Cutting Edge: The Tetraspanin CD53 Promotes CXCR4 Signaling and Bone Marrow Homing in B Cells

B cell trafficking involves the coordinated activity of multiple adhesive and cytokine-receptor interactions, and the players in this process are not fully understood. In this study, we identified the tetraspanin CD53 as a critical regulator of both normal and malignant B cell trafficking. CXCL12 is a key chemokine in B cell homing to the bone marrow and secondary lymphoid organs, and both normal and malignant B cells from Cd53-/- mice have reduced migration toward CXCL12 in vitro, as well as impaired marrow homing in vivo. Using proximity ligation studies, we identified the CXCL12 receptor, CXCR4, as a novel, to our knowledge, CD53 binding partner. This interaction promotes receptor function, because Cd53-/- B cells display reduced signaling and internalization of CXCR4 in response to CXCL12. Together, our data suggest that CD53 interacts with CXCR4 on both normal and malignant B cells to promote CXCL12 signaling, receptor internalization, and marrow homing.

Staphylococcus aureus α-toxin impairs early neutrophil localization via electrogenic disruption of store-operated calcium entry

The pore-forming S. aureus α-toxin (Hla) contributes to virulence and disease pathogenesis. While high concentrations of toxin induce cell death, neutrophils exhibit relative resistance to lysis, suggesting that the action of Hla may not be solely conferred by lytic susceptibility. Using intravital microscopy, we observed that Hla disrupts neutrophil localization and clustering early in infection. Hla forms a narrow, ion-selective pore, suggesting that Hla may dysregulate calcium or other ions to impair neutrophil function. We found that sub-lytic Hla did not permit calcium influx but caused rapid membrane depolarization. Depolarization decreases the electrogenic driving force for calcium, and concordantly, Hla suppressed calcium signaling in vitro and in vivo and calcium-dependent leukotriene B4 (LTB4) production, a key mediator of neutrophil clustering. Thus, Hla disrupts the early patterning of the neutrophil response to infection, in part through direct impairment of neutrophil calcium signaling. This early mis-localization of neutrophils may contribute to establishment of infection.

The tetraspanin CD53 protects stressed hematopoietic stem cells via promotion of DREAM complex-mediated quiescence

The hematopoietic stem cell (HSC) cycle responds to inflammatory and other proliferative stressors; however, these cells must quickly return to quiescence to avoid exhaustion and maintain their functional integrity. The mechanisms that regulate this return to quiescence are not well understood. Here, we show that tetraspanin CD53 is markedly upregulated in HSCs in response to a variety of inflammatory and proliferative stimuli and that the loss of CD53 is associated with prolonged cycling and reduced HSC function in the context of inflammatory stress. Mechanistically, CD53 promotes the activity of the dimerization partner, RB-like, E2F, and multi-vulva class B (DREAM) transcriptional repressor complex, which downregulates genes associated with cycling and division. Proximity labeling and confocal fluorescence microscopy studies showed that CD53 interacts with DREAM-associated proteins, specifically promoting the interaction between Rbl2/p130 and its phosphatase protein phosphatase 2A (PP2A), effectively stabilizing p130 protein availability for DREAM binding. Together, these data identified a novel mechanism by which stressed HSCs resist cycling.

The tetraspanin transmembrane protein CD53 mediates dyslipidemia and integrates inflammatory and metabolic signaling in hepatocytes

Tetraspanins are transmembrane signaling and proinflammatory proteins. Prior work demonstrates that the tetraspanin, CD53/TSPAN25/MOX44, mediates B-cell development and lymphocyte migration to lymph nodes and is implicated in various inflammatory diseases. However, CD53 is also expressed in highly metabolic tissues, including adipose and liver; yet its function outside the lymphoid compartment is not defined. Here, we show that CD53 demarcates the nutritional and inflammatory status of hepatocytes. High-fat exposure and inflammatory stimuli induced CD53 in vivo in liver and isolated primary hepatocytes. In contrast, restricting hepatocyte glucose flux through hepatocyte glucose transporter 8 deletion or through trehalose treatment blocked CD53 induction in fat- and fructose-exposed contexts. Furthermore, germline CD53 deletion in vivo blocked Western diet-induced dyslipidemia and hepatic inflammatory transcriptomic activation. Surprisingly, metabolic protection in CD53 KO mice was more pronounced in the presence of an inciting inflammatory event. CD53 deletion attenuated tumor necrosis factor alpha-induced and fatty acid + lipopolysaccharide-induced cytokine gene expression and hepatocyte triglyceride accumulation in isolated murine hepatocytes. In vivo, CD53 deletion in nonalcoholic steatohepatitis diet-fed mice blocked peripheral adipose accumulation and adipose inflammation, insulin tolerance, and liver lipid accumulation. We then defined a stabilized and trehalase-resistant trehalose polymer that blocks hepatocyte CD53 expression in basal and over-fed contexts. The data suggest that CD53 integrates inflammatory and metabolic signals in response to hepatocyte nutritional status and that CD53 blockade may provide a means by which to attenuate pathophysiology in diseases that integrate overnutrition and inflammation, such as nonalcoholic steatohepatitis and type 2 diabetes.

A human STAT3 gain-of-function variant confers T cell dysregulation without predominant Treg dysfunction in mice

Primary immune regulatory disorders (PIRD) represent a group of disorders characterized by immune dysregulation, presenting with a wide range of clinical disease, including autoimmunity, autoinflammation, or lymphoproliferation. Autosomal dominant germline gain-of-function (GOF) variants in STAT3 result in a PIRD with a broad clinical spectrum. Studies in patients have documented a decreased frequency of FOXP3+ Tregs and an increased frequency of Th17 cells in some patients with active disease. However, the mechanisms of disease pathogenesis in STAT3 GOF syndrome remain largely unknown, and treatment is challenging. We developed a knock-in mouse model harboring a de novo pathogenic human STAT3 variant (p.G421R) and found these mice developed T cell dysregulation, lymphoproliferation, and CD4+ Th1 cell skewing. Surprisingly, Treg numbers, phenotype, and function remained largely intact; however, mice had a selective deficiency in the generation of iTregs. In parallel, we performed single-cell RNA-Seq on T cells from STAT3 GOF patients. We demonstrate only minor changes in the Treg transcriptional signature and an expanded, effector CD8+ T cell population. Together, these findings suggest that Tregs are not the primary driver of disease and highlight the importance of preclinical models in the study of disease mechanisms in rare PIRD.

Toll-like receptor and cytokine expression throughout the bone marrow differs between patients with low- and high-risk myelodysplastic syndromes

Myelodysplastic syndromes (MDS) are hematopoietic stem cell disorders, the pathogenesis of which involves enhanced immune signaling that promotes or selects for mutant hematopoietic stem and progenitor cells (HSPCs). In particular, toll-like receptor (TLR) expression and signaling are enhanced in MDS, and their inhibition is an attractive therapeutic strategy. Although prior studies have reported increased expression of TLR2 and its binding partners TLR1 and TLR6 in the CD34+ cells of patients with MDS (especially those with low-risk disease), TLR expression in other cell types throughout the bone marrow is largely unknown. To address this, we used mass cytometry to assess the expression of TLR1, TLR2, and TLR6 and cytokines in the bone marrow hematopoietic cells of six low/intermediate-risk and six high-risk unmatched MDS bone marrow samples, as well as healthy controls, both at baseline and in response to TLR agonists. We observed several consistent differences between the groups. Most notably, TLR expression was upregulated in multiple cell populations in the low/intermediate-risk, but not high-risk, patients. In addition, many cytokines, including interleukin-6, interleukin-8, tumor necrosis factor α, transforming growth factor β, macrophage inflammatory protein 1β, and granzyme B, were highly expressed from various cell types in low/intermediate-risk patients. However, these same cytokines, with the exception of transforming growth factor β, were expressed at lower levels in high-risk MDS. Together, these findings highlight the differential role of inflammation, and specifically TLR expression, in low/intermediate- versus high-risk MDS, and suggest that elevated TLR expression and cytokine production in multiple cell types likely influences the pathogenesis of MDS in lower-risk patients.

Skull and vertebral bone marrow are myeloid cell reservoirs for the meninges and CNS parenchyma

The meninges are a membranous structure enveloping the central nervous system (CNS) that host a rich repertoire of immune cells mediating CNS immune surveillance. Here, we report that the mouse meninges contain a pool of monocytes and neutrophils supplied not from the blood but by adjacent skull and vertebral bone marrow. Under pathological conditions, including spinal cord injury and neuroinflammation, CNS-infiltrating myeloid cells can originate from brain borders and display transcriptional signatures distinct from their blood-derived counterparts. Thus, CNS borders are populated by myeloid cells from adjacent bone marrow niches, strategically placed to supply innate immune cells under homeostatic and pathological conditions. These findings call for a reinterpretation of immune-cell infiltration into the CNS during injury and autoimmunity and may inform future therapeutic approaches that harness meningeal immune cells.

Heterogeneity of meningeal B cells reveals a lymphopoietic niche at the CNS borders

The meninges contain adaptive immune cells that provide immunosurveillance of the central nervous system (CNS). These cells are thought to derive from the systemic circulation. Through single-cell analyses, confocal imaging, bone marrow chimeras, and parabiosis experiments, we show that meningeal B cells derive locally from the calvaria, which harbors a bone marrow niche for hematopoiesis. B cells reach the meninges from the calvaria through specialized vascular connections. This calvarial-meningeal path of B cell development may provide the CNS with a constant supply of B cells educated by CNS antigens. Conversely, we show that a subset of antigen-experienced B cells that populate the meninges in aging mice are blood-borne. These results identify a private source for meningeal B cells, which may help maintain immune privilege within the CNS.

Open conformation of tetraspanins shapes interaction partner networks on cell membranes

Tetraspanins, including CD53 and CD81, regulate a multitude of cellular processes through organizing an interaction network on cell membranes. Here, we report the crystal structure of CD53 in an open conformation poised for partner interaction. The large extracellular domain (EC2) of CD53 protrudes away from the membrane surface and exposes a variable region, which is identified by hydrogen-deuterium exchange as the common interface for CD53 and CD81 to bind partners. The EC2 orientation in CD53 is supported by an extracellular loop (EC1). At the closed conformation of CD81, however, EC2 disengages from EC1 and rotates toward the membrane, thereby preventing partner interaction. Structural simulation shows that EC1-EC2 interaction also supports the open conformation of CD81. Disrupting this interaction in CD81 impairs the accurate glycosylation of its CD19 partner, the target for leukemia immunotherapies. Moreover, EC1 mutations in CD53 prevent the chemotaxis of pre-B cells toward a chemokine that supports B-cell trafficking and homing within the bone marrow, a major CD53 function identified here. Overall, an open conformation is required for tetraspanin-partner interactions to support myriad cellular processes.

The Tetraspanin CD53 Regulates Early B Cell Development by Promoting IL-7R Signaling

The tetraspanin CD53 has been implicated in B cell development and function. CD53 is a transcriptional target of EBF1, a critical transcription factor for early B cell development. Further, human deficiency of CD53 results in recurrent infections and reduced serum Igs. Although prior studies have indicated a role for CD53 in regulating mature B cells, its role in early B cell development is not well understood. In this study, we show that CD53 expression, which is minimal on hematopoietic stem and progenitor cells, increases throughout bone marrow B cell maturation, and mice lacking CD53 have significantly decreased bone marrow, splenic, lymphatic, and peripheral B cells. Mixed bone marrow chimeras show that CD53 functions cell autonomously to promote B lymphopoiesis. Cd53^-/- mice have reduced surface expression of IL-7Rα and diminished phosphatidylinositol 3 kinase and JAK/STAT signaling in prepro- and pro-B cells. Signaling through these pathways via IL-7R is essential for early B cell survival and transition from the pro-B to pre-B cell developmental stage. Indeed, we find increased apoptosis in developing B cells and an associated reduction in pre-B and immature B cell populations in the absence of CD53. Coimmunoprecipitation and proximity ligation studies demonstrate physical interaction between CD53 and IL-7R. Together, these data, to our knowledge, suggest a novel role for CD53 during IL-7 signaling to promote early B cell differentiation.