B cell primary immune responses.

Abstract

B cell immune responses initiate after recognition of foreign or self antigen, often but not always in the context of an innate immune stimulus (eg, due to infection) or adjuvant (as in a vaccination). This process typically begins in B cell zones including the follicular areas of secondary lymphoid tissues and the marginal zone of the B cell. Recognition of Ag by BCR initially leads to B cell activation that triggers chemokine receptor modulation and relocation to T zone/red pulp border areas, locations at which activated B cells could encounter cognate activated T cells.1, 2 Canonically, the response continues at this site with T-B interactions leading to mutual activation, proliferation, and differentiation. Within a few days, a portion of dividing B cells differentiates into short-lived plasmablasts that secrete antibody while also continuing to divide and interact with T cells. Also around this time, a fraction of both dividing B cells and T cells reenter nearby follicles and continue to interact and differentiate toward germinal center (GC) phenotypes, ultimately generating the GC response comprised of GC B cells (GCBC) and T follicular helper cells (TFH), both of which express the signature transcription factor (TF) BCL6.3, 4 Meanwhile, within a few days, the initial response at the T-B border decays and the short-lived plasmablasts die off. Within the GC, T-B interactions continue while AID-driven somatic hypermutation of IgV regions is fully induced,5-7 resulting in nearly every daughter cell having at least one sequence difference from the parent. This diversity forms the substrate for Ag-driven selection, which may be mediated by a combination of signals from both BCR and T cell help.8-11 The latter reads out the ability of GCBC to effectively present Ag that is captured through the BCR. Via cycles of mutation, selection, and division, clones with increased affinity—as well as increased intraclonal diversity—evolve and expand within the GC.12, 13 A small fraction of GCBC differentiates into long-lived progeny: memory B cells (MBC) and long-lived plasma cells. Interestingly, recent work shows that MBC are formed during the early part of the response, including before the GC even forms, while long-lived plasma cells form only at the latter part of the GC reaction, when GCBC have more mutations and overall higher affinity.14 Notably, the GC reaction also spins off some short-lived PCs earlier in the process.2 This canonical pathway has overshadowed in terms of scientific attention several alternative immune response pathways that have also been long known but less well studied15; in fact, these pathways may be much more commonly engaged in pathogen and vaccine responses than is typically thought to be the case. These responses are loosely termed “extrafollicular,” since they represent the persistence of the initial response that had started outside of the follicle. In cases of certain infections, for example Salmonella, these EF responses can last for weeks in the absence of a GC response.16, 17 T cells can and do play a role in promoting these responses, often differentiating into a BCL6-expressing T “extrafollicular” helper cell type.18, 19 EF responses to certain types of Ags can proceed in the absence of T cells, so-called T-independent responses.20, 21 Also in the category of “alternative” pathways would be included those that induce so-called “age-related B cells” (ABCs), also termed T-bet positive B cells.22 The biological significance of these alternative pathways is less well understood. What are the conditions that preferentially induce alternative B cell responses? Do they induce memory, undergo affinity maturation, and lead to generation of long-lived plasma cells? These are all the active and reemerging areas of research. A major concept in putting together this volume of reviews was to include coverage of these types of response alongside updated reviews of processes and cells related to the classical GC pathway itself. Hence the title of this volume: Germinal Center and Extrafollicular B Cell Responses. This volume has assembled reviews from many contributors to our understanding of these modes of B cell response. They can be grouped loosely together based on specific subtopics as well as with respect to some global themes. The figure (Figure 1) represents how the reviews relate to these aspects of the B cell immune response and its malignant transformation. The stages or processes on which each review focuses are indicated on the cartoon, with some reviews appearing in multiple places. This “visual abstract” will allow the reader interested in a particular aspect to direct oneself to the relevant reviews. Below we further detail some of these topics and global themes. With apologies to co-authors who are fully credited in the reviews themselves, for clarity here we will refer to the senior authors of each article as the singular writer. Several reviews focus on aspects of the mechanics of the GC itself, albeit at different levels. Shulman, Qi, and Haberman all focus on B cell:T cell interactions in the GC. Shulman covers the key role that SLAM family members, plexins, and ultimately integrins play in shaping adhesive interactions and at the same time signaling to cells and thereby affecting other key signals such as CD40:CD40L. Qi and Haberman focus on the details of the reciprocal B-T interaction in the GC and steps leading up to the fully mature GC. Shlomchik and Shulman discuss signaling events within GC B cells that underpin positive selection; Shlomchik emphasizes how intracellular signals from both BCR and CD40 are reprogrammed in GCBC compared to naïve B cells in order to promote positive selection. Qi emphasizes reciprocal B-T interactions that lead to T cell-mediated positive selection and affinity maturation of GCBC. In a complementary way, Kurosaki discusses signals and TF networks that regulate the differentiation of GCBC into plasma cell precursors that beget LLPC. Related to this aspect, Good-Jacobson takes a “inside to out” perspective on how B cells intrinsically reprogram themselves by alterations in epigenetic DNA and chromatin marking, that in turn regulate GC, plasma, and memory B cell function and differentiation. Her review integrates work from her lab and others to both describe progressive changes in expression of key enzymes that modulate epigenetic marks as well as control B cell responses to immunization. The review is a comprehensive tour through multiple key genetic experiments that have explored the effects of genetic deletion or manipulation of these DNA and chromatin modifying systems. Multiple reviews focus on the T cell component of the GC. Qi discusses signals from B cells to TFH that either elicit direct T cell help or serve to maintain the TFH phenotype. Craft reviews the developmental processes that lead to TFH differentiation, as well as the heterogeneity among the group of T cells that express the signature TF BCL6 and are specialized to interact with responding B cells. His review discusses both cytokine signals and surface molecule expression that are key to each of these stages, linking these signals and molecules to expression of key TFs that in turn coordinate TFH differentiation. Baumjohann takes a unique perspective on TFH, using the filter of microRNA expression and function to outline how these control TFH formation, differentiation, and function. He reveals how these microRNAs interact with and regulate more well-known TF and signaling networks inherent to TFH. Finally, in this domain, Graca reviews a unique and still-mysterious subset of TFH. Graca and colleagues were among three groups that simultaneously reported the existence of BCL6-expressing TFH that also expressed FoxP3, the quintessential TF associated with regulatory T cells23-25; these TFH were designated T follicular regulatory T cells, or TFR. Overall, a predominance of evidence (but not all) suggests that TFR negatively regulate humoral immunity; nonetheless, the origins and functions of TFR remain controversial. In this volume, Graca presents a comprehensive overview of this subject, including both human and mouse and covering TFR-like cells in the blood and tissues. As alluded to, not all B cell responses take the GC path. Indeed, the literature indicates that many scenarios lead to EF or similar responses. These include Salmonella infection, Ehrlichia infection, malarial infection, multiple “T-independent” immune responses including responses to encapsulated bacteria, and lupus autoimmunity in some animal models and a significant subset of human patients. The responding B cell type for such responses has sometimes been identified as B-1 or marginal zone, though follicular B cells can also contribute to such responses. Allman reviews and provides an interesting perspective on a spectrum of such responses, proposing that—counter to most dogma—T cell-independent EF responses can and often do generate long-lived humoral immunity including LLPC and MBC. Sanz focuses on the B cell response in patients with humans, arguing cogently for a major contribution by the EF pathway. Immune responses involving T-bet expressing B cells seem in a category of their own, yet they seem to emanate from potentially different sources and result in different outputs, depending on the context. They are often associated with the same types of responses in which the EF pathway predominates, such as Ehrlichia infection or SLE. However, other types of infection, for example influenza, that elicit GC responses can also elicit T-bet expressing responding cells and longer-living progeny. In his contribution to this volume, Cancro reviews and provides a synthesis of this rapidly evolving area of B cell biology. Sanz adds the perspective of human lupus with respect to the origins and function of T-bet positive cells in that context. Plasma cells and plasmablasts (collectively termed “antibody-forming cells,” AFC) are terminally differentiated cells that are generated at various points by all of the B cell response pathways. Due to either their scarcity or in the case of plasmablasts, their ephemeral nature, insights into the origins and heterogeneity of this lineage are limited compared to the importance of these cells as the definitive source of all antibody. Yet, progress is being made in terms of understanding TF networks that generate them (as discussed by both Kurosaki and Bhattacharya). In particular, Bhattacharya reviews the metabolic underpinning of plasma cell longevity, and relates this to reported metabolic states of other differentiated cells in the B lineage, including GCBC and MBC. Sanz and Cancro focus more on short-lived AFC, especially those that express T-bet. Allman again covers the spectrum of short- and long-lived AFC. Effective B cell responses carry with them a significant liability for malignant transformation. Indeed, the vast majority of lymphomas are of B cell lineage and most are thought to derive from GCBC or post-GCBC. Aside from the intense proliferation and clonal expansion associated with GCBC responses, which in itself is a risk factor for transformation, AID is strongly expressed in GCBC, mediating class switch and somatic hypermutation of IgV regions. The off-target effects of AID, as well as aberrant class switching both contribute to meaningful genome instability that can be directly traced as causative of many types of B cell lymphoma.26, 27 The mechanisms by which GC-derived lymphomas arise, and how that relates both to lymphoma heterogeneity and subtypes as well as to normal GCBC biology, are the subjects of four reviews in this volume (Amati, Casola, Melnick, and Pasqualucci). Amati and Casola focus on Myc-driven lymphomas. Amati focuses on the mechanisms by which Myc overexpression induces transformation in lymphoma. The context is with respect to various second genetic hits, especially BCL2 overexpression, that synthetically lead to lymphoma. These insights in turn have implications for treatment strategies, as Amati explains. Casola delves into the role of BCR signals in conferring fitness to Myc-driven lymphomas in both animal models and human disease. He covers the very interesting observation that BCR signals confer advantage such that lower fitness of BCR-negative lymphomas is only revealed in the face of competitor BCR-positive cells in vitro and in vivo. There are important signaling and clinical implications and correlations to this, which are covered as well as the review concludes. Melnick and Pasqualucci, on the other hand, each review a number of genetic alterations that are commonly seen in non-Hodgkin's lymphoma, covering monumental efforts by their and multiple other labs to piece together how these mutations lead to transformation. These mutations are often in genes encoding epigenetic modifiers, including those discussed by Good-Jacobson; they are expected to have pervasive effects in altering GCBC proliferation and differentiation, thus promoting transformation. Among the additional mechanisms considered by both reviews are: TF reprogramming and networks (including MYC), BCR and innate immune signals, TNFR-family signals, evasion of immune surveillance, metabolic reprogramming, blocking apoptosis (eg, BCL2), and blocking differentiation. The dissection of these various mutations and how they function in combination has profound implications for lymphoma classification, targeted therapy development and accordingly, personalized medicine. In addition to the perspective of cellular differentiation as just discussed, it is interesting to view the collection of reviews in this volume in terms of processes that they discuss (Table 1). Signaling is a major subject in reviews by Shlomchik, Shulman, Haberman in normal GCBC and by Casola, Melnick, and Pasqualucci in GCBC-derived lymphomas. TF networks are described in multiple reviews. They are covered in normal B lineage cells by Kurosaki, Melnick, Pasqualucci, Good-Jacobson, and Cancro and in malignant cells by Amati, Melnick, and Pasqualucci. Both Craft and Graca discuss how TF networks control TFH/TFR differentiation and function. Baumjohann, Good-Jacobson, Melnick, and Pasqualucci in turn focus on the related topic of microRNA and epigenetic control of cell processes. Metabolic reprogramming is another theme found across reviews and cell types. This is the central topic of Bhattacharya with emphasis on plasma cells. Amati, Casola, Melnick, and Pasqualucci all touch on this topic in GCBC-derived lymphomas, particularly as MYC has pervasive metabolic effects. Craft, in parallel, describes how metabolism can control TFH differentiation. B-T interactions, naturally, are a subject in multiple reviews, most notably: Craft, Graca, Haberman, Kurosaki, Shulman, and Qi. All of these describe the bidirectional orchestration of differentiation that occurs into and leading up to the GC reaction. Lastly, several reviews relate their basic topics to autoimmune disease. These include contributions by Sanz, Baumjohann, Cancro, Graca, and Craft that discuss immunoregulation and how that can go wrong leading to self-reactivity and disease. Taken together the reviews in this volume thus represent an up to date and in-depth cross-section of the many related topics around B cell immune responses. They are multi-layered, like the responses themselves, and overlap in terms of cellular processes and themes that should be both informative and thought-provoking. It is hoped that this short introduction can serve as a guide and set of perspectives by which the reader can approach these reviews. At a larger scale, the insights reviewed in this volume have implications for understanding basic biology, disease, immune responses, malignancy, and therapy. The authors thank Rebecca Elsner and Danny Wikenheiser of the Shlomchik lab for their useful comments. Supported by NIH grants (1R01 AI105018-05 and R01 AI043603-18) to MJS. The authors declare no conflict of interest.