediate cytokine signaling, also regulates functional responses of these cells. In this review the role of JAK-3 in regulating various pathogenic processes in allergic asthma is discussed. We propose that targeting JAK-3 is a rationale approach to control the inflammatory responses of multiple cell types responsible for the pathogenesis of allergic asthma.

Mast cells protect against airway Mycoplasma pneumoniae under allergic conditions

BACKGROUND

Infection with Mycoplasma pneumoniae (Mp) in asthma can occur both acutely and chronically with an associated Th2 inflammatory response and/or increased numbers of bronchial mast cells. Mast cells have previously been shown to promote mycoplasma clearance in mice; however, it is unknown whether mast cells would aid Mp clearance under allergic conditions.

OBJECTIVE

Our aim was to determine the impact of allergic inflammation on mast cell-mediated lung Mp clearance. Furthermore, as we have previously demonstrated an essential role for IL-6 in lung Mp clearance we also investigated the role of mast cell-derived IL-6.

METHODS

Mast cell-deficient (WBB6F1/J-Kit(W)/Kit(W-v)) mice were challenged with ovalbumin to induce airway inflammation before Mp infection. The role of mast cell-derived IL-6 in bacterial clearance was further investigated by reconstitution of mast cell-deficient mice with IL-6(-/-) mast cells.

RESULTS

Allergic mast cell-deficient mice exhibited increased lung Mp burden compared with control littermates. Intravenous adoptive transfer of wild-type and IL-6(-/-) mast cells significantly improved Mp clearance in mast cell-deficient mice. Acutely after Mp infection, allergen-challenged mast cell-deficient mice had increased levels of the pro-inflammatory cytokines IL-6 and TNF-alpha in the bronchoalveolar lavage (BAL) fluid. The total number of neutrophils was also increased in mast cell-deficient mice.

CONCLUSIONS

Our results establish that mast cells aid host defense against Mp in an allergic setting and that while IL-6 is necessary for lung Mp clearance, mast cell-derived IL-6 is not required.

Antigen-induced increases in pulmonary mast cell progenitor numbers depend on IL-9 and CD1d-restricted NKT cells

Pulmonary mast cell progenitor (MCp) numbers increase dramatically in sensitized and aerosolized Ag-challenged mice. This increase depends on CD4(+) T cells, as no MCp increase occurs in the lungs of sensitized wild-type (WT) mice after mAb depletion of CD4(+) but not CD8(+) cells before aerosol Ag challenge. Neither the genetic absence of IL-4, IL-4Ralpha chain, STAT-6, IFN-gamma, or IL-12p40 nor mAb blockade of IFN-gamma, IL-3, IL-4, IL-5, IL-6, IL-10, IL-13, IL-17A, IL-12p40, or IL-12p40Rbeta1 before Ag challenge in WT mice reduces the pulmonary MCp increase. However, sensitized and Ag-challenged IL-9-deficient mice and sensitized WT mice given mAb to IL-9 just before Ag challenge show significant reductions in elicited lung MCp/10(6) mononuclear cells of 47 and 66%, respectively. CD1d-deficient mice and WT mice receiving anti-CD1d before Ag challenge also show significant reductions of 65 and 59%, respectively, in elicited lung MCp/10(6) mononuclear cells, revealing an additional requirement for MCp recruitment. However, in Jalpha18-deficient mice, which lack only type 1 or invariant NKT cells, the increase in the numbers of lung MCp with Ag challenge was intact, indicating that their recruitment must be mediated by type 2 NKT cells. Furthermore, anti-CD1d treatment of IL-9-deficient mice or anti-IL-9 treatment of CD1d-deficient mice does not further reduce the significant partial impairment of MCp recruitment occurring with a single deficiency. These findings implicate type 2 NKT cells and IL-9 as central regulators that function in the same pathway mediating the Ag-induced increase in numbers of pulmonary MCp.

Neither interleukin-4 receptor alpha expression on CD4+ T cells, or macrophages and neutrophils is required for protective immunity to Trichinella spiralis

The T helper type 2 (Th2) mediated expulsion of the gastrointestinal nematode Trichinella spiralis requires interleukin-4 receptor alpha (IL-4Ralpha) expression on both bone-marrow-derived and non-bone-marrow-derived cells. To more definitively investigate the role of IL-4/IL-13 responsiveness in the development of protective immunity to T. spiralis, cell-specific IL-4Ralpha signalling on CD4(+) T cells (Lck(cre) IL-4Ralpha(-/flox)) and macrophages/neutrophils (LysM(cre) IL-4Ralpha(-/flox)) was analysed on the BALB/c background. Infection of wild-type and control IL-4Ralpha(-/flox) mice induced a Th2-type immune response with elevated IL-4 cytokine production, parasite-specific immunoglobulin G1 (IgG1), total IgE, intestinal mastocytosis and enteropathy. In contrast, global IL-4Ralpha-deficient BALB/c mice showed reduced worm expulsion, antibody production, intestinal mastocytosis and gut pathology. BALB/c mice generated with cell-specific deletion of IL-4Ralpha on CD4(+) T lymphocytes or macrophages/neutrophils, controlled gastrointestinal helminth infection by eliciting a protective immune response comparable to that observed with wild-type and IL-4Ralpha(-/flox) controls. Together, this shows that the development of host protective Th2 responses accompanied by parasite loss is independent of IL-4Ralpha expression on CD4(+) T cells and macrophages/neutrophils.

Development of anaplastic lymphoma kinase (ALK) small-molecule inhibitors for cancer therapy

Anaplastic lymphoma kinase (ALK) is a receptor tyrosine kinase (RTK) involved in the genesis of several human cancers; indeed, ALK was initially identified in constitutively activated and oncogenic fusion forms--the most common being nucleophosmin (NPM)-ALK--in a non-Hodgkin's lymphoma (NHL) known as anaplastic large-cell lymphoma (ALCL) and subsequent studies identified ALK fusions in the human sarcomas called inflammatory myofibroblastic tumors (IMTs). In addition, two recent reports have suggested that the ALK fusion, TPM4-ALK, may be involved in the genesis of a subset of esophageal squamous cell carcinomas. While the cause-effect relationship between ALK fusions and malignancies such as ALCL and IMT is very well established, more circumstantial links implicate the involvement of the full-length, normal ALK receptor in the genesis of additional malignancies including glioblastoma, neuroblastoma, breast cancer, and others; in these instances, ALK is believed to foster tumorigenesis following activation by autocrine and/or paracrine growth loops involving the reported ALK ligands, pleiotrophin (PTN) and midkine (MK). There are no currently available ALK small-molecule inhibitors approved for clinical cancer therapy; however, recognition of the variety of malignancies in which ALK may play a causative role has recently begun to prompt developmental efforts in this area. This review provides a succinct summary of normal ALK biology, the confirmed and putative roles of ALK fusions and the full-length ALK receptor in the development of human cancers, and efforts to target ALK using small-molecule kinase inhibitors.

Genomic profiling reveals different genetic aberrations in systemic ALK-positive and ALK-negative anaplastic large cell lymphomas

Anaplastic large cell lymphoma (ALCL) is a T/null-cell neoplasm characterized by chromosomal translocations involving the anaplastic lymphoma kinase (ALK) gene (ALK). Tumours with similar morphology and phenotype but negative for ALK have been also recognized. The secondary chromosomal imbalances of these lymphomas are not well known. We have examined 74 ALCL, 43 ALK-positive and 31 ALK-negative, cases by comparative genomic hybridization (CGH), and locus-specific alterations for TP53 and ATM were examined by fluorescence in situ hybridization and real-time quantitative polymerase chain reaction. Chromosomal imbalances were detected in 25 (58%) ALK-positive and 20 (65%) ALK-negative ALCL. ALK-positive ALCL with NPM-ALK or other ALK variant translocations showed a similar profile of secondary genetic alterations. Gains of 17p and 17q24-qter and losses of 4q13-q21, and 11q14 were associated with ALK-positive cases (P = 0.05), whereas gains of 1q and 6p21 were more frequent in ALK-negative tumours (P = 0.03). Gains of chromosome 7 and 6q and 13q losses were seen in both types of tumours. ALCL-negative tumours had a significantly worse prognosis than ALK-positive. However no specific chromosomal alterations were associated with survival. In conclusion, ALK-positive and negative ALCL have different secondary genomic aberrations, suggesting they correspond to different genetic entities.

HMG-CoA reductase inhibitor simvastatin inhibits proinflammatory cytokine production from murine mast cells

BACKGROUND

Statins inhibit 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, a key rate-limiting enzyme in the mevalonate pathway. Accumulating data suggest that statins exhibit anti-inflammatory effects on a number of experimental models including experimental autoimmune encephalomyelitis and antigen-induced allergic airway inflammation. However, the mechanism underlying the anti-inflammatory effect of statins is still largely unknown. In this study, we examined the effect of a representative statin, simvastatin, on proinflammatory cytokine production from murine mast cells.

METHODS

Bone marrow-derived mast cells (BMMCs) were stimulated with lipopolysaccharide (LPS) in the presence or absence of simvastatin, and TNF-alpha and IL-6 production from BMMCs was evaluated at mRNA and protein levels. The effect of simvastatin on the expression of tristetraprolin, an RNA-binding protein that promotes decay of TNF-alpha mRNA, was evaluated.

RESULTS

Incubation of BMMCs with simvastatin resulted in the inhibition of LPS-induced TNF-alpha production at both mRNA and protein levels. Simvastatin also inhibited IL-6 production from LPS-stimulated BMMCs. However, simvastatin did not enhance the expression of tristetraprolin.

CONCLUSIONS

Simvastatin inhibits the production of TNF-alpha and IL-6 from activated mast cells in part by inhibiting de novo synthesis of their transcripts and the inhibition may account for the anti-inflammatory effect of simvastatin.

Thrombopoietin inhibits murine mast cell differentiation

We have recently shown that Mpl, the thrombopoietin receptor, is expressed on murine mast cells and on their precursors and that targeted deletion of the Mpl gene increases mast cell differentiation in mice. Here we report that treatment of mice with thrombopoietin or addition of this growth factor to bone marrow-derived mast cell cultures severely hampers the generation of mature cells from their precursors by inducing apoptosis. Analysis of the expression profiling of mast cells obtained in the presence of thrombopoietin suggests that thrombopoietin induces apoptosis of mast cells by reducing expression of the transcription factor Mitf and its target antiapoptotic gene Bcl2.

Long-term remission of children with relapsed and secondary anaplastic large cell non-Hodgkin lymphoma (ALCL) following treatment with pulsed dexamethasone and low dose etoposide

Anaplastic large cell lymphoma (ALCL) accounts for approximately 15% of childhood NHL. Relapsed ALCL represents a formidable challenge because outcome is poor despite the use of high-dose chemotherapy regimens. We report two patients with relapsed T-type and 0-type ALCL who achieved long-term 3rd and 4th remissions with 4-weekly oral dexamethasone (DEX) and etoposide pulses for 2 years. This regimen also induced and maintained remission in a third patient with Nijmegen breakage syndrome (NBS) with secondary T-type ALCL. These patients demonstrate that low-intensity oral chemotherapy can induce long-term remissions and offer a curative perspective in refractory, relapsed and secondary ALCL.

Need for an improved molecular/genetic classification for CD30+ lymphomas involving the skin

BACKGROUND

The spectrum of diseases that constitute the CD30+ lymphomas, with lymphomatoid papulosis (LyP) at one end, and anaplastic large-cell lymphoma (ALCL) at the other end, shows variable morphology, immunophenotype, and clinical behavior. The border between these diseases is sometimes difficult to establish and there are many grey zones in their classification.

METHODS

We reviewed the clinical and research literature and guided by our experiences attempted to discern molecular and phenotypic criteria to improve the classification and identify molecular targets for therapy of CD30-positive cutaneous lymphomas.

RESULTS

Functional studies of ALCL cell lines clonally derived from LyP have revealed loss of growth inhibition by transforming growth factor beta (TGF-beta), due to TGF-beta receptor mutations. Studies of genetic variants of the CD30 promoter showed distinct microsatellite alleles associated with development of LyP and lymphoma progression. Studies of LyP and cutaneous ALCL tissues and cell lines suggest a dual role for CD30/CD30 ligand interactions in regression of LyP and progression to lymphoma. CD30 signaling activates NF-kappaB in cell lines derived from cutaneous ALCL but not anaplastic lymphoma kinase (ALK)-positive systemic ALCL in which growth arrest occurs through cell cycle inhibitor p21WAF1/Cip1. Other likely biomarkers of disease progression include differential expression of Bcl-2, fascin, cutaneous lymphocyte antigen, and T-cell receptor clonality. These may lead to improved classification, diagnoses, and therapeutic targets.

CONCLUSIONS

The current clinicopathologic classification of CD30+ cutaneous lymphoproliferative disorders is insufficient. Incorporating genetic and molecular criteria would better define the borders between benign/ malignant and aggressive/nonaggressive disorders.

Cytotoxic molecule expression is predictive of prognosis in Hodgkin's-like anaplastic large cell lymphoma

AIMS

The Revised European American Lymphoma classification uses the term Hodgkin's-like anaplastic large cell lymphoma (HD-like ALCL) for borderline cases with features of both anaplastic large cell lymphoma (ALCL) and classical Hodgkin's lymphoma (HL). The aim of this study was to clarify the association between cytotoxic molecule (CM) expression and clinical outcome in HD-like ALCL.

METHODS AND RESULTS

Subjects were 59 patients with HD-like ALCL, defined by nodal presentation without mediastinal bulky lesions, T- or null-cell phenotype, CD30+ anaplastic lymphoma kinase (ALK)- phenotype and by confluent sheets or nodules of large cells mimicking classic Hodgkin and Reed-Sternberg cells. We evaluated the presenting features and prognosis of subjects on categorization into two defined groups, namely CM (TIA1 and/or granzyme B)-positive (n = 21) and CM-negative (n = 38). The series consisted of 18 women and 41 men ranging from 16 to 88 years of age (median 59 years). The CM+ group had poorer disease-specific survival than the CM- group (P = 0.02) despite the absence of differences in other clinical characteristics. Multivariate analysis confirmed that CM expression was an independent prognostic factor, in contrast to phenotypic categorization (T-cell vs. null-cell group), which had no prognostic impact on disease-specific survival.

CONCLUSION

CM expression is predictive of prognosis in HD-like ALCL.

Kit is essential for PMA-inflammation–induced mast-cell accumulation in the skin

Cutaneous mast cells have important pathogenic roles in skin inflammation, but the signals regulating mast-cell numbers in healthy and inflamed skin are not fully understood. Mast-cell development depends on the receptor tyrosine kinase Kit as shown by a greater than 95% reduction of mast-cell numbers in hypomorphic (KitW/Wv) mutant mice that are widely used as a mast-cell deficiency model. Mast-cell numbers are normally very low in KitW/Wv mice, but numbers can strongly increase under inflammatory conditions. It remains elusive whether this inflammation-driven mast-cell accumulation is mediated by signals transmitted via the KitWv receptor or by other, Kit-independent stimuli. We show here, using viable Kit- null mice (KitW/W), that Kit is essential for mast-cell accumulation in phorbol-12-myristate-13-acetate (PMA)–treated, chronically inflamed skin. This increase in mast- cell numbers is strongly attenuated in KitW/Wv mice lacking mature lymphocytes (T, B, and natural killer [NK] cells). These data, together with reconstitution experiments, point at a role for lymphocytes in the regulation of mast-cell compartments under limiting Kit signaling. We conclude that inflammation-induced cutaneous mast-cell accumulation is dependent on Kit signaling strength, and, under limiting Kit signals, on cells of the adaptive immune system.

Prognostic significance of NPM-ALK fusion transcript overexpression in ALK-positive anaplastic large-cell lymphoma

In anaplastic large-cell lymphomas positive for anaplastic lymphoma kinase (ALK) protein, the ALK gene is most commonly fused to the NPM gene, and less commonly to TPM3, TFG, ATIC, and other rare genes. Although this lymphoma is generally associated with a favorable clinical outcome, 25% of the patients die of the disease within 5 years. In this study, we developed three assays, all of which can be used with archival formalin-fixed, paraffin-embedded tissues: (1) a sensitive reverse transcription-polymerase chain reaction (RT-PCR) assay for various X-ALK fusion genes, (2) a 5' rapid amplification of cDNA ends (RACE) assay to identify unknown fusion partners, and (3) a real-time RT-PCR assay to quantify the amount of the NPM-ALK fusion transcript. In 26 cases of ALK(+) anaplastic large-cell lymphoma, the RT-PCR assay showed that the ALK was fused to NPM in 21 cases, to TPM3 in three, and to TFG in one. The 5' RACE assay detected ATIC-ALK fusion in the remaining case. The real-time quantitative RT-PCR assay showed that the NPM-ALK transcript was over expressed in four of 20 quantifiable cases. Patients with NPM-ALK overexpression showed a significantly unfavorable overall survival compared with those with a low expression of this transcript. The RT-PCR and 5' RACE assays developed here may be useful for identification of known and unknown gene partners fused to the ALK gene. Overexpression of the NPM-ALK fusion transcript may be associated with a poor prognosis of the patients with ALK(+) anaplastic large-cell lymphomas.

Anaplastic large cell lymphomas: a study of 75 pediatric patients

In this article, we describe the morphologic and immunophenotypic features of 75 cases of pediatric anaplastic large cell lymphoma (ALCL). According to the World Health Organization classification, 49 cases were common subtype ALCL, and respectively, 3, 6, and 17 cases were small cell, lymphohistiocytic, or mixed histologic variants. Anaplastic lymphoma kinase positivity was detected in 90.7% of the tumors and, using a panel of 9 T-cell surface markers, 88% could be assigned to the T-cell lineage. A molecular analysis for the T-cell receptor gamma (TCR- gamma) and the heavy chain of the immunoglobulin H rearrangements was performed on 6/9 ALCLs with a null immunophenotype, and a TCR clonal pattern was detected in 5/6 cases. In addition, 94.1% were immunoreactive for 1 or more cytotoxic proteins (Tia1, granzyme B, or perforin), and 15% expressed CD56. Clusterin, CD83, and Pax5, respectively, expressed in 91.3%, 1.7%, and 0% of the ALCLs, were useful biomarkers for the differential diagnosis with Hodgkin's lymphomas.

Role of thrombopoietin in mast cell differentiation

Mast cells are important elements of the body response to foreign antigens, being those represented either by small molecules (allergic response) or harbored by foreign microorganisms (response to parasite infection). These cells derive from hematopoietic stem/progenitor cells present in the marrow. However, in contrast with most of the other hematopoietic lineages, mast cells do not differentiate in the marrow but in highly vascularized extramedullary sites, such as the skin or the gut. Mast cell differentiation in the marrow is activated as part of the body response to parasites. We will review here the mast cell differentiation pathway and what is known of its major intrinsic and extrinsic control mechanisms. It will also be described that thrombopoietin, the ligand for the Mpl receptor, in addition to its pivotal rule in the control of thrombocytopoiesis and of hematopoietic stem/progenitor cell proliferation, exerts a regulatory function in mast cell differentiation. Some of the possible implications of this newly described biological activity of thrombopoietin will be discussed.