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Memory B cell

Memory B cells are long-lived, antigen-experienced lymphocytes of the that provide rapid and enhanced humoral responses upon re-exposure to previously encountered pathogens or antigens, forming a of immunological memory. They originate primarily from (GC) reactions in secondary lymphoid organs during primary immune responses, where B cells undergo , affinity maturation, and class-switch recombination with the aid of T follicular helper cells, although GC-independent pathways also contribute to their generation, particularly for T cell-independent antigens. In humans, memory B cells are typically identified by surface markers such as CD27, alongside heterogeneous subsets including unswitched (IgM+) and switched (IgG+ or IgA+) cells that differ in longevity, localization, and reactivation potential; for instance, IgG+ memory B cells often differentiate swiftly into cells for immediate production, while IgM+ cells may re-enter GCs for further maturation. These cells play a pivotal role in long-term protective immunity by persisting in quiescent states within lymphoid tissues, , or even peripheral sites like mucosal surfaces, enabling accelerated recall responses that are more potent and versatile than primary responses, including adaptation to variants through broadly neutralizing antibodies. Studies on infection or demonstrate their durability, with memory B cells remaining detectable and functional for over two years, exhibiting ongoing through to counter viral mutations. Furthermore, memory B cells contribute to efficacy by serving as precursors for high-affinity production, underscoring their importance in strategies aimed at eliciting broad-spectrum protection against evolving threats like or coronaviruses.

Overview

Definition and characteristics

Memory B cells are long-lived B lymphocytes generated following initial exposure to an antigen, persisting in the immune system to facilitate rapid and enhanced antibody responses upon re-exposure to the same antigen. These cells represent a cornerstone of humoral immunity, contributing to long-term protection by enabling quicker pathogen clearance compared to primary responses. A defining feature of memory B cells is their specificity, mediated by the (BCR), which typically bears mutations from and has undergone class-switch recombination to produce isotypes such as IgG or IgA with higher affinity. They maintain a quiescent state, residing primarily in secondary lymphoid tissues, with some also found in the , without ongoing division or secretion until reactivation. Morphologically, memory B cells are small, non-proliferating lymphocytes that express high levels of CD27 in humans, distinguishing their from other B cell populations. In comparison to naive B cells, which are antigen-inexperienced and express unmutated BCRs of lower , memory B cells are primed for superior responsiveness due to prior encounter. Unlike cells, which function as short-lived, terminally differentiated effectors focused on continuous production, memory B cells serve as dormant sentinels that can differentiate into cells or re-enter germinal centers upon stimulation.

Role in adaptive immunity

Memory B cells play a central role in establishing immunological memory within the , enabling a swift and amplified humoral response to reinfection by previously encountered pathogens. Upon secondary exposure, these long-lived, antigen-specific cells rapidly differentiate into antibody-secreting cells or re-enter germinal centers, producing antibodies at higher titers and with greater efficiency compared to the primary response. This reactive humoral memory forms a critical component of long-term protection, complementing the constitutive antibody production by long-lived cells. Through their involvement in affinity maturation, memory B cells contribute to the generation of high-affinity via and antigen-driven selection processes, ensuring more effective pathogen neutralization in subsequent encounters. Additionally, they facilitate isotype switching, transitioning from IgM to class-switched isotypes such as IgG or IgA, which enhance antibody effector functions like opsonization and mucosal immunity. These adaptations optimize the humoral response for diverse pathogenic challenges. Memory B cells provide protection against variant by leveraging , particularly through less mutated IgM-expressing subsets that recognize structurally similar but distinct antigens, thereby broadening immune coverage without requiring exact matches. This mechanism allows for rapid recall responses that mitigate the impact of , as seen in responses to variants. Furthermore, memory B cells integrate with cellular immunity through interactions with T follicular helper cells, which deliver essential signals to promote their reactivation and , thereby coordinating a robust adaptive defense.

Generation

From naive B cells

Naive B cells, which circulate through peripheral lymphoid organs such as lymph nodes and , originate memory B cells upon initial encounter with . Antigen recognition occurs via the (BCR), leading to BCR signaling that promotes antigen internalization and presentation on class II (MHC-II) molecules. This process directs activated naive B cells to the T-B cell border in secondary lymphoid organs, where they initiate interactions with CD4+ T cells or other immune components. Following , naive B cells undergo rapid and initial , often in extrafollicular foci outside of lymphoid follicles. These extrafollicular responses generate early memory B cell precursors within days of exposure, characterized by unmutated BCRs and limited . in these sites supports the expansion of -specific clones before potential migration into follicles for further maturation. is essential for this phase, including CD40 ligand (CD40L) provided by T cells or innate signals such as cytokines (e.g., IL-21) and (TLR) ligands, which enhance survival and prevent of activated B cells. Early fate decisions diverge activated naive B cells toward short-lived plasmablasts or memory precursors. High doses or strong co-stimulatory signals favor differentiation into plasmablasts for immediate production, driven by transcription factors like (Blimp-1). In contrast, limited availability or weaker T cell help promotes the formation of memory precursors, marked by higher expression of Bach2 and Foxp1, which inhibit differentiation and support long-term survival potential. These decisions occur prior to entry and lay the foundation for durable .

T cell-dependent pathways

Following activation of naive B cells by antigen, these cells migrate to the T-B cell border in secondary lymphoid organs, where they engage with activated CD4+ T cells that differentiate into T follicular helper (Tfh) cells. This interaction is mediated primarily through the ligation of CD40 on B cells with CD40L on Tfh cells, which delivers essential survival and proliferation signals to the B cells. Concurrently, Tfh cells secrete cytokines such as IL-21, which synergizes with CD40 signaling to promote B cell expansion and fate decisions toward either (GC) entry or early cell formation. The strength and duration of this T cell help determine the pathway: brief CD40-CD40L engagement favors GC-independent differentiation, while sustained signaling, often with high IL-21 levels, drives GC commitment. In the early stages of formation, affinity-based selection begins to shape memory precursor cells among the proliferating clones. Activated with higher-affinity receptors (BCRs) receive preferential Tfh help, leading to enhanced survival and selection for fate even before full GC maturation. This process occurs in nascent GCs shortly after , where memory precursors express markers of early commitment and can to form long-lived pools.00349-6) Studies in mice show that limited availability during this phase promotes the generation of these early by reducing and favoring from the GC-like structures. A distinct T cell-dependent but GC-independent route generates unswitched IgM memory B cells, which arise rapidly after initial T-B interactions without entering mature GCs. These cells develop through CD40 signaling and Tfh-derived cytokines, retaining IgM expression and providing broad, low-affinity protection against reinfection. Evidence from mouse models demonstrates that early IgM+ memory B cells emerge within days of via this pathway, distinct from later GC-derived switched memory cells. B cell-intrinsic factors, particularly the Bcl6, play a critical role in committing cells to the memory fate within T-dependent pathways. Bcl6 expression is upregulated in response to CD40 and IL-21 signals, enabling GC entry for affinity maturation while its regulated downregulation in select precursors allows exit and memory differentiation. Quantitative studies indicate that the level of Bcl6 shortly after engagement dictates whether a B cell clone pursues GC residency or memory formation, with intermediate levels favoring memory precursors. This intrinsic regulation ensures a balanced output of high-affinity memory B cells tailored to the immune challenge.

Activation and differentiation

T cell-independent pathways

Memory B cells can be activated through T cell-independent (TI) pathways in response to specific antigens that do not require T cell assistance, enabling rapid antibody production primarily against microbial components. TI antigens are classified into two types: TI-1 antigens, such as (LPS) from , which stimulate B cells at high concentrations via both the (BCR) and pattern recognition receptors like (TLR4); and TI-2 antigens, such as repetitive from encapsulated bacteria (e.g., pneumococcal capsular polysaccharides), which activate B cells through extensive BCR crosslinking due to their multivalent structures. This recognition often occurs in the splenic marginal zone, where innate-like B cells are positioned to encounter blood-borne pathogens efficiently. Activation in these pathways relies on innate signals that amplify BCR engagement, bypassing the need for T cell-derived cytokines or . Co-engagement of BCR with TLRs provides a second signal for and , while cytokines such as B cell-activating factor (BAFF) and a (APRIL), signaling through the transmembrane activator and CAML interactor (TACI) receptor, promote survival and plasmablast formation. This leads to the rapid of activated B cells into short-lived plasmablasts that secrete low-affinity IgM antibodies, typically within 1–3 days of exposure, contributing to early humoral defense against blood-borne infections. In contrast to T cell-dependent pathways, these responses are faster but generate less diverse and durable immunity. TI pathways predominantly generate unswitched IgM B cells in the splenic marginal zone, derived from marginal zone B cells or B1 cells, which express polyreactive BCRs suited for broad microbial recognition. These cells enable quicker secondary responses upon re-exposure to the same TI , producing IgM-secreting plasmablasts without the need for initial priming by T cells. However, this process is limited by the absence of class-switch recombination and , resulting in cells that retain germline-encoded BCRs with inherently lower affinity compared to their T cell-dependent counterparts. Consequently, TI provides effective but unrefined protection, particularly against polysaccharide-encapsulated bacteria, and is less efficient at generating high-affinity, isotype-switched antibodies.

Germinal center processes

Upon activation by T cell-dependent antigens, naive B cells proliferate and migrate to the borders of lymphoid follicles, where they interact with T follicular helper (Tfh) cells before entering (GCs). Within the GC, these activated B s further migrate based on gradients, primarily directing them to the (DZ) for initial proliferation and , while CCR7 and EBI2 guide movement to the light zone (LZ) for antigen-based selection. This zonal shuttling allows centroblasts in the DZ to undergo rapid divisions every 4-6 hours, expanding clones while preparing for mutational refinement. Somatic hypermutation in the DZ is driven by activation-induced cytidine deaminase (AID), which deaminates cytosines in immunoglobulin variable region genes, introducing point mutations at rates up to 10^-3 per per generation to diversify B cell receptors (BCRs). Approximately 40% of DZ B cells acquire these mutations during early , enabling affinity maturation as mutated BCRs are tested in the LZ. Following mutation, B cells transition to the LZ as centrocytes, where they compete for survival signals; (FDCs) retain native on their surface, allowing high-affinity BCRs to internalize and present peptide-MHC complexes more efficiently to Tfh cells. Tfh cells deliver CD40L and cytokines like IL-21 to selected B cells, promoting their return to the DZ for further rounds or . Memory B cell precursors emerge primarily from the LZ, where lower-affinity clones relative to plasma cell progenitors are favored for exit to maintain broad diversity. This process involves , in which one daughter cell retains polarized antigen and recycling factors to re-enter the GC, while the other downregulates DZ markers like and exits as a memory precursor. Concurrently, downregulation of the Blimp-1 (encoded by ) in these precursors prevents differentiation, sustaining a memory through sustained Bcl6 expression and metabolic shifts toward quiescence. Only about 2-3% of GC B cells ultimately exit as memory cells, marked by CCR6 expression and positioned for long-term surveillance.

Subsets

Classical switched memory B cells

Classical switched memory B cells represent the predominant subset of antigen-experienced B cells in humans, defined by their expression of CD27 and class-switched immunoglobulin isotypes such as IgG or IgA, coupled with somatically hypermutated B cell receptors (BCRs) that confer high . These cells arise primarily through T cell-dependent (GC) reactions, where naive B cells activated by and T follicular helper cell assistance undergo proliferation, , and class-switch recombination to generate this mature phenotype. They characteristically express and CD21, enabling homing to lymphoid follicles and complement-mediated enhancement of BCR signaling, respectively. Upon secondary encounter, classical switched memory B cells exhibit enhanced responsiveness, rapidly proliferating and differentiating into antibody-secreting plasma cells that produce high-affinity, class-switched antibodies to mount a swift and robust recall response. This subset demonstrates notable , allowing recognition of antigen variants due to their diversified BCR shaped by GC affinity maturation, as observed in responses to related pathogens like dengue and Zika. Post-vaccination or infection, classical switched memory B cells expand significantly in peripheral blood and secondary lymphoid tissues, comprising a substantial portion of circulating B cells (up to 20-30% in adults) and serving as a reservoir for long-term humoral . Their persistence in these compartments underscores their role in maintaining immune memory against reinfection.

Unswitched and atypical memory B cells

Unswitched memory B cells, primarily expressing IgM and retaining IgD, represent a subset of memory B cells that do not undergo class-switch recombination, distinguishing them from classical switched counterparts. These cells can be generated through both (GC)-dependent and independent pathways, including early responses to T cell-independent () antigens that do not require T cell help, as well as GC reactions that may involve limited T cell interactions. Unswitched IgM+ memory B cells often exhibit lower levels of compared to switched memory B cells, particularly those from extrafollicular or GC-independent origins, though GC-derived ones may show higher mutation levels. Functionally, unswitched IgM+ memory B cells demonstrate self-renewal capacity and rapid differentiation into antibody-secreting cells upon re-encounter, contributing to early recall responses without requiring class switching. They are often tissue-resident, particularly in mucosal sites such as the gut and lungs, where they provide localized innate-like immunity against recurring pathogens. This residency is supported by their expression of tissue-homing integrators and distinct transcriptional profiles adapted to barrier environments. Atypical memory B cells encompass heterogeneous populations, including double-negative (DN) cells characterized by a CD27- IgD- phenotype, which expand prominently under chronic antigenic stimulation such as in or infections. These cells arise via extrafollicular differentiation or early GC exit, influenced by interferon-γ and signaling, leading to a hyporesponsive or anergic state with reduced signaling. Despite this, atypical subsets retain self-renewal potential and can participate in protective recall responses, as evidenced by DN cells producing pathogen-specific antibodies in chronic settings. Recent studies have identified durable class-switched memory B cell populations with enhanced persistence post-vaccination, as well as T-bet-expressing subsets with specific tissue homing properties. Recent studies highlight atypical variants, such as CD45RBlo memory B cells specific to , which dominate circulating antigen-specific responses following or mRNA , expressing markers like CD11c+ and T-bet while showing effector-like functions including plasmablast . These cells, often lacking CD27 and CD21, contribute to sustained production and correlate with protective IgG levels, underscoring their role in immunity without classical switching.

Phenotypic and functional markers

Surface markers

Memory B cells in humans are primarily identified by the surface expression of , a pan-B cell marker, combined with , a member upregulated during reactions, and high levels of CD21, while lacking IgD on switched subsets. Classical switched memory B cells typically display a + + phenotype, whereas atypical subsets may express CD11c+ and lower CD21. In mice, memory B cells are distinguished using B220 (CD45R), a B cell lineage marker, along with low to absent GL7 (a germinal center-associated glycoform of ) and intermediate to high expression, often in IgD-low or negative populations to denote post- differentiation. These markers differentiate memory B cells from germinal center B cells, which are B220+ GL7+ CD38lo. Key surface proteins like in s provide survival signals through interaction with its ligand , promoting activation, proliferation, and immunoglobulin production essential for memory maintenance. Similarly, , expressed on both and memory B cells, facilitates homing to lymphoid follicles by binding , enabling recirculation and positioning for rapid antigen encounter. Flow cytometry protocols for identifying memory B cells typically involve multiparametric staining of peripheral blood mononuclear cells or splenocytes, gating first on viable + (human) or B220+ (mouse) lymphocytes, followed by exclusion of IgD+ naive cells and inclusion of + (human) or GL7- + (mouse) populations, often using fluorescently labeled antibodies against 8-12 markers for subset resolution. Variations occur across species, with humans relying on due to its absence of a direct murine homolog, and tissue-specific differences, such as reduced CD21 on circulating versus splenic human memory B cells or altered levels in inflamed tissues affecting homing efficiency.

Transcriptional and functional signatures

Memory B cells exhibit distinct transcriptional profiles that underscore their quiescent identity and potential for rapid differentiation. Key transcription factors such as Bach2 and Pax5 are highly expressed in memory B cells, where they promote quiescence by repressing genes associated with differentiation and maintaining lineage commitment. In contrast, levels of (encoding Blimp-1) remain low in memory B cells compared to plasma cells, as Bach2 directly represses Prdm1 transcription to prevent terminal differentiation and support long-term survival through upregulation of anti-apoptotic factors like Bcl2l1. Pax5 further reinforces this profile by counteracting plasma cell-promoting signals, such as those from Id2, ensuring memory B cells retain proliferative capacity without immediate effector commitment. Epigenetic modifications further define memory B cell identity, with open chromatin regions at loci for rapid reactivation distinguishing them from other B cell subsets. Specifically, memory B cells display accessible chromatin architecture around plasma cell-associated genes, including Prdm1, Xbp1, and Irf4, which poises these cells for swift transcriptional activation upon antigen re-exposure without requiring extensive remodeling. Integrative analyses of transcriptomes and chromatin landscapes in human memory B cells have revealed distinct enhancer and promoter accessibility patterns that correlate with their dormant yet responsive state, such as enriched open regions at metabolic and survival gene clusters. These epigenetic signatures are inherited from germinal center precursors and refined during quiescence, enabling efficient recall responses. Functionally, memory B cells demonstrate enhanced metabolic fitness and resistance to , as assessed through and bioenergetic assays. Upon exiting the , memory B cells undergo metabolic reprogramming toward and reduced , which sustains quiescence and provides a advantage via provision of prosurvival signals like BAFF-mediated Akt activation. This metabolic shift enhances their fitness, as evidenced by higher spare respiratory capacity in assays compared to B cells, allowing persistence in lymphoid niches. Regarding , memory B cells show greater resistance than naive B cells, with CD27+ populations exhibiting reduced activation and V staining in response to stress signals, mediated by elevated members and Puma regulation. Single-cell RNA sequencing (scRNA-seq) has illuminated heterogeneity in memory B cell transcriptional and functional signatures, particularly during reactivation. More recent 2025 analyses of activation dynamics via scRNA-seq identified competing regulatory networks that drive heterogeneous responses in memory versus naive B cells, highlighting subsets with enhanced metabolic adaptability for faster production. These insights, often derived from sorted populations using surface markers like and CD27, underscore the diverse reactivation potentials within memory B cell compartments.

Longevity and maintenance

Lifespan dynamics

Memory B cells display a broad spectrum of lifespans, ranging from weeks for short-lived populations to decades for long-lived clones, enabling sustained against previously encountered antigens. In humans, naive B cells typically survive only a few weeks, while memory B cell clones can persist for decades, contributing to lifelong protection. Specifically, IgG+ memory B cells generated in response to , such as , have been shown to endure for over 50 years in immunized individuals, with similar longevity observed for tetanus-specific memory B cells in cohorts spanning up to 60 years post-. The memory B cell pool consists of short-lived and long-lived subsets, with early attrition favoring the elimination of low-affinity cells during selection. Initial responses produce a heterogeneous population where low-affinity B cells form short-lived effectors, but subsequent phases prioritize high-affinity variants for long-term persistence through competitive selection processes that prune suboptimal clones. This dynamic ensures that the enduring memory compartment is enriched for cells capable of mounting robust secondary responses. Lifespan dynamics vary significantly across species, with memory B cells in mice exhibiting shorter persistence of months to a few years due to the animal's compressed lifespan, in contrast to the decades-long durability observed in humans. Tracking studies using isotope labeling, such as deuterium-glucose incorporation, reveal rapid turnover of peripheral blood memory B cells but indicate that long-lived populations establish residency in the , where they undergo slower renewal and contribute to sustained immunity.

Survival mechanisms

Memory B cells rely on specialized survival niches within lymphoid organs, such as the and , where they receive essential prosurvival signals to persist in a quiescent state. In these niches, B cell-activating factor (BAFF) and a proliferation-inducing ligand () play key roles in promoting longevity, primarily through binding to their receptors BAFF-R and TACI/, respectively, which activate anti-apoptotic pathways. For instance, BAFF signaling via BAFF-R is indispensable for maintaining memory B cell populations, as its disruption leads to rapid loss of these cells independent of exposure. Additionally, like (α4β1) facilitate adhesion to stromal cells in the , enabling retention and access to niche-derived survival factors, while LFA-1 (αLβ2) supports similar interactions in splenic environments. To counteract natural turnover and maintain pool size over time, memory B cells engage in low-level homeostatic in the absence of , driven by self-renewal signals that prevent population decline without inducing differentiation. This process involves tonic signaling through the (BCR) and associated , which sustains basal division rates estimated at around 0.02 divisions per cell per day (∼2% of cells dividing daily), ensuring stable numbers for decades. Unlike antigen-driven expansion, this is IL-7 independent and occurs primarily in recirculating memory subsets, contributing to the long-term persistence observed in human and murine models. Resistance to is a cornerstone of memory B cell survival, mediated by upregulated expression of anti-apoptotic proteins such as , Mcl-1, and , which inhibit mitochondrial outer membrane permeabilization and activation. These proteins are particularly enriched in long-lived memory B cells compared to short-lived plasmablasts, providing a against intrinsic apoptotic pressures in niche environments. For example, enforced expression enhances memory B cell recruitment and persistence, underscoring its protective role in steady-state conditions. In mucosal tissues, a of memory B cells establishes tissue residency, supported by α4β7-mediated homing to gut-associated lymphoid tissues via interaction with MAdCAM-1 on endothelial cells, allowing localized and rapid response. This residency is influenced by the , which shapes B cell maturation and selection in the gut through microbial metabolites and antigens that promote α4β7 expression and survival signals. Recent studies highlight how microbiota-driven mechanisms in the intestinal niche sustain these resident memory B cells, enhancing barrier immunity against pathogens.

Immune response contributions

Primary response involvement

During the primary , in models approximately 5-10% of B cells differentiate into memory B cell precursors, primarily identified by CCR6 expression in both and models. These precursors emerge early, often through germinal center-dependent pathways where activated B cells exit the in the light zone to adopt a quiescent state. This commitment occurs alongside the generation of antibody-secreting effector cells, setting the foundation for long-term . Memory B cell precursors play a limited role during the acute phase of the primary response, remaining quiescent while short-lived cells dominate antibody production to rapidly control replication. Their dormancy ensures preservation for future challenges, as they do not contribute significantly to immediate effector functions like high-titer secretion. The initial selection of these precursors in germinal centers shapes the quality of subsequent recall responses, with many deriving from low-affinity B s that receive reduced T follicular helper support. This process favors diversity in the memory pool, including unswitched IgM-expressing cells that provide broad reactivity against evolving antigens. For example, in primary bacterial infections such as those by , low-affinity IgM memory B cells form via T cell-dependent pathways, enabling persistent protection without extensive .

Secondary response and recall

Upon re-encounter with the same , memory B cells are reactivated primarily through cross-linking of their (BCR), which initiates intracellular signaling cascades that drive their entry into the . This reactivation occurs in specialized niches such as subcapsular proliferative foci within lymph nodes, where memory B cells rapidly proliferate and expand clonally. Unlike naive B cells, which require 7 or more days to initiate a robust response, memory B cells begin proliferating within 2–3 days, enabling a swift escalation in cell numbers and effector functions. This accelerated kinetics stems from their pre-existing affinity-matured BCRs and epigenetic priming, allowing them to outcompete naive cells for and T cell help. Following reactivation, memory B cells preferentially differentiate into antibody-secreting s, often with minimal re-entry into germinal centers, prioritizing immediate effector output over further . Transcriptional regulators such as high levels of and repression of BACH2 and BCL-6 facilitate this shift, promoting plasmablast and long-lived formation that secrete isotype-switched antibodies, predominantly IgG. These antibodies exhibit higher due to prior affinity maturation during the primary response, enhancing neutralization and opsonization efficiency compared to primary response outputs. In some cases, such as with IgG1 memory B cells, this differentiation is particularly biased toward fates, contributing to sustained humoral protection. The secondary response orchestrated by memory B cells results in an anamnestic reaction, characterized by a dramatic increase in s—often 100- to 1,000-fold higher than in the primary response—peaking within 4–7 days. This amplification arises from the rapid deployment of high-affinity plasma cells and is further boosted by cytokines from recalled T follicular helper cells, leading to profuse IgG production and enhanced immune clearance. Such responses are critical for limiting spread, as demonstrated in models where memory B cell depletion abolishes this surge. Memory B cell populations exhibit heterogeneity in secondary responses, with atypical subsets playing key roles in recognizing variant antigens. For instance, in infections, atypical memory B cells (e.g., CD27−CD21− or CD45RBlo phenotypes) expand and contribute to cross-reactive recall against variants like , providing broader protection through functional secretion despite sequence divergence. Pre-existing memory B cells specific for seasonal coronaviruses can also contribute to rapid adaptation upon booster vaccination. As of 2025, studies on repeated boosters show that phenotypic heterogeneity in responses correlates with improved neutralizing quality.

Clinical significance

Vaccination efficacy

Memory B cells play a central role in the efficacy of by providing long-term through rapid production upon re-exposure to antigens. are designed to elicit these cells primarily via T-dependent (TD) or T-independent () pathways, with TD antigens such as protein subunits or conjugated promoting class-switched, high-affinity memory B cells through (GC) reactions that involve T follicular helper cells. In contrast, TI antigens like unconjugated bacterial stimulate predominantly IgM-producing memory B cells with limited class switching and shorter-lived GC responses, resulting in protection that wanes more rapidly, often within 5 years in adults. Conjugate , which link to carrier proteins, convert TI responses to TD, enhancing memory B cell generation and durability, particularly in infants who otherwise mount poor TI responses. The longevity of vaccine-induced memory B cells is bolstered by adjuvants that amplify reactions, leading to persistent antigen-specific populations. For instance, mRNA vaccines against , such as BNT162b2, sustain at near-peak frequencies for at least 15 weeks post-vaccination, fostering long-lived memory with hypermutations that improve affinity and breadth. Adjuvants like AS03 further enhance the magnitude and persistence of these memory , increasing neutralization against variants by promoting clonal breadth in . This durability underpins sustained protection, as evidenced by cross-reactive memory that maintain efficacy against evolving pathogens for months to years post-immunization. Serial booster immunizations expand and diversify memory B cell pools, enabling adaptation to antigenic variants. In , bivalent boosters recruit pre-existing memory B cells into GCs, where refines responses to achieve broad neutralization across strains like subvariants, with up to 60% of receptor-binding domain-specific antibodies showing . Repeated dosing in previously primed individuals amplifies these pools without primarily relying on naive B cell activation, thereby strengthening recall responses and overall effectiveness against viral evolution. Despite these mechanisms, efficacy can be compromised by waning memory B cell function in vulnerable populations. In the elderly, memory B cells specific to persist post-vaccination even as neutralizing antibodies decline to undetectable levels, but overall responses are impaired due to accumulation of B cells that reduce potency and breadth after boosting. In immunocompromised patients, such as transplant recipients, initial memory B cell responses to vaccines are diminished, though repeated boosters can drive expansion of these pools to partially restore . These challenges highlight the need for tailored vaccination strategies to mitigate age- and condition-related declines in memory B cell maintenance.

Roles in disease

Memory B cells play protective roles in combating persistent and variant pathogens, particularly through atypical subsets that maintain responses against evolving threats. In infections, atypical and non-classical CD45RBlo memory B cells constitute the majority of circulating virus-specific B cells following or mRNA , enabling sustained antibody production against variants such as . Similarly, in , double-negative (DN) memory B cells, characterized by CD27-CD21- expression, expand during chronic and contribute to antiviral immunity by producing broadly neutralizing antibodies, though their functionality is often impaired by exhaustion. These atypical populations, including DN cells, have been highlighted in 2024-2025 studies as key for recall responses to viral variants, bridging gaps in classical memory B cell efficacy. In autoimmune diseases like systemic lupus erythematosus (SLE), dysregulation of memory B cells promotes through exhausted atypical subsets. Exhausted memory B cells, also termed tissue-like or atypical memory B cells marked by low CD21 expression, accumulate in SLE patients and exhibit reduced capacity to generate neutralizing antibodies, correlating with disease flares and progression. These cells display transcriptional signatures of exhaustion, including elevated T-bet and reduced proliferative potential, which perpetuate production and impair immune regulation. In SLE, DN3 B cell subsets within atypical memory populations are significantly associated with heightened disease activity, particularly in females, underscoring their role in sustaining chronic inflammation. Memory B cells have dual implications in cancer, acting protectively in antitumor immunity or pathologically as origins of malignancies. Tumor-resident memory B cells within tertiary lymphoid structures enhance responses by supporting + and promoting antibody-mediated tumor control in solid cancers like . Conversely, in B cell lymphomas such as activated B cell-like (ABC-DLBCL), memory B cells serve as the primary cells of origin, with chronic antigenic stimulation driving oncogenic transformations like pathway activation. In , dysregulated memory B cells harboring translocations evade , fueling persistence. Therapeutic strategies target B cells to modulate disease outcomes in and cancer. Rituximab, a CD20-depleting , effectively reduces pathogenic autoreactive B cells in autoimmune conditions like and SLE, leading to sustained suppression of autoantibodies and clinical remission in responsive patients. This depletion preferentially affects short-lived autoreactive precursors derived from B cells, though long-lived subsets may resist, necessitating combination therapies. For enhancement, chimeric antigen receptor () T cell therapies targeting CD19 on autoreactive s, including subsets, achieve deep B cell depletion in autoimmune diseases, resetting immunity and inducing drug-free remission by eliminating IgG+ and IgA+ B cells. In , CAR T cells indirectly bolster tumor-resident B cells by reducing immunosuppressive populations, improving overall antitumor efficacy.

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