Optimal strategies in immunology

After a first encounter with most antigens, the immune system responds to susequent encounters with a faster, more efficient and more strenuous antibody response. The memory of previous antigen contacts is carried by lymphocytes. Expanding on the model developed in Part 1 of this paper, we examine the optimal strategy available to the immune system for B memory cell production. We again find that the strategy should be of the bang-bang variety. The model we consider assumes that antigen triggers a subpopulation of B-lymphocytes. These triggered lymphocytes can proliferate and secrete modest amounts of antibody, or differentiate into non-dividing plasma cells which secrete large amounts of antibody, or differentiate into non-antibody secreting memory cells. Given injections of antigen at two widely spaced times we compute the strategy which minimizes a linear combination of the primary and secondary response times. We find that for all biologically reasonable parameter values the best strategies are ones in which memory cells are produced at the end of the primary response. Exerimental results which bear on the actual strategies employed are discussed.     

The making of a professional secretory cell: architectural and functional changes in the ER during B lymphocyte plasma cell differentiation

B lymphocytes are small cells that express antigen receptors and secrete little if any IgM. Upon encounter with antigen, they differentiate into short-lived plasma cells, which secrete large amounts of polymeric IgM. Plasma cell differentiation entails a massive development of the endoplasmic reticulum to sustain high levels of Ig production. Recent findings suggest a role for the unfolded protein response in orchestrating the architectural and functional changes during terminal plasma cell differentiation.     

Two independent pathways of helper activity provided by a single T cell clone

Data presented in this paper demonstrate the existence of two separate pathways by which a single T cell clone can induce B cell differentiation. With the use of high doses of antigen, a T cell clone can induce a primary antibody response in unprimed B cells. With the use of low doses of antigen, the same T cell clone can induce an immunoglobulin (Ig)G response in primed B cells. The primary response is accompanied by T cell proliferation and lymphokine production (interleukin 2, B cell growth factor, B cell differentiation factor for immunoglobulin M, and B cell differentiation factor for immunoglobulin G). The secondary response does not require proliferation and occurs independently of detectable lymphokine production. Variants of the wild type T cell helper clone have been isolated. One variant can provide help to unprimed B cells when high doses of antigen are used. This variant cannot provide help to primed B cells when low doses of antigen are used, nor can it provide help to CBA/N "xid" B cells at any antigen concentration tested. Additional variants have been isolated that proliferate on antigen-pulsed-presenting cells, but fail to secrete detectable lymphokines and do not induce B cell differentiation. These results suggest that a single T cell helper clone has multiple functional activities that can be independently expressed.     

Verification of immune response optimality through cybernetic modeling

An immune response cascade that is T cell independent begins with the stimulation of virgin lymphocytes by antigen to differentiate into large lymphocytes. These immune cells can either replicate themselves or differentiate into plasma cells or memory cells. Plasma cells produce antibody at a specific rate up to two orders of magnitude greater than large lymphocytes. However, plasma cells have short life-spans and cannot replicate. Memory cells produce only surface antibody, but in the event of a subsequent infection by the same antigen, memory cells revert rapidly to large lymphocytes. Immunologic memory is maintained throughout the organism's lifetime. Many immunologists believe that the optimal response strategy calls for large lymphocytes to replicate first, then differentiate into plasma cells and when the antigen has been nearly eliminated, they form memory cells. A mathematical model incorporating the concept of cybernetics has been developed to study the optimality of the immune response. Derived from the matching law of microeconomics, cybernetic variables control the allocation of large lymphocytes to maximize the instantaneous antibody production rate at any time during the response in order to most efficiently inactivate the antigen. A mouse is selected as the model organism and bacteria as the replicating antigen. In addition to verifying the optimal switching strategy, results showing how the immune response is affected by antigen growth rate, initial antigen concentration, and the number of antibodies required to eliminate an antigen are included.     

Production of a monoclonal antibody to the arylphorin of Heliothis zea

A species-specific monoclonal antibody was produced to whole plasma of fifth instar larvae of the corn earworm, Heliothis zea (Boddie) (Lepidoptera:Noctuidae). This antibody did not cross-react with proteins from the plasma of any of the other lepidopteran larvae tested, including other Heliothis spp. The antigen recognized by this antibody was characterized and found to be arylphorin, a prominent larval storage protein. This conclusion was based on the electrophoretic as well as immunological characteristics of the antigen. Polyacrylamide gel electrophoresis indicated that the antigen had an apparent native molecular weight of 460,000, and that it was composed of subunits having apparent molecular weights of 76,000. The isoelectric point of the antigen was 5.9, with some microheterogeneity being seen. Western blotting of arylphorin against the monoclonal antibody clearly identified the antigen as arylphorin. This protein was not found in egg homogenates or early instar larval plasma, but it was present in large quantities in pupal homogenates and, at trace levels, in adult homogenates. The efficient production and selection of hybridomas producing antibodies specific to H. zea arylphorin using unfractionated plasma as immunogen illustrates the fact that monoclonal antibody technology can produce highly specific antibodies using crude antigen preparations. We discuss the tradeoffs one must accept when choosing this strategy over one using purified immunogens.     

Cyclic kinetics and mathematical expression of the primary immune response to soluble antigen

The conveyer hypothesis is based on the fact that because of clone predetermination, antibody production takes place in an organism without the presence of antigen as a result of natural cell differentiation. Soluble antigen is an analogue of a specific mitogen which gives rise to reproduction mainly of cells carrying on their surface the immunoglobulin receptors to the given antigen. The mathematical model of the conveyer hypothesis takes into account the initial conditions, among them the background level of antibody-producing cells before injection of a soluble antigen, migration of precursor cells in the draining lymphoid organ, and the rate of precursor differentiation, including the rate of the change of the immunoglobulin receptor number on the cell surface. Changes of antigen concentration in blood determine the intensity of precursor proliferation. Comparison of real experiments (intraperitoneal injection of capsular antigen ofPasteurella pestis into inbred mice) with calculations done on the basis of the developed mathematical model shows a definite qualitative resemblance with the kinetics of antibody-producing cells and free antibodies as well as with the decrease of free antigen concentration in blood. In spite of some differences between model experiments and real experiments the conveyer hypothesis and its mathematical model appear suitable for describing the primary immune response of mice immunized with low doses of capsular antigen ofPasteurella pestis.     

Synergy of helper factors in the differentiation of in vivo-preactivated antigen-specific human B cells

After in vivo immunization with antigen, B cells appear in the peripheral blood which can be induced in vitro by nonspecific factors found in mixed lymphocyte culture supernatants (MLC-SN) to differentiate and secrete antibody specific for the immunizing antigen. In order to further delineate the nature of the factors involved in the differentiation of these in vivo-activated B cells, various helper factors, including interleukin 1 and interleukin 2 (IL-1 and IL-2), B-cell growth factor (BCGF), and B-cell differentiation factor (BCDF) were added separately and in combination to cultures of these preactivated B cells. T-cell-depleted fractions of peripheral blood mononuclear cells were obtained from normal individuals immunized in vivo with keyhole limpet hemocyanin. MLC-SN alone, without the addition of antigen, selectively triggered an antibody response specific for the antigen used to immunize in vivo in the absence of a polyclonal B-cell response. In order to obtain responses equal to those seen with MLC-SN, a combination of BCGF, IL-2, and BCDF was required, although any two factors partially reconstituted the response. Exogenous IL-1 had the least effect but was suppressive in the presence of optimal concentrations of monocytes. Thus, for maximal in vitro differentiation of in vivo-preactivated B cells, a combination of at least three helper factors is required and acts in a synergistic manner to induce antigen-specific antibody responses.     

Collaboration of Th1 and Th2 T cell clones in specific antibody responses: regulation of the IgM response to phosphorylcholine

Carrier (KLH)-specific type 1 T cell clones (Th1), which are defined by secretion of IL-2 and IFN-gamma but not IL-4, and type 2 (Th2) clones, which secrete IL-4, but not IL-2 or IFN-gamma, have been isolated and analyzed for their ability to collaborate in providing help for B cells to secrete phosphorylcholine-specific IgM antibodies. The resulting antibody responses exhibited a characteristic pattern suggesting two distinct regulatory interactions among the Th1, Th2, and B cells. At low doses of antigen, Th1 cells enhanced the helper function of the Th2 cells, an effect due primarily to IL-2. At high doses of antigen, Th1 cells or IFN-gamma inhibited Th2-dependent antibody responses. The inhibitory effect of Th1 or IFN-gamma affected primarily the hapten-carrier-linked portion of the response. The overall effect was a modulation of the antigen dose-response curve for antibody production, eliminating the sharp increases in dose response mediated by isolated T cell clones. The data suggest that collaborative interactions of Th1 and Th2 cells in antibody production may have important physiological consequences.     

Application of an enzyme‐labeled antigen method for visualizing plasma cells producing antibodies against Strep A,a carbohydrate antigen of Streptococcus pyogenes,in recurrent tonsillitis

Streptococcus pyogenes is the main causative pathogen of recurrent tonsillitis. Histologically, lesions of recurrent tonsillitis contain numerous plasma cells. Strep A is an antigenic carbohydrate molecule on the cell wall of S. pyogenes. As expected, plasma cells in subjects with recurrent tonsillitis secrete antibodies against Strep A. The enzyme‐labeled antigen method is a novel histochemical technique that visualizes specific antibody‐producing cells in tissue sections by employing a biotin‐labeled antigen as a probe. The purpose of the present study was to visualize plasma cells producing antibodies reactive with Strep A in recurrent tonsillitis. Firstly, the lymph nodes of rats immunized with boiled S. pyogenes were paraformaldehyde‐fixed and specific plasma cells localized in frozen sections with biotinylated Strep A. Secondly, an enzyme‐labeled antigen method was used on human tonsil surgically removed from 12 patients with recurrent tonsillitis. S. pyogenes genomes were PCR‐detected in all 12 specimens. The emm genotypes belonged to emm12 in nine specimens and emm1 in three. Plasma cells producing anti‐Strep A antibodies were demonstrated in prefixed frozen sections of rat lymph nodes, 8/12 human specimens from patients with recurrent tonsillitis but not in two control tonsils. In human tonsils, Strep A‐reactive plasma cells were observed within the reticular squamous mucosa and just below the mucosa, and the specific antibodies belonged to either IgA or IgG classes. Our technique is effective in visualizing immunocytes producing specific antibodies against the bacterial carbohydrate antigen, and is thus a novel histochemical tool for analyzing immune reactions in infectious disorders.     

Interleukin 12 and CD86 Regulate Th1 and Th2 Development Induced by a Range of Antigen Doses Presented by Peyer's Patch and Spleen Cells

In this study, we demonstrate the role of interleukin 12 (IL-12), CD80 and CD86 in T helper type 1 (Th1) and Th2 differentiation induced through antigen presentation by Peyer's patch (PP) and spleen (SPL) cells with various doses of antigen. IL-12 was found to be critical for the induction of Th1-type cytokine producing cells, while antigen-dose dependent patterns of differentiation into Th2-type cytokine producing cells were not altered by the blockade of IL-12. Further, the difference in the pattern of Th2-type cytokine producing cell differentiation induced by PP and SPL cells depending on the antigen dosage were preserved in the absence of IL-12. When the function of CD86 was blocked by specific antibody, the induction of Th1-type cytokine producing cells was kept at high levels through every antigen dose, and the difference between PP and SPL cells was abrogated. With regard to Th2 induction, CD86 enhanced the differentiation of Th2-type cytokine producing cells but it was not essential in the case of antigen presentation by SPL cells. These results suggest that antigen-dose dependent changes in Th2 cell induction are regulated by additional factors which cannot induce antigen-dose dependent changes in Th1 cell differentiation by themselves.     

Stimulation of human B cells specific for Candida albicans for monoclonal antibody production

Abstract The stimulation and immortalisation of human peripheral blood B lymphocytes specific for Candida albicans antigen were investigated. An in vitro immunisation system was employed which involved pretreatment of mononuclear cells with l -leucyl l -leucine methyl ester which removes the suppressive effects of CD8 + T cells, NK cells and monocytes. The remaining cells, CD4 + T cells, B cells and dendritic cells, were cultured with antigen and a mixture of cytokines. A mixture of IL-2, −4 and −6 was found to be optimal for antibody production as determined by an Elispot assay. Transformation of the activated B cells by Epstein Barr virus was found to be optimal after 2 days and lines secreting anti- Candida antibodies were established. These lines could form the basis for specific monoclonal antibody production by generating hybridomas, or by a newly described technique whereby cDNA encoding antibody Fab regions is transferred into phage display libraries. The overall strategy might be generally applicable for the generation of human monoclonal antibodies to infectious agents.     

Mathematical model of clonal selection and antibody production. II

Several modifications are proposed to a recent mathematical model (Bell, 1970) of the clonal selection and antibody production which take place when an adult animal is injected with an antigen. In the original model, antigen molecules were assumed univalent, i.e. to have only one combining site per molecule, and the first modification is an allowance for multivalent antigens by permitting an antigen molecule to interact with only one cell at a time. It is found that, for antigens with few (? 10) sites per molecule this modification is not important while for many sites per molecule, the modification will reduce the response as compared to that from the same number of independent antigenic sites. In section 3, it is seen that by restricting the number of cells which can arise in an immune response, much more realistic responses to high antigen doses are obtained. Moreover, the response is made more predictable by assuming that both target and proliferating cells are stimulated by antigen when a fraction of their receptors, between f_min and f_max, is bound to antigen. The parameter f_min is shown to determine the extent to which a molecule which can be recognized by antibodies will also serve as an immunogen giving rise to cellular proliferation and antibody production. In section 4, it is found that if more plasma cells than memory cells result from antigen stimulation, then weak stimulation will lead to a depletion of target plus memory cells and to at least partial immunological paralysis. Optimum antigen doses and times for the induction of such paralysis are examined. In section 5, precipitation of antigen-antibody mixtures is considered and it is shown that the number of doubly bound antibody molecules per antigen site determines whether precipitation will occur. This number is easily computed for heterogeneous bivalent antibodies.     

A monoclonal antibody with reactivity restricted to normal and neoplastic plasma cells

A monoclonal antibody that defines a new and distinct plasma cell antigen, termed PC-1, was developed against human plasmacytoma cells. This antigen is strongly expressed on normal plasma cells isolated from bone marrow and on abnormal plasma cells isolated from myelomas, plasma cell leukemias, and plasmacytomas. The antigen is not detected on normal T or B lymphocytes, granulocytes, or monocytes, and with the exception of plasma cells, is absent on malignancies of B, T, or myeloid origin. Utilizing pokeweed mitogen to induce human B lymphocyte differentiation in vitro, PC-1 is expressed when B cell determinants are lost and the plasmacytoid morphology, intracytoplasmic immunoglobulin-staining, and surface PCA-1- and T10-staining characteristic of plasma cells appear. This antigen is useful for the study of the terminal stages of normal B cell differentiation to plasma cells, and may offer insight into the heterogeneity of the plasma cell dyscrasias.     

Mucus-associated antigen in epithelial cells of the chicken digestive tract: Developmental change in expression and implications for morphogenesis-function relationships

The embryonic chicken digestive tract consists of endodermal epithelium and mesenchyme derived from splanchnic mesoderm. Interactions between these two tissues are important for the establishment of regionality and the subsequent differentiation of digestive organs. In the present study we obtained a monoclonal antibody that reacted with mucus-associated antigen and named it the MA antibody. From 6 days of incubation, this antibody reacted with the esophageal, proventricular and gizzard epithelia. In the proventriculus, the MA antigen was expressed in luminal epithelial cells, while pepsinogen-producing gland cells became MA antigen-negative. The intestinal goblet cells, which secrete mucus, became positive to the antibody from day 13 of incubation. When the esophageal, proventricular or gizzard epithelium of a 6 day embryo was associated and cultivated with the proventricular mesenchyme, the luminal epithelial cells remained reactive to the MA antibody while gland cells were negative or only weakly positive. If the small-intestinal epithelium was cultivated with the proventricular or gizzard mesenchyme, the antigen was detected on the apical surface of the epithelium, suggesting that the expression of the MA antigen was induced by mesenchymal influences in the small-intestinal epithelium. These results suggest that spatio-temporally regulated expression of the MA antigen is controlled by the epithelial-mesenchymal interactions.     

T细胞记忆的理论研究

基于CD8⁺ T记忆细胞的线性和逆线性分化假说分别建立了数学模型,并研究了各种T细胞亚类的动力学.发现在优化剂量抗原入侵的条件下,两个模型均能产生记忆,并可较好地模拟实验结果.通过进一步模拟发现CD8⁺ T细胞记忆与抗原的存在紧密相关,再次证实了抗原在维持T细胞记忆中的作用.另外还讨论了记忆细胞寿命的问题.认为逆线性假说具有更强的反应性和记忆性.     

B cell abnormalities in systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is a chronic, multisystem autoimmune disease characterized by the differentiation of short- and long-lived immunoglobulin secreting plasma cells that secrete pathogenic autoantibodies. Ectopic germinal centers and plasma cells secreting autoantibodies have been observed in lupus nephritis kidneys. Candidate genetic susceptibility loci for SLE include genes that affect differentiation and survival of plasma cells, such as those that influence activation, proliferation, cytokine and chemokine secretion/responsiveness, and apoptosis of the T and B cells that are involved in humoral immunity generated in germinal centers, as well as genes that are involved in presentation and clearance of apoptotic material and autoantigens by antigen presenting cells and other phagocytes. Emerging data have demonstrated that B lymphocytes are active participants in humoral immune responses that lead to T-dependent and T-independent differentiation of immunoglobulin-secreting plasma cells by homotypic CD154-CD40 interactions as well as continued stimulation by B cell activating factor through B cell maturation antigen, B cell activating factor receptor and transmembrane activater.     

Identification of a specialized extracellular matrix component in Drosophila imaginal discs

We have generated a monoclonal antibody that recognizes a major component of a specialized extracellular matrix in Drosophila imaginal discs. In mature larvae, antibody binding is observed almost exclusively on imaginal discs. On the basal surface of the thoracic discs, the antigen is localized to particular regions of the epithelium, and ultrastructural studies indicate that the antigen is found in a fibrous network secreted between the cells and the basal lamina. The localized expression indicates that the matrix is not simply related to disc differentiation, as all regions of the columnar disc epithelium are determined to secrete adult cuticle. A correlation of the antigen distribution with known developmental events leads us to propose that the antigen-containing network provides an extensible matrix for the rapid elongation of the disc epithelium during evagination; consistent with this, the antigen is a component of the matrix between the dorsal and ventral surfaces of the evaginated wing pouch. The antigen is very large (greater than 5 X 10(5) Da), can be labeled metabolically with methionine and sulfate, and is digested by chondroitinase ABC; these biochemical characteristics indicate that the antigen is a proteoglycan.     

The Croonian Lecture, 1992. The key role of the thymus in the body's defence strategies.

For centuries the thymus has remained a mysterious organ with largely unknown functions. The first demonstration of its crucial role in the development of the immune system was reported in 1961, when it was found that mice thymectomized at birth had poorly developed lymphoid tissues, impaired immune reactivities, and an inordinate susceptibility to develop infections. Although thymus lymphocytes were for a long time deemed immunoincompetent, it was shown in 1967 that they could respond to antigen by proliferating to give rise to a progeny of cells which did not secrete antibody (T cells), but which had a remarkable ability to induce bone marrow cells (B cells) to become antibody formers. This was the first unequivocal demonstration of a major division of labour among mammalian lymphocytes. Tremendous progress in our understanding of the function of the thymus and of the T cells derived from it followed. Distinct T cell subsets were characterized and shown to have an essential role in initiating and regulating a variety of immune responses. The ontogenetic events which occurred during their differentiation were mapped, and this allowed studies of the selection of the T cell repertoire. The major histocompatibility complex and associated peptides were shown to govern T cell selection and antigen activation, and the antigen-specific T cell receptor and the genes which code for it were characterized. Future studies should allow some insight into how to activate T cells more effectively for vaccination purposes, and how to switch them off to prevent autoimmune reactions and to induce tolerance to transplanted tissues.     

Mitogens and T-independent antigens stimulate T lymphocytes to secrete La antigens.

Previous reports from this laboratory suggest that certain I region-associated (Ia) antigens can be detected in normal mouse serum. It was found that, when mitogens are injected into mice, they produce substantial increases (up to 125-fold) in the levels of these Ia antigens in mouse serum. Similar increases were obtained when either T- or B-cell mitogens were injected. Furthermore, in vitro and in vivo studies demonstrated that the mitogens stimulated T cells to secrete Ia antigens. It appears likely, however, that the Ia antigens detected in these studies may differ from the conventional Ia glycoproteins found on the surface of B lymphocytes.All T-independent antigens tested also augmented the concentrations of Ia antigen in serum, the increases depending on the T-independent antigen injected and ranging from 3- to 125-fold. In contrast, T-dependent antigens, unless injected in large amounts, were unable to produce detectable changes in the serum levels of Ia antigen. These data indicate that an inverse relationship exists between the T dependence of an antigen and its ability to stimulate T cells to secrete Ia antigens. On the basis of this conclusion it is proposed that all antigens are T dependent and merely vary in the efficiency with which they activate T cells to release helper factors.