In vitro complement-dependent binding and in vivo kinetics of pneumococcal polysaccharide TI-2 antigens in the rat spleen marginal zone and follicle

Novel Mouse Model Reveals That Serine Phosphorylation of L-Plastin Is Essential for Effective Splenic Clearance of Pneumococcus

Asplenia imparts susceptibility to life-threatening sepsis with encapsulated bacteria, such as the pneumococcus. However, the cellular components within the splenic environment that guard against pneumococcal bacteremia have not been defined. The actin-bundling protein L-plastin (LPL) is essential for the generation of marginal zone B cells and for anti-pneumococcal host defense, as revealed by a mouse model of genetic LPL deficiency. In independent studies, serine phosphorylation of LPL at residue 5 (S5) has been described as a key "switch" in regulating LPL actin binding and subsequent cell motility, although much of the data are correlative. To test the importance of S5 phosphorylation in LPL function, and to specifically assess the requirement of LPL S5 phosphorylation in anti-pneumococcal host defense, we generated the "S5A" mouse, expressing endogenous LPL bearing a serine-to-alanine mutation at this position. S5A mice were bred to homozygosity, and LPL was expressed at levels equivalent to wild-type, but S5 phosphorylation was absent. S5A mice exhibited specific impairment in clearance of pneumococci following i.v. challenge, with 10-fold-higher bacterial bloodstream burden 24 h after challenge compared with wild-type or fully LPL-deficient animals. Defective bloodstream clearance correlated with diminished population of marginal zone macrophages and with reduced phagocytic capacity of multiple innate immune cells. Development and function of other tested leukocyte lineages, such as T and B cell motility and activation, were normal in S5A mice. The S5A mouse thus provides a novel system in which to elucidate the precise molecular control of critical immune cell functions in specific host-pathogen defense interactions.

B cell ADAM17 controls T cell independent humoral immune responses through regulation of TACI and CD138

ADAM17 is known to contribute to the immune system through its shedding of tumor necrosis factor alpha (TNFα). However, the role of ADAM17 in B cell biology is not well characterized. To determine whether B cell ADAM17 contributes to T cell-independent humoral immune responses, we crossed CD19 Cre transgenic mice with mice bearing a floxed allele of ADAM17 (ADAM17^CD19). In this study, we show a B cell intrinsic role for ADAM17 in regulating marginal zone B cell (MZB) numbers in mice. Interestingly, we demonstrate that the loss of B cell ADAM17 results in reduced MZB numbers in the naïve state and after immunization with T-independent antigen, yet enhanced humoral immunity to T cell independent antigens. We additionally find elevated TACI and CD138 levels on plasma cells following immunization in ADAM17^CD19 mice. Overall, these findings suggest that B cell ADAM17 may orchestrate T independent immune responses through both MZB numbers and plasma cell antibody production.

Altered Marginal Zone B Cell Selection in the Absence of IκBNS

Marginal zone (MZ) B cells reside in the splenic MZ and play important roles in T cell-independent humoral immune responses against blood-borne pathogens. IκBNS-deficient bumble mice exhibit a severe reduction in the MZ B compartment but regain an MZ B population with age and, thus, represent a valuable model to examine the biology of MZ B cells. In this article, we characterized the MZ B cell defect in further detail and investigated the nature of the B cells that appear in the MZ of aged bumble mice. Flow cytometry analysis of the splenic transitional B cell subsets demonstrated that MZ B cell development was blocked at the transitional-1 to transitional-2-MZ precursor stage in the absence of functional IκBNS. Immunohistochemical analysis of spleen sections from wild-type and bumble mice revealed no alteration in the cellular MZ microenvironment, and analysis of bone marrow chimeras indicated that the MZ B cell development defect in bumble mice was B cell intrinsic. Further, we demonstrate that the B cells that repopulate the MZ in aged bumble mice were distinct from age-matched wild-type MZ B cells. Specifically, the expression of surface markers characteristic for MZ B cells was altered and the L chain Igλ⁺ repertoire was reduced in bumble mice. Finally, plasma cell differentiation of sorted LPS-stimulated MZ B cells was impaired, and aged bumble mice were unable to respond to NP-Ficoll immunization. These results demonstrate that IκBNS is required for an intact MZ B cell compartment in C57BL/6 mice.

Host Resistance Assays

The goal of immunotoxicity testing is to obtain data useful for immunotoxicity safety assessment. Guidance in the performance of immunotoxicity safety evaluations is provided in documents from the US EPA for chemicals and the ICH S8 document for pharmaceuticals. The ICH S8 document outlines a tiered approach that includes (1) standard toxicity studies with associated hematology, immune system organ weights, and histopathology data; (2) functional assays, such as cytotoxic T lymphocyte (CTL) assays, natural killer (NK) cell assays, respiratory burst, phagocytosis, and T-cell-dependent antibody response (TDAR) assays; and (3) host resistance assays. Host resistance assays are considered the gold standard in immunotoxicity testing and provide a critical overview of the extent to which innate, adaptive, and homeostatic regulatory immune functions are integrated to protect the host. Both comprehensive and targeted host resistance assays are available, each with distinct advantages. This chapter serves to provide a general overview of the various assays that may be used, as well as a summary of procedures.

Emerging Role of the Spleen in the Pharmacokinetics of Monoclonal Antibodies, Nanoparticles and Exosomes

After being absorbed, drugs distribute in the body in part to reach target tissues, in part to be disposed in tissues where they do not exert clinically-relevant effects. Therapeutically-relevant effects are usually terminated by drug metabolism and/or elimination. The role that has been traditionally ascribed to the spleen in these fundamental pharmacokinetic processes was definitely marginal. However, due to its high blood flow and to the characteristics of its microcirculation, this organ would be expected to be significantly exposed to large, new generation drugs that can hardly penetrate in other tissues with tight endothelial barriers. In the present review, we examine the involvement of the spleen in the disposition of monoclonal antibodies, nanoparticles and exosomes and the possible implications for their therapeutic efficacy and toxicity. The data that we will review lead to the conclusion that a new role is emerging for the spleen in the pharmacokinetics of new generation drugs, hence suggesting that this small, neglected organ will certainly deserve stronger attention by pharmacologists in the future.

PD-1 suppresses protective immunity to Streptococcus pneumoniae through a B cell-intrinsic mechanism

Despite the emergence of the programmed cell death 1 (PD-1):PD-1 ligand (PD-L) regulatory axis as a promising target for treating multiple human diseases, remarkably little is known about how this pathway regulates responses to extracellular bacterial infections. We found that PD-1(-/-) mice, as well as wild-type mice treated with a PD-1 blocking Ab, exhibited significantly increased survival against lethal Streptococcus pneumoniae infection following either priming with low-dose pneumococcal respiratory infection or S. pneumoniae-capsular polysaccharide immunization. Enhanced survival in mice with disrupted PD-1:PD-L interactions was explained by significantly increased proliferation, isotype switching, and IgG production by pneumococcal capsule-specific B cells. Both PD-L, B7-H1 and B7-DC, contributed to PD-1-mediated suppression of protective capsule-specific IgG. Importantly, PD-1 was induced on capsule-specific B cells and suppressed IgG production and protection against pneumococcal infection in a B cell-intrinsic manner. To our knowledge, these results provide the first demonstration of a physiologic role for B cell-intrinsic PD-1 expression in vivo. In summary, our study reveals that B cell-expressed PD-1 plays a central role in regulating protection against S. pneumoniae, and thereby represents a promising target for bolstering immunity to encapsulated bacteria.

Splenic morphological changes are accompanied by altered baseline immunity in a mouse model of sickle-cell disease

Although functional asplenia from infarctions may be a major contributor to increased infectious mortality in sickle-cell disease (SCD), this relationship has not been fully defined. We used the transgenic Berkeley SCD mouse to define blood and splenic immunophenotypic differences in this model compared with C57BL/6 and hemizygous controls. In the serum of SCD mice, we found increased IgG2a and suppressed IgM, IgG2b, and IgA levels. Serum IL-6 levels in SCD mice were elevated, whereas IL-1α, CXCL10, and CCL5 levels were decreased. The blood of SCD mice had higher white blood cell counts, with an increased percentage of lymphocytes and decreases in other leukocytes. Immunophenotyping of lymphocytes revealed higher percentages of CD8(+) and T-regulatory cells and lower percentages of B cells. SCD mouse spleens exhibited histological disorganization, with reduction of defined lymphoid follicles and expansion of red pulp, a greater than fourfold increase in splenic mononuclear cells, marked expansion of the nucleated red blood cell fraction, and B-cell and CD8(+) T-cell lymphopenia. Within the splenic B-cell population, there was a significant decrease in B-1a B cells, with a corresponding decrease in IgA secreting plasma cells in the gut. Confocal microscopy of spleens demonstrated complete disruption of the normal lymphofollicular structure in the white pulp of SCD mice without distinct B, T, and marginal zones. Our findings suggest that altered SCD splenic morphological characteristics result in an impaired systemic immune response.

Enhanced immune response of T-cell independent or dependent antigens in SIGN-R1 knock-out mice

Dextran was used to explore a novel method of enhancing an immune response against T-cell independent type 2 (TI-2) polysaccharide antigens, because of its suitability as a model for the immunogenecity of many TI-2 polysaccharide antigens and its high affinity to SIGN-R1. Here we showed that the primary immune response of IgM, IgG3, and IgG2b was enhanced by dextran in SIGN-R1 knock-out (KO) mice, further evoking the induction of a secondary immune response to IgG2b in parallel. On the other hand, an immune response of IgG1 and IgG2b against T-cell dependent (TD) antigen was strongly enhanced by the administration of ovalbumin (OVA) in SIGN-R1 KO mice. These results indicate that SIGN-R1 is critical in the regulation of immune responses. Therefore, our study suggests that inhibition of TI-2 polysaccharide antigen uptake in SIGN-R1(+) macrophages contributes to the development of novel vaccination strategies against TI-2 polysaccharide antigens.

Host resistance assays including bacterial challenge models

Immunotoxicity testing is used to provide safety assessment with the major objective being the avoidance of unacceptable risk of infectious or neoplastic disease. To this end, immunotoxicity testing has employed a variety of host resistance challenge models for measuring both host resistance to disease as well as immune function. This chapter provides an overview of those viral, bacterial, fungal, and parasitic host resistance models that are most commonly used in safety assessment. It also describes in more detail the bacterial challenge models that are employed to address specific host resistance and immune function issues.

The role of complement in the success of vaccination with conjugated vs. unconjugated polysaccharide antigen

The complement system, a well-characterised arm of the innate immune system, significantly influences the adaptive immune response via direct cell-cell interaction and maintenance of lymphoid organ architecture. Development of vaccines is a major advance in modern health care. In this review, we highlight the importance of the marginal zone in response to both, polysaccharide and conjugated vaccines, and discuss the relevance of complement herein, based on findings obtained from animal models with specific deletions of certain complement components and from vaccination reports of complement-deficient individuals. We conclude that both, intactness of the complement system and maturity of expression of its components, are relatively more important to aid in the immune response to polysaccharide vaccine than to conjugated vaccines.

Low doses of antigen coupled to anti-CR2 mAbs induce rapid and enduring IgG immune responses in mice and in cynomolgus monkeys

The complement system and B cell complement receptor 2 (CR2), specific for C component C3dg, play important roles in both the innate and adaptive immune response. We used hapten and protein conjugates of anti-CR2 mAbs as models for C3dg-opsonized antigens and immune complexes to examine the handling of and immune response to these reagents in mice and in non-human primates (NHP). Mice immunized and boosted i.v. with only 100 ng of Alexa 488 rat anti-mouse CR1/2 mAb 7G6 had strong IgG immune responses to the Alexa 488 hapten and to rat IgG, compared to very weak immune responses in mice treated with a comparable isotype control; larger doses of Alexa 488 mAb 7G6 did not increase the immune response. A vaccine constructed by cross-linking anthrax protective antigen to mAb 7G6 proved to be effective at low doses in generating sufficiently high titer serum IgG antibodies to neutralize anthrax lethal toxin in vitro and to protect mice from i.v. challenge with anthrax lethal toxin. When biotinylated HB135, a mouse mAb specific for human CR2, was injected i.v. into NHP, the probe manifested the same initial marginal zone B cell binding and subsequent localization to follicular dendritic cells as we have previously reported for comparable experiments in mice. Moreover, i.v. immunization of NHP with 1 microg/kg of Alexa 488 mAb HB135 promoted an IgG immune response to the Alexa 488 hapten and to mouse IgG. Taken together, these results demonstrate the efficacy of using anti-CR2 mAbs as antigen carriers for i.v. immunization with small amounts of antigens without adjuvant.

A dominant complement fixation pathway for pneumococcal polysaccharides initiated by SIGN-R1 interacting with C1q

The intricate system of serum complement proteins provides resistance to infection. A pivotal step in the complement pathway is the assembly of a C3 convertase, which digests the C3 complement component to form microbial binding C3 fragments recognized by leukocytes. The spleen and C3 provide resistance against blood-borne S. pneumoniae infection. To better understand the mechanisms involved, we studied SIGN-R1, a lectin that captures microbial polysaccharides in spleen. Surprisingly, conditional SIGN-R1 knockout mice developed deficits in C3 catabolism when given S. pneumoniae or its capsular polysaccharide intravenously. There were marked reductions in proteolysis of serum C3, deposition of C3 on organisms within SIGN-R1(+) spleen macrophages, and formation of C3 ligands. We found that SIGN-R1 directly bound the complement C1 subcomponent, C1q, and assembled a C3 convertase, but without the traditional requirement for either antibody or factor B. The transmembrane lectin SIGN-R1 therefore contributes to innate resistance by an unusual C3 activation pathway.

Complement dependency of splenic localization of pneumococcal polysaccharide and conjugate vaccines

The immune response to polysaccharides is initiated when polysaccharides bind complement factor C3d, and these polysaccharide-C3d complexes subsequently localize on splenic marginal zone B cells strongly expressing CD21 (complement receptor 2). Infants and children under the age of 2 years have low or absent expression of CD21 on their marginal zone B cells, and consequently do not adequately respond to polysaccharides. In contrast, polysaccharide-protein conjugate vaccines are able to induce antibodies at this young age. Conjugate vaccines apparently overcome the necessity for CD21-C3d interaction for an antipolysaccharide immune response. We demonstrate in a rat model that localization of pneumococcal polysaccharides on splenic marginal zone B cells indeed is complement dependent. We also show that pneumococcal conjugates do not specifically localize on splenic marginal zone B cells and that splenic localization of polysaccharide conjugates is independent of the presence of complement. Thus, the induction of antipolysaccharide antibodies by conjugate vaccines apparently can occur independently of CD21-C3d interaction. These basic findings may explain the effectiveness of conjugated vaccines in young children and may open the way for their application in other patient groups.

MARGINAL ZONE B CELLS

Our views regarding the origins and functions of splenic marginal zone B cells have changed considerably over the past few years. Perspectives regarding the development and function of these cells vary considerably between investigators studying human and rodent immunology. Marginal zone B cells are now recognized to constitute a distinct naive B lymphoid lineage. Considerable progress has been made regarding the mechanisms involved in marginal zone B cell development in the mouse. Many of the molecular events that participate in the retention of this lineage of B cells in the marginal zone have been identified. Here, we discuss the functions of these cells in both innate and adaptive immunity. We also attempt to reconcile differing viewpoints regarding the generation and function of marginal zone B cells in rodents and primates.

Analyses of the In Vivo Trafficking of Stoichiometric Doses of an Anti-Complement Receptor 1/2 Monoclonal Antibody Infused Intravenously in Mice

Complement plays a critical role in the immune response by opsonizing immune complexes (IC) and thymus-independent type 2 Ags with C3 breakdown product C3dg, a CR2-specific ligand. We used a C3dg-opsonized IC model, anti-CR1/2 mAb 7G6, to investigate how such substrates are processed. We used RIA, whole body imaging, flow cytometry, and fluorescence immunohistochemistry to examine the disposition of 0.1- to 2-microg quantities of mAb 7G6 infused i.v. into BALB/c mice. The mAb is rapidly taken up by the spleen and binds preferentially to marginal zone (MZ) B cells; within 24 h, the MZ B cells relocate and transfer mAb 7G6 to follicular dendritic cells (FDC). Transfer occurs coincident with loss of the extracellular portion of MZ B cell CR2, suggesting that the process may be mediated by proteolysis of CR2. Intravenous infusion of an FDC-specific mAb does not induce comparable splenic localization or cellular reorganization, emphasizing the importance of MZ B cells in intrasplenic trafficking of bound substrates. We propose the following mechanism: binding of C3dg-opsonized IC to noncognate MZ B cells promotes migration of these cells to the white pulp, followed by CR2 proteolysis, which allows transfer of the opsonized IC to FDC, thus facilitating presentation of intact Ags to cognate B cells.

Age-dependent development of the splenic marginal zone in human infants is associated with different causes of death

Infants are more susceptible to infections caused by T cell- independent type 2 (TI-2) polysaccharide antigens of certain encapsulated bacteria. Immune responses against this type of antigen are related to the splenic marginal zone (MZ). However, only few data exist on the age-dependent developmental stages of the human spleen in early childhood and on their association with different diseases. Therefore, the present study aimed to investigate spleens of a large number of children at very young ages (12 days to 32 months), derived from autopsy cases. Immunohistochemical labeling was performed on paraffin sections of 34 spleens using a panel of monoclonal antibodies. The shape and size of the white pulp compartments were examined and correlated to the cause of death of the children. Results show that the development of the different compartments was statistically age-dependent, but no clear-cut time point for the maturity of each compartment was seen. Furthermore, the MZ was significantly more often missing when sudden infant death (SID) and/or infection were the cause of death, compared with other violent or traumatic reasons that served as controls. This association supports the concept that an immature state of the spleen and especially of the MZ might contribute to the increased susceptibility to bacterial infections in young infants.

The C-type lectin SIGN-R1 mediates uptake of the capsular polysaccharide of Streptococcus pneumoniae in the marginal zone of mouse spleen

SIGN-R1, a recently discovered C-type lectin expressed at high levels on macrophages within the marginal zone of the spleen, mediates the uptake of dextran polysaccharides by these phagocytes. We now find that encapsulated Streptococcus pneumoniae are rapidly cleared by these macrophages from the bloodstream, and that capture also takes place when different cell lines express SIGN-R1 after transfection. To assess the role of the capsular polysaccharide of S. pneumoniae (CPS) in the interaction of SIGN-R1 with pneumococci, we first studied binding and uptake of serotype 14 CPS in transfected cells. Binding was observed and was of a much higher avidity (3000-fold) for CPS 14 than dextran. The CPSs from four different serotypes were also cleared by marginal zone macrophages in vivo. To establish a role for SIGN-R1 in this uptake, we selectively down-regulated expression of the lectin by pretreatment of the mice with SIGN-R1 antibodies, including a newly generated hamster monoclonal called 22D1. For several days after this transient knockout, the marginal zone macrophages were unable to take up either CPSs or dextrans. Therefore, marginal zone macrophages in mice have a receptor that interacts with capsular pneumococcal polysaccharides, setting the stage for further studies of the functional consequences of this interaction.

Macrophages of the splenic marginal zone are essential for trapping of blood-borne particulate antigen but dispensable for induction of specific T cell responses

Rapid removal of pathogens from the circulation by secondary lymphoid organs is prerequisite for successful control of infection. Blood-borne Ags are trapped mainly in the splenic marginal zone. To identify the cell populations responsible for Ag trapping in the marginal zone, mice were selectively depleted of marginal zone macrophages and marginal metallophilic macrophages. In the absence of these cells, trapping of microspheres and Listeria monocytogenes organisms was lost, and early control of infection was impaired. Depletion of marginal zone macrophages and marginal metallophilic macrophages, however, did not limit Ag presentation because Listeria-specific protective T cell immunity was induced. Therefore, marginal zone macrophages and marginal metallophilic macrophages are crucial for trapping of particulate Ag but dispensable for Ag presentation.

Immunity to Streptococcus pneumoniae: Factors affecting production and efficacy

Streptococcus pneumoniae is a complex human pathogen and a major cause of morbidity and mortality. The genetic background of pneumococci and the chemical structure of their capsules is largely unraveled as well as the basic role of anticapsular antibodies and other opsonins interacting to enhance phagocytosis. Many experimental studies are improving our knowledge on the complex molecular mechanisms underlying those events. Pneumococcal optimal clearance requires the cooperation of a plethora of reactions from both innate and adaptive immunity. The last advances in the complexity of the immune response and protection are reviewed: phagocyte-pneumococcus interactions mediated by opsonins; the role of complement, reactive C protein and natural antibodies; details of novel immune evasion mechanisms; the complex role of the inflammatory mediators in the susceptibility to pneumococcal infections; why capsular polysaccharides do not yield an anamnestic response after primary immunization; the central question of whether T cells regulate in-vivo anti-polysaccharide immunoglobulin responses to intact pathogens. All of these are topics where new data and some answers are offered. The state of the art on the research of pneumococcal protein vaccines as an alternative to plain polysaccharide or conjugated vaccine and the establishment of immunologic correlates of protection to facilitate efficacy trial assessment are also reviewed.

Effects of multidose combination chemotherapy on the humoral immune system

Patients receiving multidose combination chemotherapy are at risk for severe, life-threatening infections, caused by among others encapsulated bacteria like Streptococcus pneumoniae. The splenic marginal zone is essential in the initiation of immune responses to S. pneumoniae. We analyzed effects of multidose combination chemotherapy on B-cell subpopulations. Immune response capacity was evaluated by using Pneumovax (PPS) or Tetavax (TT) as antigenic challenge. Three days after finishing therapy, all B-cell subpopulations in bone marrow and spleen were severely reduced, including the mature marginal zone B-cell population. When analyzing the anti-PPS immune response capacity at 3 days after finishing therapy, we found that the IgM antibody levels did not differ significantly from control immunized rats. The IgG antibody levels were significantly lower compared to control immunized rats but still significantly higher compared to unimmunized rats. The depletion of marginal zone B cells by multidose combination chemotherapy most likely contributes to the prolonged period that patients are at risk for developing severe infections after chemotherapy, despite the capacity to generate sufficient antibody levels. It is conceivable that the local (temporary) loss of immunological memory, together with the supposed inability to generate a humoral response in a short time frame, plays an important role in this vulnerability.

After chemotherapy, functional humoral response capacity is restored before complete restoration of lymphoid compartments

Chemotherapy has, besides the beneficial effects, several adverse effects. Suppression of the immune system is one of the most important problems. Infections caused by encapsulated bacteria like Streptococcus pneumoniae are responsible for a major part of infectious problems during and after treatment. The splenic marginal zone is essential in the initiation of an immune response to encapsulated bacteria. In this study, we analysed the effects of three different cytostatic agents on humoral immune responses. We found a reduced, but detectable immune response capacity at two days after treatment although the marginal zone B cell population is severely reduced at this time point. Twenty-four days after cessation of treatment, the immune response capacity was largely restored although lymphoid compartments were still not completely restored at that time point. Apparently, the presence of only few marginal zone B cells is sufficient to evoke a rise in antibody titres and although antibody titre increases are low, even small rises are most likely clinically relevant.

Pathogenesis and pathophysiology of pneumococcal meningitis

Until the introduction of antibiotics in the 1930s and 1940s, acute bacterial meningitis was fatal in most cases. Since then it has become curable with a variable mortality and morbidity rate for individual pathogens and patients. Neuropathological and clinical studies have shown that a fatal outcome of the disease is often due to central nervous system (CNS) complications including cerebrovascular involvement, brain oedema formation, and hydrocephalus resulting in increased intracranial pressure and seizure activity. During recent years, experimental studies with animal models have substantially increased our knowledge of the interactions of bacterial pathogens with mammalian cells and their entry into the CNS, and the complex pathophysiological mechanisms of brain dysfunction during acute bacterial meningitis. There is now a substantial body of evidence that cytokines, chemokines, proteolytic enzymes, and oxidants are involved in the inflammatory cascade that leads to tissue destruction in bacterial meningitis. Genetic targeting and/or pharmacological blockade of these pathways was beneficial in experimental bacterial meningitis. Apart from dexamethasone, these treatment strategies hold major promise for the adjunctive therapy of acute bacterial meningitis in clinical practice.

The dual function of the splenic marginal zone: essential for initiation of anti-TI-2 responses but also vital in the general first-line defense against blood-borne antigens

The splenic marginal zone (S-MZ) is especially well equipped for rapid humoral responses and is unique in its ability to initiate an immune response to encapsulated bacteria (T-cell independent type 2 (TI-2) antigens). Because of the rapid spreading through the blood, infections with blood-borne bacteria form a major health risk. To cope with blood-borne antigens, a system is needed that can respond rapidly to a great diversity of organisms. Because of a number of unique features, S-MZ B cells can respond rapid and efficient to all sorts of blood-borne antigens. These unique features include a low blood flow microenvironment, low threshold for activation, high expression of complement receptor 2 (CR2, CD21) and multireactivity. Because of the unique high expression of CD21 in a low flow compartment, S-MZ B cells can bind and respond to TI-2 antigens even with relatively low-avid B cell receptors. Although TI-2 antigens are in general poorly opsonized by classic opsonins, a particular characteristic of these antigens is their ability to bind very rapidly to complement fragment C3d without the necessity of previous immunoglobulin binding. TI-2 primed S-MZ B cells, already by first passage through the germinal centre, will meet antigen-C3d complexes bound to follicular dendritic cells, allowing unique immediate isotype switching. This explains that the primary humoral response to TI-2 antigens is unique in its characterization by a rapid increase in IgM concurrent with IgG antibody levels.

Recognition of bacterial capsular polysaccharides and lipopolysaccharides by the macrophage mannose receptor

The in vitro binding of the macrophage mannose receptor to a range of different bacterial polysaccharides was investigated. The receptor was shown to bind to purified capsular polysaccharides from Streptococcus pneumoniae and to the lipopolysaccharides, but not capsular polysaccharides, from Klebsiella pneumoniae. Binding was Ca(2+)-dependent and inhibitable with d-mannose. A fusion protein of the mannose receptor containing carbohydrate recognition domains 4-7 and a full-length soluble form of the mannose receptor containing all domains external to the transmembrane region both displayed very similar binding specificities toward bacterial polysaccharides, suggesting that domains 4-7 are sufficient for recognition of these structures. Surprisingly, no direct correlation could be made between polysaccharide structure and binding to the mannose receptor, suggesting that polysaccharide conformation may play an important role in recognition. The full-length soluble form of the mannose receptor was able to bind simultaneously both polysaccharide via the carbohydrate recognition domains and sulfated oligosaccharide via the cysteine-rich domain. The possible involvement of the mannose receptor, either cell surface or soluble, in the innate and adaptive immune responses to bacterial polysaccharides is discussed.

Structure-function relationships for human antibodies to pneumococcal capsular polysaccharide from transgenic mice with human immunoglobulin Loci

To investigate the influence of antibody structure and specificity on antibody efficacy against Streptococcus pneumoniae, human monospecific antibodies (MAbs) to serotype 3 pneumococcal capsular polysaccharide (PPS-3) were generated from transgenic mice reconstituted with human immunoglobulin loci (XenoMouse mice) vaccinated with a PPS-3-tetanus toxoid conjugate and their molecular genetic structures, epitope specificities, and protective efficacies in normal and complement-deficient mice were determined. Nucleic acid sequence analysis of three MAbs (A7, 1A2, and 7C5) revealed that they use two different V(H)3 genes (A7 and 1A2 both use V3-15) and three different V(kappa) gene segments. The MAbs were found to have similar affinities for PPS-3 but different epitope specificities and CDR3 regions. Both A7 and 7C5 had a lysine at the V(H)-D junction, whereas 1A2 had a threonine. Challenge experiments with serotype 3 S. pneumoniae in BALB/c mice revealed that both 10- and 1- micro g doses of A7 and 7C5 were protective, while only a 10- micro g dose of 1A2 was protective. Both A7 and 7C5 were also protective in mice lacking either an intact alternative (FB(-/-)) or classical (C4(-/-)) complement pathway, but 1A2 was not protective in either strain. Our data suggest that PPS-3 consists of epitopes that can elicit both highly protective and less protective antibodies and that the superior efficacies of certain antibodies may be a function of their structures and/or specificities. Further investigation of relationships between structure, specificity, and efficacy for defined MAbs to PPS may identify antibody features that might be useful surrogates for antibody (and vaccine) efficacy.

Slow recovery of follicular B cells and marginal zone B cells after chemotherapy: implications for humoral immunity

Although most chemotherapeutic agents are known to cause primarily reduction or suppression of immune responses, surprisingly little is known about the influence of cytostatic agents on lymphoid tissue compartments such as the splenic marginal zone. The marginal zone plays an important role in the defence against encapsulated bacteria, which are potential candidates for postchemotherapeutic infections. We studied the effect of three different cytostatic agents (cisplatin, methotrexate, and cyclophosphamide) on B cell subpopulations in a rat model. Rats received a single dose of a single cytostatic agent and were sacrificed at different time points after treatment. Bone marrow, blood, mesenteric lymph nodes and spleens were analysed by flow-cytometry and immunohistochemistry. All three cytostatic agents showed severe bone marrow depression. CP and MTX showed only mild reduction of cell populations in the spleen. CyPh showed a severe reduction of recirculating follicular B (RF-B) cells and marginal zone B (MZ-B) cells. At day 24 most populations were already recovered, but RF-B cells and MZ-B cells were still reduced. The reduction of the marginal zone and late recovery may imply that, beside the overall increased infection risk due to neutropenia, patients treated with chemotherapy are at risk for developing infections from encapsulated bacteria for a considerable period of time after treatment, extending beyond the period of bone marrow depression.

Spleen autotransplantation provides restoration of functional splenic lymphoid compartments and improves the humoral immune response to pneumococcal polysaccharide vaccine

After splenectomy, patients have an increased risk of overwhelming post-splenectomy infection (OPSI) or sepsis involving encapsulated bacteria such as pneumococcus. The value of spleen autotransplantation after splenectomy because of trauma has long been questioned. Much attention has been given to the restoration of mononuclear phagocyte system (MPS) function, which appeared to be similar to that of splenectomized individuals. The presence of specific anti-pneumococcal antibodies may enhance phagocytosis of opsonized bacteria by other parts of the MPS, as present in the liver. Therefore, in the present study we have evaluated the restoration of the humoral immune response after spleen autotransplantation, especially to pneumococcal capsular polysaccharides (PPS). Wistar rats were divided into three groups which were operated as follows: splenectomy, splenectomy followed by autotransplantation, and sham operation. After 12 weeks the rats were vaccinated with 23-valent pneumococcal vaccine. Blood samples were taken after 3 days, 3 and 6 weeks for anti-PPS IgM and IgG ELISA against types 3, 4, 6, 9, 14 and 23. In addition, immunohistological studies were performed on the autotransplants. Significant antibody titre rises were found in a main proportion of the autotransplanted rats, comparable to sham-operated rats. Splenectomized rats showed as well a significantly lower increase in immunoglobulin levels, as significant differences in the proportion of rats showing a minimum two-fold increase of antibody level, considered to represent an adequate response. The titres were highest 3 days after vaccination. Immunohistochemical studies demonstrated structurally functional autotransplants, including an intact marginal zone. Considering this significant anti- pneumococcal antibody response, spleen autotransplants can be expected to enable an improved humoral response to PPS, and to contribute to protection against OPSI after splenectomy.

T-independent type 2 antigens induce B cell proliferation in multiple splenic sites, but exponential growth is confined to extrafollicular foci

During the primary splenic response to the T-independent type 2 (TI-2) antigen (4-hydroxy-3-nitrophenyl) acetyl (NP)-Ficoll, small numbers of antigen-specific B cells have entered S phase of the cell cycle 24 h after intraperitoneal immunization. These are distributed in all splenic compartments (T zones, marginal zones, follicles, and red pulp), indicating early proliferation induced by NP-Ficoll does not require accessory signals delivered in a particular splenic microenvironment. Subsequently B blasts accumulate selectively in the outer T zone areas, but exponential growth leading to plasma cell production occurs only in extrafollicular foci. This growth peaks after 5 days, but 20% of peak numbers of antibody-containing cells are still present 3 months after immunization and 9% of these cells are proliferating. It is unclear if these late plasmablasts are sustained by self-renewal or continued recruitment of virgin cells into the response. Unlike TD and TI-1 responses NP-specific memory cells do not accumulate in the splenic marginal zones. The level of Cgamma3 switch transcripts increases during the first 24 h of the response, but does not increase further during exponential plasmablast growth.

Infant B cell responses to polysaccharide determinants

Newborns and infants up to the age of 1.5-2 years of age are unable to produce antibodies to bacterial capsular polysaccharides. As a consequence, children up to the age of 2 years have an increased susceptibility for infections with encapsulated bacteria. Capsular polysaccharides are classified as so-called T cell independent type 2 antigens and induce IgG2 antibodies. The mechanism of B lymphocyte activation by polysaccharides differs from that of protein antigens and involves co-stimulation by CD21 (type 2 complement receptor). Reduced expression of CD21 on neonatal B lymphocytes can explain unresponsiveness to polysaccharides. Polysaccharide protein conjugates have the ability to overcome unresponsiveness to polysaccharides early in life. The response induced is predominant IgGl.