Predicting plasma cell retention and loss over a lifetime.

Plasma cells (PCs) rely on external survival cues for persistence, which limits the size of the PC pool. How, then, are new specificities incorporated into a saturated system? In this issue of Immunity, Simons and Karin put forward a mathematical framework to explain PC retention that makes testable predictions about steady-state lifespan structure, withstands tests based on accrual and displaceability, and accounts for lifespan stratification with specificity.

Intrinsically determined turnover underlies broad heterogeneity in plasma-cell lifespan.

Antibodies produced by antibody-secreting plasma cells (ASCs) underlie multiple forms of long-lasting immunity. Here we examined the mechanisms regulating ASC turnover and persistence using a genetic reporter to time-stamp ASCs. This approach revealed ASC lifespans as heterogeneous and falling on a continuum, with only a small fraction surviving for >60 days. ASC longevity past 60 days was independent of isotype but correlated with a phenotype that developed progressively and ultimately associated with an underlying "long-lived" ASC (LL ASC)-enriched transcriptional program. While some of the differences between LL ASCs and other ASCs appeared to be acquired with age, other features were shared with some younger ASCs, such as high CD138 and CD93. Turnover was unaffected by altered ASC production, arguing against competition for niches as a major driver of turnover. Thus, ASC turnover is set by intrinsic lifespan limits, with steady-state population dynamics governed by niche vacancy rather than displacement.

B cell receptor ligation induces IgE plasma cell elimination.

The proper regulation of IgE production safeguards against allergic disease, highlighting the importance of mechanisms that restrict IgE plasma cell (PC) survival. IgE PCs have unusually high surface B cell receptor (BCR) expression, yet the functional consequences of ligating this receptor are unknown. Here, we found that BCR ligation induced BCR signaling in IgE PCs followed by their elimination. In cell culture, exposure of IgE PCs to cognate antigen or anti-BCR antibodies induced apoptosis. IgE PC depletion correlated with the affinity, avidity, amount, and duration of antigen exposure and required the BCR signalosome components Syk, BLNK, and PLCγ2. In mice with a PC-specific impairment of BCR signaling, the abundance of IgE PCs was selectively increased. Conversely, BCR ligation by injection of cognate antigen or anti-IgE depleted IgE PCs. These findings establish a mechanism for the elimination of IgE PCs through BCR ligation. This has important implications for allergen tolerance and immunotherapy as well as anti-IgE monoclonal antibody treatments.

Making sense of plasma cell heterogeneity.

Plasma cells (PCs) are essential for the quality and longevity of protective immunity. The canonical humoral response to vaccination involves induction of germinal centers in lymph nodes followed by maintenance by bone marrow-resident PCs, although there are many variations of this theme. Recent studies have highlighted the importance of PCs in nonlymphoid organs, including the gut, central nervous system, and skin. These sites harbor PCs with distinct isotypes and possible immunoglobulin-independent functions. Indeed, bone marrow now appears unique in housing PCs derived from multiple other organs. The mechanisms through which the bone marrow maintains PC survival long-term and the impact of their diverse origins on this process remain very active areas of research.

CD137L and CD4 T cells limit BCL6-expressing pre-germinal center B cell expansion and BCL6-driven B cell malignancy.

Aberrant expression of the proto-oncogene BCL6 is a driver of tumorigenesis in diffuse large B cell lymphoma (DLBCL). Mice overexpressing BCL6 from the B cell-specific immunoglobulin heavy chain µ intron promoter (Iµ-Bcl6Tg/+ ) develop B cell lymphomas with features typical of human DLBCL. While the development of B cell lymphoma in these mice is tightly controlled by T cells, the mechanisms of this immune surveillance are poorly understood. Here we show that CD4 T cells contribute to the control of lymphoproliferative disease in lymphoma-prone Iµ-Bcl6Tg/+ mice. We reveal that this CD4 T cell immuno-surveillance requires signaling by the co-stimulatory molecule CD137 ligand (CD137L; also known as 4-1BBL), which may promote the transition of pre-malignant B cells with an activated phenotype into the germinal center stage via reverse signaling, preventing their hazardous accumulation. Thus, CD137L-mediated CD4 T cell immuno-surveillance adds another layer of protection against B cell malignancy to that provided by CD8 T cell cytotoxicity.

IL-21 has a critical role in establishing germinal centers by amplifying early B cell proliferation.

The proliferation and differentiation of antigen-specific B cells, including the generation of germinal centers (GC), are prerequisites for long-lasting, antibody-mediated immune protection. Affinity for antigen determines B cell recruitment, proliferation, differentiation, and competitiveness in the response, largely through determining access to T cell help. However, how T cell-derived signals contribute to these outcomes is incompletely understood. Here, we report how the signature cytokine of follicular helper T cells, IL-21, acts as a key regulator of the initial B cell response by accelerating cell cycle progression and the rate of cycle entry, increasing their contribution to the ensuing GC. This effect occurs over a wide range of initial B cell receptor affinities and correlates with elevated AKT and S6 phosphorylation. Moreover, the resultant increased proliferation can explain the IL-21-mediated promotion of plasma cell differentiation. Collectively, our data establish that IL-21 acts from the outset of a T cell-dependent immune response to increase cell cycle progression and fuel cyclic re-entry of B cells, thereby regulating the initial GC size and early plasma cell output.

Interleukin-21, acting beyond the immunological synapse, independently controls T follicular helper and germinal center B cells.

Germinal centers (GCs), transient structures within B cell follicles and central to affinity maturation, require the coordinated behavior of T and B cells. IL-21, a pleiotropic T cell-derived cytokine, is key to GC biology through incompletely understood mechanisms. By genetically restricting production and receipt of IL-21 in vivo, we reveal how its independent actions on T and B cells combine to regulate the GC. IL-21 established the magnitude of the GC B cell response by promoting CD4+ T cell expansion and differentiation in a dose-dependent manner and with paracrine activity. Within GC, IL-21 specifically promoted B cell centroblast identity and, when bioavailability was high, plasma cell differentiation. Critically, these actions may occur irrespective of cognate T-B interactions, making IL-21 a general promoter of growth as distinct to a mediator of affinity-driven selection via synaptic delivery. This promiscuous activity of IL-21 explains the consequences of IL-21 deficiency on antibody-based immunity.

Targeting BMI-1 in B cells restores effective humoral immune responses and controls chronic viral infection.

Ineffective antibody-mediated responses are a key characteristic of chronic viral infection. However, our understanding of the intrinsic mechanisms that drive this dysregulation are unclear. Here, we identify that targeting the epigenetic modifier BMI-1 in mice improves humoral responses to chronic lymphocytic choriomeningitis virus. BMI-1 was upregulated by germinal center B cells in chronic viral infection, correlating with changes to the accessible chromatin landscape, compared to acute infection. B cell-intrinsic deletion of Bmi1 accelerated viral clearance, reduced splenomegaly and restored splenic architecture. Deletion of Bmi1 restored c-Myc expression in B cells, concomitant with improved quality of antibody and coupled with reduced antibody-secreting cell numbers. Specifically, BMI-1-deficiency induced antibody with increased neutralizing capacity and enhanced antibody-dependent effector function. Using a small molecule inhibitor to murine BMI-1, we could deplete antibody-secreting cells and prohibit detrimental immune complex formation in vivo. This study defines BMI-1 as a crucial immune modifier that controls antibody-mediated responses in chronic infection.

The concerted change in the distribution of cell cycle phases and zone composition in germinal centers is regulated by IL-21.

Humoral immune responses require germinal centres (GC) for antibody affinity maturation. Within GC, B cell proliferation and mutation are segregated from affinity-based positive selection in the dark zone (DZ) and light zone (LZ) substructures, respectively. While IL-21 is known to be important in affinity maturation and GC maintenance, here we show it is required for both establishing normal zone representation and preventing the accumulation of cells in the G1 cell cycle stage in the GC LZ. Cell cycle progression of DZ B cells is unaffected by IL-21 availability, as is the zone phenotype of the most highly proliferative GC B cells. Collectively, this study characterises the development of GC zones as a function of time and B cell proliferation and identifies IL-21 as an important regulator of these processes. These data help explain the requirement for IL-21 in normal antibody affinity maturation.

Display of Native Antigen on cDC1 That Have Spatial Access to Both T and B Cells Underlies Efficient Humoral Vaccination.

Follicular dendritic cells and macrophages have been strongly implicated in presentation of native Ag to B cells. This property has also occasionally been attributed to conventional dendritic cells (cDC) but is generally masked by their essential role in T cell priming. cDC can be divided into two main subsets, cDC1 and cDC2, with recent evidence suggesting that cDC2 are primarily responsible for initiating B cell and T follicular helper responses. This conclusion is, however, at odds with evidence that targeting Ag to Clec9A (DNGR1), expressed by cDC1, induces strong humoral responses. In this study, we reveal that murine cDC1 interact extensively with B cells at the border of B cell follicles and, when Ag is targeted to Clec9A, can display native Ag for B cell activation. This leads to efficient induction of humoral immunity. Our findings indicate that surface display of native Ag on cDC with access to both T and B cells is key to efficient humoral vaccination.

An Erg-driven transcriptional program controls B cell lymphopoiesis.

B lymphoid development is initiated by the differentiation of hematopoietic stem cells into lineage committed progenitors, ultimately generating mature B cells. This highly regulated process generates clonal immunological diversity via recombination of immunoglobulin V, D and J gene segments. While several transcription factors that control B cell development and V(D)J recombination have been defined, how these processes are initiated and coordinated into a precise regulatory network remains poorly understood. Here, we show that the transcription factor ETS Related Gene (Erg) is essential for early B lymphoid differentiation. Erg initiates a transcriptional network involving the B cell lineage defining genes, Ebf1 and Pax5, which directly promotes expression of key genes involved in V(D)J recombination and formation of the B cell receptor. Complementation of Erg deficiency with a productively rearranged immunoglobulin gene rescued B lineage development, demonstrating that Erg is an essential and stage-specific regulator of the gene regulatory network controlling B lymphopoiesis.

How intrinsic and extrinsic regulators of plasma cell survival might intersect for durable humoral immunity.

Plasma cells (PC) are key to protective immunity because they secrete antibodies. Surviving for periods ranging from days to decades in mammals, PC possess varying survival times that cannot be entirely stochastic or extrinsically set, as presumed half-lives vary with antigenic specificity. Here, we review the signals that impart survival potential to PC. These include signals provided during formation, and signals experienced once generated and embedded in the so-called long-lived niche. These signals all feed into survival by maintaining PC expression of MCL1, potentially synergistically with influences of other BCL2 family members. Herein, we propose that each formed PC has a capacity to respond to extrinsic cues that sets an upper maximum to its lifespan, but survival is also affected by variable availability of signals provided in BM survival niches. PC survival thus becomes a function of immunogen characteristics and niche anatomy, determined by the weighted survival benefit ascribed to each involved factor. Most factors, such as supporting cell types and secreted proteins, are predicted to influence survival times varying temporally by orders of magnitude, rather than absolute PC abundances measured at a single time, which may account for the variation in PC lifespan evident in the literature.

Hhex regulates murine lymphoid progenitor survival independently of Stat5 and Cdkn2a.

The transcription factor Hhex (hematopoietically expressed homeobox gene) is critical for development of multiple lymphoid lineages beyond the common lymphoid progenitor. In addition, Hhex regulates hematopoietic stem cell (HSC) self-renewal, emergency hematopoiesis, and acute myeloid leukemia initiation and maintenance. Hhex mediates its effects on HSCs and acute myeloid leukemia stem cells via repression of the Cdkn2a tumor suppressor locus. However, we report here that loss of Cdkn2a does not rescue the failure of lymphoid development caused by loss of Hhex. As loss of Hhex causes apoptosis of lymphoid progenitors associated with impaired Bcl2 expression and defective Stat5b signaling, we tested the effects of rescuing these pathways using transgenic mice. Expression of the anti-apoptotic factor Bcl2, but not activated Stat5, rescued the development of T-, B-, and NK-cell lineages in the absence of Hhex. These results indicate that Bcl2 expression, but not Stat5b signaling or loss of Cdkn2a, can overcome the lymphoid deficiencies caused by the absence of Hhex, suggesting that the primary role of this transcription factor is to promote survival of lymphoid progenitors during early lymphoid development.

Self-reactive and polyreactive B cells are generated and selected in the germinal center during γ-herpesvirus infection.

Immune responses against certain viruses are accompanied by auto-antibody production although the origin of these infection-associated auto-antibodies is unclear. Here, we report that murine γ-herpesvirus 68 (MHV68)-induced auto-antibodies are derived from polyreactive B cells in the germinal center (GC) through the activity of short-lived plasmablasts. The analysis of recombinant antibodies from MHV68-infected mice revealed that about 40% of IgG+ GC B cells were self-reactive, with about half of them being polyreactive. On the other hand, virion-reactive clones accounted for only a minor proportion of IgG+ GC B cells, half of which also reacted with self-antigens. The self-reactivity of most polyreactive clones was dependent on somatic hypermutation (SHM), but this was dispensable for the reactivity of virus mono-specific clones. Furthermore, both virus-mono-specific and polyreactive clones were selected to differentiate to B220lo CD138+ plasma cells (PCs). However, the representation of GC-derived polyreactive clones was reduced and that of virus-mono-specific clones was markedly increased in terminally differentiated PCs as compared to transient plasmablasts. Collectively, our findings demonstrate that, during acute MHV68 infection, self-reactive B cells are generated through SHM and selected for further differentiation to short-lived plasmablasts but not terminally differentiated PCs.

BAFF, IL-4 and IL-21 separably program germinal center-like phenotype acquisition, BCL6 expression, proliferation and survival of CD40L-activated B cells in vitro.

A B cell culture system using BAFF, IL-4 and IL-21 was recently developed that generates B cells with phenotypic and functional characteristics of in vivo-generated germinal center (GC) B cells. Here, we observe discrete influences of each exogenous signal on the expansion and differentiation of a CD40L-activated B cell pool. IL-4 was expressly necessary, but neither BAFF nor IL-21 was required for B cell acquisition of the GC B cell phenotypes of peanut agglutinin binding and loss of CD38 and IgD expression. Both IL-4 and IL-21 enhanced cell cycle entry upon initial activation dose-dependently, and did so additively. Importantly, while both cytokines acted in concert to increase overall BCL6 expression amounts, IL-21 exposure uniquely caused a small proportion of cells to attain a higher level of BCL6 expression, reminiscent of in vivo GC B cells. In contrast, BAFF supported survival of a fraction of memory-like B cells in extended cultures after removal of surrogate T cell-help signals. Thus, by separably programming proliferation, survival and GC phenotype acquisition, IL-4, BAFF and IL-21 drive distinct components of activated B cell fate.

Proapoptotic BIM Impacts B Lymphoid Homeostasis by Limiting the Survival of Mature B Cells in a Cell-Autonomous Manner.

The proapoptotic BH3-only protein BIM (Bcl2l11) plays key roles in the maintenance of multiple hematopoietic cell types. In mice, germline knockout or conditional pan-hematopoietic deletion of Bim results in marked splenomegaly and significantly increased numbers of B cells. However, it has remained unclear whether these abnormalities reflect the loss of cell-intrinsic functions of BIM within the B lymphoid lineage and, if so, which stages in the lifecycle of B cells are most impacted by the loss of BIM. Here, we show that B lymphoid-specific conditional deletion of Bim during early development (i.e., in pro-B cells using Mb1-Cre) or during the final differentiation steps (i.e., in transitional B cells using Cd23-Cre) led to a similar >2-fold expansion of the mature follicular B cell pool. Notably, while the expansion of mature B cells was quantitatively similar in conditional and germline Bim-deficient mice, the splenomegaly was significantly attenuated after B lymphoid-specific compared to global Bim deletion. In vitro, conditional loss of Bim substantially increased the survival of mature B cells that were refractory to activation by lipopolysaccharide. Finally, we also found that conditional deletion of just one Bim allele by Mb1-Cre dramatically accelerated the development of Myc-driven B cell lymphoma, in a manner that was comparable to the effect of germline Bim heterozygosity. These data indicate that, under physiological conditions, BIM regulates B cell homeostasis predominantly by limiting the life span of non-activated mature B cells, and that it can have additional effects on developing B cells under pathological conditions.

Plasma cell output from germinal centers is regulated by signals from Tfh and stromal cells.

Germinal centers (GCs) are the sites where B cells undergo affinity maturation. The regulation of cellular output from the GC is not well understood. Here, we show that from the earliest stages of the GC response, plasmablasts emerge at the GC-T zone interface (GTI). We define two main factors that regulate this process: Tfh-derived IL-21, which supports production of plasmablasts from the GC, and TNFSF13 (APRIL), which is produced by a population of podoplanin+ CD157high fibroblastic reticular cells located in the GTI that are also rich in message for IL-6 and chemokines CXCL12, CCL19, and CCL21. Plasmablasts in the GTI express the APRIL receptor TNFRSF13B (TACI), and blocking TACI interactions specifically reduces the numbers of plasmablasts appearing in the GTI. Plasma cells generated in the GTI may provide an early source of affinity-matured antibodies that may neutralize pathogens or provide feedback regulating GC B cell selection.

Lyn, Lupus, and (B) Lymphocytes, a Lesson on the Critical Balance of Kinase Signaling in Immunity.

Systemic lupus erythematosus (SLE) is a progressive autoimmune disease characterized by increased sensitivity to self-antigens, auto-antibody production, and systemic inflammation. B cells have been implicated in disease progression and as such represent an attractive therapeutic target. Lyn is a Src family tyrosine kinase that plays a major role in regulating signaling pathways within B cells as well as other hematopoietic cells. Its role in initiating negative signaling cascades is especially critical as exemplified by Lyn-/- mice developing an SLE-like disease with plasma cell hyperplasia, underscoring the importance of tightly regulating signaling within B cells. This review highlights recent advances in our understanding of the function of the Src family tyrosine kinase Lyn in B lymphocytes and its contribution to positive and negative signaling pathways that are dysregulated in autoimmunity.

Atypical chemokine receptor 4 shapes activated B cell fate.

Activated B cells can initially differentiate into three functionally distinct fates-early plasmablasts (PBs), germinal center (GC) B cells, or early memory B cells-by mechanisms that remain poorly understood. Here, we identify atypical chemokine receptor 4 (ACKR4), a decoy receptor that binds and degrades CCR7 ligands CCL19/CCL21, as a regulator of early activated B cell differentiation. By restricting initial access to splenic interfollicular zones (IFZs), ACKR4 limits the early proliferation of activated B cells, reducing the numbers available for subsequent differentiation. Consequently, ACKR4 deficiency enhanced early PB and GC B cell responses in a CCL19/CCL21-dependent and B cell-intrinsic manner. Conversely, aberrant localization of ACKR4-deficient activated B cells to the IFZ was associated with their preferential commitment to the early PB linage. Our results reveal a regulatory mechanism of B cell trafficking via an atypical chemokine receptor that shapes activated B cell fate.

Environmental sensing by mature B cells is controlled by the transcription factors PU.1 and SpiB.

Humoral immunity requires B cells to respond to multiple stimuli, including antigen, membrane and soluble ligands, and microbial products. Ets family transcription factors regulate many aspects of haematopoiesis, although their functions in humoral immunity are difficult to decipher as a result of redundancy between the family members. Here we show that mice lacking both PU.1 and SpiB in mature B cells do not generate germinal centers and high-affinity antibody after protein immunization. PU.1 and SpiB double-deficient B cells have a survival defect after engagement of CD40 or Toll-like receptors (TLR), despite paradoxically enhanced plasma cell differentiation. PU.1 and SpiB regulate the expression of many components of the B cell receptor signaling pathway and the receptors for CD40L, BAFF and TLR ligands. Thus, PU.1 and SpiB enable B cells to appropriately respond to environmental cues.