Complement-mediated follicular localization of T-independent type-2 antigens: the role of marginal zone macrophages revisited

Dynamic encounters with red blood cells trigger splenic marginal zone B cell retention and function

Spleen marginal zone (MZ) B cells are important for antibody responses against blood-borne antigens. The signals they use to detect exposure to blood are not well defined. Here, using intravital two-photon microscopy in mice, we observe transient contacts between MZ B cells and red blood cells that are in flow. We show that MZ B cells use adhesion G-protein-coupled receptor ADGRE5 (CD97) for retention in the spleen. CD97 function in MZ B cells depends on its ability to undergo autoproteolytic cleavage and signaling via Gα13 and ARHGEF1. Red blood cell expression of the CD97 ligand CD55 is required for MZ B cell homeostasis. Applying a pulling force on CD97-transfected cells using an optical C-trap and CD55+ beads leads to accumulation of active RhoA and membrane retraction. Finally, we show that CD97 deficiency leads to a reduced T cell-independent IgM response. Thus, our studies provide evidence that MZ B cells use mechanosensing to position in a manner that enhances antibody responses against blood-borne antigens.

Plasma Cell Differentiation Pathways in Systemic Lupus Erythematosus

Plasma cells (PCs) are responsible for the production of protective antibodies against infectious agents but they also produce pathogenic antibodies in autoimmune diseases, such as systemic lupus erythematosus (SLE). Traditionally, high affinity IgG autoantibodies are thought to arise through germinal center (GC) responses. However, class switching and somatic hypermutation can occur in extrafollicular (EF) locations, and this pathway has also been implicated in SLE. The pathway from which PCs originate may determine several characteristics, such as PC lifespan and sensitivity to therapeutics. Although both GC and EF responses have been implicated in SLE, we hypothesize that one of these pathways dominates in each individual patient and genetic risk factors may drive this predominance. While it will be important to distinguish polymorphisms that contribute to a GC-driven or EF B cell response to develop targeted treatments, the challenge will be not only to identify the differentiation pathway but the molecular mechanisms involved. In B cells, this task is complicated by the cross-talk between the B cell receptor, toll-like receptors (TLR), and cytokine signaling molecules, which contribute to both GC and EF responses. While risk variants that affect the function of dendritic cells and T follicular helper cells are likely to primarily influence GC responses, it will be important to discover whether some risk variants in the interferon and TLR pathways preferentially influence EF responses. Identifying the pathways of autoreactive PC differentiation in SLE may help us to understand patient heterogeneity and thereby guide precision therapy.

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.

Role of complement in immune regulation and its exploitation by virus

Complement is activated during the early phase of viral infection and promotes destruction of virus particles as well as the initiation of inflammatory responses. Recently, complement and complement receptors have been reported to play an important role in the regulation of innate as well as adaptive immune responses during infection. The regulation of host immune responses by complement involves modulation of dendritic cell activity in addition to direct effects on T-cell function. Intriguingly, many viruses encode homologs of complement regulatory molecules or proteins that interact with complement receptors on antigen-presenting cells and lymphocytes. The evolution of viral mechanisms to alter complement function may augment pathogen persistence and limit immune-mediated tissue destruction. These observations suggest that complement may play an important role in both innate and adaptive immune responses to infection as well as virus-mediated modulation of host immunity.

The presence of MOMA-2+ macrophages in the outer B cell zone and protection of the splenic micro-architecture from LPS-induced destruction depend on secreted IgM

The role secretory IgM has in protecting splenic tissue from LPS-induced damage was assessed in mice incapable of secreting IgM but able to express surface IgM and IgD. Within seconds after LPS challenge, 99% of the (131)I-labeled LPS was found in the liver and the spleen of both sIgM-deficient and wild-type mice. In the spleen FITC-labeled LPS was found on the surface of 2F8(+) scavenger receptor macrophages localized in the outer marginal zone, while none of the labeled LPS could be detected on marginal zone ER-TR9(+) and MOMA-1(+) macrophages. An additional population of macrophages, MOMA-2(+), were capable of producing C3 locally in the T and B cell zone after LPS challenge. Local C3 production was regulated, as no C3 was found in splenic tissue of unchallenged mice. Interestingly, in the absence of circulating and locally produced secretory IgM, MOMA-2(+) macrophages of the T and B cell zone failed to establish an additional ring of C3-producing macrophages in the outer B cell zone close to the marginal zone upon LPS challenge. The consequence was a massive destruction of the microarchitecture of the spleen where marginal zones disorganized, lymphoid follicles and T cell zones disrupted and follicular DC (FDC) networks disappeared.

Elevated levels of endogenous apoptotic DNA and IFN-alpha in complement C4-deficient mice: implications for induction of systemic lupus erythematosus

Systemic lupus erythematosus (SLE), an autoimmune disease characterized by chronic nephritis, arthritis and dermatitis, and the presence of antinuclear autoantibodies, is associated with complement factor deficiencies in the classical activation pathway. In addition, IFN-alpha seems to be a key cytokine in SLE as an activated IFN-alpha system is regularly observed in patients with SLE. Here, we demonstrate that in lupus-susceptible, complement C4-deficient mice the lack of complement results in elevated intravascular levels of apoptotic DNA. The apoptotic DNA is targeted to the splenic marginal zone where it accumulates and induces IFN-alpha. As such, we present here a unifying hypothesis for the induction of SLE that incorporates the role of complement deficiency and elevated levels of IFN-alpha.

The generation of thymus-independent germinal centers depends on CD40 but not on CD154, the T cell-derived CD40-ligand

In this report, we show that the formation of germinal center (GC)-like structures to thymus-independent type 2 antigens in mice depends on intact signals through CD40, but does not depend on T cell-derived CD40-ligand (CD154). In addition, we show that follicular dendritic cells (FDC) are also critical to thymus-independent GC formation, as their depletion by blockade of lymphotoxin-beta receptor signals abrogated GC development unless the responding B cells bound antigen with high affinity. Further evidence that immune complexes drove this CD40-dependent B cell proliferation was provided by the observation that an antibody that detects immune complexes containing complement component 4 on FDC also inhibited thymus-independent GC formation when injected in vivo at the time of immunization. Finally, we show that thymus-independent B cell proliferation was associated with class switching to IgG3, as IgG3(+) antigen-specific switched B cells could be visualized directly in GC, suggesting that immune complexes can provide the signals for class switching within GC in the absence of CD154.

Defective microarchitecture of the spleen marginal zone and impaired response to a thymus-independent type 2 antigen in mice lacking scavenger receptors MARCO and SR-A

The macrophage scavenger receptor macrophage receptor with a collagenous structure (MARCO) is expressed in mice by the marginal zone macrophages of the spleen and by macrophages of the medullary cords of lymph nodes, as well as the peritoneal macrophages. MARCO is a relative of scavenger receptor A (SR-A), the more widely expressed prototypic member of the scavenger receptor family. In the present study, we found that genetic ablation of MARCO leads to changes in the organization of the splenic marginal zone, and causes a significant reduction in the size of the resident peritoneal macrophage population, possibly due to changes in adhesion and migration capacity. In mice lacking both MARCO and SR-A these effects are even more apparent. During ontogeny, the appearance and organization of the MARCO-expressing cells in the spleen precedes the appearance of other receptors on macrophages in the marginal zone, such as SIGNR1 and Siglec-1. In the absence of MARCO, a clear delay in the organization of the marginal zone was observed. Similar findings were seen when the reappearance of the various subsets from precursors was studied after depleting macrophages from the adult spleen by a liposome treatment. When challenged with a pneumococcal polysaccharide vaccine, a T-independent type 2 Ag for which an intact marginal zone is crucial, the knockout mice exhibited a clearly impaired response. These findings suggest that both MARCO and SR-A, in addition to being important scavenger receptors, could be involved in the positioning and differentiation of macrophages, possibly through interaction with endogenous ligands.

Homeostatic and functional analysis of mature B cells in λ5-deficient mice

The peripheral B-cell pool is in dynamic equilibrium and is controlled by a variety of factors. The rate of generation of B cells can influence both the composition and size of the peripheral B-cell compartment. Mice deficient for lambda5 gene expression have a block in early B-cell development leading to a markedly reduced number of peripheral B cells. To address the question of how this early developmental block influences the composition of the B-cell pool, we have analyzed mature B-cell subpopulations in lambda5-deficient mice. In analogy with other situations of B lymphopenia, the proportion was increased but not the absolute number of marginal-zone B cells, whereas those of follicular B cells were decreased. Immunohistology revealed that B-cell follicles were smaller in overall size and contained a prominent B-cell replete marginal zone. BrdU labelling kinetics showed slower turnover of follicular as well as of marginal-zone B cells. Functionally, lambda5(-/-) mice were able to mount not only primary but also secondary thymus-dependent as well as thymus-independent responses, albeit mostly at reduced levels.

Histological analysis of CD11c-DTR/GFP mice after in vivo depletion of dendritic cells

To investigate the dependence of individual immunological processes on DC, a transgenic mouse system (CD11c-DTR/GFP mice) has been developed that allows conditional depletion of CD11c+ DC in vivo through administration of diphtheria toxin. We have performed careful histological analysis of CD11c-DTR/GFP mice at different time points after diphtheria toxin injection and confirmed the transient depletion of CD11c+ cells from lymph nodes and spleen. Unexpectedly, the injection of diphtheria toxin completely depleted marginal zone and metallophilic M(Phi) from the spleen and their sinusoidal counterparts from the lymph nodes. This finding limits the use of CD11c-DTR/GFP mice for the analysis of the role of DC to models and read outs that are proven to be independent of marginal zone and sinusoidal M(Phi).

IgM Accelerates Affinity Maturation

Secreted IgM is a potent adjuvant that concentrates antigen into secondary lymphoid organs and initiates antibody responses, germinal centre formation and is crucial in resolving infections. The current studies investigated the influence of specific IgM on both the quantity and quality of antibody produced in response to T-dependent and T-independent antigens. The addition of IgM to either antigen had no significant effect on the titre or duration of antibody responses. However, the presence of specific IgM led to accelerated affinity maturation when mice were challenged with low doses of IgM-containing immune complexes (IgM-IC) of T-dependent antigens compared with antigen alone. Interestingly, the administration of higher concentrations of IgM-IC increased follicular deposition of antigen but did not result in accelerated affinity maturation or in higher antibody affinities. The administration of IgM complexed with T-independent antigens had no effect on antibody titre, duration or affinity maturation, despite increased antigen deposition in lymphoid follicles. Together, these results demonstrate that IgM accelerates affinity maturation in immune responses to T-dependent antigens and implies an antigen optimum exists for the generation of high affinity antibodies. The data also suggest IgM plays an important role in the induction of CD4 T cells, facilitating germinal centre formation and function.

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.

B Cells Are Crucial for Both Development and Maintenance of the Splenic Marginal Zone

The splenic marginal zone is a unique compartment that separates the lymphoid white pulp from the surrounding red pulp. Due to the orchestration of specialized macrophages and B cells flanking a marginal sinus, this compartment plays an important role in uptake of blood-borne Ags and it gives the spleen its specialized function in antibacterial immunity. In this study, we demonstrate that both development and maintenance of this marginal zone is highly dependent on the presence of B cells. Spleens from B cell-deficient mice were found to lack both metallophilic and marginal zone macrophages as well as mucosal addressin cellular adhesion molecule-1+ sinus lining cells. Using an inducible Cre/loxP-driven mouse model in which mature B cells could be partially depleted by removal of the B cell receptor subunit Igalpha, we could show that the integrity and function of an established marginal zone was also dependent on the presence of B cells. This was confirmed in a transgenic model in which all B cells were gradually depleted due to overexpression of the TNF family member CD70. The loss of all cellular subsets from the marginal zone in these CD70 transgenic mice was effectively prevented by crossing these mice on a CD27(-/-) or TCRalpha(-/-) background, because this prohibited the ongoing B cell depletion. Therefore, we conclude that B cells are not only important for the development, but also for maintenance, of the marginal zone. This direct correlation between circulating B cells and the function of the spleen implies an increased risk for B cell lymphopenic patients with bacterial infections.

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.

Impaired antibody response to group B streptococcal type III capsular polysaccharide in C3- and complement receptor 2-deficient mice

Group B Streptococcus (GBS) is the foremost bacterial cause of serious neonatal infections. Protective immunity to GBS is mediated by specific Abs to the organism's capsular polysaccharide Ags. To examine the role of complement in the humoral immune response to type III GBS capsular polysaccharide (III-PS), mice deficient in C3 or in CD21/CD35 (i.e., complement receptors 1 and 2; CR1/CR2) were immunized with III-PS. Mice deficient in C3 or Cr2 had an impaired primary immune response to III-PS. The defective response was characterized by low IgM levels and the lack of an isotype switch from IgM to IgG Ab production. Compared with wild-type mice, C3- and Cr2-deficient mice exhibited decreased uptake of III-PS by follicular dendritic cells within the germinal centers and impaired localization of III-PS to the marginal zone B cells. Complement-dependent uptake of capsular polysaccharide by marginal zone B cells appears necessary for an effective immune response to III-PS. The normal immune response in wild-type mice may require localization of polysaccharide to marginal zone B cells with subsequent transfer of the Ag to follicular dendritic cells.

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.

Increased immunogenicity and induction of class switching by conjugation of complement C3d to pneumococcal serotype 14 capsular polysaccharide

Previous studies have demonstrated an adjuvant effect for the C3d fragment of complement C3 when coupled to T-dependent protein antigens. In this study, we examined the antibody response to covalent conjugates of C3d and a T-independent antigen, the capsular polysaccharide of serotype 14 Streptococcus pneumoniae (PPS14). We prepared a conjugate of mouse C3d and PPS14 and compared its immunogenicity with that of a conjugate of PPS14 and ovalbumin (OVA). When BALB/c mice were immunized with PPS14-C3d, there was a significant increase in serum anti-PPS14 concentrations compared with either native PPS14 or control PPS14-glycine conjugates. This was accompanied by a switch in anti-PPS14 from predominantly immunoglobulin M (IgM) to IgG1 by day 25 following primary immunization. Following secondary immunization with PPS14-C3d, there was a marked booster response and a further increase in the ratio of IgG1 to IgM anti-PPS14. Although the primary antibody response to the PPS14-OVA conjugate exceeded that induced by immunization with PPS14-C3d, serum anti-PPS14 concentrations after a second injection of PPS14-C3d were nearly identical to those induced by secondary immunization with PPS14-OVA. Experiments with athymic nude mice suggested that T cells were not required for the adjuvant effect of C3d on the primary immune response to PPS14 but were necessary for enhancement of the memory response after a second injection of PPS14-C3d. These studies show that the adjuvant effects of C3d extend to T-independent antigens as well as T-dependent antigens. As a means of harnessing the adjuvant potential of the innate immune system, C3d conjugates may prove useful as a component of vaccines against encapsulated bacteria.

The role of complement in the acquired immune response

Studies over the past three decades have clearly established a central role for complement in the promotion of a humoral immune response. The primary function of complement, in this regard, is to opsonize antigen or immune complexes for uptake by complement receptor type 2 (CR2, CD21) expressed on B cells, follicular dendritic cells (FDC) and some T cells. A variety of mechanisms appear to be involved in complement-mediated promotion of the humoral response. These include: enhancement of antigen (Ag) uptake and processing by both Ag-specific and non-specific B cells for presentation to specific T cells; the activation of a CD21/CD19 complex-mediated signalling pathway in B cells, which provides a stimulus synergistic to that induced by antigen interaction with the B-cell receptor (BCR); and promotion of the interaction between B cells and FDC, where C3d-bearing immune complexes participate in intercellular bridging. Finally, current studies suggest that CR2 may also play a role in the determination of B-cell tolerance towards self-antigens and thereby hold the key to the previously observed correlation between deficiencies of the early complement components and autoimmune disease.

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.

A functional soluble form of the murine mannose receptor is produced by macrophages in vitro and is present in mouse serum

A soluble form of the mannose receptor (sMR) has been found in conditioned medium of primary macrophages in vitro and in mouse serum. sMR was released as a single species, had a smaller size than the cell-associated form, and accumulated in macrophage-conditioned medium, in a cytokine-regulated manner, to levels comparable with those found for cell-associated mannose receptor. Pulse-chase experiments showed that sMR production in culture occurred by constitutive cleavage of pre-existing full-length protein. A binding assay was developed to determine the sugar specificity of sMR and its ability to interact with pathogens and particulate antigens (i.e. Candida albicans and zymosan). Protease inhibitor studies suggested that sMR was produced by cleavage of an intact mannose receptor by a matrix metalloprotease or ADAM metalloprotease. A role for sMR in the immune response is proposed based on its binding properties, regulation by cytokines, and the previous discovery of putative ligands for the cysteine-rich domain of the mannose receptor in lymph nodes and spleen.

Antigen detection in vivo after immunization with different presentation forms of rabies virus antigen, II. Cellular, but not humoral, systemic immune responses against rabies virus immune-stimulating complexes are macrophage dependent

In this paper we describe the effect of depletion of splenic macrophages on the uptake, and immune response against, different formulations of rabies virus antigen. Splenic macrophages were removed by intravenous injection with clodronate liposomes. beta-propiolacton inactivated rabies virus (RV-BPL) and immune-stimulating complexes (iscom) containing these antigens were given to macrophage-depleted and control mice. In the absence of phagocytic cells in the spleen, antigen is still trapped in the red pulp and to a lesser extent in the peri-arteriolar lymphocyte sheaths (PALS) for both antigen formulations. The localization pattern in the main area of immune response induction, namely the follicles, was unaltered after macrophage depletion. Functionally, the depletion of splenic and liver macrophages had no influence on the induction of specific antibody responses in both RV-BPL or RV-iscom immunized mice, even though the latter presentation form was clearly associated with specific localization in the marginal metallophillic macrophages. In RV-BPL immunized mice, macrophage depletion had no influence on proliferative T-cell responses. However, macrophage-depleted mice that were immunized with RV-iscom showed a significant decrease in proliferative T-cell responses. These results confirm existing ideas on the spleen as a physical filter rather than an induction site for humoral responses and shed new light on the efficient role of iscoms as antigen-presenting moieties in relation to their specific in vivo localization patterns and partial macrophage dependency.

Complement component 3 interactions with coxsackievirus B3 capsid proteins: innate immunity and the rapid formation of splenic antiviral germinal centers

Innate immunity is central to the clearance of pathogens from hosts as well as to the definition of acquired immune responses (D. T. Fearon, and R. M. Locksley, Science 272:50-53, 1996). Coxsackievirus B3 (CVB3), a human cardiopathic virus, was evaluated for the ability to activate the alternative and classical pathway of complement. CVB3 proteins interact with complement component 3 (C3, a soluble protein effector of innate immunity) after either in vitro exposure to mouse serum or in vivo murine infection and activate the alternative pathway of complement. In addition, we demonstrate that viral antigen retention and localization in germinal centers is dependent on C3, while virus antigen retention in extrafollicular regions in the spleen is not. In vivo depletion of native C3 abolished the rapid formation of virus-specific germinal centers (by day 3 post-CVB3 infection) in the absence of serum anti-CVB3 antibodies. These studies demonstrate that innate immune mechanisms, such as C3 interaction with CVB3, are essential for splenic antiviral germinal center formation in naive (antigen nonsensitized) mice resistant (C57BL/6J strain) and susceptible (A/J strain) to CVB3-induced myocarditis.

On the key role of secondary lymphoid organs in antiviral immune responses studied in alymphoplastic (aly/aly) and spleenless (Hox11(-)/-) mutant mice

The role of the spleen and of other organized secondary lymphoid organs for the induction of protective antiviral immune responses was evaluated in orphan homeobox gene 11 knockout mice (Hox11(-/-)) lacking the spleen, and in homozygous alymphoplastic mutant mice (aly/aly) possessing a structurally altered spleen but lacking lymph nodes and Peyer's patches. Absence of the spleen had no major effects on the immune response, other than delaying the antibody response by 1-2 d. In aly/aly mice, the thymus-independent IgM response against vesicular stomatitis virus (VSV) was delayed and reduced, whereas the T-dependent switch to the protective IgG was absent. Therefore, aly/aly mice were highly susceptible to VSV infection. Since aly/aly spleen cells yielded neutralizing IgM and IgG after adoptive transfer into recipients with normally structured secondary lymphoid organs, these data suggest that the structural defect was mainly responsible for inefficient T-B cooperation. Although aly/aly mice generated detectable, but reduced, CTL responses after infection with vaccinia virus (VV) and lymphocytic choriomeningitis virus (LCMV), the elimination of these viruses was either delayed (VV) or virtually impossible (LCMV); irrespective of the dose or the route of infection, aly/aly mice developed life-long LCMV persistence. These results document the critical role of organized secondary lymphoid organs in the induction of naive T and B cells. These structures also provide the basis for cooperative interactions between antigen-presenting cells, T cells, and B cells, which are a prerequisite for recovery from primary virus infections via skin or via blood.

Fc chimeric protein containing the cysteine-rich domain of the murine mannose receptor binds to macrophages from splenic marginal zone and lymph node subcapsular sinus and to germinal centers

Ligands for the cysteine-rich (CR) domain of the mannose receptor (MR) were detected by incubating murine tissues with a chimeric protein containing CR fused to the Fc region of human IgG1 (CR-Fc). In naive mice, CR-Fc bound to sialoadhesin+, F4/80low/-, macrosialin+ macrophages (M phi) in spleen marginal zone (metallophilic M phi) and lymph node subcapsular sinus. Labeling was also observed in B cell areas of splenic white pulp. Western blotting analysis of spleen and lymph nodes lysates revealed a restricted number of molecules that interacted specifically with CR-Fc. In immunized mice, labeling was upregulated on germinal centers in splenic white pulp and follicular areas of lymph nodes. Kinetic analysis of the pattern of CR-Fc labeling in lymph nodes during a secondary immune response to ovalbumin showed that CR ligand expression migrated towards B cell areas, associated with cells displaying distinctive dendritic morphology, and accumulated in developing germinal centers. These studies suggest that MR+ cells or MR-carbohydrate-containing antigen complexes could be directed towards areas where humoral immune responses take place, through the interaction of the MR CR domain with molecules expressed in specialized macrophage populations and antigen transporting cells.

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

For a better understanding of the spleen-dependent induction of the humoral immune response against thymus-independent type 2 antigens, we have studied the in vitro and in vivo localization of different capsular pneumococcal polysaccharides (PPSs) in the rat spleen. In this study, we found that in vitro binding of PPS types 3, 4, 6B, 9N/V, 14, and 23F was dependent on complement (probably a C3 fragment) and that the localization was predominantly restricted to the marginal-zone B lymphocytes and the follicular dendritic cells. In vivo, we observed with increase of time a shift of localized antigens. Shortly after injection, all PPS types localized in the marginal-zone B lymphocytes, then localized in the outer follicular mantle, and finally were found to be diffuse in the complete follicle and follicle corona. PPS types 3 and 9N/V and later also PPS type 23F localized additionally in red pulp macrophages. In particular, the localization in the marginal zone is important since the low flow in this area in combination with strongly CD21+ B cells, which are activated early, gives a maximum opportunity for the induction of a primary humoral immune response with subsequent differentiation into plasma cells or migration to the germinal center. In addition, the localization of PPSs at follicular dendritic cells should be considered important in the induction of an ongoing immune response not restricted to the spleen.

Role of apoptosis in HIV disease pathogenesis

After approximately one and a half decades of intensive studies, the exact mechanisms to explain HIV-mediated cytopathicity are still enigmatic and need closer scrutiny. There has been a dichotomy between virological and immunological viewpoints in understanding HIV-mediated cytopathicity, the former emphasizing a killing of infected cells by HIV-1 and the latter emphasizing indirect mechanisms wherein HIV or its soluble component(s) alter CD4 T-cell function and induce susceptibility to apoptosis. Accumulating evidence points to the notion that apoptosis might be a major contributor to the depletion of CD4 T-cells in HIV infection. This review summarizes current information about the regulatory mechanisms of T-cell apoptosis and the role of apoptosis in HIV pathogenesis with the goal of providing an integrated view of HIV cytopathicity.

Analysis of mutations in immunoglobulin heavy chain variable region genes of microdissected marginal zone (MGZ) B cells suggests that the MGZ of human spleen is a reservoir of memory B cells

The splenic marginal zone (MGZ), which surrounds the mantle zone (MTZ) in human splenic white pulp, contains a phenotypically and morphologically distinct population of B cells. The origin of MGZ B cells is uncertain. Whereas some experiments in rodents have suggested that they are a distinct cell lineage responsible for the immune response to T-independent type 2 antigens, others have suggested that they are memory B cells derived from a germinal center (GC) response. The progeny of a GC reaction is expected to have rearranged immunoglobulin (Ig) genes that are mutated. The distribution of mutations would be expected to reflect the selection of Ig by its affinity for antigen. We have analyzed rearranged Ig heavy chain variable region (VH) 6 and VH 4.21 genes in MGZ and MTZ B cells microdissected from frozen sections of human spleen to determine whether these genes have the properties of an affinity-selected memory B cell population. MTZ B cells contained germline Ig VH genes, confirming previous reports and providing an internal control for mutational analysis. MGZ B cells contained Ig VH genes that were mutated, showing that these cells had been subjected to a mutational mechanism characteristically active in the GC. The rearranged VH 6 genes showed patterns of mutation indicative of an antigen selection process, whereas the distribution of mutations in VH 4.21 genes was not characteristic of gene selection by conventional T-dependent antigen. Our studies provide the first evidence that the human splenic MGZ is a reservoir of memory B cells.

Germinal-center cell proliferation in response to T-independent antigens: a stathmokinetic, morphometric and immunohistochemical study in vivo

Although the nature of the germinal center reaction during responses to T-dependent antigens has been well documented, much less is known regarding the relationship between germinal centers and T-independent antigens. In this study, germinal-center cell proliferation was determined at specific time points in spleens of C3H/HeN mice following immunization with either the type-1, T-independent antigen dinitrophenol-lipopolysaccharide (DNP-LPS), or the type-2, T-independent antigen DNP-Ficoll. A stathmokinetic technique was employed to assess proliferation in terms of germinal center cell birth rate and morphometry was used to measure actual growth and regression of the germinal center cell population. An estimate of the absolute rate of germinal-center (GC) cell proliferation was derived from these two values. In addition, immunohistochemistry was performed to correlate changes in GC cell proliferation with the presence or absence of antigen within GC. Following immunization with both antigens, there was an initial reduction of proliferation within pre-existing germinal centers which manifested as either GC dissociation (DNP-LPS) or a suppression of birth rates (DNP-Ficoll). This was followed by a period of increased GC cell proliferation in animals immunized with DNP-LPS, but not in those exposed to DNP-Ficoll. GC cell proliferation was then measured in mice treated with cyclosporin A from 1 day before to 2 days after immunization with DNP-LPS. In these animals, the expected increase in GC cell birth rates did not take place. Immunohistochemistry showed that DNP-Ficoll and DNP-LPS were present in GC from 1 day after immunization until the end of the experiment on day 7. Treatment with cyclosporin A did not affect the deposition of DNP-LPS in GC. These results show that only some T-independent antigens are able to stimulate GC cell proliferation, and we propose that this is related to their ability to recruit precursors of GC B cells into the GC reaction. In addition, the results indicate that GC proliferation seen in response to a so-called T-independent antigen is at least partly driven by T cell-derived cytokines.

Antigen detection in vivo after immunization with different presentation forms of rabies virus antigen: involvement of marginal metallophilic macrophages in the uptake of immune-stimulating complexes

Several mechanisms have been postulated to explain the relatively high immunogenicity of antigens presented in immune-stimulating complexes (iscom). Their potency can in part be explained by the specific targeting of these structures to cells presenting antigens to the immune system. However, until now no method for the subcellular detection of iscom in situ was available. In the present study, a novel, fast and simple method for the detection of iscoms in situ is demonstrated. By making use of the lipophilic fluorescent carbocyanine dyes, 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) and 3,3'-dioctadecyloxacarbocyanine perchlorate (DiO), rabies virus antigen and iscom prepared with this antigen were visualized with fluorescence microscopy. The labeled antigen and iscoms were observed in macrophages of spleen and liver of mice within 1-2 h after intravenous administration. When administered intramuscularly or in the footpad, uptake in macrophages of draining lymph nodes could be demonstrated. In the spleen, labeled inactivated virus antigen localized preferentially in the marginal zone macrophages and to a lesser extent in the red pulp macrophages. In contrast, antigen presented in iscom was taken up mainly by the marginal metallophilic macrophages and to a much lesser extend by marginal zone macrophages or follicular-dendritic and -B cells. This method enables the detection of iscom and membrane viruses and allows the analysis of their relation to antigen-presenting cells in situ. Here, we demonstrate that iscom containing rabies virus antigen are taken up by a subset of macrophages in the spleen distinct from those that take up inactivated rabies virus antigen not presented in iscom, thereby possibly explaining the observed difference in immunogenicity of these antigen preparations. Furthermore, we show a lower efficiency on the induction of humoral and cellular responses after intravenous immunization for both types of antigen when compared with subcutaneous immunization.

Clonal expansion of T cells and HIV genotypes in microdissected splenic white pulps indicates viral replication in situ and infiltration of HIV-specific cytotoxic T lymphocytes

Human immunodeficiency virus (HIV) replication and T cell proliferation was investigated in situ by a PCR based analysis of individual microdissected splenic white pulps. Founder effects, revealed by an exquisite compartmentalization of HIV genotypes and T cells, indicated the recruitment of latently infected CD4+ T cells through highly localized antigen presentation, rather than the infection of CD4+ T lymphoblasts by blood borne virus or immune complexes. HIV infected white pulps could be infiltrated by HIV specific cytotoxic T lymphocytes, so implicating them in CD4+ T cell destruction in vivo. Together these data describe an iterative and deleterious mechanism of antigen driven T cell recruitment and activation, HIV replication and spread, with consequent destruction of the newly infected cells.

HIV and T cell expansion in splenic white pulps is accompanied by infiltration of HIV-specific cytotoxic T lymphocytes

Human immunodeficiency virus (HIV) replication and T cell proliferation were investigated in situ by a PCR-based analysis of individual microdissected splenic white pulps. Founder effects, revealed by an exquisite compartmentalization of HIV genotypes and T cells, indicated the recruitment of latently infected CD4+ T cells through highly localized antigen presentation rather than the infection of CD4+ T lymphoblasts by blood-borne virus or immune complexes. HIV-infected white pulps could be infiltrated by HIV-specific cytotoxic T lymphocytes, thereby implicating them in CD4+ T cell destruction in vivo. Together these data describe an iterative and deleterious mechanism of antigen-driven T cell recruitment and activation, as well as HIV replication and spread, with consequent destruction of the newly infected cells.

In vivo CD40-gp39 interactions are essential for thymus-dependent humoral immunity. I. In vivo expression of CD40 ligand, cytokines, and antibody production delineates sites of cognate T-B cell interactions

T-B cell interactions have a central role in the development of antibody responses. Upon activation, T helper (Th) cells express the ligand for CD40, gp39, which is essential for Th cell-dependent B cell activation. The cytokines produced by activated Th cells have a regulatory role in B cell differentiation. In this study, we investigated, using immunohistochemical techniques, the in vivo time course and localization of gp39 expression and cytokine production in relation to the specific antibody production. Both the immunization with keyhole limpet hemocyanin (KLH), a thymus-dependent (TD) antigen, and trinitrophenyl (TNP)-Ficoll, a thymus-independent type 2 (TI-2) antigen, induced Th cells to express gp39. The expression of gp39 was restricted to Th cells in the outer periarteriolar lymphocyte sheaths (outer-PALS) and around the terminal arterioles (TA). Incidentally, gp39+ Th cells were found in the corona of follicles, whereas gp39+ cells were never found in the germinal centers or marginal zones of the spleen. Maximum frequencies of gp39+ cells were observed 3 and 4 d after primary and secondary immunization with KLH. After injection of TNP-Ficoll, a marked increase in gp39+ cells was observed, confirming previous observations that activated T cells are involved in TI-2 antibody responses. Analysis of the in vivo cytokine production revealed that interleukin 2 (IL-2)-, IL-4- and interferon gamma (IFN-gamma)-producing cells (IFN-gamma-PC) developed according to similar kinetics as observed for gp39+ cells. IL-2-PC and IL-4-PC were present in higher frequencies as were IFN-gamma-PC in the immune response against TNP-KLH. Double staining experiments revealed gp39+ Th cells producing IL-2, IL-4, or IFN-gamma, suggesting that these cells were involved in both the initial activation as well as the differentiation process of B cells into antibody-forming cells. Dual immunohistochemical analysis revealed gp39+ T cells and cytokine-PC in close proximity to antigen-specific, antibody-forming B cells. In conclusion, this study shows that in vivo gp39 is expressed on activated Th cells after immunization with TD and TI-2 antigens. Furthermore, the time course and compartmentalization of gp39+ expression, cytokine production and antibody formation after immunization suggest that cognate T-B cell interactions and T cell-regulated B cell differentiation occur in the outer-PALS and around the TA of the spleen.

In vivo distribution of particulate antigens and liposomes in murine spleen. A possible role in the humoral immune response

Several particulate antigens and liposomes were intravenously injected in mice in order to study their localization patterns in spleen and liver. Liposomes have been proposed as promising carriers for haptens and antigens. It was studied whether the phospholipid composition, cholesterol content and charge of the liposomes played a role in their distribution within the spleen. Different thymus-independent type 1 and type 2 and thymus-dependent particulate antigens as well as liposomes were labeled with the lipophilic fluorochrome Di-I. After labeling they were intravenously injected and spleens and livers were removed at different time intervals and prepared for light- and fluorescence-microscopy. We have observed that all particulate antigens and liposomes administered to the mice localized according to the same distribution pattern in the spleen. After 2 and 4 h particles were located in macrophages of the marginal zone and after 24 h white pulp macrophages had also ingested particulate antigens and liposomes. So we conclude that the distribution of the particulate antigens and liposomes in the spleen is independent of the immunological nature of the particles. Results are discussed with respect to the question whether or not the distribution of particulate antigens and liposome associated antigens or haptens, may be a crucial factor in determining the type of immune response to be elicited.

Ia-antigen-T-cell interactions for a thymus-independent antigen composed of D amino acids

Synthetic polypeptide antigens of L amino acids, although bearing repeating sequences, are thymus-dependent (L-TD), whereas the same polymers composed of D amino acids are thymus-independent (D-TI), probably due to a slower rate of metabolism. Yet we found that lymph-node cells of BALB/c mice immunized with D-TI proliferate in response to it in vitro. To follow T-cell activation by D-TI, we established T-cell hybridomas to D-TI and to its analog composed of L isomers, L-TD, for comparison. The T-cell hybridomas express membrane alpha/beta T-cell receptors and secrete interleukin 2 upon stimulation with the respective antigen. In addition, D-TI-specific hybridomas are stimulated, to a lesser extent, by the L-TD antigen, whereas only some L-TD-specific hybridomas recognize D-TI. Moreover, biotinylated analogs of D-TI and L-TD bind to splenic antigen-presenting cells (APCs) from BALB/c mice. Binding is inhibited by an excess of nonbiotinylated L-TD, and by an excess of a peptide comprising residues 259-271 of the human acetylcholine receptor alpha subunit, which binds to I-Ad and I-Ed molecules without prior processing. Analysis of APC lysates following incubation of the APCs with biotinylated D-TI and L-TD reveals that the biotinylated antigen moiety is associated with Ia molecules. D-TI and L-TD bind to Ia molecules on intact APCs with similar KD values, 5 x 10(-8) M and 3 x 10(-8) M, respectively. However, D-TI has faster kinetics of binding than L-TD, probably due to different processing requirements. Hence, we have demonstrated a major histocompatibility complex class II-mediated T-cell response to a thymus-independent antigen.