Systematic comparison of gene expression between murine memory and naive B cells demonstrates that memory B cells have unique signaling capabilities

Antiviral memory B cells exhibit enhanced innate immune response facilitated by epigenetic memory

The long-lasting humoral immunity induced by viral infections or vaccinations depends on memory B cells with greatly increased affinity to viral antigens, which are evolved from germinal center (GC) responses. However, it is unclear whether antiviral memory B cells represent a distinct subset among the highly heterogeneous memory B cell population. Here, we examined memory B cells induced by a virus-mimicking antigen at both transcriptome and epigenetic levels and found unexpectedly that antiviral memory B cells exhibit an enhanced innate immune response, which appeared to be facilitated by the epigenetic memory that is established through the memory B cell development. In addition, T-bet is associated with the altered chromatin architecture and is required for the formation of the antiviral memory B cells. Thus, antiviral memory B cells are distinct from other GC-derived memory B cells in both physiological functions and epigenetic landmarks.

Determinants of de novo B cell responses to drifted epitopes in post-vaccination SARS-CoV-2 infections

Vaccine-induced immunity may impact subsequent de novo responses to drifted epitopes in SARS-CoV-2 variants, but this has been difficult to quantify due to the challenges in recruiting unvaccinated control groups whose first exposure to SARS-CoV-2 is a primary infection. Through local, statewide, and national SARS-CoV-2 testing programs, we were able to recruit cohorts of individuals who had recovered from either primary or post-vaccination infections by either the Delta or Omicron BA.1 variants. Regardless of variant, we observed greater Spike-specific and neutralizing antibody responses in post-vaccination infections than in those who were infected without prior vaccination. Through analysis of variant-specific memory B cells as markers of de novo responses, we observed that Delta and Omicron BA.1 infections led to a marked shift in immunodominance in which some drifted epitopes elicited minimal responses, even in primary infections. Prior immunity through vaccination had a small negative impact on these de novo responses, but this did not correlate with cross-reactive memory B cells, arguing against competitive inhibition of naïve B cells. We conclude that dampened de novo B cell responses against drifted epitopes are mostly a function of altered immunodominance hierarchies that are apparent even in primary infections, with a more modest contribution from pre-existing immunity, perhaps due to accelerated antigen clearance.

A quinazoline derivative suppresses B cell hyper-activation and ameliorates the severity of systemic lupus erythematosus in mice

Background: Aberrant autoreactive B cell responses contribute to the pathogenesis of systemic lupus erythematosus (SLE). Currently, there is no safe and effective drug for intervention of SLE. Quinazoline derivative (N4-(4-phenoxyphenethyl)quinazoline-4,6-diamine, QNZ) is a NF-κB inhibitor and has potent anti-inflammatory activity. However, it is unclear whether QNZ treatment can modulate B cell activation and SLE severity. Methods: Splenic CD19⁺ B cells were treated with QNZ (2, 10, or 50 nM) or paeoniflorin (200 μM, a positive control), and their activation and antigen presentation function-related molecule expression were examined by flow cytometry. MRL/lpr lupus-prone mice were randomized and treated intraperitoneally with vehicle alone, 0.2 mg/kg/d QNZ or 1 mg/kg/d FK-506 (tacrolimus, a positive control) for 8 weeks. Their body weights and clinical symptoms were measured and the frequency of different subsets of splenic and lymph node activated B cells were quantified by flow cytometry. The degrees of kidney inflammation and glycogen deposition were examined by hematoxylin and eosin (H&E) and PAS staining. The levels of serum autoantibodies and renal IgG, complement C3 deposition were examined by ELISA and immunofluorescence. Results: QNZ treatment significantly inhibited the activation and antigen presentation-related molecule expression of B cells in vitro. Similarly, treatment with QNZ significantly mitigated the SLE activity by reducing the frequency of activated B cells and plasma cells in MRL/lpr mice. Conclusion: QNZ treatment ameliorated the severity of SLE in MRL/lpr mice, which may be associated with inhibiting B cell activation, and plasma cell formation. QNZ may be an excellent candidate for the treatment of SLE and other autoimmune diseases.

Promoting immunity with novel targeting antigen delivery vehicle based on bispecific nanobody

As the most potent professional antigen presenting cells, dendritic cells (DCs) have been targeted in strategies to enhance vaccination efficacy. To date, targeted delivery has been mainly used for cancer therapy, with few studies focusing on vaccine antigens for animal epidemic diseases. In this study, we selected a series of mouse DC-specific nanobodies from a non-immunized camel. The four candidate nanobodies identified (Nb4, Nb13, Nb17, and Nb25), which showed efficient endocytosis of bone marrow-derived DCs, were evaluated as potential vaccine antigen targeted delivery vehicles. First, green fluorescent protein (GFP) was selected and four corresponding DCNb-GFP fusions were constructed for verification. Nb17-GFP was effective at promoting antibody production, inducing a cellular immune response, and increasing the IL-4 level. Second, foot-and-mouth disease virus (FMDV) and a FMDV-specific nanobody (Nb205) were selected and four bispecific nanobody DCNb-Nb205 fusions were generated to investigate the feasibility of a novel targeting antigen delivery vehicle. The resulting bispecific nanobody, Nb17-Nb205, could not only deliver FMDV particles instead of antigenic peptide, but also induced the production of specific antibodies, a cellular immune response, and IFN-γ and IL-4 levels upon immunization with a single subcutaneous injection. In conclusion, our results demonstrate the potential of bispecific nanobody as a novel and efficient DC-specific antigen delivery vehicle. This highlights the potential to expand targeted delivery to the field of animal epidemic diseases and provides a reference for the general application of nanotechnology in viral diseases.

B cell receptor signaling in germinal centers prolongs survival and primes B cells for selection

Germinal centers (GCs) are sites of B cell clonal expansion, diversification, and antibody affinity selection. This process is limited and directed by T follicular helper cells that provide helper signals to B cells that endocytose, process, and present cognate antigens in proportion to their B cell receptor (BCR) affinity. Under this model, the BCR functions as an endocytic receptor for antigen capture. How signaling through the BCR contributes to selection is not well understood. To investigate the role of BCR signaling in GC selection, we developed a tracker for antigen binding and presentation and a Bruton's tyrosine kinase drug-resistant-mutant mouse model. We showed that BCR signaling per se is necessary for the survival and priming of light zone B cells to receive T cell help. Our findings provide insight into how high-affinity antibodies are selected within GCs and are fundamental to our understanding of adaptive immunity and vaccine development.

Immune interference in effectiveness of influenza and COVID-19 vaccination

Vaccines are known to function as the most effective interventional therapeutics for controlling infectious diseases, including polio, smallpox, rabies, tuberculosis, influenza and SARS-CoV-2. Smallpox has been eliminated completely and polio is almost extinct because of vaccines. Rabies vaccines and Bacille Calmette-Guérin (BCG) vaccines could effectively protect humans against respective infections. However, both influenza vaccines and COVID-19 vaccines are unable to eliminate these two infectious diseases of their highly variable antigenic sites in viral proteins. Vaccine effectiveness (VE) could be negatively influenced (i.e., interfered with) by immune imprinting of previous infections or vaccinations, and repeated vaccinations could interfere with VE against infections due to mismatch between vaccine strains and endemic viral strains. Moreover, VE could also be interfered with when more than one kind of vaccine is administrated concomitantly (i.e., co-administrated), suggesting that the VE could be modulated by the vaccine-induced immunity. In this review, we revisit the evidence that support the interfered VE result from immune imprinting or repeated vaccinations in influenza and COVID-19 vaccine, and the interference in co-administration of these two types of vaccines is also discussed. Regarding the development of next-generation COVID-19 vaccines, the researchers should focus on the induction of cross-reactive T-cell responses and naive B-cell responses to overcome negative effects from the immune system itself. The strategy of co-administrating influenza and COVID-19 vaccine needs to be considered more carefully and more clinical data is needed to verify this strategy to be safe and immunogenic.

IgM+ and IgM- memory B cells represent heterogeneous populations capable of producing class-switched antibodies and germinal center B cells upon rechallenge with P. yoelii

Memory B cells (MBCs) are essential for maintaining long-term humoral immunity to infectious organisms, including Plasmodium. MBCs are a heterogeneous population whose function can be dictated by isotype or expression of particular surface proteins. Here, aided by antigen-specific B-cell tetramers, MBC populations were evaluated to discern their phenotype and function in response to infection with a nonlethal strain of P. yoelii. Infection of mice with P. yoelii 17X resulted in 2 predominant MBC populations: somatically hypermutated isotype-switched (IgM- ) and IgM+ MBCs that coexpressed CD73 and CD80 that produced antigen-specific antibodies in response to secondary infection. Rechallenge experiments indicated that IgG-producing cells dominated the recall response over the induction of IgM-secreting cells, with both populations expanding with similar timing during the secondary response. Furthermore, using ZsGreen1 expression as a surrogate for activation-induced cytidine deaminase expression alongside CD73 and CD80 coexpression, ZsGreen1+ CD73+ CD80+ IgM+ , and IgM- MBCs gave rise to plasmablasts that secreted Ag-specific Abs after adoptive transfer and infection with P. yoelii. Moreover, ZsGreen1+ CD73+ CD80+ IgM+ and IgM- MBCs could differentiate into B cells with a germinal center phenotype after adoptive transfer. A third population of B cells (ZsGreen1- CD73- CD80- IgM- ) that is apparent after infection responded poorly to reactivation in vitro and in vivo, indicating that these cells do not represent a canonical population of MBCs. Together these data indicated that MBC function is not defined by immunoglobulin isotype, nor does coexpression of key surface markers limit the potential fate of MBCs after recall.

Development and function of tissue-resident memory B cells

Barrier tissues are the primary site of infection for pathogens likely to cause future pandemics. Tissue-resident lymphocytes can rapidly detect pathogens upon infection of barrier tissues and are critical in preventing viral spread. However, most vaccines fail to induce tissue-resident lymphocytes and are instead reliant on circulating antibodies to mediate protective immunity. Circulating antibody titers wane over time following vaccination leaving individuals susceptible to breakthrough infections by variant viral strains that evade antibody neutralization. Memory B cells were recently found to establish tissue residence following infection of barrier tissues. Here, we summarize emerging evidence for the importance of tissue-resident memory B cells in the establishment of protective immunity against viral and bacterial challenge. We also discuss the role of tissue-resident memory B cells in regulating the progression of non-infectious diseases. Finally, we examine new approaches to develop vaccines capable of eliciting barrier immunity.

Resident Memory B Cells in Barrier Tissues

Epithelial barriers, which include the gastrointestinal, respiratory, and genitourinary mucosa, compose the body’s front line of defense. Since barrier tissues are persistently exposed to microbial challenges, a rapid response that can deal with diverse invading pathogens is crucial. Because B cells have been perceived as indirectly contributing to immune responses through antibody production, B cells functioning in the peripheral organs have been outside the scope of researchers. However, recent evidence supports the existence of tissue-resident memory B cells (BRMs) in the lungs. This population’s defensive response was stronger and faster than that of their circulating counterparts and could resist heterogeneous strains. With such traits, BRMs could be a promising target for vaccine design, but much about them remains to be revealed, including their locations, origin, specific markers, and the mechanisms of their establishment and maintenance. There is evidence for resident B cells in organs other than the lungs, suggesting that B cells are directly involved in the immune reactions of multiple non-lymphoid organs. This review summarizes the history of the discovery of BRMs and discusses important unresolved questions. Unique characteristics of humoral immunity that play an important role in the peripheral organs will be described briefly. Future research on B cells residing in non-lymphoid organs will provide new insights to help solve major problems regarding human health.

Instructing durable humoral immunity for COVID-19 and other vaccinable diseases

Many aspects of SARS-CoV-2 have fully conformed with the principles established by decades of viral immunology research, ultimately leading to the crowning achievement of highly effective COVID-19 vaccines. Nonetheless, the pandemic has also exposed areas where our fundamental knowledge is thinner. Some key unknowns are the duration of humoral immunity post-primary infection or vaccination and how long booster shots confer protection. As a corollary, if protection does not last as long as desired, what are some ways it can be improved? Here, I discuss lessons from other infections and vaccines that point to several key features that influence durable antibody production and the perseverance of immunity. These include (1) the specific innate sensors that are initially triggered, (2) the kinetics of antigen delivery and persistence, (3) the starting B cell receptor (BCR) avidity and antigen valency, and (4) the memory B cell subsets that are recalled by boosters. I further highlight the fundamental B cell-intrinsic and B cell-extrinsic pathways that, if understood better, would provide a rational framework for vaccines to reliably provide durable immunity.

CD40 signaling-mediated delay in terminal differentiation of B cells enables alternate fate choices during early divisions

Memory B cells and differentiated plasma cells combine to confer sustained humoral immunity. Nonetheless, we are yet to understand how B cells decide between these fates. Although pan-T cell help augments plasma cell differentiation, signaling via CD40 alone is considered to be inhibitory. Here, we examine the capacity of CD40 signaling to interfere with lipopolysaccharide-induced differentiation. Whereas lipopolysaccharide stimulation yielded only short-lived plasmablasts, co-stimulation of CD40 enhanced activation, proliferation, survival, and isotype-switching, leading to alternate fate choices such as germinal center and memory B cells during early divisions. Contrary to the notion that CD40 signaling simply arrests differentiation, the survivors, at later time points, developed into long-lived mature plasma cells, after progressively losing their ability to get restimulated. Counterintuitively, as constitutive lipopolysaccharide stimulation itself hampered differentiation, we identified that the proliferation potential of cells acted alongside CD40 signaling. Accordingly, we propose a bi-layered regulation of differentiation - CD40 signaling and proliferation potential of cells independently inhibit the commitment to and maturation of differentiation, respectively. Elucidating such cell fate decision mechanisms will aid in better vaccine design and disease management.

Surface phenotypes of naive and memory B cells in mouse and human tissues

Memory B cells (MBCs) protect the body from recurring infections. MBCs differ from their naive counterparts (NBCs) in many ways, but functional and surface marker differences are poorly characterized. In addition, although mice are the prevalent model for human immunology, information is limited concerning the nature of homology in B cell compartments. To address this, we undertook an unbiased, large-scale screening of both human and mouse MBCs for their differential expression of surface markers. By correlating the expression of such markers with extensive panels of known markers in high-dimensional flow cytometry, we comprehensively identified numerous surface proteins that are differentially expressed between MBCs and NBCs. The combination of these markers allows for the identification of MBCs in humans and mice and provides insight into their functional differences. These results will greatly enhance understanding of humoral immunity and can be used to improve immune monitoring.

Critical requirement for BCR, BAFF, and BAFFR in memory B cell survival

Memory B cells (MBCs) are long-lived cells that form a critical part of immunological memory, providing rapid antibody responses to recurring infections. However, very little is known about signals controlling MBC survival. Previous work has shown that antigen is not required for MBC survival, but a requirement for the B cell antigen receptor (BCR) has not been tested. Other studies have shown that, unlike naive B cells, MBCs do not express BAFFR and their survival is independent of BAFF, the ligand for BAFFR. Here, using inducible genetic ablation, we show that survival of MBCs is critically dependent on the BCR and on signaling through the associated CD79A protein. Unexpectedly, we found that MBCs express BAFFR and that their survival requires BAFF and BAFFR; hence, loss of BAFF or BAFFR impairs recall responses. Finally, we show that MBC survival requires IKK2, a kinase that transduces BAFFR signals. Thus, MBC survival is critically dependent on signaling from BCR and BAFFR.

Memory B cells and plasma cells: The differentiative continuum of humoral immunity

Immunological memory is a composite of lasting antibody titers maintained by plasma cells in conjunction with memory T and B cells. Memory B cells are a critical reservoir for plasma cell generation in the secondary response. Identification of memory B cells requires that they be distinguished from naïve, activated, and germinal center precursors and from plasma cells. Memory B cells are heterogeneous in isotype usage, immunoglobulin mutational content, and phenotypic marker expression. Phenotypic subsets of memory B cells are defined by PD-L2, CD80, and CD73 expression in mice, by CD27 and FCRL4 expression in humans and by T-bet in both mice and humans. These subsets display marked functional heterogeneity, including the ability to rapidly differentiate into plasma cells versus seed germinal centers in the secondary response. Memory B cells are located in the spleen, blood, other lymphoid organs, and barrier tissues, and recent evidence indicates that some memory B cells may be dedicated tissue-resident populations. Open questions about memory B cell longevity, renewal and progenitor-successor relationships with plasma cells are discussed.

Memory B Cells in Multiple Sclerosis: Emerging Players in Disease Pathogenesis

Multiple Sclerosis (MS) is an inflammatory demyelinating disease of the central nervous system. Once thought to be primarily driven by T cells, B cells are emerging as central players in MS immunopathogenesis. Interest in multiple B cell phenotypes in MS expanded following the efficacy of B cell-depleting agents targeting CD20 in relapsing-remitting MS and inflammatory primary progressive MS patients. Interestingly, these therapies primarily target non-antibody secreting cells. Emerging studies seek to explore B cell functions beyond antibody-mediated roles, including cytokine production, antigen presentation, and ectopic follicle-like aggregate formation. Importantly, memory B cells (Bmem) are rising as a key B cell phenotype to investigate in MS due to their antigen-experience, increased lifespan, and rapid response to stimulation. Bmem display diverse effector functions including cytokine production, antigen presentation, and serving as antigen-experienced precursors to antibody-secreting cells. In this review, we explore the cellular and molecular processes involved in Bmem development, Bmem phenotypes, and effector functions. We then examine how these concepts may be applied to the potential role(s) of Bmem in MS pathogenesis. We investigate Bmem both within the periphery and inside the CNS compartment, focusing on Bmem phenotypes and proposed functions in MS and its animal models. Finally, we review how current immunomodulatory therapies, including B cell-directed therapies and other immunomodulatory therapies, modify Bmem and how this knowledge may be harnessed to direct therapeutic strategies in MS.

Comprehensive analyses of B-cell compartments across the human body reveal novel subsets and a gut-resident memory phenotype

Although human B cells have been extensively studied, most reports have used peripheral blood as a source. Here, we used a unique tissue resource derived from healthy organ donors to deeply characterize human B-cell compartments across multiple tissues and donors. These datasets revealed that B cells in the blood are not in homeostasis with compartments in other tissues. We found striking donor-to-donor variability in the frequencies and isotype of CD27+ memory B cells (MBCs). A comprehensive antibody-based screen revealed markers of MBC and allowed identification of novel MBC subsets with distinct functions defined according to surface expression of CD69 and CD45RB. We defined a tissue-resident MBC phenotype that was predominant in the gut but absent in blood. RNA-sequencing of MBC subsets from multiple tissues revealed a tissue-resident MBC gene signature as well as gut- and spleen-specific signatures. Overall, these studies provide novel insights into the nature and function of human B-cell compartments across multiple tissues.

Affinity-Restricted Memory B Cells Dominate Recall Responses to Heterologous Flaviviruses

Memory B cells (MBCs) can respond to heterologous antigens either by molding new specificities through secondary germinal centers (GCs) or by selecting preexisting clones without further affinity maturation. To distinguish these mechanisms in flavivirus infections and immunizations, we studied recall responses to envelope protein domain III (DIII). Conditional deletion of activation-induced cytidine deaminase (AID) between heterologous challenges of West Nile, Japanese encephalitis, Zika, and dengue viruses did not affect recall responses. DIII-specific MBCs were contained mostly within the plasma-cell-biased CD80⁺ subset, and few GCs arose following heterologous boosters, demonstrating that recall responses are confined by preexisting clonal diversity. Measurement of monoclonal antibody (mAb) binding affinity to DIII proteins, timed AID deletion, single-cell RNA sequencing, and lineage tracing experiments point to selection of relatively low-affinity MBCs as a mechanism to promote diversity. Engineering immunogens to avoid this MBC diversity may facilitate flavivirus-type-specific vaccines with minimized potential for infection enhancement.

The transcription factor Hhex cooperates with the corepressor Tle3 to promote memory B cell development

Memory B cells (MBCs) are essential for long-lived humoral immunity. However, the transcription factors involved in MBC differentiation are poorly defined. Here, using single-cell RNA sequencing analysis, we identified a population of germinal center (GC) B cells in the process of differentiating into MBCs. Using an inducible CRISPR-Cas9 screening approach, we identified the hematopoietically expressed homeobox protein Hhex as a transcription factor regulating MBC differentiation. The corepressor Tle3 was also identified in the screen and was found to interact with Hhex to promote MBC development. Bcl-6 directly repressed Hhex in GC B cells. Reciprocally, Hhex-deficient MBCs exhibited increased Bcl6 expression and reduced expression of the Bcl-6 target gene Bcl2. Overexpression of Bcl-2 was able to rescue MBC differentiation in Hhex-deficient cells. We also identified Ski as an Hhex-induced transcription factor involved in MBC differentiation. These findings establish an important role for Hhex-Tle3 in regulating the transcriptional circuitry governing MBC differentiation.

The geography of memory B cell reactivation in vaccine-induced immunity and in autoimmune disease relapses

Memory B cells (Bmem) provide an active second layer of defense against re-infection by pathogens that have bypassed the passive first layer provided by neutralizing antibodies. Here, we review recent progress in our understanding of Bmem heterogeneity in terms of their origin (germinal center-dependent vs center-independent), phenotype (canonical vs atypical vs age-associated B cells), trafficking (recirculating vs tissue-resident), and fate (plasma cell vs germinal center differentiation). The development of transgenic models and intravital imaging technologies has made it possible to track the cellular dynamics of Bmem reactivation by antigen, their interactions with follicular memory T cells, and differentiation into plasma cells in subcapsular proliferative foci in the lymph nodes of immune animals. Such in situ studies have reinforced the importance of geography in shaping the outcome of the secondary antibody response. We also review the evidence for Bmem reactivation and differentiation into short-lived plasma cells in the pathogenesis of disease flares in relapsing-remitting autoimmune diseases. Elucidating the mechanisms that control the Bmem fate decision to differentiate into plasma cells or germinal center B cells will aid future efforts to more precisely engineer fit-for-purpose vaccines as well as to treat antibody-mediated autoimmune diseases.

Memory T cells: strategies for optimizing tumor immunotherapy

Several studies have demonstrated that memory T cells including stem cell memory (Tscm) T cells and central memory (Tcm) T cells show superior persistence and antitumor immunity compared with effector memory T (Tem) cells and effector T (Teff) cells. Furthermore, the Tcm/Teff ratio has been reported to be a predictive biomarker of immune responses against some tumors. Thus, a system-level understanding of the mechanisms underlying the differentiation of effector and memory T cells is of increasing importance for developing immunological strategies against various tumors. This review focuses on recent advances in efficacy against tumors, the origin, formation mechanisms of memory T cells, and the role of the gut microbiota in memory T cell formation. Furthermore, we summarize strategies to generate memory T cells in (ex) vivo that, might be applicable in clinical practice.

Epigenetic regulation of B cell fate and function during an immune response

The humoral immune response requires coordination of molecular programs to mediate differentiation into unique B cell subsets that help clear the infection and form immune memory. Epigenetic modifications are crucial for ensuring that the appropriate genes are transcribed or repressed during B cell differentiation. Recent studies have illuminated the changes in DNA methylation and histone post-translational modifications that accompany the formation of germinal center and antibody-secreting cells during an immune response. In particular, the B cell subset-specific expression and function of DNA methyltransferases and histone-modifying complexes that mediate epigenome changes have begun to be unravelled. This review will discuss the recent advances in this field, as well as highlight critical questions about the relationship between epigenetic regulation and B cell fate and function that are yet to be answered.

ZBTB32 restrains antibody responses to murine cytomegalovirus infections, but not other repetitive challenges

ZBTB32 is a transcription factor that is highly expressed by a subset of memory B cells and restrains the magnitude and duration of recall responses against hapten-protein conjugates. To define physiological contexts in which ZBTB32 acts, we assessed responses by Zbtb32^-/- mice or bone marrow chimeras against a panel of chronic and acute challenges. Mixed bone marrow chimeras were established in which all B cells were derived from either Zbtb32^-/- mice or control littermates. Chronic infection of Zbtb32^-/- chimeras with murine cytomegalovirus led to nearly 20-fold higher antigen-specific IgG2b levels relative to controls by week 9 post-infection, despite similar viral loads. In contrast, IgA responses and specificities in the intestine, where memory B cells are repeatedly stimulated by commensal bacteria, were similar between Zbtb32^-/- mice and control littermates. Finally, an infection and heterologous booster vaccination model revealed no role for ZBTB32 in restraining primary or recall antibody responses against influenza viruses. Thus, ZBTB32 does not limit recall responses to a number of physiological acute challenges, but does restrict antibody levels during chronic viral infections that periodically engage memory B cells. This restriction might selectively prevent recall responses against chronic infections from progressively overwhelming other antibody specificities.

Remembrance of Things Past: Long-Term B Cell Memory After Infection and Vaccination

The success of vaccines is dependent on the generation and maintenance of immunological memory. The immune system can remember previously encountered pathogens, and memory B and T cells are critical in secondary responses to infection. Studies in mice have helped to understand how different memory B cell populations are generated following antigen exposure and how affinity for the antigen is determinant to B cell fate. Additionally, such studies were fundamental in defining memory B cell niches and how B cells respond following subsequent exposure with the same antigen. On the other hand, human studies are essential to the development of better, newer vaccines but sometimes limited by the difficulty to access primary and secondary lymphoid organs. However, work using human influenza and HIV virus infection and/or immunization in particular has significantly advanced today's understanding of memory B cells. This review will focus on the generation, function, and longevity of B-cell mediated immunological memory (memory B cells and plasma cells) in response to infection and vaccination both in mice and in humans.

What B cell memories are made of

In many ways, memory B cells represent the ultimate outcome of humoral immunity. Many of these cells express exceptionally high affinity antigen-specific B cell receptors for antigen, and these cells are a critical source of the long-lived plasma cells that secrete protective serum antibodies to protect against secondary exposure to pathogens and other life-threatening antigens. Evidence is now emerging that not all memory B cells are created via the same cellular pathways and molecular events. Similarly, it is becoming clear that different memory B cells can take on different functions, with some producing IgM rather than IgG antibodies upon reactivation, and others preferentially producing plasma cells rather than additional waves of memory B cells. With this review, we discuss the conceptual ides and early studies surrounding early work on B cell memory, then discuss the many pathways and functional attributes of subpopulations of memory B cells and current approaches to characterize these cells directly.

Do Memory B Cells Form Secondary Germinal Centers? Yes and No

Memory is the defining feature of the adaptive immune system. Humoral immune memory is largely though not exclusively generated in the germinal center (GC), which spawns long-lived plasma cells that support ongoing serum antibody titers as well as "memory B cells" (MBCs) that persist in the immune host at expanded frequencies. Upon reencounter with antigen, these MBCs are reactivated and potentially can contribute to protection by further expansion, rapid differentiation to antibody-forming cells, and/or reseeding of a new round of GCs along with somatic V region mutation and selection. Here I will discuss what controls these various potential fates of MBCs and the functional significance of different types of MBC reactivation.

Blood Stage Malaria Disrupts Humoral Immunity to the Pre-erythrocytic Stage Circumsporozoite Protein

Many current malaria vaccines target the pre-erythrocytic stage of infection in the liver. However, in malaria-endemic regions, increased blood stage exposure is associated with decreased vaccine efficacy, thereby challenging current vaccine efforts. We hypothesized that pre-erythrocytic humoral immunity is directly disrupted by blood stage infection. To investigate this possibility, we used Plasmodium-antigen tetramers to analyze B cells after infection with either late liver stage arresting parasites or wild-type parasites that progress to the blood stage. Our data demonstrate that immunoglobulin G (IgG) antibodies against the pre-erythrocytic antigen, circumsporozoite protein (CSP), are generated only in response to the attenuated, but not the wild-type, infection. Further analyses revealed that blood stage malaria inhibits CSP-specific germinal center B cell differentiation and modulates chemokine expression. This results in aberrant memory formation and the loss of a rapid secondary B cell response. These data highlight how immunization with attenuated parasites may drive optimal immunity to malaria.

Loss of Xist RNA from the inactive X during B cell development is restored in a dynamic YY1-dependent two-step process in activated B cells

X-chromosome inactivation (XCI) in female lymphocytes is uniquely regulated, as the inactive X (Xi) chromosome lacks localized Xist RNA and heterochromatin modifications. Epigenetic profiling reveals that Xist RNA is lost from the Xi at the pro-B cell stage and that additional heterochromatic modifications are gradually lost during B cell development. Activation of mature B cells restores Xist RNA and heterochromatin to the Xi in a dynamic two-step process that differs in timing and pattern, depending on the method of B cell stimulation. Finally, we find that DNA binding domain of YY1 is necessary for XCI in activated B cells, as ex-vivo YY1 deletion results in loss of Xi heterochromatin marks and up-regulation of X-linked genes. Ectopic expression of the YY1 zinc finger domain is sufficient to restore Xist RNA localization during B cell activation. Together, our results indicate that Xist RNA localization is critical for maintaining XCI in female lymphocytes, and that chromatin changes on the Xi during B cell development and the dynamic nature of YY1-dependent XCI maintenance in mature B cells predisposes X-linked immunity genes to reactivation.

Females are predisposed to develop various autoimmune disorders, and the genetic basis for this susceptibility is the X-chromosome. X-linked genes are dosage compensated between sexes by X-chromosome Inactivation (XCI) during embryogenesis and maintained into adulthood. Here we show that the chromatin of the inactive X loses epigenetic modifications during B cell lineage development. We found that female mature B cells, which are the pathogenic cells in autoimmunity, have a dynamic two-step mechanism of maintaining XCI during stimulation. The transcription factor YY1, which regulates DNA looping during V(D)J recombination in B cells, is necessary for relocalizing Xist RNA back to the inactive X in activated B cells. YY1 deletion ex vivo in mature B cells impairs heterochromatin mark enrichment on the inactive X, and results in increased X-linked gene expression. We demonstrate that the DNA binding domain of YY1 is sufficient for localizing Xist RNA to the inactive X during B cell stimulation. Our study indicates that Xist RNA localization is critical for maintaining XCI in female lymphocytes. We propose that chromatin changes on the Xi during B cell development and the dynamic nature of YY1-dependent XCI maintenance in mature B cells predisposes X-linked immunity genes to reactivation.

CD11c+ T-bet+ memory B cells: Immune maintenance during chronic infection and inflammation?

CD11c+ T-bet+ B cells have now been detected and characterized in different experimental and clinical settings, in both mice and humans. Whether such cells are monolithic, or define subsets of B cells with different functions is not yet known. Our studies have identified CD11c+ IgM+ CD19^hi splenic IgM memory B cells that appear at approximately three weeks post-ehrlichial infection, and persist indefinitely, during low-level chronic infection. Although the CD11c+ T-bet+ B cells we have described are distinct, they appear to share many features with similar cells detected under diverse conditions, including viral infections, aging, and autoimmunity. We propose that CD11c+ T-bet+ B cells as a group share characteristics of memory B cells that are maintained under conditions of inflammation and/or low-level chronic antigen stimulation. In some cases, these cells may be advantageous, by providing immunity to re-infection, but in others may be deleterious, by contributing to aged-associated autoimmune responses.

Memory B Cells of Mice and Humans

We comprehensively review memory B cells (MBCs), covering the definition of MBCs and their identities and subsets, how MBCs are generated, where they are localized, how they are maintained, and how they are reactivated. Whereas naive B cells adopt multiple fates upon stimulation, MBCs are more restricted in their responses. Evolving work reveals that the MBC compartment in mice and humans consists of distinct subpopulations with differing effector functions. We discuss the various approaches to define subsets and subset-specific roles. A major theme is the need to both deliver faster effector function upon reexposure and readapt to antigenically variant pathogens while avoiding burnout, which would be the result if all MBCs generated only terminal effector function. We discuss cell-intrinsic differences in gene expression and signaling that underlie differences in function between MBCs and naive B cells and among MBC subsets and how this leads to memory responses.

Immune history and influenza virus susceptibility

Antibody responses to influenza viruses are critical for protection, but the ways in which repeated viral exposures shape antibody evolution and effectiveness over time remain controversial. Early observations demonstrated that viral exposure history has a profound effect on the specificity and magnitude of antibody responses to a new viral strain, a phenomenon called 'original antigenic sin.' Although 'sin' might suppress some aspects of the immune response, so far there is little indication that hosts with pre-existing immunity are more susceptible to viral infections compared to naïve hosts. However, the tendency of the immune response to focus on previously recognized conserved epitopes when encountering new viral strains can create an opportunity cost when mutations arise in these conserved epitopes. Hosts with different exposure histories may continue to experience distinct patterns of infection over time, which may influence influenza viruses' continued antigenic evolution. Understanding the dynamics of B cell competition that underlie the development of antibody responses might help explain the low effectiveness of current influenza vaccines and lead to better vaccination strategies.

ZBTB32 Restricts the Duration of Memory B Cell Recall Responses

Memory B cell responses are more rapid and of greater magnitude than are primary Ab responses. The mechanisms by which these secondary responses are eventually attenuated remain unknown. We demonstrate that the transcription factor ZBTB32 limits the rapidity and duration of Ab recall responses. ZBTB32 is highly expressed by mouse and human memory B cells but not by their naive counterparts. Zbtb32(-/-) mice mount normal primary Ab responses to T-dependent Ags. However, Zbtb32(-/-) memory B cell-mediated recall responses occur more rapidly and persist longer than do control responses. Microarray analyses demonstrate that Zbtb32(-/-) secondary bone marrow plasma cells display elevated expression of genes that promote cell cycle progression and mitochondrial function relative to wild-type controls. BrdU labeling and adoptive transfer experiments confirm more rapid production and a cell-intrinsic survival advantage of Zbtb32(-/-) secondary plasma cells relative to wild-type counterparts. ZBTB32 is therefore a novel negative regulator of Ab recall responses.

Naïve and memory B cells exhibit distinct biochemical responses following BCR engagement

Immunological memory is characterized by the rapid reactivation of memory B cells that produce large quantities of high-affinity antigen-specific antibodies. This contrasts the response of naïve B cells, and the primary immune response, which is much slower and of lower affinity. Memory responses are critical for protection against infectious diseases and form the basis of most currently available vaccines. Although we have known about the phenomenon of long-lived memory for centuries, the biochemical differences underlying these diverse responses of naïve and memory B cells is incompletely resolved. Here we investigated the nature of B-cell receptor (BCR) signaling in human splenic naïve, IgM(+) memory and isotype-switched memory B cells following multivalent BCR crosslinking. We observed comparable rapid and transient phosphorylation kinetics for proximal (phosphotyrosine and spleen tyrosine kinase) and propagation (B-cell linker, phospholipase Cγ2) signaling components in these different B-cell subsets. However, the magnitude of activation of downstream components of the BCR signaling pathway were greater in memory compared with naïve cells. Although no differences were observed in the magnitude of Ca(2+) mobilization between subsets, IgM(+) memory B cells exhibited a more rapid Ca(2+) mobilization and a greater depletion of the Ca(2+) endoplasmic reticulum stores, while IgG(+) memory B cells had a prolonged Ca(2+) uptake. Collectively, our findings show that intrinsic signaling features of B-cell subsets contribute to the robust response of human memory B cells over naïve B cells. This has implications for our understanding of memory B-cell responses and provides a framework to modulate these responses in the setting of vaccination and immunopathologies, such as immunodeficiency and autoimmunity.

Cellular and chromatin dynamics of antibody-secreting plasma cells

Plasma cells are terminally differentiated B cells responsible for maintaining protective serum antibody titers. Despite their clinical importance, our understanding of the linear genomic features and chromatin structure of plasma cells is incomplete. The plasma cell differentiation program can be triggered by different signals and in multiple, diverse peripheral B cell subsets. This heterogeneity raises questions about the gene regulatory circuits required for plasma cell specification. Recently, new regulators of plasma cell differentiation have been identified and the enhancer landscapes of naïve B cells have been described. Other studies have revealed that the bone marrow niche harbors heterogeneous plasma cell subsets. Still undefined are the minimal requirements to become a plasma cell and what molecular features make peripheral B cell subsets competent to become antibody-secreting plasma cells. New technologies promise to reveal underlying chromatin configurations that promote efficient antibody secretion. For further resources related to this article, please visit the WIREs website.

Reactivation of IgG-switched memory B cells by BCR-intrinsic signal amplification promotes IgG antibody production

Secondary antibody responses are marked by faster kinetics, improved antibody affinity and a switch from IgM to other immunoglobulin isotypes, most notably IgG, compared with primary responses. These changes protect from reinfection and represent the principle of most vaccination strategies. Yet, the molecular mechanisms that underlie B-cell memory responses are unclear. Here we show, by inactivating the immunoglobulin tail tyrosine (ITT) signalling motif of membrane-bound IgG1 in the mouse, that the ITT facilitates maintenance and reactivation of IgG-switched memory B cells in vivo. The ITT motif equips IgG-switched cells with enhanced BCR signalling capacity, which supports their competitiveness in secondary immune reactions and drives the formation of IgG-secreting plasma cells even in the absence of T-cell help. Our results demonstrate that ITT signalling promotes the vigorous production of IgG antibodies and thus provide a molecular basis for humoral immunological memory.

Needle-free and adjuvant-free epicutaneous boosting of pertussis immunity: Preclinical proof of concept

The limited durability of pertussis vaccine-induced protection requires novel approaches to reactivate immunity and limit pertussis resurgence in older children and adults. We propose that periodic boosters could be delivered using a novel epicutaneous delivery system (Viaskin) to deliver optimized pertussis antigens such as genetically-detoxified pertussis toxin (rPT). To best mimic the human situation in which vaccine-induced memory cells persist, whereas antibodies wane, we developed a novel adoptive transfer murine model of pertussis immunity. This allowed demonstrating that a single application of Viaskin delivering rPT and/or pertactin and filamentous hemagglutinin effectively reactivates vaccine-induced pertussis immunity and protects against Bordetella pertussis challenge. Recalling pertussis immunity without needles nor adjuvant may considerably facilitate the acceptance and application of periodic boosters.

Regulation of germinal center, B-cell memory, and plasma cell formation by histone modifiers

Understanding the regulation of antibody production and B-cell memory formation and function is core to finding new treatments for B-cell-derived cancers, antibody-mediated autoimmune disorders, and immunodeficiencies. Progression from a small number of antigen-specific B-cells to the production of a large number of antibody-secreting cells is tightly regulated. Although much progress has been made in revealing the transcriptional regulation of B-cell differentiation that occurs during humoral immune responses, there are still many questions that remain unanswered. Recent work on the expression and roles of histone modifiers in lymphocytes has begun to shed light on this additional level of regulation. This review will discuss the recent advancements in understanding how humoral immune responses, in particular germinal centers and memory cells, are modulated by histone modifiers.

The CD27+ memory B cells display changes in the gene expression pattern in elderly individuals

Memory B cells (MBCs) have a very long life-span as compared to naïve B cells (NBCs), remaining viable for years. It could predispose them to suffer misbalances in the gene expression pattern at the long term, which might be involved in the development of age-related B-cell disorders. In order to identify genes whose expression might change during life, we analyzed the gene expression patterns of CD27- NBCs versus CD27+ MBCs in young and old subjects. Using microarray assays we observed that the expression pattern of CD27- NBCs versus CD27+ MBCs is significantly different. Furthermore, in order to evaluate the age effect, we compared the gene expression pattern of young versus aged subjects in both cell populations. Interestingly, we did not find significant differences in the CD27- NBC population between young and aged individuals, whereas we found 925 genes differentially expressed in CD27+ MBCs. Among these genes, 193 were also differentially expressed in CD27+ MBCs as compared to CD27- NBCs, most of them involved in cell survival, cell growth and proliferation, cellular development and gene expression. We conclude that gene expression profiles of CD27- NBCs and CD27+ MBCs are different. Moreover, whereas the gene expression pattern of CD27+ MBCs varies with age, the same does not happen in CD27- NBCs. This suggests that MBCs undergo time-dependent changes which could underlie a higher susceptibility to dysfunction with age. This article is protected by copyright. All rights reserved.

CD40 signaling drives B lymphocytes into an intermediate memory-like state, poised between naïve and plasma cells

Immunological memory comprising of antigen-specific B and T cells contributes to the acquisition of long-term resistance to pathogens. Interactions between CD40 on B cells and CD40L on T cells are responsible for several aspects of acquired immune responses including generation of memory B cells. In order to gain insights into events leading to memory B cell formation, we analyzed the genome-wide expression profile of murine naive B cells stimulated in the presence of anti-CD40. We have identified over 8,000 genes whose expression is altered minimally 1.5-fold at least at one time point over a 3-day time course. The array analysis indicates that changes in expression level of maximum number of these genes occur within 24 h of anti-CD40 treatment. In parallel, we have studied the events following CD40 ligation by examining the expression of known regulators of naive B cell to plasma cell transition, including Pax5 and BLIMP1. The expression profile of these regulatory genes indicates firstly, that CD40 signaling activates naïve B cells to a phenotype that is intermediate between the naive and plasma cell stages of the B cell differentiation. Secondly, the major known regulator of plasma cell differentiation, BLIMP1, gets irreversibly downregulated upon anti-CD40 treatment. Additionally, our data reveal that CD40 signaling mediated BLIMP1 downregulation occurs by non-Pax5/non-Bcl6 dependent mechanisms, indicating novel mechanisms at work that add to the complexity of understanding of B cell master regulatory molecules like BLIMP1 and Pax5.

CD80 and PD-L2 define functionally distinct memory B cell subsets that are independent of antibody isotype

Memory B cells (MBCs) are long-lived sources of rapid, isotype-switched secondary antibody-forming cell (AFC) responses. Whether MBCs homogeneously retain the ability to self-renew and terminally differentiate or if these functions are compartmentalized into MBC subsets has remained unclear. It has been suggested that antibody isotype controls MBC differentiation upon restimulation. Here we demonstrate that subcategorizing MBCs on the basis of their expression of CD80 and PD-L2, independently of isotype, identified MBC subsets with distinct functions upon rechallenge. CD80(+)PD-L2(+) MBCs differentiated rapidly into AFCs but did not generate germinal centers (GCs); conversely, CD80(-)PD-L2(-) MBCs generated few early AFCs but robustly seeded GCs. The gene-expression patterns of the subsets supported both the identity and function of these distinct MBC types. Hence, the differentiation and regeneration of MBCs are compartmentalized.

Regulation of germinal center responses and B-cell memory by the chromatin modifier MOZ

Memory B cells and long-lived bone marrow-resident plasma cells maintain humoral immunity. Little is known about the intrinsic mechanisms that are essential for forming memory B cells or endowing them with the ability to rapidly differentiate upon reexposure while maintaining the population over time. Histone modifications have been shown to regulate lymphocyte development, but their role in regulating differentiation and maintenance of B-cell subsets during an immune response is unclear. Using stage-specific deletion of monocytic leukemia zinc finger protein (MOZ), a histone acetyltransferase, we demonstrate that mutation of this chromatin modifier alters fate decisions in both primary and secondary responses. In the absence of MOZ, germinal center B cells were significantly impaired in their ability to generate dark zone centroblasts, with a concomitant decrease in both cell-cycle progression and BCL-6 expression. In contrast, there was increased differentiation to IgM and low-affinity IgG1(+) memory B cells. The lack of MOZ affected the functional outcome of humoral immune responses, with an increase in secondary germinal centers and a corresponding decrease in secondary high-affinity antibody-secreting cell formation. Therefore, these data provide strong evidence that manipulating epigenetic modifiers can regulate fate decisions during humoral responses, and thus could be targeted for therapeutic intervention.

No receptor stands alone: IgG B-cell receptor intrinsic and extrinsic mechanisms contribute to antibody memory

Acquired immunological memory is a striking phenomenon. A lethal epidemic sweeps through a naïve population, many die but those who survive are never "attacked twice - never at least fatally", as the historian Thucydides observed in 430 BCE. Antibody memory is critical for protection against many human infectious diseases and is the basis for nearly all current human vaccines. Antibody memory is encoded, in part, in isotype-switched immunoglobulin (Ig)G-expressing memory B cells that are generated in the primary response to antigen and give rise to rapid, high-affinity and high-titered antibody responses upon challenge with the same antigen. How IgG-B-cell receptors (BCRs) and antigen-induced IgG-BCR signaling contribute to memory antibody responses are not fully understood. In this review, we summarize exciting new advances that are revealing the cellular and molecular mechanisms at play in antibody memory and discuss how studies using different experimental approaches will help elucidate the complex phenomenon of B-cell memory.

CD73 expression is dynamically regulated in the germinal center and bone marrow plasma cells are diminished in its absence

CD73 catalyzes the conversion of extracellular nucleosides to adenosine, modulating inflammatory and T cell responses. Elevated expression of CD73 marks subpopulations of murine memory B cells (MBC), but its role in memory development or function is unknown. Here, we demonstrate that CD73 is progressively upregulated on germinal center (GC) B cells following immunization, is expressed at even higher levels among T follicular helper cells, but is absent among plasma cells (PC) and plasmablasts (PB). We analyzed the T-dependent B cell response in CD73 knockout mice (CD73KO). During the early response, CD73KO and wild type (WT) mice formed GCs, MBCs and splenic PBs and PCs similarly, and MBCs functioned similarly in the early secondary response. Late in the primary response, however, bone marrow (BM) PCs were markedly decreased in CD73KO animals. Tracking this phenotype, we found that CD73 expression was required on BM-derived cells for optimal BM PC responses. However, deletion of CD73 from either B or T lymphocytes alone did not recapitulate the phenotype. This suggests that CD73 expression is sufficient on either cell type, consistent with its function as an ectoenzyme. Together, these findings suggest that CD73-dependent adenosine signaling is prominent in the mature GC and required for establishment of the long-lived PC compartment, thus identifying a novel role for CD73 in humoral immunity.

Recovery of an antiviral antibody response following attrition caused by unrelated infection

The homeostatic mechanisms that regulate the maintenance of immunological memory to the multiple pathogen encounters over time are unknown. We found that a single malaria episode caused significant dysregulation of pre-established Influenza A virus-specific long-lived plasma cells (LLPCs) resulting in the loss of Influenza A virus-specific Abs and increased susceptibility to Influenza A virus re-infection. This loss of LLPCs involved an FcγRIIB-dependent mechanism, leading to their apoptosis. However, given enough time following malaria, the LLPC pool and humoral immunity to Influenza A virus were eventually restored. Supporting a role for continuous conversion of Influenza A virus-specific B into LLPCs in the restoration of Influenza A virus immunity, B cell depletion experiments also demonstrated a similar requirement for the long-term maintenance of serum Influenza A virus-specific Abs in an intact LLPC compartment. These findings show that, in addition to their established role in the anamnestic response to reinfection, the B cell pool continues to be a major contributor to the maintenance of long-term humoral immunity following primary Influenza A virus infection, and to the recovery from attrition following heterologous infection. These data have implications for understanding the longevity of protective efficacy of vaccinations in countries where continuous infections are endemic.

Antibody responses to infectious pathogens are critical in host survival, recovery and protection from reinfection; they also correlate with the success of vaccination. It is currently thought that antibody serum titers are maintained at protective levels over long periods of time by specialized long-lived antibody-secreting plasma cells residing in the bone marrow. Indeed, antibodies against the original virus can still be found in survivors of the 1918 Spanish Flu, more than 90 years ago. However, it is also becoming clear that subsequent infection with heterologous pathogens may cause attrition of previously established immunological memory, in order to accommodate new lymphocyte specificities in the finite space of the host. This phenomenon is seemingly at odds with long-term maintenance of immunological memory. We also show that a single episode of malaria, caused by infection by Plasmodium chabaudi, leads to the loss of preexisting plasma cells, serum antibodies and protective immunity against Influenza A virus. However, Influenza A virus-specific immunity does eventually recover in these animals with the replenishment of plasma cells by B cells over the course of several weeks. Thus, the reported mechanism reconciles attrition of immunological memory by heterologous infection and long-term stability, and places B cells, instead of their descendant plasma cells, at the center of humoral memory.

Diversity among memory B cells: origin, consequences, and utility

Immunological memory is the residuum of a successful immune response that in the B cell lineage comprises long-lived plasma cells and long-lived memory B cells. It is apparent that distinct classes of memory B cells exist, distinguishable by, among other things, immunoglobulin isotype, location, and passage through the germinal center. Some of this variation is due to the nature of the antigen, and some appears to be inherent to the process of forming memory. Here, we consider the heterogeneity in development and phenotype of memory B cells and whether particular functions are partitioned into distinct subsets. We consider also how understanding the details of generating memory may provide opportunities to develop better, functionally targeted vaccines.

CD49b/CD69-Dependent Generation of Resting T Helper Cell Memory

In the absence of antigen, memory T helper (Th) cells are maintained in a resting state. Recently it has been shown that bone marrow (BM) is a major reservoir of resting memory Th cells. In a given immune response, less than 10% of the activated CD4 T cells are recruited to the pool of resting BM memory Th cells. Here we review recent evidence that CD69 and CD49b control homing of memory Th cell precursors to the BM. During the effector phase of an immune response, about 10% of activated CD4 T cells in the spleen express both CD69 and CD49b, and thus qualify as precursors of resting memory Th cells of BM. Loss or blockade of CD69 and CD49b expression on CD4 T cells impairs the generation of resting memory Th cells in the BM. Moreover, in the absence of BM memory Th cells in CD69-deficient mice, T-cell help for B cells is impaired, confirming the central role of BM memory Th cells in the maintenance of immunological memory.

SLE peripheral blood B cell, T cell and myeloid cell transcriptomes display unique profiles and each subset contributes to the interferon signature

Systemic lupus erythematosus (SLE) is a chronic autoimmune disease that is characterized by defective immune tolerance combined with immune cell hyperactivity resulting in the production of pathogenic autoantibodies. Previous gene expression studies employing whole blood or peripheral blood mononuclear cells (PBMC) have demonstrated that a majority of patients with active disease have increased expression of type I interferon (IFN) inducible transcripts known as the IFN signature. The goal of the current study was to assess the gene expression profiles of isolated leukocyte subsets obtained from SLE patients. Subsets including CD19⁺ B lymphocytes, CD3⁺CD4⁺ T lymphocytes and CD33⁺ myeloid cells were simultaneously sorted from PBMC. The SLE transcriptomes were assessed for differentially expressed genes as compared to healthy controls. SLE CD33⁺ myeloid cells exhibited the greatest number of differentially expressed genes at 208 transcripts, SLE B cells expressed 174 transcripts and SLE CD3⁺CD4⁺ T cells expressed 92 transcripts. Only 4.4% (21) of the 474 total transcripts, many associated with the IFN signature, were shared by all three subsets. Transcriptional profiles translated into increased protein expression for CD38, CD63, CD107a and CD169. Moreover, these studies demonstrated that both SLE lymphoid and myeloid subsets expressed elevated transcripts for cytosolic RNA and DNA sensors and downstream effectors mediating IFN and cytokine production. Prolonged upregulation of nucleic acid sensing pathways could modulate immune effector functions and initiate or contribute to the systemic inflammation observed in SLE.

Rgs13 constrains early B cell responses and limits germinal center sizes

Germinal centers (GCs) are microanatomic structures that develop in secondary lymphoid organs in response to antigenic stimulation. Within GCs B cells clonally expand and their immunoglobulin genes undergo class switch recombination and somatic hypermutation. Transcriptional profiling has identified a number of genes that are prominently expressed in GC B cells. Among them is Rgs13, which encodes an RGS protein with a dual function. Its canonical function is to accelerate the intrinsic GTPase activity of heterotrimeric G-protein α subunits at the plasma membrane, thereby limiting heterotrimeric G-protein signaling. A unique, non-canonical function of RGS13 occurs following translocation to the nucleus, where it represses CREB transcriptional activity. The functional role of RGS13 in GC B cells is unknown. To create a surrogate marker for Rgs13 expression and a loss of function mutation, we inserted a GFP coding region into the Rgs13 genomic locus. Following immunization GFP expression rapidly increased in activated B cells, persisted in GC B cells, but declined in newly generated memory B and plasma cells. Intravital microscopy of the inguinal lymph node (LN) of immunized mice revealed the rapid appearance of GFP⁺ cells at LN interfollicular regions and along the T/B cell borders, and eventually within GCs. Analysis of WT, knock-in, and mixed chimeric mice indicated that RGS13 constrains extra-follicular plasma cell generation, GC size, and GC B cell numbers. Analysis of select cell cycle and GC specific genes disclosed an aberrant gene expression profile in the Rgs13 deficient GC B cells. These results indicate that RGS13, likely acting at cell membranes and in nuclei, helps coordinate key decision points during the expansion and differentiation of naive B cells.

Multiple transcription factor binding sites predict AID targeting in non-Ig genes

Aberrant targeting of the enzyme activation-induced cytidine deaminase (AID) results in the accumulation of somatic mutations in ≈ 25% of expressed genes in germinal center B cells. Observations in Ung(-/-) Msh2(-/-) mice suggest that many other genes efficiently repair AID-induced lesions, so that up to 45% of genes may actually be targeted by AID. It is important to understand the mechanisms that recruit AID to certain genes, because this mistargeting represents an important risk for genome instability. We hypothesize that several mechanisms combine to target AID to each locus. To resolve which mechanisms affect AID targeting, we analyzed 7.3 Mb of sequence data, along with the regulatory context, from 83 genes in Ung(-/-) Msh2(-/-) mice to identify common properties of AID targets. This analysis identifies three transcription factor binding sites (E-box motifs, along with YY1 and C/EBP-β binding sites) that may work together to recruit AID. Based on previous knowledge and these newly discovered features, a classification tree model was built to predict genome-wide AID targeting. Using this predictive model, we were able to identify a set of 101 high-interest genes that are likely targets of AID.