Increased B cell activation is present in JAK2V617F-mutated, CALR-mutated and triple-negative essential thrombocythemia

Non-canonical Hedgehog signaling mediates profibrotic hematopoiesis-stroma crosstalk in myeloproliferative neoplasms

The role of hematopoietic Hedgehog signaling in myeloproliferative neoplasms (MPNs) remains incompletely understood despite data suggesting that Hedgehog (Hh) pathway inhibitors have therapeutic activity in patients. We aim to systematically interrogate the role of canonical vs. non-canonical Hh signaling in MPNs. We show that Gli1 protein levels in patient peripheral blood mononuclear cells (PBMCs) mark fibrotic progression and that, in murine MPN models, absence of hematopoietic Gli1, but not Gli2 or Smo, significantly reduces MPN phenotype and fibrosis, indicating that GLI1 in the MPN clone can be activated in a non-canonical fashion. Additionally, we establish that hematopoietic Gli1 has a significant effect on stromal cells, mediated through a druggable MIF-CD74 axis. These data highlight the complex interplay between alterations in the MPN clone and activation of stromal cells and indicate that Gli1 represents a promising therapeutic target in MPNs, particularly that Hh signaling is dispensable for normal hematopoiesis.

BAFF-driven NLRP3 inflammasome activation in B cells

BAFF supports B-cell survival and homeostasis by activating the NF-κB pathway. While NF-κB is also involved in the priming signal of NLRP3 inflammasome, the role of BAFF in NLRP3 inflammasome regulation is unknown. Here we report BAFF engagement to BAFF receptor elicited both priming and activating signals for NLRP3 inflammasomes in primary B cells and B lymphoma cell lines. This induction of NLRP3 inflammasomes by BAFF led to increased NLRP3 and IL-1β expression, caspase-1 activation, IL-1β secretion, and pyroptosis. Mechanistically, BAFF activated NLRP3 inflammasomes by promoting the association of cIAP-TRAF2 with components of NLRP3 inflammasomes, and by inducing Src activity-dependent ROS production and potassium ion efflux. B-cell receptor (BCR) stimulation on the Lyn signaling pathway inhibited BAFF-induced Src activities and attenuated BAFF-induced NLRP3 inflammasome activation. These findings reveal an additional function of BAFF in B-cell homeostasis that is associated with BCR activities.

B Lymphocytes Are the Target of Mesenchymal Stem Cells Immunoregulatory Effect in a Murine Graft-versus-Host Disease Model

There is growing clinical interest in the utilization of mesenchymal stem cells (MSCs) in the management of acute graft-versus-host disease (aGvHD), yet the effect of major histocompatibility complexes (MHCs) on B lymphocytes in this process has been less well documented. Working in an MHC fully mismatched murine aGvHD model, we found that MSC co-transfer significantly prolonged the survival time of the recipients. More interestingly, analysis on immunophenotypic profiles of posttransplant splenocytes showed that surface expression of CD69 (an early activation marker) and CD86 (a costimulatory molecule) was suppressed predominantly on donor derived B lymphocytes by MSC infusion. Additionally, mRNA level of interleukin-4, a potent B lymphocyte stimulator, was strikingly reduced from MSC-treated mice, while interleukin-10, the regulatory B lymphocytes inductor, was increased; these may underlie the lesser activation of B lymphocytes. In consistence, depletion of B lymphocytes in the transfusion inoculum further prolonged the survival time of aGvHD mice regardless of MSC administration. Therefore, B lymphocytes played an important role in the development of aGvHD, and they are targets in MSC-regulated immune response cascade in vivo. This study may provide a mechanistic clue for the treatment of human clinical aGvHD.

Lymphoproliferative disorders in patients with chronic myeloproliferative neoplasms: A systematic review

Patients with a Ph-negative myeloproliferative neoplasm (MPN) may harbor or develop lymphoproliferative disorders (LPD), however, the clinical and molecular determinants of MPN and LPD co-occurrence are still uncertain. To systematically pool the available knowledge, we conducted a scoping review of literature published since January 2005 and analyzed single-patient clinical data from 50 papers reporting 214 individuals harboring both MPN and LPD. Patients had been diagnosed essential thrombocythemia (44%), polycythemia vera (29%), or myelofibrosis (23%) at a median age of 67 years (26-94): half of them incurred a LPD after a median of 72 months from MPN diagnosis, while in 20% the LPD diagnosis was antecedent or synchronous. Patients mainly incurred indolent LPD, particularly chronic lymphocytic leukemia (CLL), while aggressive lymphomas and multiple myeloma were a relevant portion of the LPDs occurring in the follow-up of MPN. CLL was preferentially diagnosed in PV patients and was associated with a very high male-to-female ratio, as well as an older age at MPN diagnosis. On converse, multiple myeloma was rarely reported in PV patients and was preferentially diagnosed in female patients not harboring the JAK2 V617F mutation. Based on the 46 cases reporting follow-up data, median survival after MPN diagnosis was 96 months. This thorough review of published evidence confirms that LPD are relevant clinical events in the history of MPN patients. Controlled studies are needed to better refine individuals at higher risk of developing LPD, to support surveillance programs and to avoid therapies possibly favoring LPD.