Distinction of the memory B cell response to cognate antigen versus bystander inflammatory signals

Reduction of CD1d expression in vivo minimally affects NKT-enhanced antibody production but boosts B-cell memory

The CD1d-binding glycolipid α-galactosylceramide exerts potent adjuvant effects on T-dependent humoral immunity. The mechanism is driven by cognate interaction between CD1d-expressing B cells and TCR-expressing type I CD1d-restricted NKT cells. Thus, far positive effects of alpha-galactosylceramide have been observed on initial and sustained antibody titers as well as B-cell memory. Following vaccination, each of these features is desirable, but good B-cell memory is of paramount importance for long-lived immunity. We therefore tested the hypothesis that CD1d expression in vivo differentially affects initial antibody titers versus B-cell memory responses. CD1d(+/+) and CD1d(+/-) mice were generated and immunized with antigen plus CD1d ligand before analysis of cytokine expression, CD40L expression, initial and longer term antibody responses and B-cell memory. As compared with CD1d(+/+) controls, CD1d(+/-) mice had equivalent numbers of total NKT cells, lower cytokine production, fewer CD40L-expressing NKT cells, lower initial antibody responses, similar long-term antibody responses and higher B-cell memory. Our data indicate that weak CD1d antigen presentation may facilitate good B-cell memory without compromising antibody responses. This work may impact vaccine design since over-stimulation of NKT cells at the time of vaccination may not lead to optimal B-cell memory.

Induction of Bordetella pertussis-specific immune memory by DTPa vaccines

Several vaccines are available against pertussis, differing by the number of Bordetella pertussis antigens that they contain as well as their formulation. The GlaxoSmithKline Biologicals (GSK Bio) tricomponent DTPa vaccine (DTPa3, Infanrix™), and the Sanofi-Pasteur (SP) five-component formulation (DTPa5, Pediacel™) were shown to have comparable short-term efficacy in clinical trials. However, potential differences in long-term protection were recently suggested, which might reflect the elicitation of different specific immune memory by the two vaccines. Therefore, the purpose of the present study was to investigate in mice the immune responses against B. pertussis, and particularly the establishment of specific B cell memory after immunization with DTPa3 and DTPa5 vaccines. Whereas intranasal challenge experiments showed similar protection with both vaccines, DTPa3 induced higher antibody levels to FHA and PRN than DTPa5. Further, the frequency of memory B cells was investigated by B cell ELISPOT. Higher frequencies of PT- and PRN-specific memory B cells were evidenced after vaccination with DTPa3, compared with DTPa5. Although the origin of such difference is unclear, the use of two different adjuvants (aluminum phosphate versus hydroxide) is proposed as a possible explanation. In conclusion, this study proposes that the induction of higher levels of B. pertussis antigen-specific memory B cells with DTPa3 participate to the suggested longer persistence of protection observed with this vaccine, as compared with DTPa5.

V-region gene analysis of locally defined synovial B and plasma cells reveals selected B cell expansion and accumulation of plasma cell clones in rheumatoid arthritis

OBJECTIVE

To elucidate the development of synovial tissue-specific B cell immune responses, the clonality of individual naive B cells, memory B cells, and plasma cells and their organization and histologic localization in the inflamed tissue were investigated in patients with rheumatoid arthritis (RA).

METHODS

B and plasma cells were isolated by laser capture microdissection (LCM) from the synovial tissue of patients with RA. In addition, single naive B cells, memory B cells, and plasma cells were sorted from synovial tissue cell suspensions. RNA was extracted from the cells, and Ig VH genes were amplified, cloned, and sequenced.

RESULTS

Both LCM and single cell sorting analyses showed that naive and memory B cells infiltrated the RA synovial tissue. Comparison of the V-gene repertoire of B and plasma cells suggested that synovial plasma cells were generated, by and large, from locally activated B cells, indicating that a selected population of memory B cells differentiates into large plasma cell clones that then accumulate in the inflamed tissue. Clonally related plasma cells were isolated from separate and distinct localized areas of the tissue, suggesting that the newly generated plasma cells have a high migratory capacity.

CONCLUSION

These results support the idea of a continuous activation of selected B cell clones, and hence a massive accumulation of plasma cells, in RA synovial tissue. As B cells and their secreted antibodies are an important factor in controlling inflammatory processes, patients with RA displaying intensive synovial tissue lymphocytic infiltrations might benefit from B cell depletion therapy. Early treatment will prevent accumulation of pathogenic plasma cells.

Unique properties of memory B cells of different isotypes

Memory antibody responses are typically seen to T-cell-dependent antigens and are characterized by the rapid production of high titers of high-affinity antigen-specific antibody. The hallmark of T-cell-dependent memory B cells is their expression of a somatically mutated, isotype-switched B-cell antigen receptor, features that are mainly generated in germinal centers. Classical studies have focused on isotype-switched memory B cells (mainly IgG isotype) and demonstrated their unique intrinsic properties in terms of localization and responsiveness to antigen re-exposure. However, recent advances in monitoring antigen-experienced B cells have revealed the considerable heterogeneity of memory B cells, which include unswitched IgM(+) and/or unmutated memory B cells. The IgM and IgG type memory B cells reside in distinct locations and appear to possess distinct origins and effector functions, together orchestrating humoral memory responses.

The immortality of humoral immunity

Decades of high-titered antibody are sustained due to the persistence of memory B cells and long-lived plasma cells (PCs). The differentiation of each of these subsets is antigen- and T-cell driven and is dependent on signals acquired and integrated during the germinal center response. Inherent in the primary immune response must be the delivery of signals to B cells to create these populations, which have virtual immortality. Differences in biology and chemotactic behavior disperse memory B cells and long-lived PCs to a spectrum of anatomic sites. Each subset must rely on survival factors that can support their longevity. This review focuses on the generation of each of these subsets, their survival, and renewal, which must occur to sustain serological memory. In this context, we discuss the role of antigen, bystander inflammation, and cellular niches. The contribution of BAFF (B-cell activating factor belonging to the tumor necrosis factor family) and APRIL (a proliferation-inducing ligand) to the persistence of memory B cells and PCs are also detailed. Insights that have been provided over the past few years in the regulation of long-lived B-cell responses will have profound impact on vaccine development, the treatment of pre-sensitized patients for organ transplantation, and therapeutic interventions in both antibody- and T-cell-mediated autoimmunity.

Mechanisms that determine plasma cell lifespan and the duration of humoral immunity

Summary:  Humoral immunity following vaccination or infection is mainly derived from two types of cells: memory B cells and plasma cells. Memory B cells do not actively secrete antibody but instead maintain their immunoglobulin in the membrane‐bound form that serves as the antigen‐specific B‐cell receptor. In contrast, plasma cells are terminally differentiated cells that no longer express surface‐bound immunoglobulin but continuously secrete antibody without requiring further antigenic stimulation. Pre‐existing serum or mucosal antibody elicited by plasma cells (or other intermediate antibody‐secreting cells) represents the first line of defense against reinfection and is critical for protection against many microbial diseases. However, the mechanisms involved with maintaining long‐term antibody production are not fully understood. Here, we examine several models of long‐term humoral immunity and present a new model, described as the ‘Imprinted Lifespan’ model of plasma cell longevity. The foundation of this model is that plasma cells are imprinted with a predetermined lifespan based on the magnitude of B‐cell signaling that occurs during the induction of an antigen‐specific humoral immune response. This represents a testable hypothesis and may explain why some antigen‐specific antibody responses fade over time whereas others are maintained essentially for life.

Stimulation and inhibition of FVIII-specific memory B-cell responses by CpG-B (ODN 1826), a ligand for Toll-like receptor 9

Factor VIII (FVIII)-specific memory B cells are essential components for regulating anamnestic antibody responses against FVIII in hemophilia A with FVIII inhibitors. We asked how stimulation and inhibition of FVIII-specific memory B cells by low and high concentrations of FVIII, respectively, are affected by concurrent activation of the innate immune system. Using CD138(-) spleen cells from hemophilic mice treated with FVIII to study restimulation and differentiation of memory B cells in vitro, we tested modulating activities of agonists for Toll-like receptors (TLRs) 2, 3, 4, 5, 7, and 9. Ligands for TLR7 and 9 were most effective. They not only amplified FVIII-specific memory responses in the presence of stimulating concentrations of FVIII, but also countered inhibition in the presence of inhibitory concentrations of FVIII. Notably, CpG oligodeoxynucleotide (CpG-ODN), a ligand for TLR9, expressed biphasic effects. It amplified memory responses at low concentrations and inhibited memory responses at high concentrations, both in vitro and in vivo. Both stimulatory and inhibitory activities of CpG-ODN resulted from specific interactions with TLR9. Despite their strong immunomodulatory effects in the presence of FVIII, ligands for TLR induced negligible restimulation in the absence of FVIII in vitro and no restimulation in the absence of FVIII in vivo.

Preferential localization of IgG memory B cells adjacent to contracted germinal centers

It has long been presumed that after leaving the germinal centers (GCs), memory B cells colonize the marginal zone or join the recirculating pool. Here we demonstrate the preferential localization of nitrophenol-chicken gamma-globulin-induced CD38(+)IgG1(+) memory B cells adjacent to contracted GCs in the spleen. The memory B cells in this region proliferated after secondary immunization, a response that was abolished by depletion of CD4(+) T cells. We also found that these IgG1(+) memory B cells could present antigen on their surface, and that this activity was required for their activation. These results implicate this peri-GC region as an important site for survival and reactivation of memory B cells.

The Plasmodium falciparum-specific human memory B cell compartment expands gradually with repeated malaria infections

Immunity to Plasmodium falciparum (Pf) malaria is only acquired after years of repeated infections and wanes rapidly without ongoing parasite exposure. Antibodies are central to malaria immunity, yet little is known about the B-cell biology that underlies the inefficient acquisition of Pf-specific humoral immunity. This year-long prospective study in Mali of 185 individuals aged 2 to 25 years shows that Pf-specific memory B-cells and antibodies are acquired gradually in a stepwise fashion over years of repeated Pf exposure. Both Pf-specific memory B cells and antibody titers increased after acute malaria and then, after six months of decreased Pf exposure, contracted to a point slightly higher than pre-infection levels. This inefficient, stepwise expansion of both the Pf-specific memory B-cell and long-lived antibody compartments depends on Pf exposure rather than age, based on the comparator response to tetanus vaccination that was efficient and stable. These observations lend new insights into the cellular basis of the delayed acquisition of malaria immunity.

Plasmodium falciparum (Pf) is a mosquito-borne parasite that causes over 500 million cases of malaria annually, one million of which result in death, primarily among African children. The development of an effective malaria vaccine would be a critical step toward the control and eventual elimination of this disease. To date, most licensed vaccines are for pathogens that induce long-lived protective antibodies after a single infection. In contrast, immunity to malaria is only acquired after repeated infections. Antibodies play a key role in protection from malaria, yet several studies indicate that antibodies against some Pf proteins are generated inefficiently and lost rapidly. The cells that are responsible for the maintenance of antibodies over the human lifespan are memory B-cells and long-lived plasma cells. To determine how these cells are generated and maintained in response to Pf infection, we conducted a year-long study in an area of Mali that experiences a six-month malaria season. We found memory B-cells and long-lived antibodies specific for the parasite were generated in a gradual, step-wise fashion over years despite intense Pf exposure. This contrasts sharply with the efficient response to tetanus vaccination in the same population. This study lends new insights into the delayed acquisition of malaria immunity. Future studies of the cellular and molecular basis of these observations could open the door to strategies for the development of a highly effective malaria vaccine.

Use of a virus-encoded enzymatic marker reveals that a stable fraction of memory B cells expresses latency-associated nuclear antigen throughout chronic gammaherpesvirus infection

An integral feature of gammaherpesvirus infections is the ability to establish lifelong latency in B cells. During latency, the viral genome is maintained as an extrachomosomal episome, with stable maintenance in dividing cells mediated by the viral proteins Epstein-Barr nuclear antigen 1 (EBNA-1) for Epstein-Barr virus and latency-associated nuclear antigen (LANA) for Kaposi's sarcoma-associated herpesvirus. It is believed that the expression of episome maintenance proteins is turned off in the predominant long-term latency reservoir of resting memory B cells, suggesting that chronic gammaherpesvirus infection is primarily dormant. However, the kinetics of LANA/EBNA-1 expression in individual B-cell subsets throughout a course of infection has not been examined. The infection of mice with murine gammaherpesvirus 68 (MHV68, gammaHV68) provides a model to determine the specific cellular and molecular events that occur in vivo during lifelong gammaherpesvirus latency. In work described here, we make use of a heterologously expressed enzymatic marker to define the types of B cells that express the LANA homolog (mLANA) during chronic MHV68 infection. Our data demonstrate that mLANA is expressed in a stable fraction of B cells throughout chronic infection, with a prominent peak at 28 days. The expression of mLANA was detected in naïve follicular B cells, germinal-center B cells, and memory B cells throughout infection, with germinal-center and memory B cells accounting for more than 80% of the mLANA-expressing cells during the maintenance phase of latency. These findings suggest that the maintenance phase of latency is an active process that involves the ongoing proliferation or reseeding of latently infected memory B cells.

Requirement for cyclin D3 in germinal center formation and function

Germinal centers (GC) of secondary lymphoid tissues are critical to mounting a high-affinity humoral immune response. B cells within the GC undergo rapid clonal expansion and selection while diversifying their antibody genes. Although it is generally believed that GC B cells employ a unique proliferative program to accommodate these processes, little is known about how the GC-associated cell cycle is orchestrated. The D-type cyclins constitute an important component of the cell cycle engine that enables the cells to respond to physiological changes. Cell type- and developmental stage-specific roles of D-type cyclins have been described but the cyclin D requirement during GC reaction has not been addressed. In this study, we report that cyclin D3 is largely dispensable for proliferation and Ig class switching of in vitro activated B cells. In contrast, GC development in Ccnd3(-/-) mice is markedly impaired, as is the T cell-dependent antibody response. Within the GC, although both switched and unswitched B cells are affected by cyclin D3 inactivation, the IgM(-) pool is more severely reduced. Interestingly, despite a compensatory increase in cyclin D2 expression, a significant number of Ccnd3(-/-) GC B cells accumulate in quiescent G0 state. Lastly, although cyclin D3 inactivation did not disrupt BCL6 expression in GC B cells, it completely blocked the GC promoting effect of BCL6 overexpression, suggesting that cyclin D3 acts downstream of BCL6 to regulate GC formation. This is the first demonstration that cyclin D3 plays an important and unique role at the GC stage of B cell development.

Long-lived antibody and B Cell memory responses to the human malaria parasites, Plasmodium falciparum and Plasmodium vivax

Antibodies constitute a critical component of the naturally acquired immunity that develops following frequent exposure to malaria. However, specific antibody titres have been reported to decline rapidly in the absence of reinfection, supporting the widely perceived notion that malaria infections fail to induce durable immunological memory responses. Currently, direct evidence for the presence or absence of immune memory to malaria is limited. In this study, we analysed the longevity of both antibody and B cell memory responses to malaria antigens among individuals who were living in an area of extremely low malaria transmission in northern Thailand, and who were known either to be malaria naïve or to have had a documented clinical attack of P. falciparum and/or P. vivax in the past 6 years. We found that exposure to malaria results in the generation of relatively avid antigen-specific antibodies and the establishment of populations of antigen-specific memory B cells in a significant proportion of malaria-exposed individuals. Both antibody and memory B cell responses to malaria antigens were stably maintained over time in the absence of reinfection. In a number of cases where antigen-specific antibodies were not detected in plasma, stable frequencies of antigen-specific memory B cells were nonetheless observed, suggesting that circulating memory B cells may be maintained independently of long-lived plasma cells. We conclude that infrequent malaria infections are capable of inducing long-lived antibody and memory B cell responses.