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

A role for complement in feedback enhancement of antibody responses by IgG3

IgG1, IgG2a, and IgG2b, passively administered with soluble Ags, enhance specific Ab responses. The effect of IgG3 in this type of feedback regulation has not been studied previously. We immunized mice with trinitrophenyl (TNP)-coupled carrier proteins (bovine serum albumin [BSA] or ovalbumin [OVA]) alone or complexed to monoclonal TNP-specific IgG3. The carrier-specific Ab responses were enhanced by several hundred-fold by IgG3. Enhancement was significantly impaired in mice depleted of complement factor C3 and in mice lacking complement receptors 1 and 2 (Cr2-/-). In contrast, mice lacking the common Fc-receptor gamma chain (FcR gamma -/-), resulting in reduced expression of Fc gamma RI and lack of Fc gamma RIII, and mice lacking Fc gamma RIIB (Fc gamma RIIB-/-), responded equally well to immunization with IgG3-complexed Ag as wild-type controls. These findings demonstrate that IgG3 can induce feedback enhancement and that IgG3, in analogy with IgM, uses the complement system for this function.

B1 cells contribute to serum IgM, but not to intestinal IgA, production in gnotobiotic Ig allotype chimeric mice

B1 cells are a significant source of natural serum IgM, thereby serving as a first line of defense against systemic bacterial and viral infections. They can migrate to the intestinal lamina propria and differentiate into IgA-producing plasma cells and thus might play a similar role in mucosal immunity. To investigate the contribution of B1 cells to the intestinal IgA response induced by the commensal flora in immunocompetent animals, we generated gnotobiotic and conventionally reared Ig allotype chimeric mice. In this system B1- and B2-derived Abs can be distinguished based on different allotypes. FACS analysis of peritoneal cavity cells and analysis of B1- and B2-derived serum IgM indicated stable B1/B2 chimerism and the establishment of a functional B1 population. Monoassociation with either Morganella morganii, Bacteroides distasonis, or segmented filamentous bacteria induced germinal center reactions in Peyer's patches and led to the production of intestinal IgA, partially reactive with bacterial Ag. A considerable amount of serum IgM was B1 cell derived in both monoassociated and conventionally reared mice. However, most of the total as well as bacteria-specific intestinal IgA was produced by B2 cells. These data suggest that intestinal IgA production induced by commensal bacteria is mainly performed by B2, not B1, cells.

X-chromosome-linked immune-deficient mice have B-1b cells

The B lymphocyte subsets of X-chromosome-linked immune-deficient (XID) mice were examined by flow cytometric analyses of spleen and peritoneal cells. As shown in prior studies, young adult XID mice had reduced representation of the CD5+ (B-1a) subset in their peritoneal cavity. However, the CD11b+ (B-1b) B-cell subset was present and exhibited the IgM(hi) CD45(lo) CD23- phenotype characteristic of most B-1 cells. Although present at a lower frequency than that found in their normal counterparts, B-1b cells were evident in CBA/N and (XD2J)F1 male mice. With increasing age, B-1b cell number increased and in the oldest XID mice were present as B-cell chronic lymphocytic leukaemia. These results show that XID mice do have B-1 cells, particularly the B-1b subset.

The resolution of relapsing fever borreliosis requires IgM and is concurrent with expansion of B1b lymphocytes

The rate of pathogen clearance is a critical determinant of morbidity and mortality. We sought to characterize the immune response responsible for the remarkably rapid clearance of individual episodes of bacteremia caused by the relapsing fever bacterium, Borrelia hermsii. SCID or Rag(-/-) mice were incapable of resolving B. hermsii infection, indicating a critical role for T and/or B cells. TCR(-/-) mice, which lack T cells, and IL-7(-/-) mice, which are deficient in both T cells and follicular B cells, but not in B1 cells and splenic marginal zone (MZ) B cells, efficiently cleared B. hermsii. These findings suggested that B1 cells and/or MZ B cells, two B cell subsets that are known to participate in rapid, T-independent responses, might be involved. The efficient resolution of the episodes of moderate level bacteremia by splenectomized mice suggested that MZ B cells do not play the primary role in clearance of this bacterium. In contrast, xid mice, which are deficient in B1 cells, suffered more severe episodes of bacteremia than wild-type mice. The hypothesis that B1 cells are critical for clearance of B. hermsii was further supported by a selective expansion of the B1b (i.e., IgM(high), IgD(-/low), Mac1(+) CD23(-), and CD5(-)) cell subset in infected xid mice, which coincided with the eventual resolution of infection. Finally, mice selectively incapable of secreting IgM, the dominant isotype produced by B1 cells, were completely unable to clear B. hermsii. Together these results support the model that B1b cells generate the T-independent IgM required for the control and resolution of relapsing fever borreliosis.

A constitutively active aryl hydrocarbon receptor causes loss of peritoneal B1 cells

The dioxin/aryl hydrocarbon (Ah) receptor functions as a ligand-activated transcription factor that mediates toxicity of dioxins and related environmental pollutants. We have developed a transgenic mouse model that expresses a constitutively active Ah receptor. The immune system is one of the most sensitive target organs for dioxin toxicity and we have therefore investigated alterations of different lymphocyte populations in these mice. The population of mature bone-marrow derived B cells was enlarged, consistent with previous findings in dioxin exposed mice. In contrast, the peritoneal population of CD5-expressing B cells (B1 cells) was significantly diminished. This is the first study that demonstrates the effect of an activated Ah receptor on B1 cells. Since these cells are important mediators of innate immunity against pathogens such as Influenza virus, these results may explain the decreased resistance against infections that has been documented after dioxin exposure.

Single cell analysis of adenoid CD5+ B cells and their protective contributions to nasopharyngeal immunity

OBJECTIVES

CD5+ B cells are phenotypically and functionally distinct from the conventional (CD5-) B cells, and the function of CD5+ B cells in the upper respiratory tract remains unknown. A previous study showed that immunoglobulin A-producing cells in the adenoid play a protective role in the nasopharynx. In the present study, the contribution of adenoid CD5+ B cells to nasopharyngeal immunity at the single cell level was investigated.

STUDY DESIGN

In vitro laboratory study.

METHODS

Mononuclear cells were isolated from adenoids of children aged 1 to 12 years, and the frequency of CD5+ B cells was determined by flow cytometry. The numbers of cells producing immunoglobulin M, immunoglobulin G, and immunoglobulin A in sorted adenoid CD5+ B cells were determined by enzyme-linked immunospot assay. Further, to characterize adenoid CD5+ B cells, the expression of various surface molecules was analyzed by flow cytometry.

RESULTS

The results showed that adenoids of young children contain a relatively large number of CD5+ B cells, which have a greater capacity for antibody production than do CD5- B cells. CD5+ B cells also differed from CD5- B cells in the expression of interleukin receptors Il-4R, IL-5R, and IL-10R as well as CD27, B7-1, B7-2, Fas, and Bcl-2.

CONCLUSIONS

These findings suggest that adenoid CD5+ B cells contribute to protective immunity by forming a first line of defense in the upper respiratory tract of young children and that they are probably regulated in a manner that differs from that of CD5- B cells.

B cells and antibody play critical roles in the immediate defense of disseminated infection by West Nile encephalitis virus

West Nile virus (WNV) causes severe central nervous system (CNS) infection primarily in humans who are immunocompromised or elderly. In this study, we addressed the mechanism by which the immune system limits dissemination of WNV infection by infecting wild-type and immunodeficient inbred C57BL/6J mice with a low-passage WNV isolate from the recent epidemic in New York state. Wild-type mice replicated virus extraneuronally in the draining lymph nodes and spleen during the first 4 days of infection. Subsequently, virus spread to the spinal cord and the brain at virtually the same time. Congenic mice that were genetically deficient in B cells and antibody (microMT mice) developed increased CNS viral burdens and were vulnerable to lethal infection at low doses of virus. Notably, an approximately 500-fold difference in serum viral load was detected in micro MT mice as early as 4 days after infection, a point in the infection when low levels of neutralizing immunoglobulin M antibody were detected in wild-type mice. Passive transfer of heat-inactivated serum from infected and immune wild-type mice protected micro MT mice against morbidity and mortality. We conclude that antibodies and B cells play a critical early role in the defense against disseminated infection by WNV.

B cells and antibody play critical roles in the immediate defense of disseminated infection by West Nile encephalitis virus

West Nile virus (WNV) causes severe central nervous system (CNS) infection primarily in humans who are immunocompromised or elderly. In this study, we addressed the mechanism by which the immune system limits dissemination of WNV infection by infecting wild-type and immunodeficient inbred C57BL/6J mice with a low-passage WNV isolate from the recent epidemic in New York state. Wild-type mice replicated virus extraneuronally in the draining lymph nodes and spleen during the first 4 days of infection. Subsequently, virus spread to the spinal cord and the brain at virtually the same time. Congenic mice that were genetically deficient in B cells and antibody (microMT mice) developed increased CNS viral burdens and were vulnerable to lethal infection at low doses of virus. Notably, an approximately 500-fold difference in serum viral load was detected in micro MT mice as early as 4 days after infection, a point in the infection when low levels of neutralizing immunoglobulin M antibody were detected in wild-type mice. Passive transfer of heat-inactivated serum from infected and immune wild-type mice protected micro MT mice against morbidity and mortality. We conclude that antibodies and B cells play a critical early role in the defense against disseminated infection by WNV.

Positive selection of anti-thy-1 autoreactive B-1 cells and natural serum autoantibody production independent from bone marrow B cell development

A natural serum autoantibody specific for the Thy-1 glycoprotein (anti-Thy-1 autoantibody [ATA]) is produced by B-1 cells that are positively selected by self-antigen. Here, using ATA micro kappa transgenic mice we show that cells with this B cell receptor are negatively selected during bone marrow (BM) development. In a Thy-1 null environment, BM ATA B cells progress to a normal follicular stage in spleen. However, in a self-antigen-positive environment, development is arrested at an immature stage in the spleen, concomitant with induction of CD5. Such cells are tolerant and short-lived, different from B-1. Nonetheless, ATA-positive selection was evident by self-antigen-dependent high serum ATA production, comprising approximately 90% of serum immunoglobulin M in ATA micro kappa mice. Splenectomy did not eliminate ATA production and transfer of tolerant splenic B cells did not induce it. These findings demonstrate that B-1 positive selection, resulting in the production of natural serum ATA, arises independently from the major pathway of BM B cell development and selection.

Pathogenesis of flavivirus encephalitis

Within the flavivirus family, viruses that cause natural infections of the central nervous system (CNS) principally include members of the Japanese encephalitis virus (JEV) serogroup and the tick-borne encephalitis virus (TBEV) serocomplex. The pathogenesis of diseases involves complex interactions of viruses, which differ in neurovirulence potential, and a number of host factors, which govern susceptibility to infection and the capacity to mount effective antiviral immune responses both in the periphery and within the CNS. This chapter summarizes progress in the field of flavivirus neuropathogenesis. Mosquito-borne and tickborne viruses are considered together. Flavivirus neuropathogenesis involves both neuroinvasiveness (capacity to enter the CNS) and neurovirulence (replication within the CNS), both of which can be manipulated experimentally. Neuronal injury as a result of bystander effects may be a factor during flavivirus neuropathogenesis given that microglial activation and elaboration of inflammatory mediators, including IL-1β and TNF-α, occur in the CNS during these infections and may accompany the production of nitric oxide and peroxynitrite, which can cause neurotoxicity.

Human CD5 promotes B-cell survival through stimulation of autocrine IL-10 production

CD5 is a negative regulator of B-cell receptor (BCR) signaling that is up-regulated after BCR stimulation and likely contributes to B-cell tolerance in vivo. However, CD5 is constitutively expressed on the B-1 subset of B cells. Contrary to CD5(-) B-2 B cells, B-1 B cells are long-lived because of autocrine interleukin-10 (IL-10) production through unknown mechanisms. We demonstrate herein a direct relationship between CD5 expression and IL-10 production. Human peripheral blood CD5(+) B cells produce more IL-10 than CD5(-) B cells after BCR activation. Introducing CD5 into CD5(-) B cells induces the production of IL-10 by activating its promoter and the synthesis of its mRNA. The cytoplasmic domain of CD5 is sufficient for this process. CD5 also protects normal human B cells from apoptosis after BCR stimulation while reducing the BCR-induced Ca(2+) response. We conclude that CD5 supports the survival of B cells by stimulating IL-10 production and by concurrently exerting negative feedback on BCR-induced signaling events that can promote cell death.

Principles of tumor immunosurveillance and implications for immunotherapy

Although antigen loss variants, major histocompatibility (MHC) class I down-regulation, or the expression of inhibitory molecules may explain the failure of immunosurveillance against some tumors, this seems not to apply for many other solid peripheral or lymphohematopoietic tumors. Why then is immunosurveillance so ineffective and can it be improved? This review focuses on one important aspect of tumor immunity, namely the relevance of antigen dose and localization. Immune responses in vivo are induced in organized lymphoid tissues, i.e., in lymph nodes and spleen. The antigen dose that reaches secondary lymphoid organs over time is a crucial parameter that drives antiviral and antitumoral immune responses. Tumors use various strategies to prevent efficient presentation of their antigens in lymphoid organs. A major obstacle to the induction of an endogenous tumor-specific cytotoxic T lymphocyte (CTL) response is the inefficient presentation of tumor antigen on MHC class I molecules of professional antigen-presenting cells. Peripheral solid tumors that develop outside lymphoid organs are, therefore, often ignored by the immune system. In other situations, tumors - especially of lymphohematopoietic origin - may tolerize specific CTLs. Understanding tumor immunosurveillance is key to the design of efficient antitumor vaccines. Attempts to improve immunity to tumors include vaccination strategies to (a) provide the tumor antigen to secondary lymphoid organs using recombinant viruses or dendritic cells as carriers, (b) express costimulatory signals on tumor cells, or (c) improve the efficiency of cross-priming.

Functional activity of natural antibody is altered in Cr2-deficient mice

The major source of natural IgM Abs are B-1 cells, which differ from conventional B cells in their anatomic location, cell surface phenotype, restricted usage of particular V(H) genes and limited use of N-region addition during V-D-J rearrangement. The origin of B-1 cells is unclear. However, they are capable of self-renewal and their development is sensitive to signaling via the B cell receptor, as genetic defects that impair the strength of the signal often result in limited development. These findings suggest that B-1 cells require either an intrinsic signal, or contact with Ag, for positive selection and expansion and/or maintenance in the periphery. In support of interaction with cognate Ag, deficiency in the complement receptors CD21/CD35 results in a 30-40% decrease in the CD5(+) B-1 population. To determine whether this reduction reflects a loss of certain specificities or simply a proportional decline in the repertoire, we examined peritoneal B cells isolated from Cr2(+) and Cr2(def) mice for recognition of a B-1 cell Ag, i.e., phosphatidylcholine, and assayed for injury in an IgM natural Ab-dependent model of reperfusion injury. We found a similar frequency of phosphatidylcholine-specific CD5(+) B-1 cells in the two strains of mice. By contrast, the Cr2(def) mice have reduced injury in the IgM-dependent model of reperfusion injury. Reconstitution of the deficient mice with pooled IgM or adoptive transfer of Cr2(+) peritoneal B cells restored injury. These results suggest that complement receptors CD21/CD35 are important in maintenance of the B-1 cell repertoire to some, but not all, specificities.

B cell positive selection by soluble self-antigen

It is well established that autoreactive B cells undergo negative selection. This stands in paradox with the high frequency of so-called natural autoreactive B cells producing low affinity polyreactive autoantibodies with recurrent specificities, suggesting that these B cells are selected on the basis of their autoreactivity. We previously described two transgenic mouse lines (with and without IgD) producing a human natural autoantibody (nAAb) that binds ssDNA and human Fcgamma. In the absence of human IgG, nAAb-transgenic B cells develop normally. By crossing these mice with animals expressing knockin chimeric IgG with the human Fcgamma, we now show that the constitutive expression of chimeric IgG promotes the increase of nAAb-expressing B cells. This positive selection is critically dependent on the presence of IgD, occurs in the spleen, and concerns all mature B cell subsets, with a relative preferential enrichment of marginal zone B cells. These data support the view that soluble self-Ags can result in positive clonal selection.

Mice deficient in complement receptors 1 and 2 lack a tissue injury-inducing subset of the natural antibody repertoire

Intestinal ischemia-reperfusion (IR) injury is initiated when natural Abs recognize neoantigens that are revealed on ischemic cells. Cr2(-/-) mice, deficient in complement receptors (CR)1 and CR2, demonstrate defects in T-dependent B-2 B cell responses to foreign Ags and have also been suggested to manifest abnormalities of the B-1 subset of B lymphocytes. To determine whether these CRs might play a role in the generation of the natural Abs that initiate intestinal IR injury, we performed experiments in Cr2(-/-) and control Cr2(+/+) mice. We found that Cr2(-/-) mice did not demonstrate severe intestinal injury that was readily observed in control Cr2(+/+) mice following IR, despite having identical serum levels of IgM and IgG. Pretreatment of Cr2(-/-) mice before the ischemic phase with IgM and IgG purified from the serum of wild-type C57BL/6 mice reconstituted all key features of IR injury, demonstrating that the defect involves the failure to develop this subset of natural Abs. Pretreatment with IgM and IgG individually demonstrates that each contributes to unique features of IR injury. In sum, CR2/CR1 play an unanticipated but critical role in the development of a subset of the natural Ab repertoire that has particular importance in the pathogenesis of IR injury.

Differentially regulated expression and function of CD22 in activated B-1 and B-2 lymphocytes

CD22 is a B cell-restricted transmembrane protein that apparently controls signal transduction thresholds initiated through the B cell Ag receptor (BCR) in response to Ag. However, it is still poorly understood how the expression of CD22 is regulated in B cells after their activation. Here we show that the expression levels of CD22 in conventional B-2 cells are markedly down-regulated after cross-linking of BCR with anti-IgM mAb but are up-regulated after stimulation with LPS, anti-CD40 mAb, or IL-4. In contrast, treatment with anti-IgM mAb barely modulated the expression levels of CD22 in CD5(+) B-1 cells, consistent with a weak Ca(2+) response in anti-IgM-treated CD5(+) B-1 cells. Moreover, in CD22-deficient mice, anti-IgM treatment did not trigger enhanced Ca(2+) influx in CD5(+) B-1 cells, unlike CD22-deficient splenic B-2 cells, suggesting a relatively limited role of CD22 in BCR signaling in B-1 cells. In contrast, CD22 levels were markedly down-regulated on wild-type B-1 cells in response to LPS or unmethylated CpG-containing oligodeoxynucleotides. These data indicate that the expression and function of CD22 are differentially regulated in B-1 and conventional B-2 cells, which are apparently implicated in innate and adaptive immunity, respectively.

Origins and functions of B-1 cells with notes on the role of CD5

Whether B-1a (CD5+) cells are a distinct lineage derived from committed fetal/neonatal precursors or arise from follicular B-2 cells in response to BCR ligation and other, unknown signals remains controversial. Recent evidence indicates that B-1a cells can derive from adult precursors expressing an appropriate specificity when the (self-) antigen is present. Antibody specificity determines whether a B cell expressing immunoglobulin transgenes has a B-2, B-1a or marginal zone (MZ) phenotype. MZ cells share many phenotypic characteristics of B-1 cells and, like them, appear to develop in response to T independent type 2 antigens. Because fetal-derived B cell progenitors fail to express terminal deoxynucleotidyl transferase (TdT) and for other reasons, they are likely to express a repertoire that allows selection into the B-1a population. As it is selected by self-antigen, the B-1 repertoire tends to be autoreactive. This potentially dangerous repertoire is also useful, as B-1 cells are essential for resistance to several pathogens and they play an important role in mucosal immunity. The CD5 molecule can function as a negative regulator of BCR signaling that may help prevent inappropriate activation of autoreactive B-1a cells.

Cutting edge commentary: two B-1 or not to be one

B-1 cells differ from conventional B-2 cells both phenotypically and functionally. Two seemingly mutually exclusive hypotheses have been proposed to explain the origin of B-1 cells. The lineage hypothesis holds that certain B cell precursors are destined early on to become B-1 cells. The differentiation hypothesis holds that every B cell has the same potential to acquire B-1 characteristics. Reconsideration of previous studies of transgenic and knockout mice, plus recent results identifying differences between splenic and peritoneal B-1 cells, point to unexpected complexity in the pathway leading to B-1 status. A new paradigm is suggested, in which surface Ig signaling is required for B-1 cell production, but in which the signaling threshold and context that lead to B-1 cell development and/or expansion differ for particular B cell precursors. Surface Ig signaling may also produce receptor editing, apoptotic deletion, and tolerance induction; how these different outcomes are determined remains uncertain.

A model B-cell superantigen and the immunobiology of B lymphocytes

Recent reports have shown that protein A of Staphylococcus aureus (SpA) is a specific toxin for B cells by virtue of specific binding interactions with conserved sites on the V(H) region of the B-cell antigen receptor. The structural basis for these Fab-binding interactions has recently been revealed in crystallographic analyses, which have demonstrated many similarities with the interactions of T-cell superantigens. Investigations of the in vivo response to SpA have illustrated how a B-cell superantigen can be used to provide a window for examining fundamental principles that underlie the immunobiology of B lymphocytes.

Interleukin-10 modulates the sensitivity of peritoneal B lymphocytes to chemokines with opposite effects on stromal cell-derived factor-1 and B-lymphocyte chemoattractant

Interleukin-10 (IL-10) is constitutively produced by peritoneal B1a lymphocytes, and stromal cell-derived factor-1 (SDF-1) by mesothelial cells. Independent studies have shown that both IL-10 and SDF-1 are involved in the persistence of the peritoneal B-lymphocyte compartment. This study shows that IL-10 and SDF-1 act in synergy on peritoneal B lymphocytes. Indeed, autocrine production of IL-10 was absolutely required for all effects of SDF-1 on these cells, including increased proliferation, survival, and chemotaxis. Moreover, adding IL-10 to peritoneal B lymphocytes increased the effects of SDF-1. Neither IL-5, IL-6, nor IL-9 affected the response of peritoneal B lymphocytes to SDF-1. IL-10 was chemokinetic for peritoneal B lymphocytes, increasing their random mobility. It also potentiated the SDF-1-induced reorganization of the cytoskeleton without affecting CXCR4 gene expression by peritoneal B lymphocytes. Despite its chemokinetic properties, IL-10 abolished the migration of peritoneal B lymphocytes in response to B-lymphocyte chemoattractant (BLC), a chemokine targeting B lymphocytes to lymphoid organ follicles. The ability of B1a lymphocytes to produce IL-10 constitutively, combined with the opposite effects of this cytokine on the responses to SDF-1 and BLC, may account for the selective accumulation of B1 lymphocytes in body cavities.

CXCL13 is required for B1 cell homing, natural antibody production, and body cavity immunity

B1 cells are a predominant cell type in body cavities and an important source of natural antibody. Here we report that in mice lacking the chemokine, CXCL13, B1 cells are deficient in peritoneal and pleural cavities but not in spleen. CXCL13 is produced by cells in the omentum and by peritoneal macrophages, and in adoptive transfers, B1 cells home to the omentum and the peritoneal cavity in a CXCL13-dependent manner. CXCL13(-/-) mice are deficient in preexisting phosphorylcholine (PC)-specific antibodies and in their ability to mount an anti-PC response to peritoneal streptococcal antigen. These findings provide insight into the mechanism of B1 cell homing and establish a critical role for B1 cell compartmentalization in the production of natural antibodies and for body cavity immunity.

Complement activation in factor D-deficient mice

To assess the contribution of the alternative pathway in complement activation and host defense and its possible role in the regulation of systemic energy balance in vivo, factor D-deficient mice were generated by gene targeting. The mutant mice have no apparent abnormality in development and their body weights are similar to those of factor D-sufficient littermates. Complement activation could not be initiated in the serum of deficient mice by the alternative pathway activators rabbit erythrocytes and zymosan. Surprisingly, injection of cobra venom factor (CVF) caused a profound and reproducible reduction in serum C3 levels, whereas, as expected, there was no C3 reduction in factor B-deficient mice treated similarly. Studies of C3 and factor B activation in vitro by CVF demonstrated that in factor D-deficient serum the alpha chain of C3 was cleaved gradually over a period of 60 min without detectable cleavage of factor B. CVF-dependent C3 cleavage in the deficient serum required the presence of Mg(2+), whereas in normal mouse serum the presence of divalent cations was not required. These results suggest that in mouse proteolytic cleavage of factor B by factor D is not an absolute requirement for the zymogen to active enzyme conformational transition of CVF-bound factor B. Kinetics of opsonization of Streptococcus pneumoniae by C3 fragments was much slower in factor D-deficient serum, suggesting a significant contribution of the alternative pathway to antibacterial host defense early after infection.

Splenic and peritoneal B-1 cells differ in terms of transcriptional and proliferative features that separate peritoneal B-1 from splenic B-2 cells

B-1 cells constitute a distinct B cell subset with characteristic phenotypic and functional features. B-1 cells are highly represented among peritoneal lymphocytes; substantial numbers of B-1 cells are also located within splenic tissue. Here a number of differences in transcription factor and gene expression were identified that separate peritoneal B-1 and splenic B-2 cells, and then splenic B-1 cells obtained from immunoglobulin transgenic mice were tested for these parameters. Splenic B-1 cells resembled splenic B-2 cells rather than peritoneal B-1 cells in terms of nuclear expression of DNA-binding STAT3, CREB, and PU.1, with respect to transcriptional activation of IL-10, and in the failure to enter cell cycle in response to PMA. Splenic B-1 cells (B-1S) appear to constitute a unique population of B-1 cells, which, while sharing with peritoneal B-1 cells (B-1P) certain phenotypic features, differ from them in transcription factor and gene expression and in signaling for cell cycle progression.

Marginal zone and B1 B cells unite in the early response against T-independent blood-borne particulate antigens

The rate of pathogen elimination determines the extent and consequences of an infection. In this context, the spleen with its highly specialized lymphoid compartments plays a central role in clearing blood-borne pathogens. Splenic marginal zone B cells (MZ), by virtue of their preactivated state and topographical location, join B1 B cells to generate a massive wave of IgM producing plasmablasts in the initial 3 days of a primary response to particulate bacterial antigens. Because of the intensity and rapidity of this response, combined with the types of antibodies produced, splenic MZ and B1 B cells endowed with a "natural memory" provide a bridge between the very early innate and the later appearing adaptive immune response.

A genetic model of stress displays decreased lymphocytes and impaired antibody responses without altered susceptibility to Streptococcus pneumoniae

Stress pathways affect immune function, the most notable of these pathways being activation of the hypothalamic-pituitary-adrenal (HPA) axis. Although HPA activation has generally been relegated to an immunosuppressive role, recent evidence suggests that stress and HPA activation can be immunoenhancing in certain situations. To investigate specific effects of stress on immune function, we used a genetic model of chronic stress wherein transgenic mice overexpress corticotropin-releasing hormone (CRH), a primary mediator of the stress response. In these mice, CRH is overproduced in the brain, leading to chronic activation of the HPA axis. We found that CRH-transgenic mice have decreased leukocyte numbers in lymphoid compartments, with preferential loss of B lymphocytes. They also exhibit decreased Ab production and impaired isotype switching in response to immunization with a thymus-dependent Ag, phosphocholine-keyhole limpet hemocyanin. Despite these deficits, immunization protected CRH-transgenic and wild-type mice equally well against lethal challenge with Streptococcus pneumoniae, an encapsulated Gram-positive bacterium known to require Ab-mediated opsonization for clearance. While IgG responses are severely depressed in these mice, IgM titers are only modestly decreased. This fairly robust IgM response may be sufficient to protect against S. pneumoniae. Additionally, while total leukocyte numbers are decreased in these mice, neutrophil numbers are increased. This increase in number of neutrophils may compensate for the depressed IgG response, allowing adequate host defense during chronic stress.

B1 cells: similarities and differences with other B cell subsets

Since their discovery, B1 B cells' origins and developmental pathways have eluded characterization. In the past year, focus on B1 B cells has shifted dramatically from developmental to functional aspects of these cells. Most advances have been made in describing the physiological activities of B1 cells, including their migration, activation by antigen and role in both autoimmunity and malignancy.

Lupus-specific antiribonucleoprotein B cell tolerance in nonautoimmune mice is maintained by differentiation to B-1 and governed by B cell receptor signaling thresholds

Systemic lupus erythematosus is an autoimmune disease characterized by the presence of autoantibodies. One of the unique targets of the immune system in systemic lupus erythematosus is Sm, a ribonucleoprotein present in all cells. To understand the regulation of B cells specific to the Sm Ag in normal mice, we have generated an Ig H chain transgenic mouse (2-12H Tg). 2-12H Tg mice produce B cells specific for the Sm that remain tolerant due to ignorance. We demonstrate here that anti-Sm B cells of 2-12H Tg mice can differentiate into Sm-specific peritoneal B-1 cells that remain tolerant. Differentiation to B-1 and tolerance are governed by the strength of B cell receptor signaling, since manipulations of the B cell receptor coreceptors CD19 and CD22 affect anti-Sm B cell differentiation and autoantibody production. These results suggest a differentiation scheme in which peripheral ignorance to Sm is maintained in mice by the differentiation of anti-Sm B cells to B-1 cells that have increased activation thresholds.

Cutting edge commentary: origins of B-1 cells

The origin of B-1a cells, a minority population of B cells that express CD5, are abundant in coelomic cavities, and often produce autoantibodies, has been the subject of study for many years. Accumulating evidence demonstrates that the hypothesis that only B cells arising in fetal or neonatal tissues have the potential to become B-1a cells cannot be true. Rather, B cell receptor-mediated signaling initiated by ligation of autoantigen has now been shown to be required for induction of the B-1a phenotype. Furthermore, cells with a functional B-1a phenotype can be induced from adult precursors by appropriate Ag. At the same time, microenvironment-specific events may determine the likelihood that a given B cell, either adult or fetal derived, enters this pathway. CD5 expression and possibly localization to the peritoneum appear to provide some protection to autoreactive cells otherwise slated for elimination.

Neutralizing antiviral antibody responses

Neutralizing antibodies are evolutionarily important effectors of immunity against viruses. Their evaluation has revealed a number of basic insights into specificity, rules of reactivity (tolerance), and memory—namely, (1) Specificity of neutralizing antibodies is defined by their capacity to distinguish between virus serotypes; (2) B cell reactivity is determined by antigen structure, concentration, and time of availability in secondary lymphoid organs; and (3) B cell memory is provided by elevated protective antibody titers in serum that are depending on antigen stimulation. These perhaps slightly overstated rules are simple, correlate with in vivo evidence as well as clinical observations, and appear to largely demystify many speculations about antibodies and B cell physiology. The chapter also considers successful vaccines and compares them with those infectious diseases where efficient protective vaccines are lacking, it is striking to note that all successful vaccines induce high levels of neutralizing antibodies (nAbs) that are both necessary and sufficient to protect the host from disease. Successful vaccination against infectious diseases such as tuberculosis, leprosy, or HIV would require induction of additional long-lasting T cell responses to control infection.

Natural antibodies and complement link innate and acquired immunity

Natural or spontaneous antibodies are an essential part of the first line of defense against hematogenically spreading infections, including viruses. These antibodies target virus-antibody complexes and complement to the spleen. This prevents infections from reaching vital organs and enhances neutralizing antibody responses, particularly when the antibody is bound to a highly repetitive antigen.

Role of natural and immune IgM antibodies in immune responses

IgM antibodies constitute the major component of the natural antibodies and is also the first class of antibodies produced during a primary antibody response. The IgM-type antibodies differ from other classes of antibodies in that they are predominantly produced by B1 cells, in the absence of apparent stimulation by specific antigens. In addition, IgM antibodies are mostly encoded by germline V gene segments and have low affinities but broad specificites to both foreign and self structures. New developments regarding the function of both immune IgM antibodies and natural IgM antibodies will be examined here.