Kim L. Good-Jacobson

PD-1 regulates germinal center B cell survival and the formation and affinity of long-lived plasma cells

Memory B and plasma cells (PCs) are generated in the germinal center (GC). Because follicular helper T cells (TFH cells) have high expression of the immunoinhibitory receptor PD-1, we investigated the role of PD-1 signaling in the humoral response. We found that the PD-1 ligands PD-L1 and PD-L2 were upregulated on GC B cells. Mice deficient in PD-L2 (Pdcd1lg2−/−), PD-L1 and PD-L2 (Cd274−/−Pdcd1lg2−/−) or PD-1 (Pdcd1−/−) had fewer long-lived PCs. The mechanism involved more GC cell death and less TFH cell cytokine production in the absence of PD-1; the effect was selective, as remaining PCs had greater affinity for antigen. PD-1 expression on T cells and PD-L2 expression on B cells controlled TFH cell and PC numbers. Thus, PD-1 regulates selection and survival in the GC, affecting the quantity and quality of long-lived PCs.

Plasticity and Heterogeneity in the Generation of Memory B Cells and Long-Lived Plasma Cells: The Influence of Germinal Center Interactions and Dynamics

In the humoral response, short-lived plasmablasts generate an early burst of Ab that probably plays an initial protective role. Simultaneously, another arm of the response is often triggered that leads to delayed effector function but long-term protection. This arm comprises the germinal center response and its products: long-lived memory B (Bmem) cells and plasma cells (PCs). The factors that control the differentiation of PCs and Bmem cells, as well as the composition and function of the memory compartment—how it self-renews while generating rapid secondary effector function—are poorly understood. Recent work in mice and humans is beginning to illuminate these issues. We review this progress, with emphasis on events in the germinal center, especially B–T interactions, which influence the development of memory and PC compartments and on Bmem cell heterogeneity that may underlie flexibility and self-renewal of long-lived humoral immunity.

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.

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.

CD80 Expression on B Cells Regulates Murine T Follicular Helper Development, Germinal Center B Cell Survival, and Plasma Cell Generation

Germinal center (GC) B cells and T follicular helper (TFH) cells interact in the production of high-affinity long-lived plasma cells (PCs) and memory B cells, although the mechanisms regulating the formation of these long-lived populations remain unclear. Because CD80 is one of the few markers shared by human and murine memory B cells, we investigated its role in the development of GCs, memory cells, and PCs. In CD80-deficient mice, fewer long-lived PCs were generated upon immunization compared with that in B6 controls. In concert, the absence of CD80 resulted in an increase in apoptotic GC B cells during the contraction phase of the GC. CD80−/− mice had fewer TFH cells compared with that of B6, and residual TFH cells failed to mature, with decreased ICOS and PD-1 expression and decreased synthesis of IL-21 mRNA. Mixed bone marrow chimeras demonstrated a B cell-intrinsic requirement for CD80 expression for normal TFH cell and PC development. Therefore, B cell expression of CD80 plays a critical role in regulating B–T interactions in both early and late GC responses. This, in turn, results in impaired ability to produce long-lived PCs. These data provide new insights into the development of GCs and Ab-forming cells and the functions of CD80 in humoral immunity.

Transcription Factor IRF4 Regulates Germinal Center Cell Formation through a B Cell–Intrinsic Mechanism

In response to antigenic stimulation, mature B cells interact with follicular helper T cells in specialized structures called germinal centers (GCs), which leads to the development of memory B cells and Ab-secreting plasma cells. The transcription factor IFN regulatory factor 4 (IRF4) is essential for the formation of follicular helper T cells and thus GCs, although whether IRF4 plays a distinct role in GC B cells remains contentious. RNAseq analysis on ex vivo-derived mouse B cell populations showed that Irf4 was lowly expressed in naive B cells, highly expressed in plasma cells, but absent from GC B cells. In this study, we used conditional deletion of Irf4 in mature B cells as well as wild-type and Irf4-deficient mixed bone marrow chimeric mice to investigate how and where IRF4 plays its essential role in GC formation. Strikingly, GC formation was severely impaired in mice in which Irf4 was conditionally deleted in mature B cells, after immunization with protein Ags or infection with Leishmania major. This effect was evident as early as day 5 following immunization, before the development of GCs, indicating that Irf4 was required for the development of early GC B cells. This defect was B cell intrinsic because Irf4-deficient B cells in chimeric mice failed to participate in the GC in response to L. major or influenza virus infection. Taken together, these data demonstrate a B cell–intrinsic requirement for IRF4 for not only the development of Ab secreting plasma cells but also for GC formation.

Multiple routes to B-cell memory

B-cell memory describes the populations of cells that provide long-term humoral immunity: long-lived antibody-secreting plasma cells that reside mainly in the bone marrow and memory B cells. Interestingly, the memory B-cell population is heterogenous, although the importance of this heterogeneity has been unclear. Recent studies have investigated the formation and function of memory in different settings. In particular, T-independent memory-like cells and T-dependent (TD) IgM memory B cells qualitatively differ from canonical TD class-switched memory B cells; however, these studies suggest that IgM memory cells preserve the memory population over long periods of time. These subsets are evocative of the evolution of the humoral immune response, with memory-like cells appearing before acquisition of germinal centers, suggesting that there are multiple pathways to producing B-cell memory.

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

Significance Understanding the intrinsic mechanisms underlying formation of humoral memory during infection and the ability of the humoral memory population to persist for long periods of time is important for identifying potential targets for improving the efficacy of vaccines. Here we demonstrate that the chromatin modifier MOZ regulates B-cell memory formation, controlling memory compartment composition. This activity of MOZ is B cell-intrinsic and is required for establishing the germinal center gene expression program. IgM memory B cells have been implicated in maintaining the memory B-cell population over time, whereas isotype-switched memory B cells rapidly differentiate into plasmablasts upon secondary infection. Therefore, identifying potential chromatin changes that may induce differentiation into one subset over another makes MOZ a viable target for clinical translation. 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.

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.

c-Myb is required for plasma cell migration to bone marrow after immunization or infection

The transcription factor c-Myb plays a role in establishing long-lived plasma cell populations in the bone marrow by affecting migration responses to chemokine gradients. The absence of c-Myb results in an absence of IgG+ antigen-specific plasma cells in the bone marrow following immunization or virus infection.