The active form of vitamin D₃, 1α,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃), is a potent immunomodulator known to affect T-cells through targeting antigen-presenting cells such as dendritic cells (DCs). We studied the effects of a novel nonhypercalcemic 1,25(OH)₂D₃ analog, TX527, on DC differentiation, maturation, and function with respect to stimulation of a committed human GAD65-specific autoreactive T-cell clone. Continuous addition of TX527 impaired interleukin (IL)-4 and granulocyte/macrophage colony-stimulating factor (GM-CSF)-driven DC differentiation as well as lipopolysaccharide (LPS) and interferon-γ (IFN-γ)-induced maturation into Th1-promoting DC (DC1), as characterized by marked changes in DC morphology and abrogation of IL-12p70 release upon CD40 ligation. Addition of TX527 during maturation did not affect DC morphology but significantly changed DC cytokine profiles. The potential of treated DCs to alter the response pattern of committed autoreactive T-cells was found to depend on the timing of TX527 exposure. Continuously TX527-treated DCs significantly inhibited T-cell proliferation and blocked IFN-γ, IL-10, but not IL-13 production, whereas DCs treated during maturation failed to inhibit T-cell proliferation but affected IL-10 and IFN-γ production. Collectively, we provide evidence that nonhypercalcemic TX527 is a potent in vitro DC modulator, yielding DCs with the potential to change cytokine responses of committed autoreactive T-cells.

ALK-positive histiocytosis is a rare subtype of histiocytic neoplasm first described in 2008 in 3 infants with multisystemic disease involving the liver and hematopoietic system. This entity has subsequently been documented in case reports and series to occupy a wider clinicopathologic spectrum with recurrent KIF5B-ALK fusions. The full clinicopathologic and molecular spectra of ALK-positive histiocytosis remain, however, poorly characterized. Here, we describe the largest study of ALK-positive histiocytosis to date, with detailed clinicopathologic data of 39 cases, including 37 cases with confirmed ALK rearrangements. The clinical spectrum comprised distinct clinical phenotypic groups: infants with multisystemic disease with liver and hematopoietic involvement, as originally described (Group 1A: 6/39), other patients with multisystemic disease (Group 1B: 10/39), and patients with single-system disease (Group 2: 23/39). Nineteen patients of the entire cohort (49%) had neurologic involvement (7 and 12 from Groups 1B and 2, respectively). Histology included classic xanthogranuloma features in almost one-third of cases, whereas the majority displayed a more densely cellular, monomorphic appearance without lipidized histiocytes but sometimes more spindled or epithelioid morphology. Neoplastic histiocytes were positive for macrophage markers and often conferred strong expression of phosphorylated extracellular signal-regulated kinase, confirming MAPK pathway activation. KIF5B-ALK fusions were detected in 27 patients, whereas CLTC-ALK, TPM3-ALK, TFG-ALK, EML4-ALK, and DCTN1-ALK fusions were identified in single cases. Robust and durable responses were observed in 11/11 patients treated with ALK inhibition, 10 with neurologic involvement. This study presents the existing clinicopathologic and molecular landscape of ALK-positive histiocytosis and provides guidance for the clinical management of this emerging histiocytic entity.

Bidirectional cell transfer during pregnancy frequently leads to postpartum persistence of allogeneic cells and alloimmune responses in both the mother and in her offspring. The life-long consequences of naturally acquired alloimmune reactivity are probably of importance for the outcome of allogeneic stem cell transplantation. We investigated the presence of CD8pos minor histocompatibility (H) antigen–specific cytotoxic T lymphocytes (TCTL) and CD8pos minor H antigen–specific T regulator cells (TREG) in peripheral blood cells obtained from 17 minor H antigen–disparate mother-offspring pairs. Absence of minor H antigen–specific TREG, as marked by the feasibility to expand TCTL from isolated tetramerpos populations, was observed in 6 mothers and 1 son. The presence of minor H alloantigen–specific TREG was observed in 4 mothers and 5 sons. These TREG were detected within isolated tetramerdim staining fractions and functioned in a CTLA-4–dependent fashion. Our study indicates that both TCTL and TREG mediated alloimmunity against minor H antigens may be present in healthy female and male hematopoietic stem cell donors, potentially influencing graft-versus-host reactivity in different ways.

Histiocytic disorders are a spectrum of rare diseases characterised by the accumulation of macrophage‐, dendritic cell‐, or monocyte‐differentiated cells in various tissues and organs. The discovery of recurrent genetic alterations in many of these histiocytoses has led to their recognition as clonal neoplastic diseases. Moreover, the identification of the same somatic mutation in histiocytic lesions and peripheral blood and/or bone marrow cells from histiocytosis patients has provided evidence for systemic histiocytic neoplasms to originate from haematopoietic stem/progenitor cells (HSPCs). Here, we investigated associations between histiocytic disorders and additional haematological malignancies bearing the same genetic alteration(s) using the nationwide Dutch Pathology Registry. By searching on pathologist‐assigned diagnostic terms for the various histiocytic disorders, we identified 4602 patients with a putative histopathological diagnosis of a histiocytic disorder between 1971 and 2019. Histiocytosis‐affected tissue samples of 187 patients had been analysed for genetic alterations as part of routine molecular diagnostics, including from nine patients with an additional haematological malignancy. Among these patients, we discovered three cases with different histiocytic neoplasms and additional haematological malignancies bearing identical oncogenic mutations, including one patient with concomitantKRASp.A59E mutated histiocytic sarcoma and chronic myelomonocytic leukaemia (CMML), one patient with synchronousNRASp.G12V mutated indeterminate cell histiocytosis and CMML, and one patient with subsequentNRASp.Q61R mutated Erdheim–Chester disease and acute myeloid leukaemia. These cases support the existence of a common haematopoietic cell‐of‐origin in at least a proportion of patients with a histiocytic neoplasm and additional haematological malignancy. In addition, they suggest that driver mutations in particular genes (e.g.N/KRAS) may specifically predispose to the development of an additional clonally related haematological malignancy or secondary histiocytic neoplasm. Finally, the putative existence of derailed multipotent HSPCs in these patients emphasises the importance of adequate (bone marrow) staging, molecular analysis and long‐term follow‐up of all histiocytosis patients.