Progressive differentiation toward the long-lived plasma cell compartment in the bone marrow

Analysis of Normal Plasma Cell Distribution Across Distinct Age Cohorts Reveals Age-Dependent Changes

Hematopoietic and stromal cells within the bone marrow (BM) provide membrane-bound and/or soluble factors that are vital for the survival of plasma cells (PCs). Recent reports in murine BM demonstrated the dynamic formation and dispersion of PC clusters. To date, PC clustering in normal human BM has yet to be thoroughly examined. The goal of this study was to determine whether PC clusters are present in human BM and whether clustering changes as a function of age. Quantification of PCs and clustering in BM sections across six different age groups revealed that fewer PCs and PC clusters were observed in the youngest and oldest age groups. PC clustering increased with age until the sixth decade and then began to decrease. A positive correlation between the number of PCs and PC clusters was observed across all age groups. PC clusters were typically heterogeneous for immunoglobulin heavy- and light-chain expression. Taken together, these data demonstrate that PC clusters are present in human BM and that PC clustering increases until middle adulthood and then begins to diminish. These results suggest the spatial distribution of BM PC-supportive stromal cells changes with age.

Beyond the skin: B cells in pemphigus vulgaris, tolerance and treatment

Pemphigus vulgaris (PV) is a rare autoimmune bullous disease characterized by blistering of the skin and mucosa owing to the presence of autoantibodies against the desmosome proteins desmoglein 3 and occasionally in conjunction with desmoglein 1. Fundamental research into the pathogenesis of PV has revolutionized its treatment and outcome with rituximab, a B-cell-depleting therapy. The critical contribution of B cells to the pathogenesis of pemphigus is well accepted. However, the exact pathomechanism, mechanisms of onset, disease course and relapse remain unclear. In this narrative review, we provide an overview of the fundamental research progress that has unfolded over the past few centuries to give rise to current and emerging therapies. Furthermore, we summarize the multifaceted roles of B cells in PV, including their development, maturation and antibody activity. Finally, we explored how these various aspects of B-cell function contribute to disease pathogenesis and pave the way for innovative therapeutic interventions.

New insights into the mechanisms regulating plasma cell survival and longevity

Plasma cells correspond to the last stage of B cell differentiation and are professional antibody-secreting cells. While most persist for only few days, some may survive for weeks to years in dedicated survival niches. The determination of plasma cell survival rate seems to rely both on intrinsic and extrinsic factors. Although often opposed, the deterministic and environmental models for plasma cell longevity are certainly overlapping. Understanding the contribution and the regulation of these different factors is paramount to develop better vaccines but also to target malignant plasma cells. Here, we review recent literature highlighting new findings pertaining to plasma cell survival rate, intrinsic regulation of plasma cell persistence and function, as well as the plasma cell/niche dialogue. Moreover, the now well-recognised heterogeneity observed among plasma cells is also discussed.

New insights into the ontogeny, diversity, maturation and survival of long-lived plasma cells

Plasma cells are unique immune effectors, capable of producing large amounts of high-affinity antibodies that protect against pathogenic infections. Although most plasma cells have short lifespans, certain conditions or vaccinations can give rise to long-lived plasma cells (LLPCs) that provide individuals with lifelong protection against pathogen exposure. The nature of these LLPCs is poorly understood; however, recent studies have shed new light on the ontogeny, diversity, maturation and survival of these unique cells. Whereas LLPCs had been thought to arise preferentially from germinal centres, novel genetic tools have revealed that they can originate from various stages throughout the humoral response. Furthermore, new single-cell analyses have shown that mouse and human plasma cells are heterogeneous and may undergo further maturation in situ in the bone marrow niche. Finally, plasma cells were previously considered to be sessile cells maintained in fixed survival niches, but new data show that plasma cell subsets can differentially migrate and organize into clusters that may be associated with survival niches. These descriptive findings provide new insights into how cell-intrinsic programmes and extrinsic factors may regulate the longevity of plasma cells in various contexts, which suggest new research avenues for their functional validation.

Fine-tuning spatial-temporal dynamics and surface receptor expression support plasma cell-intrinsic longevity

Durable serological memory following vaccination is critically dependent on the production and survival of long-lived plasma cells (LLPCs). Yet, the factors that control LLPC specification and survival remain poorly resolved. Using intravital two-photon imaging, we find that in contrast to most plasma cells (PCs) in the bone marrow (BM), LLPCs are uniquely sessile and organized into clusters that are dependent on APRIL, an important survival factor. Using deep, bulk RNA sequencing, and surface protein flow-based phenotyping, we find that LLPCs express a unique transcriptome and phenotype compared to bulk PCs, fine-tuning expression of key cell surface molecules, CD93, CD81, CXCR4, CD326, CD44, and CD48, important for adhesion and homing. Conditional deletion of Cxcr4 in PCs following immunization leads to rapid mobilization from the BM, reduced survival of antigen-specific PCs, and ultimately accelerated decay of antibody titer. In naïve mice, the endogenous LLPCs BCR repertoire exhibits reduced diversity, reduced somatic mutations, and increased public clones and IgM isotypes, particularly in young mice, suggesting LLPC specification is non-random. As mice age, the BM PC compartment becomes enriched in LLPCs, which may outcompete and limit entry of new PCs into the LLPC niche and pool.

A guide to adaptive immune memory

Immune memory - comprising T cells, B cells and plasma cells and their secreted antibodies - is crucial for human survival. It enables the rapid and effective clearance of a pathogen after re-exposure, to minimize damage to the host. When antigen-experienced, memory T cells become activated, they proliferate and produce effector molecules at faster rates and in greater magnitudes than antigen-inexperienced, naive cells. Similarly, memory B cells become activated and differentiate into antibody-secreting cells more rapidly than naive B cells, and they undergo processes that increase their affinity for antigen. The ability of T cells and B cells to form memory cells after antigen exposure is the rationale behind vaccination. Understanding immune memory not only is crucial for the design of more-efficacious vaccines but also has important implications for immunotherapies in infectious disease and cancer. This 'guide to' article provides an overview of the current understanding of the phenotype, function, location, and pathways for the generation, maintenance and protective capacity of memory T cells and memory B cells.

Metabolic fitness of IgA+ plasma cells in the gut requires DOCK8

Dedicator of cytokinesis 8 (DOCK8) mutations lead to a primary immunodeficiency associated with recurrent gastrointestinal infections and poor antibody responses but, paradoxically, heightened IgE to food antigens, suggesting that DOCK8 is central to immune homeostasis in the gut. Using Dock8-deficient mice, we found that DOCK8 was necessary for mucosal IgA production to multiple T cell-dependent antigens, including peanut and cholera toxin. Yet DOCK8 was not necessary in T cells for this phenotype. Instead, B cell-intrinsic DOCK8 was required for maintenance of antigen-specific IgA-secreting plasma cells (PCs) in the gut lamina propria. Unexpectedly, DOCK8 was not required for early B cell activation, migration, or IgA class switching. An unbiased interactome screen revealed novel protein partners involved in metabolism and apoptosis. Dock8-deficient IgA+ B cells had impaired cellular respiration and failed to engage glycolysis appropriately. These results demonstrate that maintenance of the IgA+ PC compartment requires DOCK8 and suggest that gut IgA+ PCs have unique metabolic requirements for long-term survival in the lamina propria.

Jchain-DTR Mice Allow for Diphtheria Toxin-Mediated Depletion of Antibody-Secreting Cells and Evaluation of Their Differentiation Kinetics

Antibody-secreting cells (ASCs) are generated following B cell activation and constitutively secrete antibodies. As such, ASCs are key mediators of humoral immunity whether it be in the context of pathogen exposure, vaccination or even homeostatic clearance of cellular debris. Therefore, understanding basic tenants of ASC biology such as their differentiation kinetics following B cell stimulation is of importance. Towards that aim, we developed a mouse model which expresses simian HBEGF (a.k.a., diphtheria toxin receptor (DTR)) under the control of the endogenous Jchain locus (or J-DTR). ASCs from these mice expressed high levels of cell surface DTR and were acutely depleted following diphtheria toxin treatment. Furthermore, proof-of-principle experiments demonstrated the ability to use these mice to track ASC reconstitution following depletion in 3 distinct organs. Overall, J-DTR mice provide a new and highly effective genetic tool allowing for the study of ASC biology in a wide range of potential applications.

Fine-tuning spatial-temporal dynamics and surface receptor expression support plasma cell-intrinsic longevity

Durable serological memory following vaccination is critically dependent on the production and survival of long-lived plasma cells (LLPCs). Yet, the factors that control LLPC specification and survival remain poorly resolved. Using intra-vital two-photon imaging, we find that in contrast to most plasma cells in the bone marrow, LLPCs are uniquely sessile and organized into clusters that are dependent on April, an important survival factor. Using deep, bulk RNA sequencing, and surface protein flow-based phenotyping, we find that LLPCs express a unique transcriptome and proteome compared to bulk PCs, fine tuning expression of key cell surface molecules, CD93, CD81, CXCR4, CD326, CD44 and CD48, important for adhesion and homing, and phenotypically label LLPCs within mature PC pool. Conditional deletion of Cxcr4 in PCs following immunization leads to rapid mobilization from the BM, reduced survival of antigen-specific PCs, and ultimately accelerated decay of antibody titer. In naive mice, the endogenous LLPCs BCR repertoire exhibits reduced diversity, reduced somatic mutations, and increased public clones and IgM isotypes, particularly in young mice, suggesting LLPC specification is non-random. As mice age, the BM PC compartment becomes enriched in LLPCs, which may outcompete and limit entry of new PC into the LLPC niche and pool.

Tuning of plasma cell lifespan by competition explains the longevity and heterogeneity of antibody persistence

Plasma cells that emerge after infection or vaccination exhibit heterogeneous lifespans; most survive for days to months, whereas others persist for decades, providing antigen-specific long-term protection. We developed a mathematical framework to explore the dynamics of plasma cell removal and its regulation by survival factors. Analyses of antibody persistence following hepatitis A and B and HPV vaccination revealed specific patterns of longevity and heterogeneity within and between responses, implying that this process is fine-tuned near a critical "flat" state between two dynamic regimes. This critical state reflects the tuning of rates of the underlying regulatory network and is highly sensitive to variation in parameters, which amplifies lifespan differences between cells. We propose that fine-tuning is the generic outcome of competition over shared survival signals, with a competition-based mechanism providing a unifying explanation for a wide range of experimental observations, including the dynamics of plasma cell accumulation and the effects of survival factor deletion. Our theory is testable, and we provide specific predictions.

Lack of affinity signature for germinal center cells that have initiated plasma cell differentiation

Long-lived plasma cells (PCs) secrete antibodies that can provide sustained immunity against infection. High-affinity cells are proposed to preferentially select into this compartment, potentiating the immune response. We used single-cell RNA-seq to track the germinal center (GC) development of Ighg2A10 B cells, specific for the Plasmodium falciparum circumsporozoite protein (PfCSP). Following immunization with Plasmodium sporozoites, we identified 3 populations of cells in the GC light zone (LZ). One LZ population expressed a gene signature associated with the initiation of PC differentiation and readily formed PCs in vitro. The estimated affinity of these pre-PC B cells was indistinguishable from that of LZ cells that remained in the GC. This remained true when high- or low-avidity recombinant PfCSP proteins were used as immunogens. These findings suggest that the initiation of PC development occurs via an affinity-independent process.

Unraveling the diversity and functions of tissue-resident plasma cells

Antibody-secreting plasma cells (PCs) are generated in secondary lymphoid organs but are reported to reside in an emerging range of anatomical sites. Analysis of the transcriptome of different tissue-resident (Tr)PC populations revealed that they each have their own transcriptional signature indicative of functional adaptation to the host tissue environment. In contrast to expectation, all TrPCs were extremely long-lived, regardless of their organ of residence, with longevity influenced by intrinsic factors like the immunoglobulin isotype. Analysis at single-cell resolution revealed that the bone marrow is unique in housing a compendium of PCs generated all over the body that retain aspects of the transcriptional program indicative of their tissue of origin. This study reveals that extreme longevity is an intrinsic property of TrPCs whose transcriptome is imprinted by signals received both at the site of induction and within the tissue of residence.

The NFκB signaling system in the generation of B-cell subsets: from germinal center B cells to memory B cells and plasma cells

Memory B cells and antibody-secreting cells are the two prime effector B cell populations that drive infection- and vaccine-induced long-term antibody-mediated immunity. The antibody-mediated immunity mostly relies on the formation of specialized structures within secondary lymphoid organs, called germinal centers (GCs), that facilitate the interactions between B cells, T cells, and antigen-presenting cells. Antigen-activated B cells may proliferate and differentiate into GC-independent plasmablasts and memory B cells or differentiate into GC B cells. The GC B cells undergo proliferation coupled to somatic hypermutation of their immunoglobulin genes for antibody affinity maturation. Subsequently, affinity mature GC B cells differentiate into GC-dependent plasma cells and memory B cells. Here, we review how the NFκB signaling system controls B cell proliferation and the generation of GC B cells, plasmablasts/plasma cells, and memory B cells. We also identify and discuss some important unanswered questions in this connection.

Minimally invasive longitudinal intravital imaging of cellular dynamics in intact long bone

Intravital two-photon microscopy enables deep-tissue imaging at high temporospatial resolution in live animals. However, the endosteal bone compartment and underlying bone marrow pose unique challenges to optical imaging as light is absorbed, scattered and dispersed by thick mineralized bone matrix and the adipose-rich bone marrow. Early bone intravital imaging methods exploited gaps in the cranial sutures to bypass the need to penetrate through cortical bone. More recently, investigators have developed invasive methods to thin the cortical bone or implant imaging windows to image cellular dynamics in weight-bearing long bones. Here, we provide a step-by-step procedure for the preparation of animals for minimally invasive, nondestructive, longitudinal intravital imaging of the murine tibia. This method involves the use of mixed bone marrow radiation chimeras to unambiguously double-label osteoclasts and osteomorphs. The tibia is exposed by a simple skin incision and an imaging chamber constructed using thermoconductive T-putty. Imaging sessions up to 12 h long can be repeated over multiple timepoints to provide a longitudinal time window into the endosteal and marrow niches. The approach can be used to investigate cellular dynamics in bone remodeling, cancer cell life cycle and hematopoiesis, as well as long-lived humoral and cellular immunity. The procedure requires an hour to complete and is suitable for users with minimal prior expertise in small animal surgery.

The origins and longevity of IgE responses as indicated by serological and cellular studies in mice and humans

The existence of long-lived IgE antibody-secreting cells (ASC) is contentious, with the maintenance of sensitization by the continuous differentiation of short-lived IgE+ ASC a possibility. Here, we review the epidemiological profile of IgE production, and give an overview of recent discoveries made on the mechanisms regulating IgE production from mouse models. Together, these data suggest that for most individuals, in most IgE-associated diseases, IgE+ ASC are largely short-lived cells. A subpopulation of IgE+ ASC in humans is likely to survive for tens of months, although due to autonomous IgE B cell receptor (BCR) signaling and antigen-driven IgE+ ASC apoptosis, in general IgE+ ASC probably do not persist for the decades that other ASC are inferred to do. We also report on recently identified memory B cell transcriptional subtypes that are the likely source of IgE in ongoing responses, highlighting the probable importance of IL-4Rα in their regulation. We suggest the field should look at dupilumab and other drugs that prohibit IgE+ ASC production as being effective treatments for IgE-mediated aspects of disease in most individuals.

Genetic Reduction of IRF5 Expression after Disease Initiation Reduces Disease in a Mouse Lupus Model by Impacting Systemic and End-Organ Pathogenic Pathways

Gain-of-function polymorphisms in the transcription factor IFN regulatory factor 5 (IRF5) are associated with an increased risk of developing systemic lupus erythematosus. Global homozygous or heterozygous deficiency of IRF5 from birth confers protection in many lupus mouse models. However, less is known about the effects of IRF5 targeting after autoimmunity has already developed. This is an important point to clarify when considering IRF5 as a potential therapeutic target in lupus. In this study, we demonstrate that genetic reduction of IRF5 expression after disease initiation reduces disease severity in the FcγRIIB-/- Y-linked autoimmune accelerating mouse lupus model. Reduction of IRF5 expression resulted in a decrease in splenomegaly and lymphadenopathy and a reduction in splenic B cell activation and plasmablast numbers. Splenic T cell activation and differentiation were also impacted as demonstrated by an increase in the number of naive CD4+ and CD8+ T cells and a reduction in the number of memory/effector CD4+ and CD8+ T cells. Although serum antinuclear autoantibody levels were not altered, reduction in IRF5 expression led to decreased immune complex deposition and complement activation, diminished glomerular and interstitial disease, and a reduction in immune cell infiltrate in the kidney. Mechanistically, myeloid cells in the kidney produced less inflammatory cytokines after TLR7 and TLR9 activation. Overall, we demonstrate that genetic reduction of IRF5 expression during an active autoimmune process is sufficient to reduce disease severity. Our data support consideration of IRF5 as a therapeutic target and suggest that approaches targeting IRF5 in systemic lupus erythematosus may need to impact IRF5 activity both systemically and in target organs.

450 million years in the making: mapping the evolutionary foundations of germinal centers

Germinal centers (GCs) are distinct microanatomical structures that form in the secondary lymphoid organs of endothermic vertebrates (i.e., mammals and some birds). Within GCs, B cells undergo a Darwinian selection process to identify clones which can respond to pathogen insult as well as affinity mature the B cell repertoire. The GC response ultimately generates memory B cells and bone marrow plasma cells which facilitate humoral immunological memory, the basis for successful vaccination programs. GCs have not been observed in the secondary lymphoid organs of ectothermic jawed vertebrates (i.e., fishes, reptiles, and amphibians). However, abundant research over the past decades has indicated these organisms can produce antigen specific B cell responses and some degree of affinity maturation. This review examines data demonstrating that the fundamentals of B cell selection may be more conserved across vertebrate phylogeny than previously anticipated. Further, research in both conventional mammalian model systems and comparative models raises the question of what evolutionary benefit GCs provide endotherms if they are seemingly unnecessary for generating the basic functional components of jawed vertebrate humoral adaptive immune responses.

Putting a stamp on plasma cells' age certificate

Long-lived plasma cells (LLPCs) play a central role in protective immunity after vaccination and infection. In this issue of Immunity, Robinson, Ding, et al. utilize a timestamping approach to fate map and characterize the LLPC compartment and demonstrate that PC longevity in the bone marrow is independent of competition for niches with newly generated incoming PCs.

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.