Increasing evidence suggests that Rho family GTPases could have a critical role in the biology of T-cell lymphoma. In ALK-rearranged anaplastic large cell lymphoma (ALCL), a specific subtype of T-cell lymphoma, the Rho family GTPases Cdc42 and Rac1 are activated by the ALK oncogenic activity. In vitro studies have shown that Cdc42 and Rac1 control rather similar phenotypes of ALCL biology such as the proliferation, survival, and migration of lymphoma cells. However, their role and possible redundancy in ALK-driven lymphoma development in vivo are still undetermined. We genetically deleted Cdc42 or Rac1 in a mouse model of ALK-rearranged ALCL to show that either Cdc42 or Rac1 deletion impaired lymphoma development, modified lymphoma morphology, actin filament distribution, and migration properties of lymphoma cells. Cdc42 or Rac1 deletion primarily affected survival rather than proliferation of lymphoma cells. Apoptosis of lymphoma cells was equally induced following Cdc42 or Rac1 deletion, was associated with upregulation of the proapoptotic molecule Bid, and was blocked by Bcl2 overexpression. Remarkably, Cdc42/Rac1 double deletion, but not Cdc42 or Rac1 single deletions, completely prevented NPM-ALK lymphoma dissemination in vivo. Thus, Cdc42 and Rac1 have nonredundant roles in controlling ALK-rearranged lymphoma survival and morphology but are redundant for lymphoma dissemination, suggesting that targeting both GTPases could represent a preferable therapeutic option for ALCL treatment.

Platelets are essential for hemostasis, and thrombocytopenia is a major clinical problem. Megakaryocytes (MKs) generate platelets by extending long processes, proplatelets, into sinusoidal blood vessels. However, very little is known about what regulates proplatelet formation. To uncover which proteins were dynamically changing during this process, we compared the proteome and transcriptome of round vs proplatelet-producing MKs by 2D difference gel electrophoresis (DIGE) and polysome profiling, respectively. Our data revealed a significant increase in a poorly-characterized MK protein, myristoylated alanine-rich C-kinase substrate (MARCKS), which was upregulated 3.4- and 5.7-fold in proplatelet-producing MKs in 2D DIGE and polysome profiling analyses, respectively. MARCKS is a protein kinase C (PKC) substrate that binds PIP2. In MKs, it localized to both the plasma and demarcation membranes. MARCKS inhibition by peptide significantly decreased proplatelet formation 53%. To examine the role of MARCKS in the PKC pathway, we treated MKs with polymethacrylate (PMA), which markedly increased MARCKS phosphorylation while significantly inhibiting proplatelet formation 84%, suggesting that MARCKS phosphorylation reduces proplatelet formation. We hypothesized that MARCKS phosphorylation promotes Arp2/3 phosphorylation, which subsequently downregulates proplatelet formation; both MARCKS and Arp2 were dephosphorylated in MKs making proplatelets, and Arp2 inhibition enhanced proplatelet formation. Finally, we used MARCKS knockout (KO) mice to probe the direct role of MARCKS in proplatelet formation; MARCKS KO MKs displayed significantly decreased proplatelet levels. MARCKS expression and signaling in primary MKs is a novel finding. We propose that MARCKS acts as a "molecular switch," binding to and regulating PIP2 signaling to regulate processes like proplatelet extension (microtubule-driven) vs proplatelet branching (Arp2/3 and actin polymerization-driven).

Epstein-Barr virus (EBV) is a ubiquitous virus that establishes a latent infection within the host and in some cases can lead to the development of EBV-associated lymphomas, lymphoproliferative disorders, hemophagocytic lymphohistiocytosis, solid tumors, and other diseases. We studied the clinical significance of detecting EBV DNA in the plasma and peripheral blood mononuclear cells (PBMCs) of 2146 patients who had blood specimens sent to the Johns Hopkins Hospital clinical laboratory for viral quantitative real-time polymerase chain reaction assay over a 5-year period. Within this largely immunocompromised and hospitalized cohort, 535 patients (25%) had EBV detected in plasma or PBMCs. When EBV was detected in the absence of an EBV(+)disease (n = 402), it was present only in PBMCs in 69% of cases. Immunocompromised patients were less likely to have EBV in plasma than in PBMCs in the absence of EBV(+)disease. In patients with active, systemic EBV(+)diseases (n = 105), EBV was detected in plasma in 99% of cases but detected in PBMCs in only 54%. Across a range of copy number cutoffs, EBV in plasma had higher specificity and sensitivity for EBV(+)disease as compared with EBV in PBMCs. EBV copy number in plasma distinguished untreated, EBV(+)lymphoma from EBV(+)lymphoma in remission and EBV(-)lymphoma, and also distinguished untreated, EBV(+)posttransplantation lymphoproliferative disorder (PTLD) from EBV(+)PTLD in remission and EBV(-)PTLD. EBV copy number quantification is a useful diagnostic marker across the spectrum of EBV(+)diseases, even among immunocompromised patients, with plasma specimens more indicative of EBV(+)disease than PBMCs.

Genetic disorders affecting biogenesis and transport of lysosome-related organelles are heterogeneous diseases frequently associated with albinism. We studied a patient with albinism, neutropenia, immunodeficiency, neurodevelopmental delay, generalized seizures, and impaired hearing but with no mutation in genes so far associated with albinism and immunodeficiency. Whole exome sequencing identified a homozygous mutation in AP3D1 that leads to destabilization of the adaptor protein 3 (AP3) complex. AP3 complex formation and the degranulation defect in patient T cells were restored by retroviral reconstitution. A previously described hypopigmented mouse mutant with an Ap3d1 null mutation (mocha strain) shares the neurologic phenotype with our patient and shows a platelet storage pool deficiency characteristic of Hermansky-Pudlak syndrome (HPS) that was not studied in our patient because of a lack of bleeding. HPS2 caused by mutations in AP3B1A leads to a highly overlapping phenotype without the neurologic symptoms. The AP3 complex exists in a ubiquitous and a neuronal form. AP3D1 codes for the AP3δ subunit of the complex, which is essential for both forms. In contrast, the AP3β3A subunit, affected in HPS2 patients, is substituted by AP3β3B in the neuron-specific heterotetramer. AP3δ deficiency thus causes a severe neurologic disorder with immunodeficiency and albinism that we propose to classify as HPS10.

Myeloid-derived suppressor cells (MDSCs) are heterogeneous immature cells and natural inhibitors of adaptive immunity. In this study, the MDSC population was evaluated in adult patients with primary immune thrombocytopenia (ITP), where cell-mediated immune mechanisms are involved in platelet destruction. Our data demonstrated that both the numbers and suppressive functions of MDSCs were impaired in the peripheral blood and spleens of patients with ITP compared with healthy control patients. High-dose dexamethasone (HD-DXM) treatment rescued MDSC numbers in patients with ITP. And DXM modulation promoted the suppressive function of MDSCs induced in vitro. Moreover, the expression of interleukin 10 and transforming growth factor β was significantly upregulated in DXM-modulated MDSCs compared with the unmodulated cultures. DXM-modulated MDSCs inhibited autologous CD4(+)T-cell proliferation and significantly attenuated cytotoxic T lymphocyte-mediated platelet lysis, further indicating enhanced control over T-cell responses. Elevated expression of the transcription factor Ets1 was identified in DXM-modulated MDSCs. Transfection of Ets-1 small interfering RNA efficiently blocked regulatory effects of MDSCs, which almost offset the augmentation of MDSC function by DXM. Meanwhile, splenocytes from CD61 knockout mice immunized with CD61(+)platelets were transferred into severe combined immunodeficient (SCID) mouse recipients (C57/B6 background) to induce a murine model of severe ITP. We passively transferred the DXM-modulated MDSCs induced from bone marrow of wild-type C57/B6 mice into the SCID mouse recipients, which significantly increased platelet counts in vivo compared with those receiving splenocyte engraftment alone. These findings suggested that impaired MDSCs are involved in the pathogenesis of ITP, and that HD-DXM corrected MDSC functions via a mechanism underlying glucocorticoid action and Ets1.

In this issue of Blood, Palmer et al provide encouragement that important chronic graft-versus-host disease (GVHD) patient outcomes (such as overall survival [OS] and failure-free survival [FFS]) are predicted by clinician-assessed response, patient-reported outcomes, and 2014 National Institutes of Health (NIH)-response criteria.

In this issue of Blood, Yu et al describe a novel anti–Fcγ receptor III (FcγRIII)-albumin fusion protein that inhibits the development of thrombocytopenia in a murine model of immune thrombocytopenia (ITP).1 The unique aspect of this protein is that it blocks FcγRIII-mediated uptake of antibody-coated platelets without activating FcγRIII and the associated inflammatory response.

In this issue of Blood, Melenotte and colleagues provide an interesting and provocative analysis of a potential novel risk factor for B-cell non-Hodgkin lymphoma (NHL).

Autologous hematopoietic stem cell transplantation (HSCT) is increasingly used for severe autoimmune and inflammatory diseases, but the mechanisms involved have yet to be elucidated. In this issue of Blood, Delemarre et al report their findings in both animal and human models which provide insights into restoration of functionality and diversity within the regulatory T-cell (Treg) compartment following HSCT.

In this issue of Blood, Byrd et al present data from a randomized phase 2 study in which 78 previously untreated patients with chronic lymphocytic leukemia (CLL) received 8 cycles of either 1000 mg (the current standard dose) or 2000 mg of the anti-CD20 monoclonal antibody (mAb) obinutuzumab. The authors report a higher overall response rate with higher doses of obinutuzumab (67% vs 49%), but there was no significant difference in progression-free survival (PFS) between groups.

In this issue of Blood, Bride et al report results of the first prospective multi-institutional trial of a long-term single-agent therapy for refractory cytopenias using rapamycin in 30 patients and show remarkable efficacy in children with autoimmune lymphoproliferative syndrome (ALPS).

This paper, by one of the legends of hematology, William Dameshek, and his colleague Edward Miller, is from the inaugural issue of Blood. By studying bone marrow specimens from controls, patients with acute or chronic immune thrombocytopenia, or patients with other thrombocytopenic disorders, the authors concluded that, in idiopathic thrombocytopenic purpura (ITP), production of platelets from megakaryocytes is defective, even while marrow megakaryocytes are greatly increased in number. This defect resolved after splenectomy. The authors appropriately credit E. Frank with having proposed defective platelet production from megakaryocytes in ITP in 1915. The idea that platelet production was defective in ITP was superseded or ignored for decades, but it has now been validated by the therapeutic effectiveness of the thrombopoietin mimetics in ITP.

Platelet and megakaryocyte endocytosis is important for loading certain granule cargo (ie, fibrinogen [Fg] and vascular endothelial growth factor); however, the mechanisms of platelet endocytosis and its functional acute effects are understudied. Adenosine 5'-diphosphate-ribosylation factor 6 (Arf6) is a small guanosine triphosphate-binding protein that regulates endocytic trafficking, especially of integrins. To study platelet endocytosis, we generated platelet-specific Arf6 knockout (KO) mice. Arf6 KO platelets had less associated Fg suggesting that Arf6 affects αIIbβ3-mediated Fg uptake and/or storage. Other cargo was unaffected. To measure Fg uptake, mice were injected with biotinylated- or fluorescein isothiocyanate (FITC)-labeled Fg. Platelets from the injected Arf6 KO mice showed lower accumulation of tagged Fg, suggesting an uptake defect. Ex vivo, Arf6 KO platelets were also defective in FITC-Fg uptake and storage. Immunofluorescence analysis showed initial trafficking of FITC-Fg to a Rab4-positive compartment followed by colocalization with Rab11-positive structures, suggesting that platelets contain and use both early and recycling endosomes. Resting and activated αIIbβ3 levels, as measured by flow cytometry, were unchanged; yet, Arf6 KO platelets exhibited enhanced spreading on Fg and faster clot retraction. This was not the result of alterations in αIIbβ3 signaling, because myosin light-chain phosphorylation and Rac1/RhoA activation were unaffected. Consistent with the enhanced clot retraction and spreading, Arf6 KO mice showed no deficits in tail bleeding or FeCl3-induced carotid injury assays. Our studies present the first mouse model for defining the functions of platelet endocytosis and suggest that altered integrin trafficking may affect the efficacy of platelet function.

Natural killer (NK) cells exist as subsets based on expression of inhibitory receptors that recognize major histocompatibility complex I (MHCI) molecules. NK cell subsets bearing MHCI binding receptors for self-MHCI have been termed as "licensed" and exhibit a higher ability to respond to stimuli. In the context of bone marrow transplantation (BMT), host licensed-NK (L-NK) cells have also been demonstrated to be responsible for the acute rejection of allogeneic and MHCI-deficient BM cells (BMCs) in mice after lethal irradiation. However, the role of recipient unlicensed-NK (U-NK) cells has not been well established with regard to allogeneic BMC resistance. After NK cell stimulation, the prior depletion of host L-NK cells resulted in a marked increase of donor engraftment compared with the untreated group. Surprisingly, this increased donor engraftment was reduced after total host NK cell depletion, indicating that U-NK cells can actually promote donor allogeneic BMC engraftment. Furthermore, direct coculture of U-NK cells with allogeneic but not syngeneic BMCs resulted in increased colony-forming unit cell growth in vitro, which was at least partially mediated by granulocyte macrophage colony-stimulating factor (GM-CSF) production. These data demonstrate that host NK cell subsets exert markedly different roles in allogeneic BMC engraftment where host L- and U-NK cells reject or promote donor allogeneic BMC engraftment, respectively.

Previous studies have shown that hemostatic thrombi formed in response to penetrating injuries have a core of densely packed, fibrin-associated platelets overlaid by a shell of less-activated, loosely packed platelets. Here we asked, first, how the diverse elements of this structure combine to stem the loss of plasma-borne molecules and, second, whether antiplatelet agents and anticoagulants that perturb thrombus structure affect the re-establishment of a tight vascular seal. The studies combined high-resolution intravital microscopy with a photo-activatable fluorescent albumin marker to simultaneously track thrombus formation and protein transport following injuries to mouse cremaster muscle venules. The results show that protein loss persists after red cell loss has ceased. Blocking platelet deposition with an αIIbβ3antagonist delays vessel sealing and increases extravascular protein accumulation, as does either inhibiting adenosine 5'-diphosphate (ADP) P2Y12receptors or reducing integrin-dependent signaling and retraction. In contrast, sealing was unaffected by introducing hirudin to block fibrin accumulation or a Gi2α gain-of-function mutation to expand the thrombus shell. Collectively, these observations describe a novel approach for studying vessel sealing after injury in real time in vivo and show that (1) the core/shell architecture previously observed in arterioles also occurs in venules, (2) plasma leakage persists well beyond red cell escape and mature thrombus formation, (3) the most critical events for limiting plasma extravasation are the stable accumulation of platelets, ADP-dependent signaling, and the emergence of a densely packed core, not the accumulation of fibrin, and (4) drugs that affect platelet accumulation and packing can delay vessel sealing, permitting protein escape to continue.

Cutaneous T-cell lymphoma (CTCL) is characterized by proliferation of malignant T cells in a chronic inflammatory environment. With disease progression, bacteria colonize the compromised skin barrier and half of CTCL patients die of infection rather than from direct organ involvement by the malignancy. Clinical data indicate that bacteria play a direct role in disease progression, but little is known about the mechanisms involved. Here, we demonstrate that bacterial isolates containing staphylococcal enterotoxin A (SEA) from the affected skin of CTCL patients, as well as recombinant SEA, stimulate activation of signal transducer and activator of transcription 3 (STAT3) and upregulation of interleukin (IL)-17 in immortalized and primary patient-derived malignant and nonmalignant T cells. Importantly, SEA induces STAT3 activation and IL-17 expression in malignant T cells when cocultured with nonmalignant T cells, indicating an indirect mode of action. In accordance, malignant T cells expressing an SEA-nonresponsive T-cell receptor variable region β chain are nonresponsive to SEA in monoculture but display strong STAT3 activation and IL-17 expression in cocultures with SEA-responsive nonmalignant T cells. The response is induced via IL-2 receptor common γ chain cytokines and a Janus kinase 3 (JAK3)-dependent pathway in malignant T cells, and blocked by tofacitinib, a clinical-grade JAK3 inhibitor. In conclusion, we demonstrate that SEA induces cell cross talk-dependent activation of STAT3 and expression of IL-17 in malignant T cells, suggesting a mechanism whereby SEA-producing bacteria promote activation of an established oncogenic pathway previously implicated in carcinogenesis.