The unresponsiveness of aged mice to polysaccharide antigens is a result of a defect in macrophage function

A reduction in macrophage (MPhi) function with aging makes mice less responsive to bacterial capsular polysaccharides, such as those present in the pneumococcal polysaccharide vaccine, a model of thymus independent (TI) antigen (Ag). Using trinitrophenol (TNP)-lipopolysaccharide (LPS) and TNP-Ficoll, two other well-studied TI Ag, we studied the mechanistic basis of reduced MPhi function in the aged. We show that aged mice are profoundly hyporesponsive to these TI Ag. As a result of a requirement for MPhi, highly purified B cells from young-adult mice do not respond to TI Ag. When purified, young B cells were immunized with TNP-Ficoll, the antibody production from those cultures reconstituted with MPhi from aged mice was significantly lower than that seen with young MPhi. Consequently, this unresponsiveness can be overcome by a mixture of interleukin (IL)-1beta and IL-6. Upon stimulation with LPS, in comparison with young MPhi, aged MPhi secreted reduced amounts of IL-6, tumor necrosis factor alpha, IL-1beta, and IL-12, cytokines necessary for B cells to respond to TI Ag. LPS also induced aged MPhi to produce an excess of IL-10. Neutralization of IL-10 enhanced the production of proinflamatory cytokines by MPhi upon LPS stimulation and also induced Ab production by aged splenocytes. Thus, the inability of aged MPhi to help the B cell response appears to be caused by an excess of IL-10. As aged MPhi have a reduced number of cells expressing Toll-like receptor 4 and CD14, the imbalance in cytokine production might be partly a result of fewer cells expressing key components of the LPS receptor complex.     

Defective antigen-presenting cell function in human neonates

Immaturity of the immune system has been suggested as an underlying factor for the high rate of morbidity and mortality from infections in newborns. Functional impairment of neonatal T cells is frequently quoted as the main underlying mechanism for such immaturity. However, recent studies suggest that neonatal antigen-presenting cells (APCs) also exhibit functional alterations, which could lead to secondary defects of adaptive T-cell responses. In this review, we summarize what is known on the functionality of APC at birth and during early childhood. Compared to adults, neonatal APCs display markers of immaturity and produce low levels of cytokines. Multiple factors could be involved in neonatal APC alteration, such as intrinsic immaturity, defective interaction between APCs and T cells and regulatory T-cell-mediated inhibition. Characterization of the relative contribution of each mechanism is clearly needed to better understand the functional capability of the neonatal immune system.     

Murine macrophage interleukin-1 release by capsularlike serotype-specific polysaccharide antigens of Actinobacillus actinomycetemcomitans.

Serotype-specific polysaccharide antigens (SPAs) were extracted from whole cells of Actinobacillus actinomycetemcomitans ATCC 29523 (serotype a), Y4 (serotype b), and NCTC 9710 (serotype c) by autoclaving and purified by chromatography on DEAE-Sephadex A-25 and Sephacryl S-300 columns. Y4 SPA induced interleukin-1 (IL-1) release by P388D1 murine macrophages. Polymyxin B had virtually no effect on the release of IL-1. Rabbit anti-murine IL-1 serum strongly suppressed the proliferation of C3H/HeJ mouse thymocytes induced with the culture supernatants of Y4 SPA-stimulated P388D1 cells and a submitogenic dose of concanavalin A. Gel filtration of the culture supernatants of Y4 SPA-stimulated macrophages on Sephacryl S-200 showed that an IL-1 peak at a point corresponding to approximately 16.5 kDa was eluted. The ability of SPAs from strains ATCC 29523 and NCTC 9710 to induce the release of IL-1 was lower than that of Y4 SPA. The IL-1-releasing ability of serotype a and c antigens was enhanced by deacetylation of both polysaccharides, suggesting that acetyl groups of these antigens might hinder the interaction between the antigens and macrophages.     

Inter-mouse strain differences in macrophage function and its relationship to antibody responses

Macrohage function, as measured by carbon clearance, differs in different inbred strains. Low affinity (K_R) antibody response to HST is associated either with poor carbon clearance, or slow recovery from carbon blockade by macrophages.     

Immunological unresponsiveness to protein antigens in rabbits: I. The duration of unresponsiveness following a single injection at birth

The immunological responses of rabbits to HSA, HGG or BSA were tested at various times later in animals which had received the corresponding antigens before or shortly after birth. As judged by the criterion of failure to show immune elimination of antigen, a high proportion of the rabbits remained unresponsive at times when it was calculated that all the originally administered antigen would have been eliminated from the circulation. Furthermore, removal of antigen by passively administered antibody failed to restore the capacity to respond. It is concluded that, in respect of the antigens used, their persistence in the extracellular body fluids is not a prerequisite for maintenance of immunological unresponsiveness.

Further administration of the same antigen to rabbits which had escaped from a state of specific immunological unresponsiveness generally produced a very weak response, and in a few instances resulted in a return to the unresponsive state.

When the cross-reacting antigens HSA and BSA were administered adsorbed on alum to rabbits made unresponsive by neonatal contact with BSA and HSA respectively, and at the same time a further dose of the original antigen was given, antibodies were formed which were specific for the second antigen and did not cross-react with the first. In only 1/9 animals was responsiveness to the first antigen restored. The significance of these results is discussed.

     

Immunoglobulin isotype in the murine response to polysaccharide antigens

Murine antisera specific for the α (1→3) and α (1→6)-linked glucosyl determinants of dextran, as well as for meningococcal polysaccharide group C, have been examined for the distribution of their immunoglobulin classes and subclasses. Whereas the thymus-independent anti-α (1→3) dextran response in BALB/c mice was found to be IgM > IgG₃ > IgA, thus corresponding to previously published work, neither the α (1→6) response in its thymus-dependent or-independent form, nor the response to purified meningococcal polysaccharide, corresponded to this pattern. No preference for any of the IgG subclasses appeared for these antigens when given as thymus-independent carbohydrates. On the other hand, thymus-dependent forms of α (1→6) dextran showed an IgG₁ > IgG₃ > IgG₂ pattern.     

Monoclonal antibodies to human macrophage and leucocyte common antigens

Three monoclonal antibodies have been made to identify cells of the human mononuclear phagocyte system in fluids and tissues. The first, PHM 1, recognises a surface antigen common to all leucocytes. The other 2 antibodies, PHM 2 and PHM 3, bind to monocytes and macrophages but not to polymorphonuclear cells (PMN). PHM 2 labels all monocytes, macrophages and a small population of T-cells. PHM 3 labels most monocytes and macrophages but no other blood cells. The application of these monoclonal antibodies to the identification of mononuclear phagocytes in blood and liver using an unlabelled antibody immunoperoxidase (PAP) technique is demonstrated.     

Defective macrophage antigen presentation following haemorrhage is associated with the loss of MHC class II (Ia) antigens

Although recent studies indicate that severe and prolonged haemorrhage, despite adequate fluid resuscitation, induces profound depression of cell-mediated immunity, the mechanism of this remains unknown. Since macrophages (M phi) play a key role in the development of a competent immune response by the presentation of antigens, the study investigated (i) whether the capacity of the M phi to present antigen is altered even following mild hypotension, and (ii) what effects do different degrees of hypotension have on the M phi-mediated processes associated with antigen presentation (i.e. the expression Ia antigen, membrane-associated IL-1 or the secretion of IL-1). The results indicate that a minimal drop in blood pressure to approximately 50 mmHg (1 hr duration) was sufficient to depress M phi antigen presentation (AP). Similarly, even a transient hypotensive episode of 15 min duration at 35 mmHg was sufficient to produce a pronounced decline in AP. Decreased AP was observed as early as 12 hr after the haemorrhagic episode (35 mmHg; 1 hr) and remained detectable for at least 120 hr thereafter. The reductions in AP capacities were qualitatively similar in both peritoneal and splenic populations, and were not attributable to surgical stress, heparinization or ether anaesthesia. Determination of IL-1 production, as well as membrane-bound IL-1 levels, in these cell populations showed no significant difference from controls. However, a significant decrease was observed in the percentage of Ia antigen (MHC class II)-positive M phi, suggesting that reduced AP following haemorrhage may be related to the inability of these cells to express Ia.     

Immunological unresponsiveness to protein antigens in rabbits: II. The nature of the subsequent antibody response

Rabbits made immunologically unresponsive by neonatal administration of HSA, HGG or BSA were given a course of intravenous injections of the respective antigens, adsorbed on alum, after a lapse of 13–27 months since the last administration of antigen. 8/12 responded to HSA, 4/5 to HGG, 9/10 to BSA, as judged by immune elimination of antigen, but this was delayed in onset and slow compared with that in previously untreated rabbits. The antibody formed was small in quantity and usually failed to precipitate with antigen.

The sedimentation coefficients of ¹³¹I-labelled antigens, in the presence of excess antibody, were measured by ultracentrifugation through a sucrose density gradient. These showed that only small complexes were formed in some of the non-precipitating antisera. In one instance the diffusion coefficient of the complex was also measured, by a technique based on diffusion through agar gel. The calculated molecular weight of the complex, 330,000 indicated the presence of only two combining sites on the antigen.

Combination of the anti-HSA sera with an HSA fragment was also measured. Whereas the amount of the fragment bound by ordinary hyperimmune anti-HSA sera was about one-fifth the HSA bound, the amounts bound by the test sera were relatively much less. Some non-precipitating sera failed to bind the fragment, although they bound HSA.

These findings indicate that following neonatally induced immunological unresponsiveness the capacity to respond to antigen returns piecemeal in respect of different parts of the antigenic mosaic, and that it may be severely restricted. The theoretical implications are discussed.

     

Modulation of human macrophage activity by Ascaris antigens is dependent on macrophage polarization state

Parasitic worms (helminths) are known to actively modulate host immune responses and inflammation. The aim of this study was to investigate if adult body fluid (ABF) from the helminth Ascaris suum has immunomodulatory effects on different subtypes of human monocyte-derived macrophages (M?) in vitro. M?s were exposed to A. suum ABF at different stages of their differentiation and/or polarization. M? were first differentiated from monocytes into either uncommitted (M-), classically activated (M(GM-CSF)) or alternatively activated (M(M-CSF)) phenotypes and then stimulated with lipopolysaccharide (LPS). ABF strongly suppressed LPS-induced TNF-α, IL-6 and IL-10 secretion in M(GM-CSF)s, however in M(M-CSF)s only TNF-α was suppressed, with these cells secreting high levels of IL-10 which was not affected by ABF treatment. To determine if ABF modulated the differentiation of previously uncommitted M? to either type 1 or type 2 M?, monocytes were differentiated with human serum into (M-)s and then polarized by IFN-γ/LPS or IL-4 treatment in the presence of ABF. Under these conditions, ABF did not modulate cytokine secretion but did reduce CD80 expression in IFNγ/LPS-polarized cells but not IL-4-polarized cells. Finally, we demonstrate that when monocytes are differentiated into M(GMCSF)s in the presence of ABF, subsequent inflammatory responses are markedly suppressed. Our data suggest that ABF inhibits cytokine secretion and co-stimulatory molecule expression in classically activated M? but not in alternatively activated M?, indicating selective action of ABF depending on M? subtype. Moreover, ABF appears to exert stronger activity when acting upon M? that have already been polarized to the type 1 phenotype, rather than influencing the polarization process per se.     

T-cell modulation of the antibody response to bacterial polysaccharide antigens.

Pretreatment of mice with subimmunogenic doses of meningococcal polysaccharide (MP), Pseudomonas aeruginosa lipopolysaccharide (PA), or Streptococcus mutans polysaccharide (SM) resulted in suppression of antibody response. The transfer of putative suppressor T cells (Ts cells) from donor mice primed with a subimmunogenic dose of MP to naive recipients at the time of immunization with MP substantially reduced the magnitude of the antibody response. Also, the infusion of B cells taken from animals immunized with either MP or PA suppressed the antibody response of naive recipients to MP or PA, respectively, relative to controls, suggesting that Ts cells respond to determinants on immune B cells. We observed that the injection of concanavalin A or phytohemagglutinin (two lectins known to augment the activity of amplifier T cells [Ta cells]) 2 days postimmunization enhanced the antibody response to MP and SM. In addition, Ta-cell activity was transferred to naive animals by using spleen cells. Although the administration of phytohemagglutinin at the time of immunization with MP also resulted in increased antibody response, the injection of concanavalin A simultaneous with immunization resulted in a suppression of the antibody response to MP. Although Ts cells generated in response to pneumococcal polysaccharide type III were found to respond to monoclonal antibody Ly-m22, Ta cells responded to monoclonal antibodies L3T4 and Ia but not to Ly-m22. These studies suggest that Ta and Ts cells can modulate the antibody response to MP, SM, and PA in a positive and negative manner, respectively.     

Contribution of metabolic reprogramming to macrophage plasticity and function

Macrophages display a spectrum of functional activation phenotypes depending on the composition of the microenvironment they reside in, including type of tissue/organ and character of injurious challenge they are exposed to. Our understanding of how macrophage plasticity is regulated by the local microenvironment is still limited. Here we review and discuss the recent literature regarding the contribution of cellular metabolic pathways to the ability of the macrophage to sense the microenvironment and to alter its function. We propose that distinct alterations in the microenvironment induce a spectrum of inducible and reversible metabolic programs that might form the basis of the inducible and reversible spectrum of functional macrophage activation/polarization phenotypes. We highlight that metabolic pathways in the bidirectional communication between macrophages and stromals cells are an important component of chronic inflammatory conditions. Recent work demonstrates that inflammatory macrophage activation is tightly associated with metabolic reprogramming to aerobic glycolysis, an altered TCA cycle, and reduced mitochondrial respiration. We review cytosolic and mitochondrial mechanisms that promote initiation and maintenance of macrophage activation as they relate to increased aerobic glycolysis and highlight potential pathways through which anti-inflammatory IL-10 could promote macrophage deactivation. Finally, we propose that in addition to their role in energy generation and regulation of apoptosis, mitochondria reprogram their metabolism to also participate in regulating macrophage activation and plasticity.     

Effect of lead on macrophage function

Lead (Pb) has been shown to alter various parameters of immune function such as host resistance and antibody formation. In addition, various heavy metals have been implicated as inducers of autoimmunity. In these experiments, macrophages, isolated from the peritoneal cavity of mice exposed to various doses of lead in vivo as well as cells exposed in vitro were tested for the following immunologic parameters: phagocytosis, antigen presentation, interleukin 1 production, and their ability to stimulate the autologous mixed lymphocyte reaction (AMLR). The results obtained indicate that Pb appears to alter the ability of macrophages to present antigen by enhancing the AMLR while having no effect on phagocytosis or IL-1 production. These data suggest that Pb may interfere with antigen-specific interactions between macrophages and T cells.