Extrafollicular B cell responses – is one tent big enough?

Abstract

Initial description of extrafollicular B cell responses (EF) identified them as short-lived, clusters of rapidly proliferating B cells and plasmablasts in splenic bridging channels and red pulp as well as lymph node medullary cord areas. Their physical location guided the nomenclature: outside, or, at the edges of B cell follicles and near T cell zones of secondary lymphoid organs, thus distinct from the follicular situated germinal centers (GCs). Because EFs are often induced transiently and to both T-dependent and T-independent antigens, and because they generate IgM and class-switched antibodies often with no or little signs of somatic hypermutations, they were thought to be less impactful than GC-derived antibodies. However, highly protective antibodies are generated by these EFs rapidly after acute infections and their induction often correlate with pathogen clearance, while in autoimmunity EF-derived antibodies have been implicated as pathogenic drivers of disease. Moreover, subsets of memory B cells, including some CD11c+ “atypical” B cells/ABCs are generated independently of GC. Their diverse appearance and impact have initiated an ongoing debate about whether all “non-GC responses” are necessarily EF responses. The current debate is reflected also in the articles compiled for this issue of Immunological Reviews considering EF responses. Here, I briefly summarize the steps leading to B cell activation and EF and GC formation, providing context for the contributed reviews that span a breadth of topics from descriptions of non-GC responses in jawed non-mammalian vertebrates as possible orthologues of mammalian EF, to the molecular and metabolic requirements and CD4 T cell helper quality of EF, tissue-specific B cell responses, and discussions on the origins and classifications of “atypical” memory B cells in mice and man.

Definitions of extrafollicular responses

Following antigen- induced activation of naïve, mature follicular B cells in man and mice in secondary lymphoid organs (SLO), such as the splenic white pulp and lymph nodes (LNs), B cells undergo initial proliferation and class-switch recombination, and are then thought to undergo one of two differentiation pathways. These involve the CD4 T cell-dependent formation of germinal centers (GCs) in B cell follicles, or further B cell clonal expansion and differentiation outside the follicle, in so-called extrafollicular (EF) B cell responses ¹. The EF response is now understood to generate much of the early-protective antibody response to many infections, including protective responses against both T-dependent and T-independent antigens. In conditions of chronic activation and inflammation, however, these responses also generate pathogenic autoantibodies, such as those observed in SLE ².

While the initial description of EF only included the development of foci of rapidly differentiating antibody-secreting cells (ASCs; plasmablasts or post-proliferative plasma cells) ³, it is now understood that memory B cells (MBCs) can also form independent of GCs, generating MBCs of various effector functions and distinct responses to immunological recall (Fig. 1) ^4–6. Whether the GC-independent formation of MBCs is generated as part of the originally described EF response remains to be more clearly established. This has led to some confusion in the field, since EF origin of certain MBCs and plasmablasts, especially in human studies, often is assumed rather than demonstrated ⁷.

Figure 1. Examples of activation events considered to result in extrafollicular B cell responses.

a) 1- Activation of LN follicular B cells by antigen entering the LN subcapsular sinus (SCS) via afferent lymphatics, trapped and presented via SCM macrophages; 2 – migration of antigen-stimulated B cells towards the outer edges of the follicles, where they may engage with CD4 T cells, which have migrated from the paracortex to that area following their stimulation by antigen-carrying DC in the paracortex. 3- B cell stimulation leads to intense B cell proliferation and differentiation into antibody-secreting plasmablast/plasma cells or memory B cells in the interfollicular area and medullary cords; 4 – alternatively, B cells return to the follicular where they interact with antigen-bearing FDC as well as T_FH during germinal center responses. Similar responses can occur in the spleen but are not outlined here. b) 1- Activation by antigen and/or TLR stimulation in the periphery (body cavities) results in B-1 cell migration from those sides 2- EBI2+ cells may enter inflamed LN via HEV producing oxysterol (Ch25h); 3- undergo further proliferation and 4 – differentiate into antibody-producing cells in the medullary cords. c) 1- B cells and primed CD4 T cells may enter inflamed peripheral tissues of various organs; 2- in the local presence of antigen and CXCL13 producing CD4 T peripheral helper (T_PH) cells B cells are further activated; 3- they can proliferate and differentiate into 4- tissue resident plasma cells or 5 – memory B cells (MBC).

Research over the last 30+ years has led to a much better understanding of GC responses. Including their induction, the transcriptional profiles of light and dark zone B cells, their regulation, the CD4 helper subsets, i.e. T follicular helper (T_FH) and T follicular regulatory (T_FR) cells that control their responses, as well as their outputs, somatically hypermutated (SMH) long-lived plasma cells and MBCs ^8,9. Much less is known about EF responses, despite their increasingly recognized impact on humoral immunity and health. So much so, that what is currently labeled as “EF response” is often mostly understood at what it is not, namely a GC response. Eisenbarth and colleagues have explored this argument by reviewing (IMR-2025–041.R1)¹⁰ the evolution of B cell responses that occur among jawed vertebrates, and the emergence of GC responses. In the absence of sufficiently detailed information on what constitutes EF responses, they define the fundamental components that define GC responses and distinguish them from all other B cell responses, which they term “nonGC”. By defining GCs in that way, non-mammalian jawed vertebrates have B cell responses that demonstrate some but never all fundamental characteristics of GCs. They conclude that GC responses only occur in mammals. Importantly, this does not preclude the establishment in many species of successful humoral immunity, including affinity maturation, immune memory and the differentiation of B cells into ASCs. Some or perhaps even all of those nonGC responses might be considered the orthologues of EF responses of mammals.

This is an interesting point of view from which it could be argued that “nonGC” (including EF) responses represent the “canonical”, evolutionary conserved, B cell response type, while the more recently emerging GC responses constitute a non-canonical response. This would also be consistent with the functional impact of GC responses, which are often less about ensuring survival from an ongoing immune response, much of which is achieved through the more rapid formation of ASCs via EF responses, but more about broadening the B cell repertoire that becomes increasingly responsive to potential future exposures against similar pathogens or the same pathogen carrying several mutations ^1,8.

Common and unique pathways of extrafollicular and germinal center B cell responses

Current understanding suggests that EF and GC responses emerge initially from identical B cell activation steps, although to my knowledge it has not been experimentally proven that other pathways may also exist that would favor one or the other response. Naïve follicular B cell activation in SLO begins with antigen-induced BCR-mediated signaling in B cell follicles, resulting in antigen uptake via BCR internalization and processing for MHCII presentation, as well as initiation of B cell proliferation and upregulation of surface receptors such as CD40, ICOS and MHCII that facilitate cognate T cell interaction. Critically, this initial activation step also leads to the repositioning of follicular B cells from the interior of the follicle towards the follicular outer edges, and into the interfollicular areas of lymph nodes, or towards the bridging channels of the spleen ¹¹ (Fig. 1a). The movement is supported by upregulation of the orphan G protein-coupled receptor Epstein-Barr virus-induced gene 2 (Ebi2, also known as GPR183)¹², whose ligand 7α,25-dihydroxycholesterol¹³ is abundant in the outer zones of the B cell follicle¹⁴ and interfollicular areas of inflamed LN ¹⁵, as well as CCR7, the chemokine receptor for CCL19/21 that supports cell movement towards the T cell zone¹⁶, and the downregulation of CXCR5, the follicular homing receptor recognizing CXCL13 produced by follicular stromal cells¹⁷.

Following this relocation, B cells undergo a second phase of activation, where their antigen-only induced and thus limited activation is enhanced by receipt of co-stimulatory signals that strongly increase B cell proliferation, i.e. clonal expansion. Such signals also enhance B cell survival and the expression of activation-induced cytidine deaminase (AID), responsible for class-switch recombination (CSR) and somatic hyper-affinity maturation (SHM). Kuan & Chaudhuri et al. [IMR-2025–031.R1]¹⁸ review the role and function of the activation-induced deaminase (AID) and its impact on B cell activation and functionality during both EF and GC responses. Indeed, CSR but not SMH, two processes controlled by AID, often occurs during this second phase of B cell activation^19,20. This is also supported and directed through the local production of cytokines by CD4 T cells or/and other cells, whose identity remains to be further explored. IL21, the canonical cytokine of T_FH cells, provides critical and distinct signals to B cells, depending on their activation state, as also discussed by Kuan & Chaudhuri ¹⁸.

It is thought that during the critical third phase of B cell differentiation a variety of signals affect activated B cells to differentiate along the EF or GC pathway. For EF responses, B cells remain CXCR5^neg and either remain in the outer regions of the follicle or further migrate into the interfollicular and outer T cell zones, supported by expression of Ebi2 and CCR7. This may also be supported also by antigen-presenting cells and/or CD4 T cells, inducing rapid proliferation and eventual differentiation into Blimp-1+ Bcl6− IRF4hi ASCs ¹¹. How these responses lead to MBC formation is incompletely resolved ^5,6,21. The Ebi2-mediated positioning of B cells in the T-B border zone might be critical, as in the absence of Ebi2, EF responses are greatly curtailed ¹². However, a recent study by Cyster and colleagues identified a new role for Ebi2 in the recruitment of naïve B cells via oxysterol 7α,25-dihydroxycholesterol (Ch25h)-secreting HEVs into inflamed LN ¹⁵, raising other possible scenarios for an Ebi2-dependent establishment of EF (Fig. 1b). For GC development, activated B cells must upregulate Bcl6 which suppresses Blimp-1 induction, re-express CXCR5, downregulate Ebi2 and CCR7 as well as upregulate S1PR2, in support of their return to the B cell follicle together with Bcl6+ CXCR5+ CD4 T_FH ^9,22. Here they locate close to CXCL13 producing antigen-presenting follicular dendritic cells, and form GC light and dark zones further supported by CXCL12 production and surface expression of CXCR4 ²³.

The signals that favor the EF over the GC differentiation pathway or vice versa are focus of ongoing research. To-date, B cell fate decisions have been shown to be affected by at least four variables briefly described below and discussed in greater detail by Staniek & Rizzi ²⁴ [IMR-2024–122]. They address a) the type of B cell involved in the response, b) the availability and perhaps quality of sustained CD4 T cell help, c) the presence and quality of innate signals as well as d) the type and affinity of BCR-antigen interactions.

B cell subsets:

Mouse B-1 cells and splenic marginal zone (MZ) B cells undergo predominantly, albeit perhaps not exclusively, T-independent EF responses, perhaps driven by their high sensitivity to TLR-mediated signaling ^25,26, and for B-1 cells, inability to respond to BCR-signaling with clonal expansion due to inhibitory receptor expression ²⁷. Since these cells are not activated in B cell follicles, thus at least their initial activation events are distinct from those described above (Fig. 1b). The question arises to what extent the resulting responses are ”EF responses”, and whether they are regulated similarly to those of follicular B cells that have undergone the phased activation processes outlined (Fig. 1a). Indeed, it remains to be ascertained whether follicular B cells generating EF responses may or may not (or not always) follow the above outlined activation steps. In their review article, Carroll and colleagues suggest that the participation of MZB cells in EF responses may contribute to the heterogeneity of the EF B cell pools observed after B cell immunization affecting the spleen ²⁸ [IMR-2024–121.R1].

T cell help:

While GC pathways require interaction of follicular B cells with CD4 T helper cells, EF responses can be induced in an either T-dependent or T-independent manner. The GC pathway requires sustained T cell interaction with B cells²⁹, an interaction that also leads to further T_FH differentiation and migration into a nascent secondary B cell follicle. Antigen-specific, primed CD4 T cells interact with B cells via TCR-MHCII, ICOSL-ICOS and CD40L-CD40 signaling ³⁰. These CD4 T cells are recruited to the T-B border through some of the same chemokine receptors regulating the movement of B cells: Ebi2³¹, CXCR5^32,33 and the downregulation of CCR7, releasing T cells from the T cell zone ³⁰. The functions and interactions of primed CD4 T cells and lymph tissue T_FH and their migration and distinct support for GC responses versus extrafollicular B cell activation events are outlined in detail by Chen & Craft [IMR-2025–041.R1] ³⁴. They argue that cells other than classical Bcl6-dependent T_FH may mediate B cell help under certain conditions. A focus of their review is a relatively recently identified discrete CD4 T cell subset, the CXCL13 producing “T peripheral helper” (T_PH) cells, found in autoimmunity and virus infections, as well as in tumors, where their frequencies correlate with that of peripheral blood plasmablasts and active disease. Common to these T helper cells is their support of B cell responses in peripheral tissues, outside of distinct tertiary lymphoid structures (Fig. 1c). As Chen & Craft describe, additional B cell helper T cells exist, and thus further work is required to understand their relationship to each other. The extent to which B cell responses in peripheral tissues are akin to or distinct from extrafollicular B cell responses was not addressed by the authors, but remains a topic of discussion ⁷.

B cell intrinsic and extrinsic innate signals:

Several studies of models of autoimmunity and infection have demonstrated a role for B cell intrinsic innate signaling, especially delivered either via TLRs and/or complement receptors, as interconnected drivers of B cell activation and EF plasmablast differentiation ^35–39, and as detailed by Carroll and colleagues ²⁸ [IMR-2024–121.R1]. Their review emphasizes the impact of a chronic inflammatory milieu with aberrant innate signals and the ongoing presence of self-antigens as critical drivers of autoimmunity. They further argue that a less stringent selection of B cell clones in EF versus GC responses in such an environment might be responsible for the “breaking” of tolerance and induction of autoreactive antibody-producing EF plasmablasts in diseases such as SLE. Furthermore, they review evidence from mouse models of autoimmune diseases that implicate TLR signaling in driving plasma cell differentiation and thus the pathological consequences of EF. It is important to note that similar inflammatory signals that may cause a break of tolerance in autoimmune prone individuals and mice are required for the reorganization of the SLOs architecture ^40–42 and for induction of strong protective antibody responses to infection ^35,43, highlighting the “good and bad” of EF responses.

Integrated BCR signaling:

Strong, high-affinity and/or high avidity interactions between BCR and cognate antigen drives the upregulation of IRF4, the master transcriptional regulator of the plasma cell fate ^44,45. Thus, despite prevailing views and the lack of SHM, one should expect EF responses to contain particularly high affinity B cell clones. Indeed, earlier studies of serum antibody responses in vesicular stomatitis virus (VSV) infection demonstrated that the collective avidity of the serum antibodies is fairly high both early and late during the anti-VSV immune response, despite the later development of GCs ^46,47. The data could indicate that early high-affinity B cell-generated antibodies in EF might have been replaced over time by GC derived antibodies that achieve high-affinity due to SHM.

Consistent with those reports, a study by Brink and colleagues using hen-egg lysozyme (HEL) transgenic mice reported that lowering the very high avidity of antibody-HEL interactions by using variant antigens causes reduced EF formation ⁴⁸. Although later studies by that group let to the conclusion that the reduction in EF formation observed may have been due mainly to the number of B cells initially activated, rather than fate decisions to enter EF versus GC responses ⁴⁹. A potential shift in the repertoire of B cells drawn into EF versus GC responses was suggested by studies of Gerhard and colleagues in BALB/c mice immunized with influenza A/Puerto Rico 8/34. In these seminal early studies, they generated hybridomas from activated B cells early and later in the response and then sequenced their BCRs. In so doing they identified distinct repertoires of BCR gene usage in early versus later-appearing antibody-producing cells ⁵⁰, a finding more recently confirmed ⁵¹. One of the prominent hemagglutinin (HA)-specific early activated B cell clones in BALB/c (but not C57BL/6) mice encoded by the C12Id was indeed shown subsequently to bind HA with high affinity. B cells with those BCR only generate EF plasmablasts but not GC B cells after influenza infection of BALB/c mice ⁵² (and unpubl. observations). Thus, supporting the idea that above a certain threshold of BCR signaling, and thereby strong induction of IRF4, B cells are differentiating rapidly into plasmablasts. Such conditions might be met especially frequently in the context of acute infections where innate costimulatory signals known to drive IRF4 induction and Blimp-1 upregulation, such as TLR stimulation ³⁵, may suppress Bcl6 induction to fate high affinity B cells towards EF responses ^53–55. This is an attractive model of B cell activation that explains the signals for induction of plasmablast generation in early infection, where such innate signals are abundant. However, whether EF-derived MBCs emerging from the same early B cell responses show lower BCR affinity for their cognate antigen remains to be ascertained.

Not mutually exclusive but reaching distinct conclusions from the above outlined studies, however, is earlier work by Kelsoe et al. ⁵⁶. They demonstrated that in C57BL/6 mice NP-specific B cells expressing identical BCR when activated by the hapten 4-hydroxy-3-nitrophenyl)acetyl (NP) coupled to a protein carrier, generated both, EF plasmablasts and GC B cells. Given the above findings, existing evidence supports the hypothesis that BCR signaling strength alone is insufficient for B cell fate determination. Carroll and colleagues, who in their review summarize the impact of inflammatory signals, especially TLR signaling, on EF development [IMR-2024–121.R] come to similar conclusions ³⁵. Staniek & Rizzi ²⁴ [IMR-2024–122] provide a comprehensive overview of the multiple signaling pathways that promote or suppress GC and EF responses by regulating B cell activation, cell growth, proliferation and cell differentiation. They emphasize a need for the integration of multiple signals, both positive and negative, such as provided through the BCR-, co-stimulatory and inflammatory- stimuli to guide B cell fate decisions. In their view, the net outcome (and thus B cell fate) of such integrated signals is critically affected by the differentiation state of the B cell and by signal convergence on the PI3K/AKT/mTOR pathway. They further highlight their recent studies on the importance of non-apoptotic FAS signaling in directing B cell fate decisions ⁵⁷.

The products of extrafollicular B cell responses

While the original description of EF B cell responses considered only the generation of rapidly differentiating plasmablasts ³, more recent data have firmly established the presence of MBC development prior to and/or independent of GC development. Whether these MBCs develop from the same responses that give rise to EF-ASCs is currently unknown. Yet, these cells are largely referred to as EF-derived and the nomenclature has been adopted here also. Elsner & Shlomchik (IMR-2025–15)⁵⁸ provide an extensive overview of EF responses and the emergence of both plasmablasts/cells and the developmental paths of EF-MBCs, including a brief description of the complexities of assigning definitive functions to EF and GC-MBCs in the absence of lineage tracing, as well as the emergence of “ABCs”, which will be discussed further below.

Extrafollicular plasmablasts/cells

Arguably the most striking aspect of EF B cell responses is the rapid differentiation of clusters of B cells into ASCs, which has been reported following both T-dependent and T-independent B cell activation, most frequently resulting in apparent short-lived plasmablasts ^3,56,59. As discussed by Elsner and Shlomchik (IMR-2025–15)⁵⁸, while ASCs developing in EF responses are considered to be only short-lived, whether this is truly an intrinsic property of these EF-derived cells or instead due to the environment in which they are generated, remains an important outstanding question for the field. As they suggest, that does not mean, however, that EF responses themselves are always short-lived, as in situations of chronic activation and autoimmunity ongoing differentiation can provide continuous autoantibody production.

The understanding of the B cell fate decision between EF and GC has been focused on understanding the signals that drive the formation of plasmablasts and terminally differentiated plasma cells instead of GC B cells. With regards to the signals that induce EF plasmablasts, Elsner & Shlomchik summarize their recent work leading to the identification of the IL-12/IFNγ axis as critical direct and indirect signals promoting EF plasmablast development in the context of “TH1” inflammation (induced in vitro via TLR4 stimulation). Collectively their work demonstrates the importance of an inflammatory milieu for efficient induction of B cell differentiation, acting both on B ⁶⁰ and on CD4 T cells ⁶¹. Thus, their studies add an additional critical signaling pathway that can integrate with innate TLR-mediated signals to drive EF plasmablast formation. Whether these signals define a fate-decision between GC versus EF responses, or the difference between continued B cell turnover versus plasmablast differentiation will be an important future area of inquiry, as will be an assessment whether cytokine signals other than those identified by their studies can drive plasmablast responses in other inflammatory contexts.

Extrafollicular memory B cells

Early studies of MBCs in mice relied on the identification of class-switched, hapten-binding B cells by flow cytometry. More recently, however, MBCs were identified that express IgM, using methods of isolation that relied on antigen-binding alone, independent of the isotype of the selected B cells ^4,5,62. The finding of IgM+ unswitched memory B cells in mice was consistent with the presence of CD27+ non-switched MBCs in human PBMCs ⁶³. Recent studies have confirmed and expanded these earlier studies, revealing the presence of a complex set of distinct MBCs emerging during primary B cell activation. They differ in origins, phenotype and differentiation potential based on their epigenetic state ⁶. MBC differentiation, often non-class switched, seems to occur early during EF responses ^4,6,64,65. These early induced, T-dependent MBCs are generated independent of GC responses. They typically lack expression of both CD80 and PD-L2 (double negative MBCs), although CD80+ PD-L2+ double positive, GC-dependent and GC-independent MBCs were also found ⁶. Upon secondary challenge, (double-positive) GC-derived MBCs seemed to differentiate preferentially into EF-plasmablasts and to provide cross-reactive memory, while EF-derived MBCs were effective at seeding GC responses ^66–69. A third population of “single positive” (CD80+ PD-L2-) MBCs had an intermediate phenotype ⁶⁶. The presence of early EF-MBC development thus extends the impact and longevity of early-induced B cell responses beyond that of an immediate “emergency” antibody response.

T-bet dependent B cells in mice

The description of CD11c expressing plasmablasts and MBCs launched a major research effort to understand their origins in both mice and humans. CD11c was initially found only on myeloid lineage cells. As outlined by Winslow & Levack [IMR-2024–118 ]⁷⁰, they first reported the emergence of a population of T cell-independent CD11c+ EF plasmablasts in spleens of mice infected with intracellular bacteria Ehrlichia spp. in 2008 ⁷¹, but later also MBCs with that phenotype [IMR-2024–118 ]⁷⁰. More recent studies have identified these cells arising in a variety of infections, most notable chronic viral infections, where they were shown to be critical for protection ⁷². Consistent with these findings, Philippa Marrack and colleagues first described B cell intrinsic expression of the Th1 defining transcriptional regulator T-bet as critical for the emergence of these CD11c+ cells ⁷³. Subsequent work, including that by Michael Cancro and Garry Winslow and colleagues further demonstrated the importance of T-bet expression and TLR and cytokine signaling for the development, functionality and regulation of these MBCs ^{70 74}.

The emergence of CD11c+ T-bet-dependent B cells occurs in the context of a Th1 skewed inflammatory milieu in a variety of diseases associated with chronic inflammation, including infections, autoimmunity as well as in aging, where they have significant effects of both protective and disease-causing antibodies. The appearance of T-bet dependent, mostly CD11c+ MBC subsets in autoimmunity and aging, respectively, led to the now widely adopted (albeit confusing) nomenclature of age- or autoimmunity associated B cells (ABCs) ^37,75. T-bet is the defining transcription factor required for the development of ABCs ⁷³, while other surface phenotypes, including markers such as CD11c, CD11b, CD73, FcRL4 and FcRL5 may be expressed on only a subset of these cells, depending on their activation state and tissue location ^{76 77}. In addition to reports of their rapid differentiation and effector potential ⁷⁸, these memory B cells strongly support T cell activation via antigen-presentation ⁷⁹. Given their impact on a variety of disease states it is not surprising that recent work has focused on identifying therapeutic targets affecting these cells ^75–77. Winslow and Levack IMR-2024–118 ]⁷⁰ report in their review one potential target for such therapeutic intervention, namely the adenosine 2a receptor, a receptor selectively expressed by T-bet+ MBCs ⁸⁰.

“Atypical” memory B cells in humans

Initial descriptions as early as 1992 ⁸¹, and by Moir and colleagues in 2001 ⁸², reported HIV infection-induced perturbations in the peripheral blood of chronically infected patients, and an emergence of a CD21^low B cell subset correlating with high HIV viremia. These cells could rapidly differentiate into ASCs following in vitro restimulation but could not undergo sustained proliferation ⁸². Because CD27 was considered a canonical MBC marker in humans and these B cells lack expression of CD27, they were initially considered both “atypical” and dysfunctional. Inaki Sanz [IMR-2025–044.R1]⁸³, who first described the presence of significant numbers of CD27− IgD− “double negative (DN)” B cells in patients suffering from autoimmune diseases, clarifies the nomenclature of these human B cells. His review includes tables and flow cytometry gating strategies to delineate the phenotypic heterogeneity of “DN” cell populations with regards to other surface markers expressed. They appear to represent orthologues of the T-bet dependent MBCs of mice. As discussed by Sanz, this heterogenous group of MBCs is often assumed to be EF derived, but evidence for it is rarely provided. Thus, much work remains to be done to understand their regulation in humans and how these DN B cells might be exploited therapeutically.

Of importance, there appears to be significant plasticity among activated B cells and MBCs, and some B cells may express a particular phenotype and undergo a distinct differentiation pathway during a primary response but acquire another during recall responses. This should be considered when drawing conclusions of the origins of B cells and B cell responses from studies of MBCs with a particular phenotype and function based on the frequency of BCR mutations. For example, GC-derived MBC, which often have undergone extensive SHM during a primary response, may participate in EF plasmablast responses during a secondary response. This could falsely indicate that the EF response leads to extensive SMH, when instead it is a reflection of MBC priming.

This compilation of review articles outlines how ASCs and MBCs emerge in both mice and humans via multiple, as yet incompletely resolved differentiation pathways. Particularly vague is our understanding of responses generated outside of GCs, which can be important sources of antibodies and MBCs with a variety of effector functions during primary and recall responses. The compilation highlights the need to clarify whether a canonical “EF differentiation pathway” exists that regulates B cell responses outside of GC, or whether the label “EF response” maybe applied too broadly, possibly obscuring the presence of several distinct biological processes that affect cell fate and function. The answer to this question will be critical for learning how to exploit these different B cell activation pathways to help the development of improved therapies and prophylaxes.