A B cell population that dominates the primary response to influenza virus hemagglutinin does not participate in the memory response

Germinal Centre Shutdown

Germinal Centres (GCs) are transient structures in secondary lymphoid organs, where affinity maturation of B cells takes place following an infection. While GCs are responsible for protective antibody responses, dysregulated GC reactions are associated with autoimmune disease and B cell lymphoma. Typically, ‘normal’ GCs persist for a limited period of time and eventually undergo shutdown. In this review, we focus on an important but unanswered question – what causes the natural termination of the GC reaction? In murine experiments, lack of antigen, absence or constitutive T cell help leads to premature termination of the GC reaction. Consequently, our present understanding is limited to the idea that GCs are terminated due to a decrease in antigen access or changes in the nature of T cell help. However, there is no direct evidence on which biological signals are primarily responsible for natural termination of GCs and a mechanistic understanding is clearly lacking. We discuss the present understanding of the GC shutdown, from factors impacting GC dynamics to changes in cellular interactions/dynamics during the GC lifetime. We also address potential missing links and remaining questions in GC biology, to facilitate further studies to promote a better understanding of GC shutdown in infection and immune dysregulation.

Single-cell BCR and transcriptome analysis after influenza infection reveals spatiotemporal dynamics of antigen-specific B cells

B cell responses are critical for antiviral immunity. However, a comprehensive picture of antigen-specific B cell differentiation, clonal proliferation, and dynamics in different organs after infection is lacking. Here, by combining single-cell RNA and B cell receptor (BCR) sequencing of antigen-specific cells in lymph nodes, spleen, and lungs after influenza infection in mice, we identify several germinal center (GC) B cell subpopulations and organ-specific differences that persist over the course of the response. We discover transcriptional differences between memory cells in lungs and lymphoid organs and organ-restricted clonal expansion. Remarkably, we find significant clonal overlap between GC-derived memory and plasma cells. By combining BCR-mutational analyses with monoclonal antibody (mAb) expression and affinity measurements, we find that memory B cells are highly diverse and can be selected from both low- and high-affinity precursors. By linking antigen recognition with transcriptional programming, clonal proliferation, and differentiation, these finding provide important advances in our understanding of antiviral immunity.

The Shaping of a B Cell Pool Maximally Responsive to Infections

B cell subsets differ in development, tissue distribution, and mechanisms of activation. In response to infections, however, all can differentiate into extrafollicular plasmablasts that rapidly provide highly protective antibodies, indicating that these plasmablasts are the main humoral immune response effectors. Yet, the effectiveness of this response type depends on the presence of antigen-specific precursors in the circulating mature B cell pool, a pool that is generated initially through the stochastic processes of B cell receptor assembly. Importantly, germinal centers then mold the repertoire of this B cell pool to be increasingly responsive to pathogens by generating a broad array of antimicrobial memory B cells that act as highly effective precursors of extrafollicular plasmablasts. Such B cell repertoire molding occurs in two ways: continuously via the chronic germinal centers of mucosal lymphoid tissues, driven by the presence of the microbiome, and via de novo generated germinal centers following acute infections. For effectively evaluating humoral immunity as a correlate of immune protection, it might be critical to measure memory B cell pools in addition to antibody titers.

Innate and adaptive immune responses in respiratory virus infection: implications for the clinic

INTRODUCTION

The innate immune response is the first line of defense and consists of physical, chemical and cellular defenses. The adaptive immune response is the second line of defense and is pathogen-specific. Innate immunity occurs immediately while adaptive immunity develops upon pathogen exposure, and is long-lasting, highly specific, and sustained by memory T cells. Respiratory virus infection typically induces effective immunity but over-exuberant responses are associated with pathophysiology. Cytokines expressed in response to viral infection can enhance biological responses, activate, and trigger signaling pathways leading to adaptive immunity Vaccines induce immunity, specifically B and T cell responses. Vaccination is generally efficacious, but for many viruses, our understanding of vaccination strategies and immunity is incomplete or in its infancy. Studies that examine innate and adaptive immune responses to respiratory virus infection will aid vaccine development and may reduce the burden of respiratory viral disease.

AREAS COVERED

A literature search was performed using PubMed. The search covered: innate, adaptive, respiratory virus, vaccine development, B cell, and T cell.

EXPERT OPINION

Immunity rests on two pillars, i.e. the innate and adaptive immune system, which function together on different tasks to maintain homeostasis. a better understanding of immunity is necessary for disease prevention and intervention.

Innate Immune Responses to Highly Pathogenic Coronaviruses and Other Significant Respiratory Viral Infections

The new pandemic virus SARS-CoV-2 emerged in China and spread around the world in <3 months, infecting millions of people, and causing countries to shut down public life and businesses. Nearly all nations were unprepared for this pandemic with healthcare systems stretched to their limits due to the lack of an effective vaccine and treatment. Infection with SARS-CoV-2 can lead to Coronavirus disease 2019 (COVID-19). COVID-19 is respiratory disease that can result in a cytokine storm with stark differences in morbidity and mortality between younger and older patient populations. Details regarding mechanisms of viral entry via the respiratory system and immune system correlates of protection or pathogenesis have not been fully elucidated. Here, we provide an overview of the innate immune responses in the lung to the coronaviruses MERS-CoV, SARS-CoV, and SARS-CoV-2. This review provides insight into key innate immune mechanisms that will aid in the development of therapeutics and preventive vaccines for SARS-CoV-2 infection.

B Cell Activation and Response Regulation During Viral Infections

Acute viral infections are characterized by rapid increases in viral load, leading to cellular damage and the resulting induction of complex innate and adaptive antiviral immune responses that cause local and systemic inflammation. Successful antiviral immunity requires the activation of many immune cells, including T cells, natural killer cells, and macrophages. B cells play a unique part through their production of antibodies that can both neutralize and clear viral particles before virus entry into a cell. Protective antibodies are produced even before the first exposure of a pathogen, through the regulated secretion of so-called natural antibodies that are generated even in the complete absence of prior microbial exposure. An early wave of rapidly secreted antibodies from extrafollicular (EF) responses draws on the preexisting naive or memory repertoire of B cells to induce a strong protective response that in kinetics tightly follows the clearance of acute infections, such as with influenza virus. Finally, the generation of germinal centers (GCs) provides long-term protection through production of long-lived plasma cells and memory B cells, which shape and broaden the B cell repertoire for more effective responses following repeat exposures. In this study, we review B cell responses to acute viral infections, primarily influenza virus, from the earliest nonspecific B-1 cell to early, antigen-specific EF responses and finally to GC responses. Throughout, we address known factors that lead to distinct B cell response outcomes and discuss how their functions effect viral clearance, highlighting the critical contributions of each response type to the induction of highly protective antiviral humoral immunity.

Influenza-infected newborn and adult monkeys exhibit a strong primary antibody response to hemagglutinin stem

The specificity of antibodies (Abs) generated against influenza A virus (IAV) infection can significantly alter protection and viral clearance. At present, the impact of age upon this process is relatively unexplored. Here, we evaluated the Ab response in newborn and adult African green monkeys following infection with IAV using a strain that enables us to determine the immunodominance (ID) hierarchy of the Ab response to hemagglutinin (HA), the principal target of protective Abs. This revealed altered ID patterns in the early IgM anti-HA response in newborns versus adults that converged over time. While the IgG ID profiles for HA in newborn and adult monkeys were similar, this was not the case for IgA. Importantly, HA stem-specific Abs were generated robustly and similarly in newborns and adults in terms of quality and quantity. Together, these results demonstrate that newborns and adults can differ in the Ab ID pattern established following infection and that the ID pattern can vary across isotypes. In addition, newborns have the ability to generate potent HA stem-specific Ab responses. Our findings further the understanding of the newborn response to IAV antigens and inform the development of improved vaccines for this at-risk population.

IgM, IgG, and IgA Influenza-Specific Plasma Cells Express Divergent Transcriptomes

Ab-secreting cells (ASC) or plasma cells are essential components of the humoral immune system. Although Abs of different isotypes have distinct functions, it is not known if the ASC that secrete each isotype are also distinct. ASC downregulate their surface BCR upon differentiation, hindering analyses that couple BCR information to other molecular characteristics. In this study, we developed a methodology using fixation, permeabilization, and intracellular staining coupled with cell sorting and reversal of the cross-links to allow RNA sequencing of isolated cell subsets. Using hemagglutinin and nucleoprotein Ag-specific B cell tetramers and intracellular staining for IgM, IgG, and IgA isotypes, we were able to derive and compare the gene expression programs of ASC subsets that were responding to the same Ags following influenza infection in mice. Intriguingly, whereas a shared ASC signature was identified, each ASC isotype-specific population expressed distinct transcriptional programs controlling cellular homing, metabolism, and potential effector functions. Additionally, we extracted and compared BCR clonotypes and found that each ASC isotype contained a unique, clonally related CDR3 repertoire. In summary, these data reveal specific complexities in the transcriptional programming of Ag-specific ASC populations.

The Multifaceted B Cell Response to Influenza Virus

Protection from yearly recurring, highly acute infections with a pathogen that rapidly and continuously evades previously induced protective neutralizing Abs, as seen during seasonal influenza virus infections, can be expected to require a B cell response that is too highly variable, able to adapt rapidly, and able to reduce morbidity and death when sterile immunity cannot be garnered quickly enough. As we outline in this Brief Review, the influenza-specific B cell response is exactly that: it is multifaceted, involves both innate-like and conventional B cells, provides early and later immune protection, employs B cells with distinct BCR repertoires and distinct modes of activation, and continuously adapts to the ever-changing virus while enhancing overall protection. A formidable response to a formidable pathogen.

Absence of anti-hypocretin receptor 2 autoantibodies in post pandemrix narcolepsy cases

Background

A recent publication suggested molecular mimicry of a nucleoprotein (NP) sequence from A/Puerto Rico/8/1934 (PR8) strain, the backbone used in the construction of the reassortant strain X-179A that was used in Pandemrix^® vaccine, and reported on anti-hypocretin (HCRT) receptor 2 (anti-HCRTR2) autoantibodies in narcolepsy, mostly in post Pandemrix^® narcolepsy cases (17 of 20 sera). In this study, we re-examined this hypothesis through mass spectrometry (MS) characterization of Pandemrix^®, and two other pandemic H1N1 (pH1N1)-2009 vaccines, Arepanrix^® and Focetria^®, and analyzed anti-HCRTR2 autoantibodies in narcolepsy patients and controls using three independent strategies.

Methods

MS characterization of Pandemrix^® (2 batches), Arepanrix^® (4 batches) and Focetria^® (1 batch) was conducted with mapping of NP 116I or 116M spectrogram. Two sets of narcolepsy cases and controls were used: 40 post Pandemrix^® narcolepsy (PP-N) cases and 18 age-matched post Pandemrix^® controls (PP-C), and 48 recent (≤6 months) early onset narcolepsy (EO-N) cases and 70 age-matched other controls (O-C). Anti-HCRTR2 autoantibodies were detected using three strategies: (1) Human embryonic kidney (HEK) 293T cells with transient expression of HCRTR2 were stained with human sera and then analyzed by flow cytometer; (2) In vitro translation of [³⁵S]-radiolabelled HCRTR2 was incubated with human sera and immune complexes of autoantibody and [³⁵S]-radiolabelled HCRTR2 were quantified using a radioligand-binding assay; (3) Optical density (OD) at 450 nm (OD450) of human serum immunoglobulin G (IgG) binding to HCRTR2 stably expressed in Chinese hamster ovary (CHO)-K1 cell line was measured using an in-cell enzyme-linked immunosorbent assay (ELISA).

Results

NP 116M mutations were predominantly present in all batches of Pandemrix^®, Arepanrix^® and Focetria^®. The wild-type NP_109-123 (ILYDKEEIRRIWRQA), a mimic to HCRTR2_34-45 (YDDEEFLRYLWR), was not found to bind to DQ0602. Three or four subjects were found positive for anti-HCRTR2 autoantibodies using two strategies or the third one, respectively. None of the post Pandemrix^® narcolepsy cases (0 of 40 sera) was found positive with all three strategies.

Conclusion

Anti-HCRTR2 autoantibody is not a significant biological feature of narcolepsy or of post Pandemrix^® autoimmune responses.

Defining B cell immunodominance to viruses

Immunodominance (ID) defines the hierarchical immune response to competing antigens in complex immunogens. Little is known regarding B cell and antibody ID despite its importance in immunity to viruses and other pathogens. We show that B cells and serum antibodies from inbred mice demonstrate a reproducible ID hierarchy to the five major antigenic sites in the influenza A virus hemagglutinin globular domain. The hierarchy changed as the immune response progressed, and it was dependent on antigen formulation and delivery. Passive antibody transfer and sequential infection experiments demonstrated 'original antigenic suppression', a phenomenon in which antibodies suppress memory responses to the priming antigenic site. Our study provides a template for attaining deeper understanding of antibody ID to viruses and other complex immunogens.

Complex Antigens Drive Permissive Clonal Selection in Germinal Centers

Germinal center (GC) B cells evolve toward increased affinity by a Darwinian process that has been studied primarily in genetically restricted, hapten-specific responses. We explored the population dynamics of genetically diverse GC responses to two complex antigens-Bacillus anthracis protective antigen and influenza hemagglutinin-in which B cells competed both intra- and interclonally for distinct epitopes. Preferred VH rearrangements among antigen-binding, naive B cells were similarly abundant in early GCs but, unlike responses to haptens, clonal diversity increased in GC B cells as early "winners" were replaced by rarer, high-affinity clones. Despite affinity maturation, inter- and intraclonal avidities varied greatly, and half of GC B cells did not bind the immunogen but nonetheless exhibited biased VH use, V(D)J mutation, and clonal expansion comparable to antigen-binding cells. GC reactions to complex antigens permit a range of specificities and affinities, with potential advantages for broad protection.

IL-21R signaling suppresses IL-17+ gamma delta T cell responses and production of IL-17 related cytokines in the lung at steady state and after Influenza A virus infection

Influenza A virus (IAV) infection of the respiratory tract elicits a robust immune response, which is required for efficient virus clearance but at the same time can contribute to lung damage and enhanced morbidity. IL-21 is a member of the type I cytokine family and has many different immune-modulatory functions during acute and chronic virus infections, although its role in IAV infection has not been fully evaluated. In this report we evaluated the contributions of IL-21/IL-21 receptor (IL-21R) signaling to host defense in a mouse model of primary IAV infection using IL-21R knock out (KO) mice. We found that lack of IL-21R signaling had no significant impact on virus clearance, adaptive T cell responses, or myeloid cell accumulations in the respiratory tract. However, a subset of inflammatory cytokines were elevated in the bronchoalveolar lavage fluid of IL-21R KO mice, including IL-17. Although there was only a small increase in Th17 cells in the lungs of IL-21R KO mice, we observed a dramatic increase in gamma delta (γδ) T cells capable of producing IL-17 both after IAV infection and at steady state in the respiratory tract. Finally, we found that IL-21R signaling suppressed the accumulation of IL-17+ γδ T cells in the respiratory tract intrinsically. Thus, our study reveals a previously unrecognized role of IL-21R signaling in regulating IL-17 production by γδ T cells.

Coinfection with Streptococcus pneumoniae modulates the B cell response to influenza virus

Pathogen-specific antibodies (Abs) protect against respiratory infection with influenza A virus (IAV) and Streptococcus pneumoniae and are the basis of effective vaccines. Sequential or overlapping coinfections with both pathogens are common, yet the impact of coinfection on the generation and maintenance of Ab responses is largely unknown. We report here that the B cell response to IAV is altered in mice coinfected with IAV and S. pneumoniae and that this response differs, depending on the order of pathogen exposure. In mice exposed to S. pneumoniae prior to IAV, the initial virus-specific germinal center (GC) B cell response is significantly enhanced in the lung-draining mediastinal lymph node and spleen, and there is an increase in CD4(+) T follicular helper (TFH) cell numbers. In contrast, secondary S. pneumoniae infection exaggerates early antiviral antibody-secreting cell formation, and at later times, levels of GCs, TFH cells, and antiviral serum IgG are elevated. Mice exposed to S. pneumoniae prior to IAV do not maintain the initially robust GC response in secondary lymphoid organs and exhibit reduced antiviral serum IgG with diminished virus neutralization activity a month after infection. Our data suggest that the history of pathogen exposures can critically affect the generation of protective antiviral Abs and may partially explain the differential susceptibility to and disease outcomes from IAV infection in humans. Importance: Respiratory tract coinfections, specifically those involving influenza A viruses and Streptococcus pneumoniae, remain a top global health burden. We sought to determine how S. pneumoniae coinfection modulates the B cell immune response to influenza virus since antibodies are key mediators of protection.

How specific is too specific? B-cell responses to viral infections reveal the importance of breadth over depth

Influenza virus infection induces robust and highly protective B-cell responses. Knowledge gained from the analysis of such protective humoral responses can provide important clues for the design of successful vaccines and vaccination approaches and also provides a window into the regulation of fundamental aspects of B-cell responses that may not be at play when responses to non-replicating agents are studied. Here, I review features of the B-cell response to viruses, with emphasis on influenza virus infection, a highly localized infection of respiratory tract epithelial cells, and a response that is directed against a virus that continuously undergoes genetic changes to its surface spike protein, a major target of neutralizing antibodies. Two aspects of the B-cell response to influenza are discussed here, namely polyreactive natural antibodies and the role and function of germinal center responses. Both these features of the B-cell response raise the question of how important antibody fine-specificity is for long-term protection from infection. As outlined, the pathogenesis of influenza virus and the nature of the antiviral B-cell response seem to emphasize repertoire diversity over affinity maturation as driving forces behind the influenza-specific B-cell immunity.

Protective B cell responses to flu--no fluke!

The mechanisms regulating the induction and maintenance of B lymphocytes have been delineated extensively in immunization studies using proteins and hapten-carrier systems. Increasing evidence suggests, however, that the regulation of B cell responses induced by infections is far more complex. In this study, we review the current understanding of B cell responses induced following infection with influenza virus, a small RNA virus that causes the flu. Notably, the rapidly induced, highly protective, and long-lived humoral response to this virus is contributed by multiple B cell subsets, each generating qualitatively distinct respiratory tract and systemic responses. Some B cell subsets provide extensive cross-protection against variants of the ever-mutating virus, and each is regulated by the quality and magnitude of infection-induced innate immune signals. Knowledge gained from the analysis of such highly protective humoral response might provide a blueprint for successful vaccines and vaccination approaches.

Immune complex-mediated enhancement of secondary antibody responses

Immunologic memory is a hallmark of the vertebrate immune system. The first antigenic exposure leads to a slow and modest immune response, whereas repeated exposure, even many years later, leads to a rapid and exaggerated response that is two to three orders of magnitude greater than the primary. In the case of humoral immunity, the increased efficacy of recall responses is due to the production of amplified levels of Ag-specific Ab, as well as the accelerated kinetics of their production. Current thinking suggests that this is due to selective activation of long-lived, Ag-specific memory B cells. A downside of restricting secondary responses solely to memory cells is that the repertoire of the memory B cell pool remains static while pathogens continue to evolve. In this study, we propose that during secondary responses, naive Ag-specific B cells participate alongside memory cells. We show that immune complexes formed in vivo between the Ag and pre-existing Abs from the primary response activate these naive B cells, inducing them to respond with accelerated kinetics and increased magnitude. Thus, the continued recruitment of new B cell clones after each antigenic exposure enables the immune system to stay abreast of rapidly changing pathogens.

B-cell fate decisions following influenza virus infection

Rapidly induced, specific Ab generated in extrafollicular foci are important components of early immune protection to influenza virus. The signal(s) that prompt B cells to participate in extrafollicular rather than germinal center responses are incompletely understood. To study the regulation of early B-cell differentiation events following influenza infection, we exploited earlier findings of a strong contribution of C12 idiotype-expressing B cells to the primary HA-specific response against influenza A/PR/8/34. Using an idiotype-specific mAb to C12 and labeled HA, in conjunction with multicolor flow cytometry, we followed the fate of C12Id-expressing influenza HA-specific B cells in WT BALB/c mice, requiring neither genetic manipulation nor adoptive cell transfer. Our studies demonstrate that HA-specific C12Id(+) B cells are phenotypically indistinguishable from follicular B cells. While they induced both extrafollicular and germinal center responses, extrafollicular responses were strongly predominant. Provision of increased HA-specific T-cell help increased the magnitude of the extrafollicular response, but did not shift the C12Id(+) response toward germinal center formation. Collectively the data are consistent with the hypothesis that B-cell fate determination following activation is a stochastic process in which infection-induced innate signals might drive the preferential expansion of the early extrafollicular response.

Lambda light chain revision in the human intestinal IgA response

Revision of Ab L chains by secondary rearrangement in mature B cells has the potential to change the specific target of the immune response. In this study, we show for the first time that L chain revision is normal and widespread in the largest Ab producing population in man: intestinal IgA plasma cells (PC). Biases in the productive and non-productive repertoire of lambda L chains, identification of the circular products of rearrangement that have the characteristic biases of revision, and identification of RAG genes and protein all reflect revision during normal intestinal IgA PC development. We saw no evidence of IgH revision, probably due to inappropriately orientated recombination signal sequences, and little evidence of kappa-chain revision, probably due to locus inactivation by the kappa-deleting element. We propose that the lambda L chain locus is available and a principal modifier and diversifier of Ab specificity in intestinal IgA PCs.

B cell lineage contributions to antiviral host responses

B cell responses are a major immune protective mechanism induced against a large variety of pathogens. Technical advances over the last decade, particularly in the isolation and characterization of B cell subsets by multicolor flow cytometry, have demonstrated the multifaceted nature of pathogen-induced B cell responses. In addition to participation by the major follicular B cell population, three B cell subsets are now recognized as key contributors to pathogen-induced host defenses: marginal zone (MZ) B cells, B-1a and B-1b cells. Each of these subsets seems to require unique activation signals and to react with distinct response patterns. Here we provide a brief review of the main developmental and functional features of these B cell subsets. Furthermore, we outline our current understanding of how each subset contributes to the humoral response to influenza virus infection and what regulates their differential responses. Understanding of the multilayered nature of the humoral responses to infectious agents and the complex innate immune signals that shape pathogen-specific humoral responses are likely at the heart of enhancing our ability to induce appropriate and long-lasting humoral responses for prophylaxis and therapy.

Spontaneous autoreactive memory B cell formation driven by a high frequency of autoreactive CD4+ T cells

Although somatically mutated autoantibodies are characteristic of many autoimmune diseases, the processes that can lead to their development remain poorly understood. We have examined the formation of autoreactive memory B cells in PevHA mice, which express the influenza virus PR8 hemagglutinin (HA) as a transgenic membrane bound neo-self-Ag. Using a virus immunization strategy, we show that PR8 HA-specific memory B cell formation can occur in PevHA mice, even though a major subset of PR8 HA-specific B cells is negatively selected from the primary repertoire. Moreover, PR8 HA-specific memory B cells develop spontaneously in TS1 x PevHA mice, which coexpress a transgenic PR8 HA-specific TCR and contain a high frequency of HA-specific CD4(+) T cells. Notably, autoreactive memory B cell formation occurred in TS1 x PevHA mice even though approximately half of the HA-specific CD4(+) T cells were CD25(+)Foxp3(+) cells that could significantly attenuate, but did not completely abolish HA-specific autoantibody production in an adoptive transfer setting. The findings provide evidence that a high frequency of autoreactive CD4(+) T cells can be sufficient to promote autoreactive memory B cell formation in the absence of signals provided by overt immunization or infection and despite the presence of abundant autoantigen-specific CD4(+)CD25(+)Foxp3(+) regulatory T cells.

Enhancement of neutralizing activity of influenza virus-specific antibodies by serum components

The role of serum components in enhancing virus neutralizing (VN) activity of influenza virus A/PR/8/34 hemagglutinin (HA)-specific MAbs in vitro was investigated. The degree of enhancement depended on the MAb's fine specificity and heavy chain isotype and on type of serum. Greatest enhancement (>100-fold) was seen with sera from immunodeficient mice that lacked serum immunoglobulin. At least two serum components were involved: C1q and a heat-resistant factor. C1q was mandatory for enhancement, and other components of the complement system were not required. C1q appeared to operate by improving MAb-mediated inhibition of virus attachment to host cells and was most effective with MAbs that inhibited virus attachment poorly on their own. The heat-resistant factor enhanced VN activity only in the presence of C1q and appeared to operate by enhancing VN activity at a post-attachment stage.

Specificity-Based Negative Selection of Autoreactive B Cells during Memory Formation

Autoreactive B cells are not completely purged from the primary B cell repertoire, and whether they can be prevented from maturation into memory B cells has been uncertain. We show here that a population of B cells that dominates primary immune responses of BALB/c mice to influenza virus A/PR/8/34 hemagglutinin (HA) are negatively selected in transgenic mice expressing PR8 HA as an abundant membrane-bound Ag (HACII mice). However, a separate population of B cells that contains precursors of memory B cells is activated by PR8 virus immunization and is subsequently negatively selected during the formation of the memory response. Negative selection of PR8 HA-specific B cells altered the specificity of the memory B cell response to a mutant virus containing a single amino acid substitution in a B cell epitope. Strikingly, this skewed reactivity resulted from an increase in the formation of memory B cells directed to non-self-epitopes on the mutant virus, which increased 8-fold in HACII mice relative to nontransgenic mice and precisely compensated for the absence of autoreactive PR8 HA-specific memory B cells. Negative selection of PR8 HA-specific B cells was a dominant process, since B cells from HACII mice could induce negative selection of PR8 HA-specific B cells from BALB/c mice. Lastly, HA-specific memory responses were unaffected by self-tolerance in another lineage of HA-transgenic mice (HA104 mice), indicating that the amount and/or cell type in which self-Ags are expressed can determine their ability to prevent autoreactive memory B cell formation.

Complement component C1q enhances the biological activity of influenza virus hemagglutinin-specific antibodies depending on their fine antigen specificity and heavy-chain isotype

We have previously observed that selected influenza virus hemagglutinin (HA)-specific monoclonal antibodies (MAbs) with poor virus-neutralizing (VN) activity in vitro exhibited greatly enhanced VN activity in vivo after administration to SCID mice. The same Abs displayed improved VN activity also when tested in vitro in the presence of noninactivated serum from SCID mice. To identify Ab-dependent properties and serum components that contributed to enhancement of Ab activity, we screened a large panel of HA-specific MAbs for hemagglutination inhibition (HI) in the presence of noninactivated serum from naive mice (NMS). We found that HI activity was enhanced by NMS depending on the Ab's fine specificity (antigenic region Cb/E > Ca/A,D > Sa,Sb/B), its heavy-chain isotype (immunoglobulin G2 [IgG2] > IgG3; IgG1 and IgM negative), and to some extent also on its derivation (primary response > memory response). On average, the HI activity of Cb/E-specific MAbs of the IgG2 isotype isolated from the primary response was enhanced by 20-fold. VN activity was enhanced significantly but less strongly than HI activity. Enhancement (i) was destroyed by heat inactivation (30 min, 56 degrees C); (ii) did not require C3, the central complement component; (iii) was abolished by treatment of serum with anti-C1q; and (iv) could be reproduced with purified C1q, the binding moiety of C1, the first complement component. We believe that this is the first description of a direct C1q-mediated enhancement of antiviral Ab activities.

Canonical germinal center B cells may not dominate the memory response to antigenic challenge

Spleen and bone marrow (BM) are the major sites of antibody production and anamnestic response in systemically immunized mice. We examined the VDJ segment repertoire of antibody plaque-forming cells (APFC) in those two sites in the course of antibody responses to the hapten nitrophenyl (NP). Individual IgG APFC expressed any one of 10 V(H) segments of the V186.2/V3 (J558) gene family: 186.2, 102, 23, C1H4, 165.l, CH10, 3, 593.3, 24.8 and 671.5. The majority of cells in both spleen and BM expressed the V186.2 gene joined to a D segment with Tyr95. During a 2-month period after a single immunization, the V186.2(+) APFC in BM accumulated 3 times as many somatic mutations than splenic APFC (average 8.5 versus 3 mutations/V(H)); this process was T(h) dependent as shown by in vivo depletion of CD4(+) lymphocytes. However, the V186.2(+) APFC in both spleen and BM shared a recurrent W33L replacement, indicating their common origin from germinal centers. The APFC expressing the other (analogue) V(H) segments were evenly represented in the spleen and BM, but they accumulated few, if any, mutations. The anamnestic V186.2(+) APFC were highly mutated both in the spleen and BM; they represented a new and unexpected clonotype. The V/D segments were joined by Gly95 instead of Tyr95, the W33L was absent and a new shared K58R replacement appeared. The APFC expressing the 'analogue' V(H) genes comprised approximately 20% of the anamnestic response and did not accumulate more mutations, but their affinities were in the range of the memory V186.2(+) cells. These data suggest that the late primary and secondary responses to a hapten may be born by different B cell lineages, and that some clonotypes may reach the memory pool without an extensive mutation and expansion.

Virus-induced maturation and activation of autoreactive memory B cells

We have examined B cell populations that participate in distinct phases of the immune response to the influenza virus A/PR/8/34 hemagglutinin (HA) for their susceptibility to negative selection in mice that express the HA as a neo-self-antigen (HA104 mice). We demonstrated previously that specificity for the neo-self-HA causes a population of immunoglobulin G antibody-secreting cells, which dominate the primary response to virus immunization in BALB/c mice, to be negatively selected in HA104 mice. We find here that in contrast to these primary response B cells, HA-specific memory response B cells developed equivalently in HA104 and nontransgenic (BALB/c) mice. Indeed, there was no indication that HA-specific B cells were negatively selected during memory formation in influenza virus-immunized HA104 mice, even though the neo-self-HA can be recognized by memory B cells. Furthermore, HA-specific autoantibodies were induced in the absence of virus immunization by mating HA104 mice with mice transgenic for a CD4(+) HA-specific T cell receptor. These findings indicate that specificity for a self-antigen does not prevent the maturation of autoreactive B cells in the germinal center pathway. Rather, the availability of CD4(+) T cell help may play a crucial role in regulating autoantibody responses to the HA in HA104 mice.

Contrasting the In Situ Behavior of a Memory B Cell Clone During Primary and Secondary Immune Responses

Whether memory B cells possess altered differentiative potentials and respond in a qualitatively distinct fashion to extrinsic signals as compared with their naive precursors is a current subject of debate. We have investigated this issue by examining the participation of a predominant anti-arsonate clonotype in the primary and secondary responses in the spleens of A/J mice. While this clonotype gives rise to few Ab-forming cells (AFC) in the primary response, shortly after secondary immunization its memory cell progeny produce a massive splenic IgG AFC response, largely in the red pulp. Extensive clonal expansion and migration take place during the secondary AFC response but Ab V region somatic hypermutation is not reinduced. The primary and secondary germinal center (GC) responses of this clonotype are both characterized by ongoing V gene hypermutation and phenotypic selection, little or no inter-GC migration, and derivation of multiple, spatially distinct GCs from a single progenitor. However, the kinetics of these responses differ, with V genes containing a high frequency of total as well as affinity-enhancing mutations appearing rapidly in secondary GCs, suggesting either recruitment of memory cells into this response, or accelerated rates of hypermutation and selection. In contrast, the frequency of mutation observed per V gene does not increase monotonically during the primary GC response of this clonotype, suggesting ongoing emigration of B cells that have sustained affinity- and specificity-enhancing mutations.

Reconciling Repertoire Shift with Affinity Maturation: The Role of Deleterious Mutations

The shift in Ab repertoire, from Abs dominating certain primary B cell responses to genetically unrelated Abs dominating subsequent “memory” responses, challenges the accepted paradigm of affinity maturation. We used mathematical modeling and computer simulations of the dynamics of B cell responses, hypermutation, selection, and memory cell formation to test hypotheses attempting to explain repertoire shift. We show that repertoire shift can be explained within the framework of the affinity maturation paradigm, only when we recognize the destructive nature of hypermutation: B cells with a high initial affinity for the Ag are less likely to improve through random mutations.

The Dual Phases of the Response to Neonatal Exposure to a VH Family-Restricted Staphylococcal B Cell Superantigen

In vitro studies of several naturally occurring proteins have characterized VH family-specific B lymphocyte binding and stimulatory properties that appear analogous to those of T cell superantigens. To examine the in vivo consequences of exposure to a putative B cell superantigen, we treated neonatal BALB/c mice with a form of staphylococcal protein A (MS) devoid of Fcγ binding activity, which retains the clan VHIII Fab binding specificity. In naive adults, about 5% of peripheral B cells and &gt;13% of splenic IgM-secreting cells display MS binding activity, in association with high IgM and low IgG circulating anti-MS Ab titers. Neonatal exposure to MS elicited two distinct temporal phases of immune responsiveness. The early phase, representing the first approximately 5 wk of life, was associated with MS-specific B cell and T cell tolerance. Microfluorometric assays revealed that exposure caused a dramatic MS-specific B cell clonal loss in bone marrow and spleen, but levels normalized by about 3 wk of life. The late phase (&gt;6 wk of age) was associated with spontaneous priming for MS-specific T cell responses and production of MS-specific IgG1 Abs despite long term persistently depressed in vivo and in vitro MS-specific IgM responses. In vivo challenge during the late phase induced high frequencies of MS-specific IgG-secreting cells, indicating recruitment of highly focused Ab responses that were predominantly encoded by rearrangements of the S107 family, a member of the VHIII clan. These studies document the immunodominance of the VH-restricted Fab binding site on staphylococcal protein A and demonstrate the diverse effects of a B cell superantigen on the emerging peripheral B cell compartment.

The roles of antibody variable region hypermutation and selection in the development of the memory B-cell compartment

Somatic hypermutation and selection of immunoglobulin (Ig) variable (V)-region genes, working in concert, appear to be essential for memory B-cell development in mammals. There has been substantial progress on the nature of the cis-acting DNA elements that regulate hypermutation. The data obtained suggest that the mechanisms of Ig gene hypermutation and transcription are intimately intertwined. While it has long been appreciated that stringent phenotypic selection forces are imposed on the somatically mutated Ig V regions generated during a T-cell dependent B-cell response, the mechanisms involved in this selection have remained enigmatic. Our studies have questioned the role of foreign antigen deposited on follicular dendritic cells in affinity-based positive selection of V regions, and have shown that this selection takes place in a "clone-autonomous" fashion. In addition, our data strongly suggest that affinity for antigen alone is not the driving force for selection of B-cell clones into the memory compartment. In contrast, we suggest that a combination of positive selection for increased foreign antigen binding, and negative selection of antibody V regions that are autoreactive at the onset of the response, or have acquired autoreactivity via hypermutation, results in the "specificity maturation" of the memory B-cell response.

Immune recognition of influenza hemagglutinin as a viral and a neo-self-antigen

To analyze mechanisms governing tolerance and autoimmunity to self-antigens, we have generated lineages of transgenic mice that express the influenza virus PR8 hemagglutinin (HA) as a neo-self-antigen. By comparing the HA-specific T and B cell responses that can be induced in HA Tg mice with those that are induced in non-Tg (BALB/c) mice, the specificity and genetic basis with which tolerance is induced to the HA has been examined. This article summarizes studies using lineages of HA Tg mice that express different forms and amounts of the HA under the control of the SV40 promoter/enhancer. Our studies have revealed that specific subsets of HA-specific T and B cells are negatively selected from the primary repertoires of HA Tg mice. However, substantial populations of HA-specific T and B cells evade negative selection and can be activated by virus immunization. Understanding the capacity of these autoreactive lymphocytes to differentiate and participate in antigen-specific immune responses will provide important insights into mechanisms by which autoimmunity might be induced by viruses bearing structural similarities with self-antigens.

A Periarteriolar Lymphoid Sheath-Associated B Cell Focus Response Is Not Observed During the Development of the Anti-Arsonate Germinal Center Reaction

The behavior of p-azophenylarsonate (Ars)-specific B cell clones during the primary T cell-dependent splenic response of A/J mice was investigated using an immunohistochemical approach. The earliest Ars-specific B cells were observed as isolated cells in the red pulp by day 3 after immunization with Ars-keyhole limpet hemocyanin, (KLH) and at day 6, large clusters of Ars-specific B cells were first detected in germinal centers, which continued to be observed for an additional 8 to 15 days. Surprisingly, no Ars-specific B cell foci were observed in or near the CD4 T cell-rich periarteriolar lymphoid sheath (PALS) during the entire primary response. Nevertheless, A/J mice immunized with (4-hydroxy-3-nitrophenyl)acetyl-chicken gamma globulin (NP-CGG) or Ars-CGG mounted robust splenic (4-hydroxy-3-nitrophenyl)acetyl or CGG-specific PALS-associated focus reactions, respectively. In contrast, no Ars-specific PALS B cell foci were detected in A/J mice immunized with Ars-CGG. These data add to a growing body of evidence indicating that B cell proliferation and differentiation in CD4 T cell-rich microenvironments are not prerequisites for the GC reaction. Taken together with previous results obtained using other model Ags, the data suggest that the specificity of the B cell Ag receptor may strongly influence the lymphoid microenvironment in which a B cell clone first undergoes Ag-driven clonal expansion and differentiation.

The role of germinal centers for antiviral B cell responses

Germinal centers (GCs) are crucially involved in T cell-dependent B cell responses. B cells rapidly proliferate within GCs and their Ig variable region genes undergo hypermutation. Cognate T helper cells and antigen presented in native form on follicular dendritic cells (FDCs) select B cells expressing high-affinity Igs, leading to affinity maturation and the generation of memory B cells. In addition to these well-established functions of GCs, this article presents evidence that they also play a crucial role for the maintenance of specific memory Ig titers and for the prevention of viral antibody escape mutants.

CD4+ T cells are ineffective in clearing a pulmonary infection with influenza type A virus in the absence of B cells

Recovery from influenza virus infection is dependent on T cell functions which can be provided either by CD8 or CD4 T cells. To identity the functions involved in recovery promoted by CD4 T cells, we have studied the course of the infection in B-cell deficient micro MT mice which had been depleted of CD8 T cells by antibody treatment. Upon infection with PR8 [A/PR/8/34(H1N1)], such B- and CD8 T cell-deficient mice mounted strong CD4 T cell responses that were comparable in size and cytokine secretion to those seen in intact mice. Yet, these B- and CD8 T cell-deficient mice could not clear the infection, in contrast to (CD8-depleted) mice containing both B- and CD4 T cells. These findings indicate that the promotion of the T-dependent antibody response is an indispensable component in the CD4 T cell-dependent recovery process.

Role of the B-cell response in recovery of mice from primary influenza virus infection

Recovery from influenza virus infection has long been known to require an intact T-cell compartment. More recent studies revealed that CD8 and CD4 T cells can promote recovery through independent mechanisms. The CD4 T-cell-dependent recovery process appears to operate primarily through promotion of the T-dependent antibody response as B-cell-deficient microMT mice cannot recover from infection if they have been depleted of CD8 T cells. The potential therapeutic activity of the B-cell response was further studied by transfer of antibodies into infected SCID mice. At the dose of 200 micrograms/mouse, most antibodies (of IgG2a isotype) to the viral transmembrane protein HA cured the infection, while those to the transmembrane proteins NA and M2 suppressed virus titers in the lung but failed to clear the infection. The ability of passive antibody to resolve the infection was closely related to its prophylactic activity, suggesting that neutralization of progeny virus (VN) played an important role in the process of virus clearance in vivo, while reaction of antibodies with infected host cells contributed to but was insufficient, on its own, for cure. HA-specific antibodies of IgM and IgA isotypes were therapeutically ineffective against pulmonary infection, presumably because of a preferential delivery into the upper respiratory tract, while IgG exhibited highest activity against pulmonary and minimal activity against nasal infection. B cells appear to be of similar importance for recovery from primary infection as CD8 T cells.

Neutralizing antiviral B cell responses

Neutralizing antiviral B cell responses differ in various aspects from the many usually measured B cell responses specific for protein in adjuvants. In particular, such neutralizing antiviral B cell responses are more rapidly induced, reach higher titers, are longer lived, and are efficiently generated without adjuvants. Evidence is summarized here that the repetitiveness of many viral antigens is a key factor responsible for the efficiency of these B cell responses, amplifying B cells early and rapidly for potent IgM responses and also for efficient switching to IgG. The data reviewed indicate that B cells discriminate antigen patterns via the degree of surface Ig-cross-linking and use antigen repetitiveness as a self/nonself discriminator.

Correlation between immune maturation and idiotypic network recognition

The maturation of T-dependent humoral immune responses is mediated by somatic mutations. Antigen selection is one mechanism for the activation of B cell clones which express antibodies with progressively increased affinity and which are derived as somatic variants from germ-line-encoded genes. However, the emergence of B cell clones secreting rather low-affinity antibodies and the shift to alternative germ-line V region gene combinations during secondary and tertiary responses cannot be explained by antigen selection. It has been considered that idiotypic suppression may favor this clonal shift. Such an involvement would require that idiotypic recognition in the syngeneic host must be highly restricted to private idiotopes of each clone sequentially activated during immune maturation. To test this possibility, we produced 19 syngeneic anti-idiotypic antibodies to the germ-line-encoded major Ox1 idiotype (IgM-IdOx1 H11.5) of the anti-2-phenyl-oxazolone (phOx) immune response in BALB/c mice. The fine specificity of these anti-IdOx1 was tested with a set of anti-phOx monoclonal antibodies, representing the first steps of maturation. About half of the anti-IdOx1 showed almost no reactivity with the IdOx1 after the switch to IgG and none of the anti-IdOx1 reacted with anti-phOx antibodies which carried a glycine or histidine instead of arginine as the middle amino acid of the D region. These observations suggest a strong correlation between immune maturation and the idiotypic network. A model is presented in which idiotypic suppression may function as a driving force for diversification and maturation of the antigen-induced immune response.

Activation and negative selection of functionally distinct subsets of antibody-secreting cells by influenza hemagglutinin as a viral and a neo-self antigen

We have compared transgenic mice that express the influenza virus PR8 hemagglutinin (PR8 HA) as a membrane-bound neo-self antigen (HA104 mice) with nontransgenic (non-Tg) mice for their ability to generate HA-specific B cell responses after primary immunization with PR8 virus. HA-specific, IgM-secreting B cells were induced with similar frequencies in HA104 and non-Tg mice. In addition, a B cell clonotype (C4) that is characteristic of anti-HA immune responses of BALB/c mice was identified among HA-specific IgM hybridomas from HA104 mice. A subset of HA-specific, IgG-secreting B cells that arises rapidly after primary virus immunization in non-Tg mice, however, was substantially reduced in HA104 mice. Likewise, a B cell clonotype (C12) that dominates HA-specific IgG hybridomas generated after primary immunization of non-Tg mice was present at greatly reduced frequencies among hybridomas from HA104 mice. Because HA-specific, IgG-secreting B cells were generated by HA104 mice in response to a mutant HA containing an amino acid interchange in a B cell antigenic site, we conclude that these PR8 HA-specific, IgG-secreting B cells are negatively selected in HA104 mice as a result of their specificity for the neo-self PR8 HA. The findings demonstrate that HA-specific B cells that display distinct phenotypic potentials in non-Tg mice also differ in their susceptibility to negative selection from the primary B cell repertoire of HA104 mice: a subset of B cells that undergo rapid differentiation to become HA-specific IgG antibody-secreting cells (ASC) after activation in non-Tg mice is negatively selected in HA104 mice. By contrast, a subset that gives rise to HA-specific, IgM-secreting ASC persists in the primary repertoire of HA104 mice and can be activated by virus immunization.

Virus-neutralizing antibodies of immunoglobulin G (IgG) but not of IgM or IgA isotypes can cure influenza virus pneumonia in SCID mice

The ability of monoclonal antibodies (MAbs) to passively cure an influenza virus pneumonia in the absence of endogenous T- and B-cell responses was investigated by treating C.B-17 mice, homozygous for the severe combined immunodeficiency (SCID) mutation, with individual monoclonal antiviral antibodies 1 day after pulmonary infection with influenza virus PR8 [A/PR/8/34 (H1N1)]. Less than 10% of untreated SCID mice survived the infection. By contrast, 100% of infected SCID mice that had been treated with a single intraperitoneal inoculation of at least 175 micrograms of a pool of virus-neutralizing (VN+) antihemagglutinin (anti-HA) MAbs survived, even if antibody treatment was delayed up to 7 days after infection. The use of individual MAbs showed that recovery could be achieved by VN+ anti-HA MAbs of the immunoglobulin G1 (IgG1), IgG2a, IgG2b, and IgG3 isotypes but not by VN+ anti-HA MAbs of the IgA and IgM isotypes, even if the latter were used in a chronic treatment protocol to compensate for their shorter half-lives in vivo. Both IgA and IgM, although ineffective therapeutically, protected against infection when given prophylactically, i.e., before exposure to virus. An Fc gamma-specific effector mechanism was not an absolute requirement for antibody-mediated recovery, as F(ab')2 preparations of IgGs could cure the disease, although with lesser efficacy, than intact IgG. An anti-M2 MAb of the IgG1 isotype, which was VN- but bound well to infected cells and inhibited virus growth in vitro, failed to cure. These observations are consistent with the idea that MAbs of the IgG isotype cure the disease by neutralizing all progeny virus until all productively infected host cells have died. VN+ MAbs of the IgA and IgM isotypes may be ineffective therapeutically because they do not have sufficient access to all tissue sites in which virus is produced during influenza virus pneumonia.

IgG isotype distribution of local and systemic immune responses induced by influenza virus infection

The IgG isotype profile of the influenza virus-specific immune response was studied by quantitation of serum antibody (Ab) levels in correlation with the enumeration of antibody-secreting cells (ASC) detected in the lung, spleen, mediastinal lymph nodes (MLN), Peyer's patches and bone marrow (BM). Distinct isotypic patterns for serum Ab and Ab produced by cells present at or close to the site of infection were found after primary or repeated infections. An elevated number of IgM ASC was found after primary challenge in the spleen, lung and MLN. In contrast, the site of IgA and IgG production is restricted to the lung and lymph nodes draining the site of infection. In these organs IgA, IgG2a and IgG1 ASC are found as a result of primary virus infection while viral challenge induces mostly activation of IgA-producing cells and secretion of IgA to the lung lavage. In contrast, the majority (80-90%) of Ab detected in the serum belong to the IgG2a subclass and their serum level is maintained at a high level during the whole period of the response. The relative level of virus-specific serum IgG2a in correlation with the production of IgG2a Ab found predominantly in MLN and lung is highly dependent on the viral dose used for priming or challenge. As IgG2a ASC can be detected at relatively low numbers in the spleen and BM these results suggest that the production of the dominant IgG2a isotype of serum Ab occurs close to the viral challenge site. These data, however, point to distinct isotypic regulation in systemic versus local virus-specific Ab responses.

Persistence of intestinal antibody response to heterologous rotavirus infection in a murine model beyond 1 year

We used an ELISPOT (enzyme-linked immunosorbent spot) assay to quantitate the long-term rotavirus-specific intestinal antibody response in a murine model. The frequency of murine intestinal antibody-secreting cells (ASCs) was followed for a period of 1 year after a single dose of rhesus rotavirus (10(6) PFU) was administered at 10 days of age. Some animals were boosted at that time with a second dose. One year after infection, virus-specific ASCs declined from acute-phase levels, but they were still present at significant levels (1.32 x 10(4) virus-specific ASCs per 10(6) intestinal mononuclear cells; approximately 17% of the previously reported response at 1 month after infection). A booster dose 1 year after the primary infection produced a 100% increase in virus-specific ASCs but did not restore the response to that of the primary infection.