SUMMARY The germinal center (GC) reaction produces high-affinity antibodies by random mutation and selective clonal expansion of B cells with high-affinity receptors. However, the mechanism by which B cells are selected remains unclear, as does the role of the two anatomically-defined areas of the GC, light zone (LZ) and dark zone (DZ). We combined a new transgenic photoactivatable green fluorescent protein (PA-GFP) tracer with multiphoton laser-scanning microscopy and flow cytometry to examine anatomically defined LZ and DZ B cells and GC selection. We find that B cell division is restricted to the DZ, and that there is a net vector of B cell movement from the DZ to the LZ. The decision to return from the LZ to the DZ and undergo clonal expansion is controlled by T cells, which discern between LZ B cells based on the amount of antigen captured, providing a mechanism for GC selection.

Summary The molecular complexity of the bone marrow (BM) microenvironment and its response to stress are incompletely understood, despite its key role in the regulation of hematopoiesis. Here we map the transcriptional landscape of BM vascular, perivascular, and osteoblast niche populations at single-cell resolution at both homeostasis and under stress hematopoiesis. This analysis revealed a previously unappreciated level of cellular heterogeneity within the BM niche, identified novel cellular subsets, and resolved cellular sources of pro-hematopoietic growth factors, chemokines, and membrane-bound ligands. Under conditions of stress, our studies revealed a significant transcriptional remodeling of these niche elements, including an adipocytic skewing of the perivascular cells. Among the stress-induced changes, we observed that vascular Notch ligand delta-like ligands (Dll1,4) were downregulated. In the absence of vascular Dll4, hematopoietic stem cells (HSC) prematurely induced a myeloid transcriptional program. These findings refine our understanding of the cellular architecture of the BM niche, reveal a dynamic and heterogeneous molecular landscape that is highly sensitive to stress, and illustrate the utility of single cell transcriptomic data in systematically evaluating the regulation of hematopoiesis by discrete niche populations.

Responses to cholesterol depletion are often taken as evidence of a role for lipid rafts in cell function. Here, we show that depletion of cell cholesterol has global effects on cell and plasma membrane architecture and function. The lateral mobility of membrane proteins is reduced when cell cholesterol is chronically or acutely depleted. The change in mobility is a consequence of the reorganization of the cell actin. Binding of a GFP-tagged pleckstrin homology domain specific for phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] to the plasma membrane is reduced after cholesterol depletion. This result implies that the reorganization of cytoskeleton depends on the loss or redistribution of plasma membrane PI(4,5)P2. Consistent with this observation, agents that sequester plasma membrane PI(4,5)P2 mimic the effects of cholesterol depletion on actin organization and on lateral mobility.T here is a growing consensus that cell surface membranes are patchworks of domains, local concentrations of membrane proteins, and lipids quite different from the average for an entire membrane (1). Cholesterol is important in organizing some types of domains, usually termed lipid rafts (2, 3). These lipid rafts are thought to be required for cell functions, including directed mobility and capping of membrane proteins, receptor-mediated signaling, entry and exit of pathogens and membrane trafficking (reviewed in ref. 4). Lipid rafts are dispersed when cell cholesterol is extracted (3). Hence, an effect of cholesterol depletion on a particular function is usually assumed to show that lipid rafts are required for this function (5-11). This assumption neglects the way in which the effects of cholesterol depletion ramify beyond local membrane environments and so have global effects on membrane and cell properties.Our starting point for considering global effects of cholesterol depletion is recent work showing that the key regulatory phospholipid, phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] is concentrated in cholesterol-dependent domains in proximity to concentrations of F actin, and other components of membrane trafficking (12-16). The localization of PI(4,5)P2 is consistent with its regulated involvement in a wide variety of cell functions (17), particularly regulation of the cytoskeleton (18). Availability of PI(4,5)P2 modulates the cytoskeleton͞membrane interaction (19), the stability of cortical actin, and the turnover of cytoplasmic stress fibers (20).Here, we connect the requirement for cholesterol in organizing plasma membrane PI(4,5)P2 with the role of PI(4,5)P2 in organizing the cytoskeleton. We found that the lateral mobility of plasma membrane proteins was restricted after cholesterol depletion. This effect was reversed by cytochalasin D and was paralleled by changes in organization and turnover of cell actin. The level of PI(4,5)P2 in the plasma membrane was reduced after cholesterol depletion, and the effects of cholesterol depletion were mimicked by sequestering plasma membrane PI(4,5)P2. Thus, cholesterol depleti...

T cell activation and function require a structured engagement of antigen-presenting cells. These cell contacts are characterized by two distinct dynamics in vivo: transient contacts resulting from promigratory junctions called immunological kinapses or prolonged contacts from stable junctions called immunological synapses. Kinapses operate in the steady state to allow referencing to selfpeptide-MHC (pMHC) and searching for pathogen-derived pMHC. Synapses are induced by T cell receptor (TCR) interactions with agonist pMHC under specific conditions and correlate with robust immune responses that generate effector and memory T cells. High-resolution imaging has revealed that the synapse is highly coordinated, integrating cell adhesion, TCR recognition of pMHC complexes, and an array of activating and inhibitory ligands to promote or prevent T cell signaling. In this review, we examine the molecular components, geometry, and timing underlying kinapses and synapses. We integrate recent molecular and physiological data to provide a synthesis and suggest ways forward.

Entry into the germinal center requires antigen-bearing B cells to compete for cognate T cell help at the T–B border.

SUMMARY The role of the microenvironment in T cell acute lymphoblastic leukemia (T-ALL), or any acute leukemia, is poorly understood. Here we demonstrate that T-ALL cells are in direct, stable contact with CXCL12-producing bone marrow stroma. Cxcl12 deletion from vascular endothelial, but not perivascular, cells impeded tumor growth, suggesting a vascular niche for T-ALL. Moreover, genetic targeting of CXCR4 in murine T-ALL after disease onset led to rapid, sustained disease remission, and CXCR4 antagonism suppressed human T-ALL in primary xenografts. Loss of CXCR4 targeted key T-ALL regulators, including the MYC pathway, and decreased leukemia initiating cell activity in vivo. Our data identify a T-ALL niche, and suggest targeting CXCL12/CXCR4 signaling as a powerful therapeutic approach for T-ALL.

The cellular inhibitor of apoptosis cIAP1 and −2 are amplified in about 3% of cancers, and were identified in multiple malignancies as potential therapeutic targets due to their role in evasion of apoptosis. Consequently, small molecule IAP antagonists, like LCL161, have entered clinical trials for their ability to induce TNF-mediated apoptosis of cancer cells. However, cIAP1 and −2 are recurrently homozygously deleted in multiple myeloma resulting in constitutive activation of the non-canonical NFkB pathway. It was therefore counterintuitive to observe a robust in vivo anti-myeloma activity of LCL161 in a transgenic myeloma mouse model and patients with relapsed-refractory myeloma, where addition of cyclophosphamide resulted in a median progression free survival of 10 months. This effect is not due to direct induction of tumor cell death, but rather to upregulation of a tumor cell autonomous type I interferon signaling and a strong inflammatory response with activation of macrophages and dendritic cells resulting in phagocytosis of tumor cells. Treatment with LCL161 established long-term anti-tumor protection and cure in a fraction of transgenic Vk*MYC mice. Remarkably, combination of LCL161 with the immune-checkpoint inhibitor anti-PD1 was curative in all treated mice.

Summary In this study, we imaged the differentiation and migratory behavior of nascent plasma cells (PCs) in mouse lymph nodes by intravital microscopy.. Pre-PCs exhibited a unique migration pattern characterized by long, linear paths that were randomly oriented. Although chemotaxis via Gαi coupled-receptors has been implicated in PC migration, treatment with Pertussis toxin (Ptx), which ablates these signals, did not prevent movement of pre-PCs while it arrested other lymphocytes. In vitro, pre-PCs displayed processive amoeboid locomotion on surfaces coated with integrin ligand, while fully-differentiated PC moved slowly or were arrested. Both PC arrest and differentiation occurred in the medullary cords. Ptx treatment before PC differentiation blocked their accumulation in the medullary cords but pre-PCs still differentiated in other lymph node regions. Taken together, we suggest pre-PCs undergo a persistent random walk to find the medullary cords, where localized chemokines help retain these cells until they undergo differentiation and arrest in situ.