B-1 and B-2 cell-derived immunoglobulin M antibodies are nonredundant components of the protective response to influenza virus infection

Natural antibodies and cancer

The innate or natural immunity is the basis and key for all immune processes. Specific receptors on macrophages, dendrites, NK cells and natural antibodies producing B cells act as a first line defense and remove all 'foreign' and potentially harmful substances, that is, bacteria, viruses, cellular waste, modified molecules and, most importantly, cancer cells. Recognition and removal of transformed cells is a lifelong task of immune surveillance processes. Antibodies are hallmark components of this anti-cancer activity. To investigate their nature, specificity, and function, we used the human hybridoma technology for isolating antibodies from cancer patients. These were then tested with a panel of assays against cancer cell lines in vitro and in vivo. Interestingly, all the tumor-specific antibodies we found were germ-line coded and belonged nearly exclusively to the IgM class. Furthermore, they all bound to new carbohydrates on post-translationally modified cell surface receptors on malignant cells. So far no affinity maturated immunoglobulins detecting tumor-specific peptides were found. However, only the presentation of peptide motifs can create an immunological memory. In general malignant cells are detected at very early precursor stages and manifest tumors can be considered as exceptional events. In addition, malignant cells are neither infectious nor hide intracellularly like viruses and some bacteria. Therefore, it makes sense that anti-tumor immunity seems to be solely a part of the natural immunity and a memory is not needed and therefore not induced. This indicates that the tumor immunity seems to be restricted to innate immune mechanisms and the instruments used by nature, like natural antibodies, are obviously excellent therapeutics.

Expression of IL-5Ralpha on B-1 cell progenitors in mouse fetal liver and involvement of Bruton's tyrosine kinase in their development

B-1 cells are a subset of B cells responsible for the production of natural antibodies. Although the amount of natural antibody is tightly regulated, how this regulation occurs remains unknown. We examined the expression of IL-5 receptor, a cytokine receptor critical for homeostatic proliferation of B-1 cells, on B-1 cell progenitors in the fetal liver. We identified B-1 progenitors expressing low levels of IL-5 receptor alpha chain (IL-5Ralpha) and eosinophil progenitors expressing higher levels of IL-5Ralpha in the fetal liver. Moreover, the number of these B-1 progenitors were significantly reduced in the fetuses of mice deficient in Bruton's tyrosine kinase (Btk), even though IL-5 and thymic stroma lymphopoietin signaling are intact in early B lineage cells in Btk-deficient mice. These data suggest that IL-5 is possibly involved in B-1 cell development and an uncharacterized, Btk-dependent regulatory signaling pathway is involved in unexpectedly early stages of B-1 cell differentiation.

The Human Antibody Response Against WNV

Experimental evidence has shown that antibody responses to West Nile virus (WNV) are critical for protection from WNV-mediated disease. Antibody responses are also an important immune correlate of protection for the clinical evaluation of WNV vaccines. However, little direct study has been carried out on the characteristics of the human antibody response to natural WNV infection. Preliminary evidence suggests that there are important differences in the way humans and experimental animals mount humoral responses to WNV. In humans, IgM is remarkably persistent in the serum and specific IgG is slow to appear. In addition, mapping of the IgG response to the functionally relevant E-protein suggests that it directed away from critical protective epitopes and towards weakly neutralizing immunodominant epitopes. These findings have important implications for vaccine design and testing.

Heterosubtypic neutralizing monoclonal antibodies cross-protective against H5N1 and H1N1 recovered from human IgM+ memory B cells

BACKGROUND

The hemagglutinin (HA) glycoprotein is the principal target of protective humoral immune responses to influenza virus infections but such antibody responses only provide efficient protection against a narrow spectrum of HA antigenic variants within a given virus subtype. Avian influenza viruses such as H5N1 are currently panzootic and pose a pandemic threat. These viruses are antigenically diverse and protective strategies need to cross protect against diverse viral clades. Furthermore, there are 16 different HA subtypes and no certainty the next pandemic will be caused by an H5 subtype, thus it is important to develop prophylactic and therapeutic interventions that provide heterosubtypic protection.

METHODS AND FINDINGS

Here we describe a panel of 13 monoclonal antibodies (mAbs) recovered from combinatorial display libraries that were constructed from human IgM(+) memory B cells of recent (seasonal) influenza vaccinees. The mAbs have broad heterosubtypic neutralizing activity against antigenically diverse H1, H2, H5, H6, H8 and H9 influenza subtypes. Restriction to variable heavy chain gene IGHV1-69 in the high affinity mAb panel was associated with binding to a conserved hydrophobic pocket in the stem domain of HA. The most potent antibody (CR6261) was protective in mice when given before and after lethal H5N1 or H1N1 challenge.

CONCLUSIONS

The human monoclonal CR6261 described in this study could be developed for use as a broad spectrum agent for prophylaxis or treatment of human or avian influenza infections without prior strain characterization. Moreover, the CR6261 epitope could be applied in targeted vaccine strategies or in the design of novel antivirals. Finally our approach of screening the IgM(+) memory repertoire could be applied to identify conserved and functionally relevant targets on other rapidly evolving pathogens.

Dual role for B-1a cells in immunity to influenza virus infection

B-1 cells are known to contribute most of the “natural antibodies” that are secreted in the steady state, antibodies which are crucial for protection against many pathogens including influenza virus. Whether the CD5⁺ B-1a subset plays a role during an active immune response is incompletely understood. In contrast to recent data suggesting a passive role for B-1a cells, data provided here show strong highly localized activation of B-1 cells in the draining lymph nodes of the respiratory tract after influenza infection. B-1 cells are identified as a major source for both steady state and infection-induced local virus-neutralizing IgM. The CD5⁺ B-1a subset is the main B-1 cell subset generating this response. B-1a cell responses are generated by their increased local accumulation rather than by antigen-specific expansion. Our study reveals that during infection with influenza, CD5-expressing B-1a cells respond to and contribute to protection, presumably without the need for B cell receptor–mediated antigen-specific signals, which are known to induce the death of B-1a cells rather than activation. With that, our data reveal fundamental differences in the response regulation of B-1 and B-2 cells during an infection.

A novel role for non-neutralizing antibodies against nucleoprotein in facilitating resistance to influenza virus

Current influenza vaccines elicit Abs to the hemagglutinin and neuraminidase envelope proteins. Due to antigenic drift, these vaccines must be reformulated annually to include the envelope proteins predicted to dominate in the following season. By contrast, vaccination with the conserved nucleoprotein (NP) elicits immunity against multiple serotypes (heterosubtypic immunity). NP vaccination is generally thought to convey protection primarily via CD8 effector mechanisms. However, significant titers of anti-NP Abs are also induced, yet the involvement of Abs in protection has largely been disregarded. To investigate how Ab responses might contribute to heterosubtypic immunity, we vaccinated C57BL/6 mice with soluble rNP. This approach induced high titers of NP-specific serum Ab, but only poorly detectable NP-specific T cell responses. Nevertheless, rNP immunization significantly reduced morbidity and viral titers after influenza challenge. Importantly, Ab-deficient mice were not protected by this vaccination strategy. Furthermore, rNP-immune serum could transfer protection to naive hosts in an Ab-dependent manner. Therefore, Ab to conserved, internal viral proteins, such as NP, provides an unexpected, yet important mechanism of protection against influenza. These results suggest that vaccines designed to elicit optimal heterosubtypic immunity to influenza should promote both Ab and T cell responses to conserved internal proteins.

CD11c expression identifies a population of extrafollicular antigen-specific splenic plasmablasts responsible for CD4 T-independent antibody responses during intracellular bacterial infection

Although T-independent immunity is known to be generated against bacterial capsular and cell wall polysaccharides expressed by a number of bacterial pathogens, it has not been studied in depth during intracellular bacterial infections. Our previous study demonstrated that Ehrlichia muris, an obligate intracellular tick-borne pathogen, generates protective classical TI responses in CD4 T cell-deficient C57BL/6 mice. We found that E. muris T-independent immunity is accompanied by the expansion of a very large extrafollicular spleen population of CD11c(low)-expressing plasmablasts that exhibit characteristics of both B-1 and marginal zone B cells. The plasmablasts comprised up to 15% of the total spleen lymphocytes and approximately 70% of total spleen IgM(high)IgD(low) cells during peak infection in both wild-type and MHC class II-deficient mice. The CD11c(low) cells exhibited low surface expression of B220, CD19, and CD1d, high expression of CD11b, CD43, but did not express CD5. Approximately 50% of the CD11c(low) cells also expressed CD138. In addition to CD11b and CD11c, the plasmablasts expressed the beta(1) (CD29) and alpha4 (CD49d) integrins, as well as the chemokine receptor CXCR4, molecules which may play roles in localizing the B cells extrafollicular region of the spleen. During peak infection, the CD11c(low) cells accounted for the majority of the IgM-producing splenic B cells and nearly all of the E. muris outer membrane protein-specific IgM-secreting cells. Thus, during this intracellular bacterial infection, CD11c expression identifies a population of Ag-specific spleen plasmablasts responsible for T-independent Ab production.

Regulatory B and T cells in infections

The role of T and B lymphocytes, as antigen-specific effector immune cells playing an essential role in host defense against pathogens, is well recognized. Over the last decade, these lymphocytes have however also emerged as key regulatory components of the immune system, able to prevent various immunopathologies due to excessive inflammatory responses. These regulatory T (Treg) and B (Breg) cells, endowed with anti-inflammatory properties, operate via both antigen-specific and non-specific mechanisms and mainly develop during chronic infections. Here, we discuss the role of Treg and Breg lymphocytes in various infectious diseases, in experimental murine models and in human.

Alternate hypothesis on the pathogenesis of dengue hemorrhagic fever (DHF)/dengue shock syndrome (DSS) in dengue virus infection

Dengue fever, caused by infection with dengue virus, is not a new disease, but recently because of its serious emerging health threats, coupled with possible dire consequences including death, it has aroused considerable medical and public health concerns worldwide. Today, dengue is considered one of the most important arthropod-borne viral diseases in humans in terms of morbidity and mortality. Globally, it is estimated that approximate 50 to 100 million new dengue virus infections occur annually. Among these, there are 200,000 to 500,000 cases of potential life-threatening dengue hemorrhagic fever (DHF)/dengue shock syndrome (DSS), characterized by thrombocytopenia and increased vascular permeability. The death rate associated with the more severe form DHF/DSS is approximately 5%, predominantly in children under the age of 15. Although intensive efforts have been made to study the early clinical pathophysiology of dengue infection with the objective to identify the potential cause of DHF, results or data that have accumulated from different regions of the world involving studies of different ethnicity groups are inconsistent at present in terms of identifying a unified hypothesis for the pathogenesis of DHF/DSS. Thus, the potential mechanisms involved in the pathogenesis of DHF and DSS remain elusive. The purpose of this review is to identify alternate factors, such as innate immune parameters, hyper-thermal factors, conditioning of neutralizing antibody, concept of vector transmission, and physical status of virus in viremic patients that may play a role in the induction of DHF and DSS, which might have directly or indirectly contributed to the discrepancies that are noted in the literature reported to date. It is the hope that identification of an alternative explanation for the pathogenesis of DHF/DSS will pave the way for the institution of new strategies for the prevention of this complicated disease.

Genetic imprinting of autoantibody repertoires in systemic lupus erythematosus patients

Systemic lupus erythematosus (SLE) is an autoimmune disease distinguished by great heterogeneity in clinical manifestations and autoantibody expression. While only a handful of autoantibody specificities have proved useful for clinical diagnosis, to characterize complex lupus-associated autoantibody profiles more fully we have applied proteome microarray technology. Our multiplex microarrays included control ligands and 65-autoantigens, which represent diverse nuclear and cytoplasmic antigens recognized by disease-associated and natural autoantibodies. From longitudinal surveys of unrelated SLE patients, we found that autoantibody profile patterns can be patient-specific and highly stable overtime. From profiles of 38 SLE patients that included 14 sets of SLE twins, autoantibodies to the phospholipid neo-determinants, malondialdehyde (MDA) and phosphorylcholine (PC), which are exposed on apoptotic but not healthy cells, were among the most prevalent and highly expressed. We also found that immunoglobulin M (IgM) reactivity to MDA and PC ligands had significant direct correlations with DNA-containing antigens, while such a general relationship was not found with a panel of RNA-related antigens, or for IgG-autoantibodies. Significantly, hierarchical analysis revealed co-distribution/clustering of the IgM autoantibody repertoire patterns for six of 14 twin sets, and such patterns were even more common (10 of 14) for IgG autoantibody profiles. Our findings highlight the potentially distinct roles of IgM and IgG autoantibodies, as we postulate that the direct correlations for IgM autoantibodies to DNA antigens with apoptosis-related determinants may be due to co-expression arising from common pro-homeostatic protective roles. In contrast, the sharing of IgG autoantibody fingerprints by monozygotic twins suggests that lupus IgG autoantibodies can arise in predisposed individuals in genetically determined patterns.

The broad antibacterial activity of the natural antibody repertoire is due to polyreactive antibodies

Polyreactive antibodies bind to a variety of structurally unrelated antigens. The function of these antibodies, however, has remained an enigma, and because of their low binding affinity their biological relevance has been questioned. Using a panel of monoclonal polyreactive antibodies, we showed that these antibodies can bind to both Gram-negative and Gram-positive bacteria and acting through the classical complement pathway can inhibit bacterial growth by lysis, generate anaphylatoxin C5a, enhance phagocytosis, and neutralize the functional activity of endotoxin. Polyreactive antibody-enriched, but not polyreactive antibody-reduced, IgM prepared from normal human serum displays antibacterial activity similar to that of monoclonal polyreactive IgM. We conclude that polyreactive antibodies are a major contributor to the broad antibacterial activity of the natural antibody repertoire.

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.

The Bw cells, a novel B cell population conserved in the whole genus Mus

In common laboratory mouse strains, which are derived from the crossing between three subspecies, peritoneal B cells are enriched in B-1a cells characterized by the CD5(+)Mac-1(+)B220(low)IgM(high)IgD(low)CD43(+)CD9(+) phenotype. Intriguingly in other vertebrates, CD5(+)Mac-1(+) cells have never been found in a specific anatomic site. To ascertain the peculiarity of the CD5(+) peritoneal B cells in laboratory mice, we analyzed the peritoneal B cell subsets in 9 inbred and 39 outbred wild-derived mouse strains belonging to 13 different species/subspecies. We found that most of these strains do not have the CD5(+) B-1a cell population. However, all of these strains including classical laboratory mouse strains, have variable proportions of a novel B cell population: Bw, which is characterized by a unique phenotype (CD5(-)Mac-1(+)B220(high)IgM(high)IgD(high)CD43(-)CD9(-)) and is not restricted to the peritoneal cavity. Bw cells are also distinct from both B-1 and B-2 cells from a functional point of view both by proliferative responses, cytokine secretion and Ab synthesis. Moreover, transfer experiments show that bone marrow and fetal liver cells from wild mice can give rise to Bw cells in alymphoid mice. The conservation of this B cell population, but not of the CD5(+) B-1a, during evolution of the genus Mus, its readiness to respond to TLR ligands and to produce high concentration of autoantibodies suggest that Bw cells play a key role in innate immunity.

The acquisition of narrow binding specificity by polyspecific natural IgM antibodies in a semi-physiological environment

Natural IgM antibodies (Abs) play an important role in clearing pathogens, enhancing immune responses, and preventing autoimmunity. However, the molecular mechanisms that mediate the functions of natural IgM Abs are understood only to a limited degree. This shortcoming is largely due to the fact that isolated natural IgM Abs are commonly polyspecific and recognize a variety of antigens (Ags) with no apparent structural homology. It is generally believed that polyspecificity is an inherent property of natural Abs. However, there is increasing evidence that polyspecificity may be induced by mild denaturing conditions. In this study, we compared the specificity of three polyspecific IgM Abs in conventional buffers and undiluted sera deficient in immunoglobulins. All three Abs lost their polyspecificity in serum. They no longer reacted with conventional screening Ags, including hapten-BSA conjugates, ssDNA, thyroglobulin and myosin, but fully retained their reactivity with cognate peptide Ags selected from a T7 phage library. The acquisition of narrow specificity by polyspecific IgM in serum was also observed with muscle tissue sections used as a source of endogenous Ags. The loss of polyspecificity by different Abs was apparently dependent on the presence of different serum constituents. The results of this study suggest that the seemingly inherent polyspecificity of many natural IgM Abs may be largely an in vitro phenomenon related to the lack of normal serum components in the medium. Potential mechanisms underlying the loss of polyreactivity are discussed.

PD-L2 expression extends beyond dendritic cells/macrophages to B1 cells enriched for V(H)11/V(H)12 and phosphatidylcholine binding

B1 B cells are the major source of natural antibody that is essential for innate immunity. The B1 repertoire is skewed toward production of phosphatidylcholine (PtC)-binding V(H)11 and V(H)12 immunoglobulin that plays a key role in immune defense against bacterial infection. Programmed death-ligand 2 (PD-L2) is a ligand for the immunosuppressive receptor programmed death-1 (PD-1). It has been reported that expression of PD-L2 is restricted to dendritic cells and macrophages in mice. Here we show that 50-70% of resting peritoneal B1 cells express PD-L2, which is not present or inducible on conventional B2 B cells or PD-L2(-) B1 cells. Although PD-L2(+) and PD-L2(-) B1 cells are similar in proliferative responses and spontaneous immunoglobulin secretion, PD-L2(+) B1 cells are highly enriched for expression of V(H)11 and V(H)12 genes and encompass the bulk of PtC-binding B1 cells. These findings extend the range of known PD-L2 expression to B cells and show that B1 cells identified by this marker express a specific repertoire associated with anti-bacterial immunity.

Reciprocal generation of Th1/Th17 and Treg cells by B1 and B2 B cells

Regulatory T (T(reg)) cells are indispensable for maintaining peripheral tolerance, whereas T helper (Th)1 and Th17 cells induce inflammation and tissue destruction. Using Foxp3-GFP knock-in mice, we report a novel regulatory role for B cell subsets in influencing the differentiation of T(reg) versus Th1/Th17 cells. Peritoneal B1 cells strongly promoted T cell proliferation and cytokine secretion when presenting nominal or allogeneic antigens, as compared to conventional follicular B (B2) cells. However, peritoneal B1 cells largely failed to convert naive Foxp3(-)CD4(+) T cells into Foxp3(+) T(reg) cells in the presence of TGF-beta and IL-2, in marked contrast to conventional B2 cells, which excelled in T(reg) conversion. Interestingly, under the same T(reg) conversion conditions, peritoneal B1 cells preferentially promoted Th1 and Th17 cell differentiation. Blockade of CD86 but not CD80 costimulation markedly enhanced T(reg) cell induction by B1 cells. Thus, B cell antigen presentation function is inversely correlated with de novo T(reg) cell induction for these B cell subsets. Our findings suggest that B1 and B2 cell subsets play distinct roles in immune regulation by promoting reciprocal differentiation of T cell lineages.

Induction of epitope-specific neutralizing antibodies against West Nile virus

Previous studies have established that an epitope on the lateral ridge of domain III (DIII-lr) of West Nile virus (WNV) envelope (E) protein is recognized by strongly neutralizing type-specific antibodies. In contrast, an epitope against the fusion loop in domain II (DII-fl) is recognized by flavivirus cross-reactive antibodies with less neutralizing potential. Using gain- and loss-of-function E proteins and wild-type and variant WNV reporter virus particles, we evaluated the expression pattern and activity of antibodies against the DIII-lr and DII-fl epitopes in mouse and human serum after WNV infection. In mice, immunoglobulin M (IgM) antibodies to the DIII-lr epitope were detected at low levels at day 6 after infection. However, compared to IgG responses against other epitopes in DI and DII, which were readily detected at day 8, the development of IgG against DIII-lr epitope was delayed and did not appear consistently until day 15. This late time point is notable since almost all death after WNV infection in mice occurs by day 12. Nonetheless, at later time points, DIII-lr antibodies accumulated and comprised a significant fraction of the DIII-specific IgG response. In sera from infected humans, DIII-lr antibodies were detected at low levels and did not correlate with clinical outcome. In contrast, antibodies to the DII-fl were detected in all human serum samples and encompassed a significant percentage of the anti-E protein response. Our experiments suggest that the highly neutralizing DIII-lr IgG antibodies have little significant role in primary infection and that the antibody response of humans may be skewed toward the induction of cross-reactive, less-neutralizing antibodies.

Virus recognition by specific natural antibodies and complement results in MHC I cross-presentation

Natural antibodies (NAb) and complement (C’) are important regulators of immune system activation. We have shown previously that the galactosyl‐α1,3‐galactosyl (Galα1,3Gal) xenoantigen and the similar ABO histo‐blood group antigens are transferred onto virus from the producer cell, resulting in sensitisation of the virus to the respective NAb in a C’‐dependent manner. Here we show that measles virus (Mv) that expresses Galα1,3Gal termini can drive the proliferation of human T cells in the presence of serum and autologous DC, whereas without such targets, measles, as expected, suppress T cell reactivity. The use of affinity‐purified NAb to Galα1,3Gal and rabbit C’ demonstrated the components in human serum responsible for this effect. Proteasome inhibition and blocking of antigen presentation showed that the increased T cell proliferation was mediated by MHC class I cross‐presentation of immune complexes. These results lend further support to the idea that polymorphic carbohydrates of the Galα1,3Gal/ABO type serve as important targets for NAb and C’ and that their expression on virus has influenced their evolution by contributing to protection against viral transmission within as well as between species. The adjuvance effect of this recognition, acting as a bridge between the natural innate and adaptive immune systems, also has important implications for vaccine development.

Heterosubtypic immunity to influenza A virus infection requires a properly diversified antibody repertoire

Heterosubtypic immunity (HSI) is defined as cross-protection to infection with an influenza A virus serotype other than the one used for primary infection. Although HSI has been thought to be mediated by serotype cross-reactive cytotoxic T lymphocytes (CTL) that recognize conserved epitopes of structural proteins, recent studies suggest that antibodies (Abs) may make a significant contribution. In this study, we provide further evidence for the role of Abs in HSI using transgenic mice lacking terminal deoxyribonucleotidyltransferase (TdT), which adds N nucleotides to V-D and D-J junctions of the complementary determining region 3 (CDR3) (TdT(-/-)) and mice with altered Ab repertoires due to replacement of the complete locus of heavy chain diversity segments (D(H)) with an altered D(H) segment (namely, Delta D-iD). Both types of mice failed to generate complete HSI, although they were able to mount protective immunity to a homologous challenge. Lower levels of virus-specific antibodies along with more severely impaired HSI were observed in TdT(-/-) mice compared to those in Delta D-iD mice, while CTL activity remained unchanged in both types of mice. These findings indicate that a properly diversified antibody repertoire is required for HSI and that N addition by TdT is a more effective mechanism in the induction of a properly diversified antibody repertoire and, therefore, complete HSI. The results suggest that the diversity of the antibody repertoire as determined by the composition of the D region of HCDR3 and by N addition are among the mechanisms selected for in evolution to create a favorable environment to resolve infections with mutated viruses.

Early events in innate immunity in the recognition of microbial pathogens

Innate immunity is characterised by a rapid action of host effector molecules and leukocytes aimed at limiting the multiplication of invading microbial organisms and destroying them. The recognition and destruction of microorganisms involves humoral factors (e.g., the complement system and natural antibodies) and different cell types (e.g., phagocytic cells, mast cells, natural killer cells). Microbial detection by cells involves germ line-encoded pattern-recognition receptors such as Toll-like receptors and nucleotide-binding oligomerization domain-like receptors. Cellular activation by pathogens leads to the release of antimicrobial peptides (e.g., defensins and peptidoglycan recognition proteins) and cytokines that orchestrate the anti-infectious response. Cytokines enhance phagocytosis and leukocyte microbicidal activity, allow cellular recruitment into the infectious focus, boost hematopoiesis, induce fever and lead to the production of acute phase proteins.

The Protean Nature of Cells in the B Lymphocyte Lineage

The subdivision of bone marrow (BM) with surface markers and reporter systems and the use of multiple culture and transplantation assays to assess differentiation potential have led to extraordinary progress in defining stages of B lymphopoiesis between the hematopoietic stem cell and B cell receptor (BCR)-expressing lymphocytes. Despite the lack of standard nomenclature and a series of technical issues that still need to be resolved, there seems to be a general consensus regarding the major route to becoming a B cell. Nevertheless, evidence that additional, minor pathways through which B lineage cells are generated exists, and a new appreciation that lymphoid progenitors are protean and able to alter their differentiation potential during embryogenesis and after birth in response to infections suggests that a full understanding of B cell development and how it is regulated has not yet been attained.

Type I interferons protect neonates from acute inflammation through interleukin 10-producing B cells

Newborns and infants are highly susceptible to viral and bacterial infections, but the underlying mechanism remains poorly understood. We show that neonatal B cells effectively control the production of proinflammatory cytokines by both neonatal plasmacytoid and conventional dendritic cells, in an interleukin (IL) 10-dependent manner, after Toll-like receptor (TLR) 9 triggering. This antiinflammatory property of neonatal B cells may extend to other TLR agonists (Pam3CSK4, lipopolysaccharide, and R848) and viruses. In the absence of B cells or of CD5(+) B cell subsets, neonatal mice developed stronger inflammatory responses and became lethally susceptible to CpG challenge after galactosamine sensitization, whereas wild-type (WT) mice were resistant. Paradoxically, interferon (IFN)-alpha/beta enhanced the inflammatory response to CpG challenge in adult mice, whereas they helped to control neonatal acute inflammation by stimulating the secretion of IL-10 by neonatal B cells. Finally, WT neonatal B cells rescued IL-10(-/-) neonates from a lethal CpG challenge, whereas IFN-alpha/beta receptor-deficient B cells did not. Our results show that type I IFNs support a negative regulatory role of neonatal B cells on TLR-mediated inflammation, with important implications for neonatal inflammation and infection.

Autoreactivity in human IgG+ memory B cells

More than half of the nascent B cells in humans initially express autoreactive antibodies. However, most of these autoantibodies are removed from the repertoire at two checkpoints before maturation into naive B cells. A third checkpoint excludes remaining autoantibodies from the antigen-experienced IgM(+) memory B cell pool. Nevertheless, low-affinity self-reactive antibodies are frequently found in the serum of normal humans. To determine the source of these antibodies, we cloned and expressed antibodies from circulating human IgG(+) memory B cells. Surprisingly, we found that self-reactive antibodies including anti-nuclear antibodies were frequently expressed by IgG(+) memory B cells in healthy donors. Most of these antibodies were created de novo by somatic hypermutation during the transition between mature naive and IgG(+) memory B cells. We conclude that deregulation of self-reactive IgG(+) memory B cells may be associated with autoimmunity.

Natural antibody and complement mediate neutralization of influenza virus in the absence of prior immunity

Early control of virus replication by the innate immune response is essential to allow time for the generation of a more effective adaptive immune response. As an important component of innate immunity, complement has been shown to be necessary for protection against numerous microbial infections. This study was undertaken to investigate the role of complement in neutralizing influenza virus. Results demonstrated that the classical pathway of complement mediated serum neutralization of influenza virus. Although nonimmune serum neutralized influenza virus, the mechanism of virus neutralization (VN) required antibody, as sera from RAG1-deficient mice lacked VN activity; moreover, purified natural immunoglobulin M (IgM) restored VN activity to antibody-deficient sera. The mechanism of VN by natural IgM and complement was associated with virion aggregation and coating of the viral hemagglutinin receptor; however, viral lysis did not significantly contribute to VN. Additionally, reconstitution of RAG1-deficient mice with natural IgM resulted in delayed morbidity during influenza virus infection. Collectively, these results provide evidence that natural IgM and the early components of the classical pathway of complement work in concert to neutralize influenza virus and that this interaction may have a significant impact on the course of influenza viral pneumonia.

Unraveling B-1 progenitors

B-1 cells comprise a small percentage of the B lymphocytes that reside in multiple tissues in the mouse, including the peritoneal and pleural cavities. Functionally, B-1 cells participate in innate immunity by producing the majority of the natural IgM in serum, which protects against invading pathogens before the onset of the adaptive immune response. B-1 cells arise from fetal and neonatal progenitors and are distinct from the adult bone marrow progenitors that give rise to follicular and marginal zone B-2 cells. Recent studies have attempted to delineate the progenitors of B-1 cells from those of B-2 cells. Notably, the identification of CD45R(-/lo)CD19(+) B-1 progenitors and expression of two surface determinants, CD138 and major histocompatibility class II antigens, distinguish developing B-1 cells from B-2 cells.

Monolayers assembled from a glycolipid biosurfactant from Pseudozyma (Candida) antarctica serve as a high-affinity ligand system for immunoglobulin G and M

A carbohydrate ligand system has been developed which is composed of self-assembled monolayers (SAMs) of mannosylerythritol lipid-A (MEL-A) from Pseudozyma antarctica, serving for human immunoglobulin G and M (HIgG and HIgM). The estimated binding constants from surface plasmon resonance (SPR) measurement were Ka = 9.4 x 10(6) M(-1) for HIgG and 5.4 x 10(6) M(-1) for HIgM, respectively. The binding site was not in the Fc region of immunoglobulin but in the Fab region. Large amounts of HIgG and HIgM bound to MEL-A SAMs were directly observed by atomic force microscopy.

Division and differentiation of natural antibody-producing cells in mouse spleen

B-1a cells reside in both the peritoneal cavity and the spleen. LPS stimulates splenic B-1a to differentiate to plasma cells producing natural IgM specific for microbial and self antigens. However, there are conflicting views as to whether the B-1a cells divide before this differentiation occurs, and hence how the resident B-1a population is maintained in the spleen. Studies here resolve this dispute in favor of both sides: we show that (some or all) B-1a cells resident in the spleen respond to LPS by differentiating to plasma cells immediately, without dividing; however, we also show that additional B-1a cells immigrate into the spleen after LPS stimulation and divide at least once before differentiating. Importantly, the studies we presently describe reveal the complex cell migration and differentiation events that collectively underlie the rapid production of natural antibodies in response to in vivo LPS stimulation. Thus, the studies present a different view of the roles that B-1a cells play in the early phases of the innate immune response.

Innate immune sensing of pathogens and danger signals by cell surface Toll-like receptors

The Toll family of receptors consists of cell surface TLRs (TLR4/MD-2, TLR1, TLR2, and TLR6) and intracellular TLRs (TLR3, TLR7, TLR8, and TLR9). Cell surface TLRs sense microbial membranes such as lipopolysaccharide or lipopeptides. Recognition by TLRs is the frontline where pathogens and a host try to take the control of immune responses. Bacteria can modulate the structure of a TLR ligand lipid A to subvert host responses. Cell surface TLRs also sense endogenous ligands which are released in tissue damages as danger signals and induce inflammation in infectious and non-infectious condition. The availability of endogenous ligands and the amount of cell surface TLRs are both tightly limited to keep TLR responses sufficient for containment of pathogens without detrimental responses to the host.

Influenza Virus Infection Causes Global Respiratory Tract B Cell Response Modulation via Innate Immune Signals

Induction of primary B cell responses requires the presence of Ag and costimulatory signals by T cells. Innate signals further enhance B cell activation. The precise nature and kinetics of such innate immune signals and their functional effects are unknown. This study demonstrates that influenza virus-induced type I IFN is the main innate stimulus affecting local B cells within 48 h of infection. It alters the transcriptional profile of B cells and selectively traps them in the regional lymph nodes, presumably via up-regulation of CD69. Somewhat paradoxically, innate B cell stimulation inhibited the ability of regional lymph node B cells to clonally expand following BCR-mediated stimulation. This inhibition was due to IFNR-signaling independent B cell intrinsic, as well as IFNR-dependent B cell extrinsic, regulation induced following influenza infection. IFNR-mediated signals also reduced B cell migration to various chemotactic agents. Consistent with the lack of responsiveness to CCR7 ligands, unaltered or reduced expression of MHC class II and genes associated with MHC class II Ag processing/presentation and CD40, B cells were unable to induce proliferation of naive CD4 T cells. Instead, they showed increased expression of a subset of nonclassical MHC molecules that facilitate interaction with gammadelta T cells and NK T cells. We conclude that type I IFN is the main "third" B cell signal following influenza infection causing early trapping of B cells in regional lymph nodes and, at a time when cognate T cell help is rare, enhancing their propensity to interact with innate immune cells for noncognate stimulation.

Btk regulates multiple stages in the development and survival of B-1 cells

B-1 cells are important players in the first line of defense against pathogens. According to current models for the origin of B-1 cells, they either represent a separate lineage from conventional B-2 cells or differentiate from conventional B-2 cells via an intermediate, B-1(int), in response to positive selection by antigen. Here we show that Btk, a Tec family kinase that mediates B cell antigen receptor (BCR) signaling, is required at multiple stages of B-1 cell development. VH12 anti-phosphatidylcholine (PtC) IgH transgenic mice provide a model for the induced differentiation of B-1 cells. This transgene selects for PtC-reactive cells and induces them to adopt a B-1 phenotype. Both processes have been shown to depend on Btk. To determine whether this is secondary to a requirement for Btk in the development of mature B-2 cells, we crossed VH12 transgenic mice to mice expressing low levels of Btk. B-2 cell development occurs normally in Btk(lo) mice despite reduced responsiveness to BCR crosslinking. Analysis of VH12.Btk(lo) mice reveals that Btk regulates the B-1(int) to B-1 transition and/or the survival of splenic B-1 cells, in part via a mechanism independent of its role in BCR signaling. We also show that Btk mediates the survival of, and expression of IL-10 by, those B-1 cells that do develop and migrate to the peritoneum. Multiple roles for Btk in B-1 cell development and maintenance may explain the particular sensitivity of this population to mutations in components of Btk signaling pathways.

Identification of poly-reactive natural IgM antibody that recognizes late apoptotic cells and promotes phagocytosis of the cells

Natural IgM can recognize apoptotic cells, but the molecular structure and the role in macrophage phagocytosis of apoptotic cells remain unclear.

OBJECTIVES

(1) To examine the binding of previously isolated natural IgM (3B4) to apoptotic cells and its effects on phagocytosis of apoptotic cells. (2) To characterize the molecular structure of 3B4.

METHODS

3B4 binding to apoptotic thymocytes was examined by flow cytometry. Polyreactivity of 3B4 was assayed by ELISA. PKH26-labeled Macrophages were incubated with PKH67-stained apoptotic cells in the presence of 3B4. Macrophages phagocytosis of apoptotic cell was evaluated by flow cytometry. The DNA segments of 3B V(H) and V(K) were sequenced and analyzed.

RESULTS

3B4 IgM recognized late apoptotic cells. Polyreactive-recognitions of lysophosphatidylcholine (LPC) as well as some autoantigens were observed in 3B4. Phagocytosis of late apoptotic cells was increased in the presence of 3B4. The V(H) and V(K) genes of 3B4 showed a germline gene context, while N-sequences and nucleotide loss were observed in CDR3.

CONCLUSION

3B4 promotes macrophage phagocytosis of late apoptotic cells in a complement-independent process. 3B4 has a germline configuration and is possibly ligand-selected. Out experiments suggest an independent role of natural IgM as opsonin in clearance of late apoptotic cells.

Natural antibodies, autoantibodies and complement activation in tissue injury

Ischemia/reperfusion-induced tissue damage is a significant problem occurring in multiple clinical conditions. Antibodies and complement activation contribute significantly to this pathology. Mice deficient in complement receptors 1 and 2 fail to produce a component of the natural antibody repertoire that binds to ischemia-conditioned tissues and activate complement. In contrast, mice prone to autoimmunity display accelerated tissue injury that results from the binding of autoantibodies to injured tissues. The specificity and production of natural antibodies, their role in autoimmunity and the mode of complement activation are reviewed from the perspective of the processes involved in ischemia/reperfusion-induced tissue damage.

Regulation of B1 cell migration by signals through Toll-like receptors

Peritoneal B1 cells are known to generate large amounts of antibodies outside their residential site. These antibodies play an important role in the early defense against bacteria and viruses, before the establishment of adaptive immune responses. Although many stimuli, including antigen, lipopolysaccharide, or cytokines, have been shown to activate B1 cells and induce their differentiation into plasma cells, the molecular signals required for their egress from the peritoneal cavity are not understood. We demonstrate here that direct signals through Toll-like receptors (TLRs) induce specific, rapid, and transient down-regulation of integrins and CD9 on B1 cells, which is required for detachment from local matrix and a high velocity movement of cells in response to chemokines. Thus, we revealed an unexpected role for TLRs in governing the interplay between integrins, tetraspanins, and chemokine receptors required for B1 cell egress and, as such, in facilitating appropriate transition from innate to adaptive immune responses.

Antibody-mediated regulation of the immune response

Antibodies administered in vivo together with the antigen they are specific for can regulate the immune response to that antigen. This phenomenon is called antibody-mediated feedback regulation and has been known for over 100 years. Both passively administered and actively produced antibodies exert immunoregulatory functions. Feedback regulation can be either positive or negative, resulting in >1000-fold enhancement or >99% suppression of the specific antibody response. Usually, the response to the entire antigen is up- or downregulated, regardless of which epitope the regulating antibody recognizes. IgG of all isotypes can suppress responses to large particulate antigens like erythrocytes, a phenomenon used clinically in Rhesus prophylaxis. IgG suppression works in mice lacking the known Fc-gamma receptors (FcgammaR) and a likely mechanism of action is epitope masking. IgG1, IgG2a and IgG2b administered together with soluble protein antigens will enhance antibody and CD4+ T-cell responses via activating FcgammaR, probably via increased antigen presentation by dendritic cells. IgG3 as well as IgM also enhance antibody responses but their effects are dependent on their ability to activate complement. A possible mechanism is increased B-cell activation caused by immune complexes co-crosslinking the B-cell receptor with the complement-receptor 2/CD19 receptor complex, known to lower the threshold for B-cell activation. IgE-antibodies enhance antibody and CD4+ T-cell responses to small soluble proteins. This effect is entirely dependent on the low-affinity receptor for IgE, CD23, the mechanism probably being increased antigen presentation by CD23+ B cells.

B-1 B cell development

CD5+ B cells have attracted considerable interest because of their association with self-reactivity, autoimmunity, and leukemia. In mice, CD5+ B cells are readily generated from fetal/neonatal precursors, but inefficiently from precursors in adult. One model proposed to explain this difference is that their production occurs through a distinctive developmental process, termed B-1, that enriches pre-B cells with novel germline VDJs and that requires positive selection of newly formed B cells by self-Ag. In contrast, follicular B cells are generated throughout adult life in a developmental process termed B-2, selecting VDJs that pair well with surrogate L chain, and whose maturation appears relatively independent of antigenic selection. In the present study, I focus on processes that shape the repertoire of mouse CD5+ B cells, describing the differences between B-1 and B-2 development, and propose a model encompassing both in the generation of functional B cell subpopulations.

B-1 B lymphocytes require Blimp-1 for immunoglobulin secretion

B-1 B cells produce circulating natural antibodies that provide “innate-like” protection against bacterial and viral pathogens. They also provide adaptive responses to blood and air-borne pathogens. B lymphocyte–induced maturation protein 1 (Blimp-1) is a transcriptional repressor that is required for the formation of B-2–derived antibody-secreting plasma cells. In this study, we used mice lacking Blimp-1 in the B cell lineage to show that Blimp-1 is not necessary for the formation or self-renewal of B-1 B cells but that Blimp-1 is required for normal immunoglobulin (Ig) secretion by B-1 cells. B-1 cells lacking Blimp-1 do not repress Pax5 mRNA and do not induce X-box binding protein 1, and μ secreted mRNA normally, showing that B-1 and B-2 cells both use a common pathway for Ig secretion. Blimp-1–deficient B-1 B cells are also defective in providing early protection against influenza infection.

Multiple antigenic mimotopes of HIV carbohydrate antigens: relating structure and antigenicity

Carbohydrate mimetic peptides are designable, and they can carry T-cell epitopes and circumvent tolerance. A mimic-based human immunodeficiency virus (HIV) vaccine can be a viable alternative to carbohydrate-based antigens if the diversity of epitopes found on gp120 can be recapitulated. To improve existing mimics, an attempt was made to study the structural correlates of the observed polyspecificity of carbohydrate mimetic peptides based on the Y(P/R)Y motif in more detail. A carbohydrate mimetic peptide, D002 (RGGLCYCRYRYCVCVGR), bound a number of lectins with different specificities. Although this peptide reacted strongly with both lotus and concanavalin A (ConA) lectins, it bound to lotus stronger than ConA. By varying the central motif RYRY, five versions were produced in multiple antigen peptide format, and their avidity for lotus and ConA lectins was tested by surface plasmon resonance. Although the kinetic parameters were similar, the version based on the sequence YPYRY had an optimal affinity for both lectins as well as improved avidity for wheat germ agglutinin and phytohemagglutinin. Thus, as far as lectin specificity is concerned, YPYRY had improved multiple antigenic properties. Both RYRY and YPYRY precipitated antibodies from human IgG for intravenous use that bound to gp120 in vitro and immunoprecipitated gp120 from transfected CHO-PI cells. Thus, Y(P/R)Y motifs mimic multiple carbohydrate epitopes, many of which are found on HIV, and preimmune human IgG antibodies that bind to HIV carbohydrates cross-react to a comparable extent with both RYRY and YPYRY carbohydrate mimetic peptides.

Complement receptor 2, natural antibodies and innate immunity: Inter-relationships in B cell selection and activation

Complement receptor type 2 (CR2) is a receptor that serves as an important interface between the complement system and adaptive immunity. Recent studies have shown that CR2 is also centrally involved in innate immunity, and one key area is the development of potentially pathogenic natural antibodies that target neo-epitopes revealed in ischemic tissue undergoing reperfusion. Mice lacking either total immunoglobulins or CR2 alone are protected from the development of ischemia-reperfusion injury, and this effect can be reversed by introducing CR2-sufficient B-1 cells or by transferring polyclonal natural IgM antibody from wild type mice as well as monoclonal antibodies that recognize phospholipids, DNA or non-muscle myosin. We will report at the XXI ICW an additional membrane-associated protein to which pathogenic IgM antibodies are directed. Whether B cells producing these natural antibodies are differentially selected in CR2-deficient mice is as yet not well understood, and the complement-related mechanism(s) whereby this differential repertoire selection process could occur have yet to be explored in any detail. In addition to this important role in innate immunity, CR2 can also act as a receptor for other components or activators of innate immunity. One such component is interferon-alpha, an anti-viral cytokine that binds CR2 and induces a component of its mRNA signature in B cells through this receptor. Other potential CR2 ligands are DNA and DNA-containing complexes such as chromatin. The biologic role of these CR2 interactions with interferon-alpha and DNA-containing complexes is not well understood, but may be important in the development of the autoimmune disease systemic lupus erythematosus that is characterized by enhanced interferon-alpha levels and loss of self tolerance to DNA-containing self antigens.

New perspectives in B-1 B cell development and function

In contrast to the predominant population of B-2 B cells produced in the bone marrow, B-1 B cells are a minor population of B lymphocytes that are found in multiple tissues, including the pleural and peritoneal cavities in mice. Although the role of B-1 B cells as effectors of innate-like immunity is widely accepted, their developmental origin has been controversial. This review highlights recent experimental data from murine studies supporting the hypothesis that B-1 B cells belong to a developmental lineage distinct from B-2 B cells, and draws attention to recent studies that have defined new roles for the B-1a and B-1b B-cell subsets in the response to bacteria and self-antigens.

Evaluation of intranuclear BrdU detection procedures for use in multicolor flow cytometry

BACKGROUND

Measurement of cell proliferation via BrdU incorporation in combination with multicolor cell surface staining would facilitate studies on cell subsets that require multiple markers for their identification. However, the extent to which the often harsh cell preparation procedures required affect the staining quality of more recently developed fluorescent dyes has not been assessed.

METHODS

Three cell preparation protocols for BrdU measurement were compared for their ability to maintain fluorescent surface staining and scatter parameters of in vivo BrdU-labeled cells by flow cytometry. A 10-color fluorescent panel was developed to test the quality of surface staining, following cell treatment and the ability to perform BrdU measurements on even small B lymphocyte subsets.

RESULTS

All cell preparation procedures affected the quality of fluorescent and/or scatter parameters to varying degrees. Paraformaldehyde/saponin-based procedures preserved sufficient fluorescent surface staining to determine BrdU incorporation rates among all splenic B cell subsets, including B-1a cells, which constitute roughly 0.5% of cells. Turnover rates of B-1a cells were similar to immature B cells and higher than those of the other mature B cell subsets.

CONCLUSION

Paraformaldehyde/saponin-based cell preparation procedures facilitate detailed cell turnover studies on small cell subsets in vivo, revealing new functional information on rare cell populations.

Type I IFN receptor signals directly stimulate local B cells early following influenza virus infection

Rapidly developing Ab responses to influenza virus provide immune protection even during a primary infection. How these early B cell responses are regulated is incompletely understood. In this study, we show that the first direct stimulatory signal for local respiratory tract B cells during influenza virus infection is provided through the type I IFNR. IFNR-mediated signals were responsible for the influenza infection-induced local but not systemic up-regulation of CD69 and CD86 on virtually all lymph node B cells and for induction of a family of IFN-regulated genes within 48 h of infection. These direct IFNR-mediated signals were shown to affect both the magnitude and quality of the local virus-specific Ab response. Thus, ligand(s) of the type I IFNR are direct nonredundant early innate signals that regulate local antiviral B cell responses.