Hairy cell leukemia (HCL) represents 2% of all leukemia cases, with men aged above 55 years being the most affected. The most common symptoms of this type of leukemia include splenomegaly, monocytopenia, and neutropenia. In the basic blood count examination, leukopenia with monocytopenia and granulocytopenia, as well as aplastic anemia and/or thrombocytopenia occur. The mutation of β-rapidly accelerated fibrosarcoma (BRAF) proto-oncogene, which can be found in nearly 100% of patients, is an important feature of HCL. Immunophenotypic analysis of the HCL cells reveals high expression of B-lineage antigens, including CD19, CD20, and CD22. Additionally, CD11c, CD25, CD103, and CD123 belong to specific markers of HCL. Lactate dehydrogenase activity and β-2-microglobulin concentration are also important in the patient\'s assessment. The differential diagnosis between HCL, hairy cell leukemia variant (HCL-V) and splenic marginal zone lymphoma (SMZL) is of first importance. Currently, the main treatment for HCL involves the use of purine analogues, excluding pregnant women, individuals with severe infections, and those with relapsing HCL.

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The association between Hairy Cell Leukemia (HCL) and non-tuberculous mycobacterial infections (NTMs) is well described, most notably Mycobacterium kansasii. The exact pathophysiology is not known. We report a case of a 31-year-old male with concomitantly diagnosed HCL and disseminated M kansasii infection who presented with rash, pancytopenia, and bulky axillary lymphadenopathy. The M kansasii was initially diagnosed through use of cell-free DNA detection and confirmed by bone marrow and lymph node cultures. Hairy Cell Leukemia was diagnosed with peripheral flow cytometry and confirmed via the same bone marrow sample. His HCL was put into remission with a single course of cladribine and rituximab chemotherapy; however, his M kansasii infection persisted for 6 months despite aggressive antimicrobial and surgical therapy. It was finally controlled using high-dose rifampin in combination with azithromycin and ethambutol. This case highlights the known link between HCL and M kansasii. Furthermore, it hints at potential causes beyond chemotherapy-induced immunocompromise. Notable possibilities include HCL cells acting as sanctuary sites for M kansasii to evade the immune system, and subclinical M kansasii infections causing NLRP3 inflammasome overactivation to trigger the oncogenic transformation to HCL. More research into the pathophysiologic link between HCL and M kansasii infections would allow for more effective prevention, diagnosis, and treatment of these severe atypical infections which are the major cause of morbidity in the cladribine era of HCL treatment.

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BACKGROUND: Hairy cell leukemia (HCL) is characterized by the underlying genetic lesion of BRAFV600E and responsiveness to BRAF inhibitors. We assessed the safety and activity of the BRAF inhibitor vemurafenib combined with obinutuzumab in patients with previously untreated HCL. METHODS: We conducted a single-arm, multicenter clinical study of vemurafenib plus obinutuzumab. Vemurafenib 960 mg twice daily was administered for four cycles, and obinutuzumab was administered in cycles 2 to 4. The primary end point was complete remission (CR). Secondary end points included assessment of safety, minimal residual disease (MRD), and BRAF allele burden according to digital droplet polymerase chain reaction (ddPCR). RESULTS: Thirty patients were enrolled in the study, and 27 patients completed all four cycles of treatments and achieved CR (90%; 95% confidence interval [CI], 73 to 98). Three patients discontinued the study early because of adverse events and were not evaluable for response. Of the 27 patients who achieved CR, 26 patients (96%; 95% CI, 81 to 99) achieved MRD negativity. BRAFV600E allele was undetectable by ddPCR in all 21 evaluable patients. At a median follow-up of 34.9 months (95% CI, 29.6 to 36.9), no patient experienced disease relapse. The most common vemurafenib-related adverse events were rash and arthralgia. Febrile neutropenia occurred in two patients, and blood or platelet transfusions were required in two patients. CONCLUSIONS: Combined time-limited vemurafenib and obinutuzumab achieved CR in more than 90% of patients with previously untreated HCL. In this small study, acquired vemurafenib resistance or dose-limiting toxicity was not observed. Patients were not observed long enough to reveal secondary malignancies. (Funded by the National Cancer Institute and others; ClinicalTrials.gov number, NCT03410875.)

The variant form of hairy cell leukaemia (HCL-V) is a rare disease very different from hairy cell leukaemia (HCL), which is a very well-defined entity. The 5th WHO edition (Leukemia, 36, 2022 and 1720) classification (WHO-HAEM5) introduced splenic lymphomas/leukaemias including four different entities: (1) HCL, (2) splenic marginal zone lymphoma (SMZL) with circulating villous cells in the peripheral blood, (3) splenic lymphoma with prominent nucleolus (SLPN), which replaced HCL-V and CD5 negative B-prolymphocytic leukaemia (B-PLL), and (4) splenic diffuse red pulp lymphoma (SDRPL). All these entities have to be distinguished because of a different clinical course and the need for a different treatment. The diagnosis can be challenging because of complex cases and overlap and/or grey zones between all the entities and needs integrating clinical, histologic, immunophenotypic, cytogenetic and molecular data. We review the diagnostic criteria including clinical, immunophenotypic and molecular characteristics of patients with HCL-V and other HCL-like disorders including HCL, SDRPL, SMZL, B-PLL and the Japanese form of HCL. We also discuss the different criteria allowing us to separate these different entities and we will update the recent therapeutic options that have emerged, in particular the advances with chemoimmunotherapy and/or targeted therapies.

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Introduction: Hairy cell leukemia (HCL) is an indolent B-cell lymphoproliferative disease. BRAF V600E mutation is detected in nearly all classical HCL cases which offers the possibility of targeted therapy. Objective: The aim of our study was to assess the efficacy of low-dose vemurafenib as well as to assess the long term outcome of HCL patients treated with this drug at the Department of Internal Medicine and Oncology at Semmelweis University. Methods: We report on 10 patients with classical HCL treated with low-dose vemurafenib at our Department between 2013 and 2022. Results: As a result of fixed time low-dose vemurafenib treatment, 5 of 10 patients (5/10) achieved partial remission, 4 (4/10) had stable disease, and 1 (1/10) had MRD positivity. No patients achieved complete remission. The median progression-free survival was 28.5 months while the overall survival was 82 months. Conclusion: We confirm that low dose of vemurafenib is effective and safe in the vast majority of patients with HCL. This small-molecule oral treatment allows to gain valuable time-months or even years-before further, usually parenteral treatment options have to be given or before previous treatment has to be repeated. There are also promising data supporting the combination of vemurafenib with other drugs for the treatment of HCL patients which could provide even further possibility to bridge treatment.

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Hairy cell leukemia (HCL), similar to its variant HCLv, is a B-cell malignancy associated with decreased humoral immunity. We prospectively monitored the largest cohort of patients with HCL/HCLv to date (n = 503) for COVID-19 by symptoms, antibody, and polymerase chain reaction (PCR) and/or antigen positivity. Fifty percent (253 of 503) of the patients with HCL/HCLv (238 HCL and 15 HCLv) had evidence of COVID-19, with 210 (83%) testing positive by PCR or rapid-antigen test. Of the 43 patients without positive tests, all had nucleocapsid antibodies indicating COVID-19 exposure, 7 recalled no symptoms, and 36 had mild symptoms. Of the 210 who tested positive, 23, 46, 129, and 12 cases occurred in 2020, 2021, 2022, and 2023, respectively. Among them, 175 began treatment for HCL/HCLv 0.4 to 429 (median, 66) months before, and 132 had their last dose of anti-CD20 monoclonal antibody 0.2 to 229 (median, 63) months before. Two patients died, including a young woman who began rituximab 2 months after first-line cladribine before vaccine availability. Nearly all patients with HCL/HCLv recovered uneventfully from COVID-19 including those without vaccination or those with significant immunosuppression and recent treatment. However, decreased normal B cells from HCL or treatment was associated with lower spike antibody levels as a response to COVID-19 (P = .0094) and longer recovery time (P = .0036). Thus, in a large cohort of patients with HCL/HCLv and in the first to determine relationships between COVID-19 outcome and immune markers, mortality was relatively low (∼1%), sequelae were uncommon, and recovery from COVID-19 was longer if normal B cells were low after recent treatment. The trials are registered at www.clinicaltrials.gov as #NCT01087333 and #NCT04362865.