Tolerize one, tolerize them all: tolerance is self-assertion

Individuation and the Organization in Complex Living Ecosystem: Recursive Integration and Self-assertion by Holon-Lymphocytes

Individuation and organization in complex living multi-level ecosystem occurs as dynamical processes from early ontogeny. The notion of living "holon" displaying dynamic self-assertion and integration is used here to explain the ecosystems dynamic processes. The update of the living holon state according to the continuous change of the dynamic system allows for its viability. This is interpreted as adaptation, selection and organization by the human that observes the system a posteriori from its level. Our model concerns the complex dynamics of the adaptive immune system, integrating holon-lymphocytes that collectively preserve the identity and integrity of the organism. Each lymphocyte individualizes as a dynamic holon-lymphocyte, with somatic gene individuation leading to an individual, singular antigen immunoreceptor type, promoting the self-assertion. In turn, the "Immunoception" allows for perception of the environmental antigenic context, thus integration of the holon in its environment. The self-assertion/integration of holon-lymphocyte starts from fetal stages and is influenced by mother Lamarckian acquired historicity transmissions, a requisite for the integrity of the holobiont-organism. We propose a dynamic model of the perception by holon-lymphocyte, and at the supra-clonal level of the immune system functions that sustain the identity and integrity of the holon-holobiont organism.

Grafts of supplementary thymuses injected with allogeneic pancreatic islets protect nonobese diabetic mice against diabetes

In nonobese diabetic (NOD) mice, the autoimmune attack of the beta-cells in pancreatic islets is now believed to result from abnormal thymic selection. Accordingly, grafts of thymic epithelium from NOD donors to athymic recipients promote autoimmune islet inflammation in normal strains, and intrathymic islet grafts decrease the incidence of disease in NOD animals. Two competing hypotheses of abnormal thymic selection in diabetic mice have been proposed: deficient negative selection with poor elimination of aggressive organ-specific T cells vs. deficient positive selection of protective T regulatory cells. We have now addressed these alternatives by grafting, into young NOD mice whose own thymus was left intact, newborn NOD thymuses containing allogeneic pancreatic islets. If the NOD defect represented poor negative selection, these animals would develop disease at control rates, as the generation of autoreactive T cells proceeds undisturbed in the autologous thymus. In contrast, if NOD thymuses are defective in the production of T regulatory cells, lower disease incidence is expected in the chimeras, as more protective cells can be produced in the grafted thymus. The results show a reduced incidence of diabetes in the chimeras (24%) as compared with control (72%) NOD mice, throughout adult life. We conclude that amelioration of NOD mice by intrathymic islet grafts is not caused by enhanced negative selection and suggest that autoimmune diabetes in this system is the result of inefficient generation of T regulatory cells in the thymus.

Ontogeny of B cells expressing IgM in embryonic and larval tissues of the American grass frog, Rana pipiens

Affinity-purified, fluorochrome-tagged F(ab')(2) antibody fragments specific for heavy (mu) chains of Rana pipiens IgM were prepared from hyperimmune rabbit sera. By using two-color immunofluorescent procedures we observed that (1) the first cells expressing IgM, termed pre-B cells, lack detectable quantities of membrane or surface IgM but contain detectable quantities of cytoplasmic IgM (smu(-)/cmu(+)), (2) sIgM(+) B cells were the second type of IgM containing cell to appear in development, and (3) plasma cells, which contain copious quantities of cIgM, were the final phenotype to appear in the development of B cells expressing IgM. These cells were first observed in the pronephros of the developing urogenital system. Shortly after their appearance in the pronephros, cells in B lineages were observed in the liver. These observations (1) are consistent with recent studies of B lymphopoiesis in the aorta-gonad-mesonephros (AGM) region in endothermic vertebrates, including mice, (2) suggest that there are fundamental ontogenetic and phylogenetic similarities between cells and tissues of developing vertebrate immune systems, and (3) evoke questions concerning the possible function(s) of lymphocytes in developing anurans up to metamorphosis and beyond.

Leishmania amazonensis infection in nude mice

Leishmania amazonensis is an intracellular protozoan parasite of macrophages. Cutaneous leishmaniasis in an immunocompetent host begins as papules or nodules followed by ulceration at the site of promastigote inoculation. In this study, the pathological changes of cutaneous leishmaniasis lesions in T cell deficient nude mice were examined. When infected with L. amazonensis promastigotes, nude mice developed non-ulcerative cutaneous nodules. By histological examination of cutaneous lesions, massive accumulation of vacuolated histiocytes containing amastigotes was observed in all the nude mice. Although infiltration of mononuclear and polymorphonuclear cells was seen in the lesions of immunocompetent mice, few such cells were observed in the lesions of nude mice. These results indicate the importance of T cells on the ulcer formation in cutaneous leishmaniasis.

Prevention of Transplant Rejection: Can Tolerance be Achieved with Immunosuppressive Treatment?

Successful solid organ transplantation is generally attributed to the increasingly precise ability of drugs to control rejection. However, it was recently shown that a few donor haematolymphoid cells can survive for decades in recipients of successful organ allografts, a phenomenon called microchimaerism. The association for decades of haematolymphoid chimaerism with allograft tolerance in experimental transplantation suggests that immunosuppressive drugs merely create a milieu that enables an allograft and its complement of passenger leucocytes to prime the recipient for graft acceptance.Exploitation of this concept requires a fundamental shift in the classical view of passenger leucocytes only as initiators of rejection. Microchimaerism has taught us that solid organ transplantation involves the transfer of two donor organ systems to the recipient: the allograft parenchyma and the donor haematolymphoid system in the form of donor stem cells contained within the passenger leucocyte compartment. Each has the potential to integrate with the corresponding recipient system and carry out normal physiological functions, such as immunological self definition. Resistance to initial integration by mature T cells requires some form of immunosuppression, but maintenance of donor immune system function will depend on renewable supply of cells, which can be provided by engrafted progenitors. Successful clinical application will depend on the development of low morbidity methods to enhance engraftment of donor haemopoietic stem cells.

Signal minus 1: a key factor in immunological tolerance to tissue-specific self antigens?

Recent data suggest that many autoreactive T cells, particularly to tissue-specific self antigens, can escape thymic deletion. The current dogma is that these autoreactive T cells are silenced by the failure of most tissues to provide co-stimulation (signal 2), antigen alone (signal 1) inducing T cell unresponsiveness. However, I propose that activation of autoreactive T cells frequently occurs but autodestruction by effector T cells is tightly regulated. This phenomenon is most evident with lymph node metastasizing tumour cells where the regional lymph node can mount a vigorous response to the invading tumour cells but tumour growth is unimpaired. I suggest that autodestruction is prevented by inhibitory receptors on T cells which recognize class I MHC structures on target cells. These receptors, which I propose deliver 'signal minus 1' to T cells, were recently described on NK cells and a subpopulation of peripheral T cells. They are also strikingly similar to a family of anti-self receptors that my laboratory described on murine T and B cells 15 years ago. In the 'signal minus 1' model, antigen-activated T cells acquire the inhibitory receptors when they become co-stimulation independent and gain the ability to exit lymphoid organs and enter non-lymphoid tissues. Thus, if autoreactive effector T cells encounter autoantigen in tissues they are functionally silenced by inhibitory receptor engagement and signal minus 1 delivery. In contrast, I propose that in response to intracellular infections, cells down-regulate expression of their ligands for inhibitory receptors. Such a model allows infected cells to be selectively eliminated by effector T cells. If correct, the model predicts that effector T cells, whether foreign-antigen- or autoantigen-specific, can selectively respond to infected cells. This apparent 'usefulness' of autoreactive T cells may explain their observed persistence even after an encounter with autoantigen. It is also suggested that signal minus 1 may silence autoreactive B cells specific for tissue-specific cell surface antigens and lack of signal minus 1 may partially explain the vigorous T cell response to allogeneic MHC. Finally, it is hypothesized that, in evolutionary terms, inhibition of autodestruction by the recognition of a 'self marker' and delivery of signal minus 1 is an ancient process which probably emerged in early metazoans.

A model for developmentally acquired thymus-dependent tolerance to central and peripheral antigens

Current models of tolerance to peripheral, tissue-specific antigens contain some major caveats. First, they consider peripheral tolerance independently from intrathymic T cell selection, a dichotomy that is challenged by observations on TE-induced tolerance. Second, they do not account for the fact that vertebrates are more readily tolerised in development than in adult life. Third, they do not explain the fact that embryonic/neonatal tolerance to foreign tissues can only be induced by HC or TE. A model of thymic selection and peripheral tolerance is developed here that resolves those problems, by assuming two classes of T cell effector functions, one being regulatory and the other aggressive. Three postulates are required: (1) both epithelial and hemopoietic cellular compartments of the thymic stroma can support both positive and negative selection of T cells, but with vastly different avidity requirements and efficiency; (2) positively selected T cells with the highest avidity that escape deletion are activated intrathymically and irreversibly committed for regulatory effector functions; (3) the functional phenotype of all other thymic emigrants is determined in the periphery upon encounter with antigen. Functional commitment in the periphery depends on the maturity stage (RTE or PMR) of the immunocompetent cell, on the nature of the antigen-presenting cells, and on the effector classes of other T lymphocytes interacting on the same presenting cell. This model explains a number of observations on experimental autoimmune disease and transplantation tolerance, and it contains several readily testable predictions.

Lymphocytes selected in allogeneic thymic epithelium mediate dominant tolerance toward tissue grafts of the thymic epithelium haplotype

Athymic mice grafted at birth with allogeneic thymic epithelium (TE) from day 10 embryos before hematopoietic cell colonization reconstitute normal numbers of T cells and exhibit full life-long tolerance to skin grafts of the TE haplotype. Intravenous transfers of splenic cells, from these animals to adult syngeneic athymic recipients, reconstitute T-cell compartments and the ability to reject third-party skin grafts. The transfer of specific tolerance to skin grafts of the TE donor strain, however, is not observed in all reconstituted recipients, and the fraction of nontolerant recipients increases with decreasing numbers of cells transferred. Furthermore, transfers of high numbers of total or CD4+ T cells from TE chimeras to T-cell receptor-anti-H-Y antigen transgenic immunocompetent syngeneic hosts specifically hinder the rejection of skin grafts of the TE haplotype that normally occurs in such recipients. These observations demonstrate (i) that mice tolerized by allogeneic TE and bearing healthy skin grafts harbor peripheral immunocompetent T cells capable of rejecting this very same graft; and (ii) that TE selects for regulatory T cells that can inhibit effector activities of graft-reactive cells.

Thymic epithelium induces full tolerance to skin and heart but not to B lymphocyte grafts

Athymic nude mice reconstituted at birth with allogeneic thymic epithelia (TE) from day 10 embryos (E10), show life-long specific tolerance to skin and heart grafts, but eliminate B lymphocytes of the TE donor haplotype, nearly as well as those from a third strain. Previous immunizations with B cells do not alter the state of tolerance to skin grafts, but specifically accelerate elimination of lymphocytes. In contrast, transplantation of E15 allogeneic thymuses already seeded by hematopoietic cells resulted in chimeras tolerant to both skin and B lymphocytes. In vitro reactivities towards stimulator spleen cells of the haplotype of the thymus were observed in both E10 TE and E15 thymus chimeras. We conclude that induction of full in vivo tolerance to B cells requires hematopoietic cells, while this is not the case for induction of tolerance to skin and heart tissues; furthermore, in vitro reactivity to stimulator spleen cells of the tolerized haplotype is independent of in vivo tolerance.

Prospects for induction of tolerance in renal transplantation

Immunological tolerance is the ultimate goal of transplantation immunobiology. Current therapies involve nonspecific immunosuppression with concomitant risks for infection, malignancy, and drug-specific side effects. By inducing specific immune unresponsiveness to the graft it should be possible to maintain transplants without the need for chronic drug administration and without the risk of nonspecific immunosuppression. This review highlights recent progress in the understanding of immunological tolerance, with special attention to the long-term prospects for successful induction of tolerance in renal transplant patients.

Mechanisms of autoimmunity and AIDS: prospects for therapeutic intervention

The network theory of autoimmunity is presented with recent experimental data relevant to the understanding of the pathogenesis of AIDS. Schematically, effector T cells specific for self-antigens exist normally, but their activity is modulated and prevented by networks of regulatory T cells. As a result of mimicry between molecular components of microorganisms and self-antigens, autoimmune disease can be triggered by specific foreign pathogens which alter the state of activity of the network from suppression to activation. Conversely, by a procedure known as T-cell vaccination, autologous effector T cells re-injected after in vitro stimulation and attenuation may alter the state of the network from an activation to a suppression. Numerous observations are reviewed that support the concept of autoimmune activity in the destruction of non-infected T4 cells. Such activity is presumed to be triggered by an antigen of viral origin, the most likely, but not the only one, being the envelope protein gp 120. Based on this hypothesis, a T-cell vaccination procedure against effector T cells responsible for autoimmunopathic activity in HIV-seropositive patients is proposed, similar to the one known from experimental study of autoimmunity and presently being tested in human autoimmune diseases. Its purpose would be to prevent T-cell loss and the onset of immunodeficiency disease in HIV-seropositive patients. Apart from its potential therapeutic value, this procedure will have use as a therapeutic test from which insight will be gained about the immunopathogenesis of AIDS.

Thymus-derived cytokine(s) including interleukin-7 induce increase of T cell receptor alpha/beta+ CD4-CD8- T cells which are extrathymically differentiated in athymic nude mice

Extrathymic T cell differentiation pathways have been reported, although the thymus is the main site of T cell differentiation. The thymus is also known to produce several cytokines that induce proliferation of thymocytes. In the present study, we investigated the influence of thymus-derived cytokines on extrathymic T cell differentiation by intraperitoneal implantation with a diffusion chamber which encloses fetal thymus (we named it fetal thymus-enclosed diffusion chamber, FTEDC) in athymic BALB/c nu/nu mice. Increase in number of T cells bearing T cell receptor (TcR) alpha/beta was detected in lymph nodes and spleens of FTEDC-implanted nude mice 1 week after implantation, whereas no such increase was detected in control nude mice implanted with a diffusion chamber without thymus. The FTEDC-induced increase of T cells was suppressed by intraperitoneal injection of anti-interleukin-7 monoclonal antibody (mAb). The TcR alpha/beta T cells in FTEDC-implanted BALB/c nu/nu mice preferentially expressed V beta 11, although V beta 11-positive T cells are deleted in the thymus of euthymic BALB/c mice by clonal elimination of self-super-antigen Dvb11-specific T cells. TcR alpha/beta T cells in FTEDC-implanted nude mice were of CD4-CD8- phenotype and showed no proliferative response against anti-TcR monoclonal antibody stimulation. These results suggest that the thymus can induce extrathymic T cell differentiation through the influence of thymus-derived cytokine(s) including interleukin-7, and that such extrathymically differentiated T cells have acquired only a little or no ability for proliferation when they recognize antigen by their TcR.

The role of thymic epithelium in the establishment of transplantation tolerance

From experimental observations on induction of transplantation tolerance, we discuss a model that accounts for tissue-specific tolerance to antigens not expressed inside the thymus. It is postulated that antigens presented to differentiating T cells by thymic epithelium (or at large within the thymic environment) positively select and activate self-reactive T cells. A developmental program and/or prevalent conditions in the thymic environment restrict the proliferative potential and the class of effector functions that can be exerted by differentiating T cells activated in the thymus. These do not mediate inflammatory or cytolytic activities, but instead will produce the appropriate mediators to inhibit aggressive effector activities by other T cells activated in their proximity. Such "regulatory" functions will be locally expressed at the periphery upon recognition of tissue antigens shared with the thymus, towards newly formed thymic emigrants directed at tissue-specific antigens expressed by the same "target" cells. This mechanism imposes "dominant tolerance", based on specific self-recognition and predominantly established in the embryonic and neonatal period. Throughout life, the process of thymic positive selection results in all newly-formed T cells being susceptible to such suppressive mechanisms, but becoming increasingly refractory with time in the resting, post-differentiative stage. Absence of antigen (nonself) in the embryonic and neonatal life therefore allows for the accumulation of such "suppression-resistant" antigen-reactive T cells that will mount aggressive responses upon antigenic exposure. Tolerance or immunity thus represent two classes of specific immune responses, the relative predominance of which is determined by the frequency of each type of effector T cell, representing the antigenic overlap between thymic and peripheral tissues, as well as the frequency of tissue-specific T-cell generation, and the kinetics of peripheral antigenic exposure. Tolerance induced by hemopoietic cells to all other tissues is also "dominant" and based on thymic colonization and persistence of antigenic cells, with the consequent positive selection of regulatory T cells and peripheral conditions for the establishment of suppression. Upon this simple model, that ensures "interclonal class regulation" by "bridging" regulatory and effector T cells through the recognition of different antigens on the same target cell, other mechanisms which are based on V-region interactions among T cells (Ben-Nun et al. 1981, Pereira et al. 1989, Webb & Sprent 1990, Gaur et al. 1993) might well operate to ensure "dominant tolerance" by self-reactivity and class regulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Multiple levels of peripheral tolerance

The establishment and maintenance of self tolerance is based on multiple events in the thymus and periphery, resulting in either deletion of self-reactive T cells or induction of nonresponsiveness. Here, Bernd Arnold and colleagues propose that, depending on the tolerogenic signals, peripheral T cells can reach different levels of tolerance with regard to their capacity for reactivation. In addition, it appears that tolerant T cells are still susceptible to further tolerogenic signals, driving them into a deeper state of tolerance. Thus, induction of T-cell tolerance can be viewed as a multistep mechanism.

Dissociation of autoaggression and self-superantigen reactivity

Self-superantigens have been described as products of endogenous retroviruses of the mouse ('minor lymphocyte stimulating loci') that are capable of interacting without prior processing with conserved domains of TCR V beta chains, causing the activation and deletion of most T cells expressing products of determined V beta gene families [1-4]. The fact that superantigens activate a far higher percentage of T cells (1-20%) than conventional, peptidic antigens (< 0.1%) provides the methodological advantage that the degree of clonal deletion may be measured by the analysis of the TCR repertoire using appropriate anti-V beta antibodies. Although much information on the spatio-temporal organization of repertoire-purging has been gathered by virtue of self-superantigens, serious doubts exist as to the possibility that such structures serve as pathogenetically relevant autoantigens. Thus, certain inbred mice spontaneously develop autoimmune diseases, although they bear T-cell repertoires that appear to be purged from self-superantigen-reactive V beta products. In addition, therapeutic interventions targeted to V beta gene products that are not specific for self-superantigens are successful in preventing disease development. The lack of correlation between superantigen-related V beta deletions and autoimmune disease development is substantiated in further models of murine autoimmunity. Based on these observations, we formulate the hypothesis that self-superantigen-reactive T cells are not involved in the development of autoimmune diseases.

Transplantation tolerance is unrelated to superantigen-dependent deletion and anergy

C57BL/6 (B6; I-E-, Mls-2b) nude mice, reconstituted at birth with thymic epithelium (TE) from BALB/c (BA; I-E+, Mls-2a) day 10 embryos (E10), permanently accepted BALB/c skin, when grafted as adults. T-cell receptor repertoire analyses in the periphery of these mice revealed no difference in frequencies of I-E/superantigen-reactive T-cell receptor V beta families, as compared to chimeras constructed with syngeneic B6 E10 TE. T lymphocytes bearing V beta 3, V beta 5, and V beta 11 T-cell receptors, from either allogeneic or syngeneic TE chimeras, responded equally well to in vitro receptor-dependent stimulation. Similar results were obtained with nude mice reconstituted at birth with E14 thymuses, already colonized by hemopoietic cells. These observations indicate that neither TE cells nor the progenies of hemopoietic precursors that colonize the thymus up to E14 express or functionally present the superantigens addressed here; it follows that tolerance to skin grafts and superantigen-related T-cell deletions are unrelated phenomena.

Thymic epithelium induces neither clonal deletion nor anergy to Mls 1a antigens

Grafting of thymic anlagen from day-10 DBA/2 (H-2d; Mls-1a) embryos to newborn athymic BALB/c (H-2d; Mls-1b) mice leads to reconstitution of T cell populations in the recipients. Analysis of adult chimeras shows that their V beta T cell receptor (TcR) repertoires, particularly V beta 6 and V beta 8.1, do not significantly differ in most animals (10 out of 13) from those scored in control chimeras that received syngeneic thymic anlagen. In all cases analyzed, such Mls-1a-reactive T cells could be stimulated at levels comparable to control responses, both in vitro and in vivo. The few cases in which Mls-1a reactive V beta TcR were reduced seem to reflect the variability in TcR V beta repertoires found in this experimental system. In contrast, BALB/c mice, injected at birth with DBA/2 spleen cells show a marked, albeit variable, reduction in the frequencies of V beta 6- and V beta 8.1-bearing CD4+ T cells, and lower frequencies of Mls-1a-reactive T cells in limiting dilution analyses. It appears, however, that V beta 6- and V beta 8.1-bearing T cells remaining in these mice are functionally competent. We conclude that Mls-1 antigens are not expressed by thymic epithelium.

Second generation immune networks

Network approaches have had little impact on immunology because they have addressed the wrong questions. They have concentrated on the regulation of clonal immune responses rather than on the supraclonal properties of the immune system that emerge from its network organization, such as natural tolerance and memory. Theoretical advances, observations in unimmunized mice and humans, and the success of novel therapeutics in autoimmune diseases have recently promoted a new burst of research on the structure, temporal dynamics and metadynamical plasticity of immune networks.

Autoimmunity, microbial immunity and the immunological homunculus

Clonal deletion and anergy are believed by many immunologists to be the fundamental mechanisms responsible for self tolerance. Nevertheless, as Irun Cohen and Douglas Young point out, such notions of nonreactivity cannot explain certain key features of immune behaviour: the immunological dominance of microbial antigens that mimic self, the uniformity of autoimmune diseases and the prevalence of natural autoimmunity among the healthy. The theory of the immunological homunculus is presented here as a unifying principle.

Stress proteins, autoimmunity, and autoimmune disease

At birth, the immune system is biased toward recognition of microbial antigens in order to protect the host from infection. Recent data suggest that an important initial line of defense in this regard involves autologous stress proteins, especially conserved peptides of hsp60, which are presented to T cells bearing gamma delta receptors by relatively nonpolymorphic class lb molecules. Natural antibodies may represent a parallel B cell mechanism. Through an evolving process of "physiological" autoreactivity and selection by immunodominant stress proteins common to all prokaryotes, B and T cell repertoires expand during life to meet the continuing challenge of infection. Because stress proteins of bacteria are homologous with stress proteins of the host, there exists in genetically susceptible individuals a constant risk of autoimmune disease due to failure of mechanisms for self-nonself discrimination. That stress proteins actually play a role in autoimmune processes is supported by a growing body of evidence which, collectively, suggests that autoreactivity in chronic inflammatory arthritis involves, at least initially, gamma delta cells which recognize epitopes of the stress protein hsp60. Alternate mechanisms for T cell stimulation by stress proteins undoubtedly also exist, e.g., molecular mimicry of the DR beta third hypervariable region susceptibility locus for rheumatoid arthritis by a DnaJ stress protein epitope in gram-negative bacteria. While there still is confusion with respect to the most relevant stress protein epitopes, a central role for stress proteins in the etiology of arthritis appears likely. Furthermore, insight derived from the work thus far in adjuvant-induced arthritis already is stimulating analyses of related phenomena in autoimmune diseases other than those involving joints. Only limited data are available in the area of humoral autoimmunity to stress proteins. Autoantibodies to a number of stress proteins have been identified in SLE and rheumatoid arthritis, but their pathogenetic significance remains to be established. Nevertheless, the capacity of certain stress proteins to bind to multiple proteins in the nucleus and cytoplasm both physiologically and during stress or injury to cells, suggests that stress proteins may be important elements in the "immunogenic particle" concept of the origin of antinuclear and other autoantibodies. In short, this fascinating group of proteins, so mysterious only a few years ago, has impelled truly extraordinary new lines of investigation into the nature of autoimmunity and autoimmune disease.

Identification of a low-affinity subset of protamine-reactive IgM antibodies present in normal, deficient in AIDS, sera: implications for HIV latency

We demonstrate here that the protamine-reactive IgM antibodies previously shown to be present in normal adult sera include two subsets differing in binding affinity. The principal, high-affinity subset was detected in AIDS and ARC as well as normal sera. The secondary, low-affinity subset, however, was absent or markedly deficient in AIDS or ARC sera. Protamine-reactive IgM antibodies were also detected in normal pediatric sera, suggesting that one subset of that class of antibodies may be "natural," i.e., not antigenically induced. The proportionate titer of the low-affinity protamine-reactive IgM antibodies was determined for HIV-positive males who were asymptomatic or mildly immune deficient at specimen collection. Of those who subsequently remained AIDS free for 18 months to 7 years, more than 90% had titers in the range established for the normal sera, while of those diagnosed with AIDS or ARC within 12 months, more than 80% had titers below the normal range. We propose that the low-affinity subset of adult sera corresponds to the natural antibodies of pediatric sera and that a relationship of those natural antibodies to resistance to progression of HIV pathogenesis is suggested.

Inside the thymus, Mls antigen is exclusively presented by B lymphocytes

The ability to stimulate an Mls-1 mixed lymphocyte reaction (MLR) is predominantly expressed by low density B lymphocytes in the spleen and peritoneal cavity of normal adult mice, and is absent in splenic B cells 1 month after lethal irradiation and reconstitution from autologous bone marrow. Coreconstitution of these mice with normal syngeneic peritoneal cells restores the stimulatory potential of splenic B cells, but sorted CD5+ or CD5- IgM+ lymphocytes from peritoneum are equally good stimulators, suggesting that functional Mls-1 expression may require long life spans and selection. Bone-marrow-reconstituted DBA/2 mice that fail to express Mls-1 antigens in the periphery nevertheless maintain T-cell receptor V beta 6 and 8.1 deletions among the newly formed T cells. These findings led us to directly investigate the Mls stimulatory ability of purified antigen-presenting cell populations inside the thymus. We report here that thymic B lymphocytes seem to represent the only intrathymic cell population able to stimulate Mls-1 MLR.

The role of thymic epithelium in the acquisition of tolerance

There are two separate mechanisms of induction of T-cell tolerance in the thymus. First, MHC molecules expressed on bone-marrow-derived cells can cause clonal deletion of autoreactive cells. Second, as discussed here by Elisabeth Houssaint and Martin Flajnik, thymic epithelial cells can generate a form of tolerance that does not eliminate self-reactive clones. This nondeletional mechanism, which is also a feature of the other MHC class-II-bearing epithelia, may contribute to the establishment of tolerance-maintaining regulatory networks.

Presentation of endogenous immunoglobulin determinant to immunoglobulin-recognizing T cell clones by the thymic cells

Using immunoglobulin (Ig)-recognizing T helper clones the expression of Ig peptide/major histocompatibility complex class II complexes derived by the processing of endogeneous Ig molecules in the thymus was demonstrated. It was found that thymic B cells but not "classic" thymic antigen-presenting cells and macrophages represent the major antigen-presenting cell type of determinants of endogenously synthesized surface Ig (Ig kappa-1b) and anti-surface Ig antibodies (IdC3B9). The Ig kappa-1b-presenting activity in the thymus appears relatively late, only after 3 weeks of postnatal life, while in the spleen an efficient presentation of endogenous Ig kappa-1b epitope is observed very early after birth. This difference between thymic and peripheral presentation of endogeneous Ig determinant could be important for understanding the mechanisms of T cell tolerance to self Ig and the role of self Ig in negative and positive selection of T cell repertoire.

Immunoglobulin-specific T-B cell interaction. IV. B cell presentation of idiotypic determinant(s) of monoclonal anti-surface immunoglobulin antibody to idiotope-recognizing helper T clones

Previously, we have demonstrated T-B cell interactions mediated by T cell recognition of immunoglobulin (Ig) peptide/major histocompatibility complex (MHC) class II complexes derived by the B cell processing of endogenously synthesized Ig molecules. In this report Ig-specific T-B cell interaction mediated by B cell presentation of idiotopes (Id) of anti-sIg antibodies to Id-specific T cell clones has been studied in Ig kappa-1-congenic rat strains. A panel of August (RT-1c; Ig kappa-1a) rat T helper clones specific for Id of syngeneic anti-Ig kappa-1b C3B9 monoclonal antibody (mAb) has been developed to study IdC3B9 presentation by Ig kappa-1b-bearing B cells from congenic August.1b (RT-1c; Ig kappa-1b) rats. Five of seven IdC3B9-specific T clones responded even at very low concentrations (100-200 pg/ml) of C3B9 mAb presented by Ig kappa-1b+ B cells. In contrast, the presentation of intact C3B9 mAb by nonspecific antigen-presenting cells (macrophages, Ig kappa-1a+ B cells, etc.) to IdC3B9-specific T cells was of low efficiency. The IdC3B9-specific T cell response to idiotopes of anti-Ig kappa-1b C3B9 mAb was found to be restricted by RT-1B molecule and required the processing of intact C3B9 molecule. IdC3B9 epitope recognized by C31 and C5 clones was mapped to the heavy chain of C3B9 mAb. Thus, B cells are able to present peptides related to the V region of anti-sIg Ab, i.e. Id peptide/MHC class II complexes, to Id-recognizing T cells. IdC3B9-presenting B cells are specifically activated both to proliferation and Ig production upon interaction with IdC3B9-specific T clones. Based on the results of our studies on B cell presentation of Ig epitopes to T cells a hypothetical model of Ig peptide-driven T-B cell interaction has been proposed.

The primary lesion theory of autoimmunity: a speculative hypothesis

Autoaggression and autoimmune attack are established vernacular in the literature of autoimmune disease, and reflect the popular view that autoimmunity is a disease of immune dysregulation in which the immune system inappropriately attacks healthy tissues. The aim of this article is to focus attention on an alternative, indeed opposite, view--that autoimmunity represents the response to a primary lesion in the target tissue, rather than its cause, and that like alloimmunity, autoimmunity is physiological appropriate and protective. The cell death and tissue damage which results is characteristic of an immune response programmed to eliminate immunogen, remove detritus and isolate the lesion.

Immune networks: getting on to the real thing

After some ten years of faltering development, the network approach in immunology is finally heading toward steady ground. At a recent international workshop (*), experimentalists and theoreticians discussed some of the latest developments, including an impressive array of novel results and applications to fundamental properties of biological immune networks: connectivity, patterns of dynamic activity, ontogenesis, and tolerance.