A natural model of immunologic tolerance. Tolerance to murine C5 is mediated by T cells, and antigen is required to maintain unresponsiveness

Application of central immunologic concepts to cancer: helping T cells and B cells become intolerant of tumors

CD4-mediated T-cell help in the activation of CD8(+) T cells and B cells, through linked-recognition of antigenic determinants, is a long-standing concept foundational to our understanding of immunity (presence of help) versus tolerance (lack of help). Surprisingly, this function of CD4(+) T cells has not been extensively examined as a means to overcome immune tolerance of the self-antigens made by tumor cells. Hesitation to employ this powerful mechanism may be due to the potential to cause unwanted autoimmune pathology. In this issue of the European Journal of Immunology, Snook et al. [Eur. J. Immunol. 2014. 44: 1956-1966] identify a state of split tolerance, showing that CD4(+) T cells specific for a number of tumor-associated self-antigens are robustly tolerant, while their CD8(+) T-cell and B-cell counterparts are far less tolerant. Furthermore, the authors demonstrate that provision of linked foreign helper epitopes, such as influenza hemagglutinin, substantially enhances both CD8(+) T-cell and B-cell responses to tumor self-antigens without causing any overt autoimmune pathology. These findings provide a strong rationale to employ foreign helper epitopes in cancer vaccines and highlight the need to fully explore therapeutic strategies that are based on well-established immunologic concepts.

On ‘Credo 2004’ as Viewed Under the ‘Development-Context’ Model of Colin Anderson

In analysing the Zinkernagel and Hengartner's 'Credo 2004,' Anderson introduces his 'development-context model' for the immunity-tolerance discrimination. He compares this model with the 'geographical model of Credo 2004' and our 'time-based two-signal model'. The discussion here deals with the advantages and limitations of the Anderson model considered largely at the level of principle. A meaningful discussion requires that we agree on the principle which separates the pathway of the effector output into two decision steps, the sorting of the repertoire and the regulation of effector class. The mechanism for the sorting of the repertoire is what might be referred to as the Self-Nonself discrimination. The black box approach, antigen-in, effector response-out, is what is referred to as the immunity-tolerance discrimination which includes the sorting of the repertoire. If this point of principle is accepted then we are left with a 'time-based two signal default model'.

Time, space and contextual models of the immunity tolerance decision: bridging the geographical divide of Zinkernagel and Hengartner's 'Credo 2004'

In Credo 2004, Zinkernagel and Hengartner (Z&H) have continued their challenge to the immunological community to reconsider assumptions regarding the most fundamental aspects of adaptive immunity. They have appropriately championed the role of persistent, widely distributed antigen in tolerance induction, parameters that do not figure prominently in most other models. The global theory of immunity they have developed is predominantly based on observations from studies with viruses and tumours. I suggest here that a more successful approach to generating a theory of the default rules of immunity can be obtained through the study of immunity versus tolerance in the setting of transplantation. Transplantation studies lack the confounding variable of competing evolution present in responses to specific infectious agents and tumours and, therefore, more clearly elucidate default rules of immunity. The geographical model in Credo 2004, primarily a one-signal model regulated by antigen, is contrasted with (1) Cohn's time-based two-signal model and (2) a development-context model that postulates distinct central and peripheral tolerance mechanisms.

Thymic commitment of regulatory T cells is a pathway of TCR-dependent selection that isolates repertoires undergoing positive or negative selection

The seminal work of Le Douarin and colleagues (Ohki et al. 1987; Ohki et al. 1988; Salaun et al. 1990; Coutinho et al. 1993) first demonstrated that peripheral tissue-specific tolerance is centrally established in the thymus, by epithelial stromal cells (TEC). Subsequent experiments have shown that TEC-tolerance is dominant and mediated by CD4 regulatory T cells (Treg) that are generated intrathymically by recognition of antigens expressed on TECs (Modigliani et al. 1995; Modigliani et al. 1996a). From these and other observations, in 1996 Modigliani and colleagues derived a general model for the establishment and maintenance of natural tolerance (MM96) (Modigliani et al. 1996b), with two central propositions: (1) T cell receptor (TCR)-dependent sorting of emergent repertoires generates TEC-specific Treg displaying the highest TCR self-affinities below deletion thresholds, thus isolating repertoires undergoing positive and negative selection; (2) Treg are intrathymically committed (and activated) for a unique differentiative pathway with regulatory effector functions. The model explained the embryonic/perinatal time window of natural tolerance acquisition, by developmental programs determining (1) TCR multireactivity, (2) the cellular composition in the thymic stroma (relative abundance of epithelial vs hemopoietic cells), and (3) the dynamics of peripheral lymphocyte pools, built by accumulation of recent thymic emigrants (RTE) that remain recruitable to regulatory functions. We discuss here the MM96 in the light of recent results demonstrating the promiscuous expression of tissue-specific antigens by medullary TECs (Derbinski et al. 2001; Anderson et al. 2002; Gotter et al. 2004) and indicating that Treg represent a unique differentiative pathway (Fontenot et al. 2003; Hori et al. 2003; Khattri et al. 2003), which is adopted by CD4 T cells with high avidity for TEC-antigens (Bensinger et al. 2001; Jordan et al. 2001; Apostolou et al. 2002). In the likelihood that autoimmune diseases (AID) result from Treg deficits, some of which might have a thymic origin, we also speculate on therapeutic strategies aiming at selectively stimulating their de novo production or peripheral function, within recent findings on Treg responses to inflammation (Caramalho et al. 2003; Lopes-Carvalho et al., submitted, Caramalho et al., submitted). In short, the MM96 argued that natural tolerance is dominant, established and maintained by the activity of Treg, which are selected upon high-affinity recognition of self-ligands on TECs, and committed intrathymically to a unique differentiative pathway geared to anti-inflammatory and antiproliferative effector functions. By postulating the intrathymic deletion of self-reactivities on hemopoietic stromal cells (THC), together with the inability of peripheral resident lymphocytes to engage in the regulatory pathway, the MM96 simultaneously explained the maintenance of responsiveness to non-self in a context of suppression mediating dominant self-tolerance. The major difficulty of the MM96 is related to the apparent tissue specificity of Treg repertoires generated intrathymically. This difficulty has now been principally solved by the work of Hanahan, Kyewski and others (Jolicoeur et al. 1994; Derbinski et al. 2001; Anderson et al. 2002; Gotter et al. 2004), demonstrating the selective expression of a variety of tissue-specific antigens by TECs, in topological patterns that are compatible with the MM96, but difficult to conciliate with recessive tolerance models (Kappler et al. 1987; Kisielow et al. 1988). While the developmentally regulated multireactivity of TCR repertoires (Gavin and Bevan 1995), as well as the peripheral recruitment of Treg among RTE (Modigliani et al. 1996a) might add to this process, it would seem that the establishment of tissue-specific tolerance essentially stems from the "promiscuous expression of tissue antigens" by TEC. The findings of AID resulting from natural mutations (reviewed in Pitkanen and Peterson 2003) or the targeted inactivation (Anderson et al. 2002; Ramsey et al. 2002) of the AIRE transcription factor that regulates promiscuous gene expression on TECs support this conclusion. The observations on the correlation of natural or forced expression of the Foxp3 transcription factor in CD4 T cells with Treg phenotype and function (Fontenot et al. 2003; Hori et al. 2003; Khattri et al. 2003) provided support for the MM96 contention that Treg represent a unique differentiative pathway that is naturally established inside the thymus. Furthermore, Caton and colleagues (Jordan et al. 2001), as well as several other groups (Bensinger et al. 2001; Apostolou et al. 2002), have provided direct evidence for our postulate that Treg are selected among differentiating CD4 T cells with high affinity for ligands expressed on TECs (Modigliani et al. 1996b). Finally, the demonstration by Caramalho et al. that Treg express innate immunity receptors (Caramalho et al. 2003) and respond to pro-inflammatory signals and products of inflammation (Caramalho et al., submitted) brought about a new understanding on the peripheral regulation of Treg function. Together with the observation that Treg also respond to ongoing activities of "naïve/effector" T cells--possibly through the IL-2 produced in these conditions--these findings explain the participation of Treg in all immune responses (Onizuka et al. 1999; Shimizu et al. 1999; Annacker et al. 2001; Curotto de Lafaille et al. 2001; Almeida et al. 2002; Shevach 2002; Bach and Francois Bach 2003; Wood and Sakaguchi 2003; Mittrucker and Kaufmann 2004; Sakaguchi 2004), beyond their fundamental role in ensuring self-tolerance (e.g., Modigliani et al. 1996a; Shevach 2000; Hori et al. 2003; Sakaguchi 2004; Thompson and Powrie 2004). Thus, anti-inflammatory and anti-proliferative Treg are amplified by signals that promote or mediate inflammation and proliferation, accounting for the quality control of responses (Coutinho et al. 2001). In turn, such natural regulation of Treg by immune responses to non-self may well explain the alarming epidemiology of allergic and AID in wealthy societies (Wills-Karp et al. 2001; Bach 2002; Yazdanbakhsh et al. 2002), where a variety of childhood infections have become rare or absent. Thus, it is plausible that Treg were evolutionarily set by a given density of infectious agents in the environment. With hindsight, it is not too surprising that natural Treg performance falls once hygiene, vaccination, and antibiotics suddenly (i.e., 100 years) plunged infectious density to below some critical physiological threshold. As the immune system is not adapted to modern clean conditions of postnatal development, clinical immunologists must now deal with frequent Treg deficiencies (allergies and AID) for which they have no curative or rational treatments. It is essential, therefore, that basic immunologists concentrate on strategies to selectively stimulate the production, survival, and activity of this set of lymphocytes that is instrumental in preventing immune pathology. We have argued that the culprit of this inability of basic research to solve major clinical problems has been the self-righteousness of recessive tolerance champions, from Ehrlich to some of our contemporaries. It is ironical, however, that none of us--including the heretic opponents of horror autotoxicus--had understood that self-tolerance, or its robustness at least, is in part determined by the frequency and intensity of the responses to non-self. In the evolution of ideas on immunological tolerance, the time might be ripe for some kinds of synthesis. First, conventional theory reduced self-tolerance to negative selection and microbial defense to positive selection, while the MM96 solution was the precise opposite: positive selection of autoreactivities for self-tolerance (Treg) and negative selection (of Treg) for ridding responses. In contrast, it would now appear that positive and negative selection of autoreactive T cells are both necessary to establish either self-tolerance or competence to eliminate microbes, two processes that actually reinforce each other in the maintenance of self-integrity. Second, V-region recognition has generally been held responsible for specific discrimination between what should be either tolerated or eliminated from the organism. In contrast again, it would now seem that both processes of self-tolerance and microbial defense (self/non-self discrimination) also operate on the basis of evolutionarily ancient, germ-line-encoded innate, nonspecific receptors (Medzhitov and Janeway 2000) capable of a coarse level of self/non-self discrimination (Coutinho 1975). It could thus be interesting to revisit notions of cooperativity between V-regions and such mitogen receptors, both in single cell functions (Coutinho et al. 1974) and in the system's evolution (Coutinho 1975, 1980) as well. After all, major transitions in evolution were cooperative (Maynard-Smith and Szathmary 1995).

The primary IgM antibody repertoire: a source of potent idiotype immunogens

A widely held view is that, to elicit adaptive immune responses, most protein antigens must be given with adjuvants that activate the innate immune system. It has also been proposed that the immune system is tolerant to idiotypes (Id) of the syngeneic primary antibody (Ab) repertoire. We now show that among 73 purified noncomplexed secretory IgM monoclonal antibodies (mAb), 4 (5.5%) elicited high levels of IgG Ab against the Id even though no adjuvant was added. The responses were controlled by H2-linked immune response genes. IgG1, but no IgG2a or IgG2b, anti-Id Ab were detected, indicating involvement of T helper type 2 (Th2) cells. All 4 IgM mAb are likely germ-line gene-encoded, and 1 was shown to represent a recurrent Id. After endotoxin depletion the most potent immunogen of the 4 still provoked robust humoral anti-Id responses. The results suggest that a natural protein of the primary IgM Ab repertoire can be immunogenic without an adjuvant.

Dynamics of serum IgM autoreactive repertoires following immunization: strain specificity, inheritance and association with autoimmune disease susceptibility

Immunization of Lewis rats with myelin basic protein (MBP) in complete Freud's adjuvant (CFA) provokes experimental allergic encephalomyelitis (EAE). Here we compare, irrespective of antigen specificity, the structure and dynamics of serum IgM autoreactive repertoires following immunization with MBP/CFA in EAE-susceptible Lewis and relatively resistant Fischer rats. Prior to the appearance of clinical symptoms, Lewis rats developed a specific modification of serum IgM autoreactivities that, scored on other determinants than MBP itself, showed a prognostic association with EAE symptoms. Although comparable in their production of MBP-specific serum IgM and IgG antibodies, Fischer rats did not share these MBP/CFA-induced IgM autoreactivities of Lewis rats when immunized in the same manner. Moreover, while the Lewis-type repertoire reaction was specific for MBP/CFA alone, the respective Fischer reaction was not qualitatively different from that observed in this strain upon non-pathogenic immunization with self-related or -unrelated antigens. In general, the repertoire reactions differed qualitatively between the strains, consisting of components with typical behavior and strain preferences. The EAE-associated, as well as the other components of both Lewis- and Fischer-type repertoire reactions were usually co-dominantly inherited in F1 animals. These results indicate that a global antibody repertoire analysis may serve as a tool to describe prototypical response structures, possibly involved in immune regulation and susceptibility to pathogenic autoimmunity.

Neutralizing antiviral B cell responses

Neutralizing antiviral B cell responses differ in various aspects from the many usually measured B cell responses specific for protein in adjuvants. In particular, such neutralizing antiviral B cell responses are more rapidly induced, reach higher titers, are longer lived, and are efficiently generated without adjuvants. Evidence is summarized here that the repetitiveness of many viral antigens is a key factor responsible for the efficiency of these B cell responses, amplifying B cells early and rapidly for potent IgM responses and also for efficient switching to IgG. The data reviewed indicate that B cells discriminate antigen patterns via the degree of surface Ig-cross-linking and use antigen repetitiveness as a self/nonself discriminator.

T helper cell unresponsiveness: rapid induction in antigen-transgenic and reversion in non-transgenic mice

T cell tolerance is usually established by clonal deletion of self-specific T cells in the thymus, or some times, in the periphery. Alternatively, tolerance may also be achieved by induction of clonal T cell unresponsiveness by a poorly understood mechanism called "anergy". We found that transgenic mice expressing a soluble form of vesicular stomatitis virus (VSV) glycoprotein (G) predominantly in liver and kidney exhibited normal B cell responses. VSV-G-specific T help-independent neutralizing IgM responses were within normal ranges, but no T help-dependent neutralizing IgG antibodies were generated upon immunization with recombinant VSV-G protein and recombinant vaccinia virus expressing VSV-G. This demonstrated absence of B cell tolerance but presence of T helper cell unresponsiveness. After adoptive transfer of transgenic spleen cells into thymectomized immuno-incompetent hosts, the unresponsive T helper cells regained function and switched the neutralizing IgM response to IgG, comparably to control T helper cells, within 7 days. Conversely, when naive non-transgenic spleen cells were transferred into transgenic mice, VSV-G-specific T helper cells became unresponsive within 3-4 days. These results suggest that VSV-G-specific T helper cells are rendered unresponsive within a few days in the VSV-G transgenic host also outside of the thymus and that this unresponsiveness was reversed by transfer into antigen-free recipients.

Virus or a hapten-carrier complex can activate autoreactive B cells by providing linked T help

We investigated the mechanism leading to an IgG autoantibody response in two transgenic mouse lines expressing the glycoprotein of vesicular stomatitis virus (VSV-G). Previous experiments have shown that these animals do not mount a transgene-specific IgG response upon stimulation with purified VSV-G or infection with recombinant vaccinia virus expressing VSV-G. However, infection of VSV-G transgenic animals with wild-type vesicular stomatitis virus, serotype Indiana, readily induced VSV-G-specific, neutralizing IgG autoantibodies. We have tested whether this labile state of tolerance reflected differential availability of VSV-G-specific T help. For this, we immunized transgenic mice with the self-antigen VSV-G covalently coupled to sperm-whale myoglobulin (VSV-G-SWM), to provide new T helper epitopes that are linked to the B cell epitope; co-injected uncoupled VSV-G and SWM served as control. High titers of VSV-G specific IgG autoantibodies were detected in serum of mice immunized with VSV-G-SWM but not after co-injection of uncoupled VSV-G and SWM. Transgenic animals depleted of CD4+ T cells prior to injection of VSV-G-SWM failed to mount an IgG response. Priming of transgenic mice with the foreign carrier did not accelerate the IgG autoantibody response to VSV-G-SWM, suggesting that B cells were limiting the rate of the response. Thus, self-reactive B cells could be triggered to produce IgG, if they received linked T help specific for a foreign carrier determinant provided either by a classical carrier determinant or a virus.

Global analysis of antibody repertoires. II. Evidence for specificity, self-selection and the immunological "homunculus" of antibodies in normal serum

The serum IgM repertoires of C57BL/6, DBA/2 and BALB/c mouse strains were analyzed using a recently developed global and quantitative assay that measures antibody reactivities to a very large number of antigens. A characteristic repertoire could be assigned to each strain. The different repertoires could be successfully classified with multivariate statistics. Many common reactivities were also observed among the different strains, which allows the definition of a mouse-specific repertoire. Analysis of human sera support this notion. To investigate the impact of minor genetic differences on the serum IgM repertoire, the congenic strains B10.D2/oSn and B10.D2/nSn, which differ in the expression of the C5 component of complement, were analyzed. The two strains could be separated based on the reactivity profiles obtained. The analysis of the results reveals that many antigenic proteins are not recognized at all by natural antibodies, while others are disproportionately reactive, the resulting patterns giving rise to what could be the definition of an "immunological homunculus". The relevance of this type of analysis for clinical applications is discussed.

Self-nonself discrimination and tolerance in T and B lymphocytes

The immune system must not only fight off infections, but also ensure that it does not react against its own body tissues. Since clones of lymphocytes have predetermined reactivities, some will be self-reactive and have the potential to cause damage. They should therefore be neutralized in some way. In a system as complex and important as that governing self-tolerance, many mechanisms must exist to neutralize autoaggressive lymphocytes. They may be classified under two main groups. In one the tolerant state arises from the physical or functional silencing of potentially autoaggressive lymphocytes after antigen encounter. This may involve clonal deletion, clonal abortion or clonal anergy. In the second, regulatory mechanisms of the immune system itself may hold autoreactive lymphocytes in check, for example through the operation of idiotypic network interactions and the action of specialized suppressor cells. Much evidence has accumulated for the physical deletion of autoreactive T cells as they mature in the thymus. The fate of any that escape thymus censorship has been the subject of recent research and is discussed here. Under certain conditions, self-tolerance must also be imposed at the B-cell level to prevent the production of potentially damaging autoantibodies. Although the mechanisms which silence self-reactive lymphocytes are very efficient, self-tolerance can break down, and autoimmunity will thus ensue. The main factors responsible for this are briefly described here.

Presentation of a circulating self protein (C5) to MHC class II restricted T cells

Presentation of a soluble self antigen, the fifth component of complement (C5), is discussed with emphasis on the differential ability of presentation by subpopulations of APC (dendritic cells, macrophages, B cells, fibroblasts, B cell lines and bone marrow macrophages). Constitutive presentation of C5 in C5 sufficient mice is a prerequisite for tolerance induction in MHC class II restricted T cells and can be directly demonstrated by the ability of ex-vivo APC from C5 sufficient, but not C5 deficient mice, to activate C5 specific T cells in vitro in the absence of added antigen. C5 presentation and tolerance induction in MHC class II restricted T cells is strictly dependent on an exogenous source of self antigen. C5 biosynthesized, but not secreted by macrophages is ignored by MHC class II restricted cells and induces neither tolerance nor autoimmunity. C5 presentation for tolerance induction depends largely on the efficiency of antigen uptake by APC, a property which varies within different APC subpopulations and with the nature of the antigen.

Localization of self antigen: implications for antigen presentation and induction of tolerance

The fifth component of complement (C5) is a self antigen expressed in serum of normal mice at a concentration of about 50 micrograms/ml. We have previously shown that C5 is constitutively processed and presented by antigen-presenting cells (APC) in normal mice to induce and maintain complete tolerance in major histocompatibility complex (MHC) class II-restricted T cells. This report addresses the question of whether C5 presentation involves exogenous antigen which has been internalized for processing or whether intracellular, biosynthesized C5 is being presented with MHC class II. Macrophages were found to synthesize, but not secrete C5 in bone marrow chimeras made from irradiated C5-deficient [C5(-)] hosts reconstituted with C5-sufficient [C5(+)] bone marrow [C5(+)-->C5(-)]. In these mice, macrophages are the only source of C5. [C5(+)-->C5(-)] chimeras are not tolerant of C5 and generate C5-specific T and B cell responses upon immunization indistinguishable from those of C5(-) mice. Macrophages from [C5(+)-->C5(-)] chimeras are unable to activate C5-specific T cell hybrids in vitro unlike macrophages from a C5(-) strain that has matured in a C5-expressing environment [C5(-)-->C5(+) chimeras]. This shows that under physiological conditions in vivo intracellular C5 does not get access to the class II presentation pathway and thus, does not induce tolerance in class II-restricted T cells.

The effect of neonatal tolerance to bovine gamma globulin (BGG) on BGG-reactive CD4+ T lymphocytes

Compelling evidence has been presented that tolerance to major histocompatibility complex (MHC) and other antigens expressed on cells of the thymus is due to clonal deletion. However, it has not been determined conclusively how tolerance to diverse antigens, which may initially interact with the immune system in the periphery, is induced and maintained. Considerable evidence exists to suggest that mechanisms other than deletion, such as clonal anergy or immunoregulation, may be involved. We have previously shown that the clonal deletion of CD4+ cells specific for bovine gamma globulin (BGG) does not occur during induction of tolerance to this soluble antigen in adult life. In this study we have extended our previous findings by examining unresponsiveness to BGG which had been established during early ontogeny, when natural tolerance of self-antigens is likely to develop. It is demonstrated here that BGG-reactive, CD4+8- T cells, which proliferate and secrete interleukin-2 on stimulation with BGG in vitro, can be obtained from mice rendered tolerant of BGG as neonates. Even though the mice from which they were derived were unable to respond to BGG, these BGG-reactive T cells, by the parameters tested here, could not readily be distinguished from the corresponding cells in BGG-immune and non-immune animals. It is therefore evident that this tolerant state is not simply the result of clonal deletion of BGG-reactive CD4+ T cells but is more likely to be due to a reversible mechanism which controls their responsiveness in vivo.

Induction of active immunological hypo/non-responsiveness to C5 in adult C5-deficient DBA/2 mice

Injection of C5-sufficient BALB/c serum rendered DBA/2 mice (C5-deficient) immunologically hypo- or non-responsive to C5. This was indicated by C5-elimination studies in the C5-deficient mice showing similar half-lives for C5 upon single and repeated BALB/c serum injection. Concrete evidence for C5 non-responsiveness came from experiments showing that C5-injected DBA/2 mice were unable to mount an anti-C5 antibody response after active immunization with C5-sufficient serum in Freund's complete adjuvant. C5 hypo/non-responsiveness could be induced in DBA/2 mice via the intravenous as well as the intraperitoneal route, provided the C5-sufficient serum was administered in the very narrow dose range of 10-100 microliters (approximately 0.3-3 micrograms of C5). Upon i.v. C5 injection, C5 non-responsiveness was nearly complete on Day 4 and lasted about 3 weeks. Hyporesponsiveness was still present 6 weeks after serum injection. C3-/C5-depleting cobra venom factor reversed tolerization for C5, at least when applied within 48 hr after i.v. C5 injection. Similarity between the acquired C5 hypo/non-responsiveness of DBA/2 mice and the established C5 tolerance of BALB/c mice was suggested by adoptive cell transfer experiments: spleen cells from naive DBA/2 mice stimulated B cells of C5-sufficient nude mice to produce C5-neutralizing antibodies. In contrast, splenocytes from C5-tolerized DBA/2 mice, like those of BALB/c mice, did not decrease haemolytic C5 levels in C5-sufficient nude mice.

Is there really a "lack of natural tolerance to allotypic gamma-globulins in rabbits"?

We present data of extended studies on the possibility of maternally derived allotype Ig inducing a state of natural immunological tolerance to a non-inherited allotype in the offspring. Rabbits homozygous at the a locus, encoding allotypes in the variable region of immunoglobulin heavy chains, and rabbits homozygous at the unliked b locus, encoding allotypes of the constant region of kappa 1 chains, were immunized at the age of 2 months against the non-inherited allotype of their heterozygous mothers to which they had been exposed in utero and in early life. As control, we immunized rabbits of the same Ig phenotype but born to homozygous mothers, and therefore not exposed to that allotype. Immunization was done in a3/a3 offspring of either a1/a3 or a3/a3 mothers, by injecting a 1 IgG, and in b6/b6 offspring of b4/b6 or b6/b6 mothers, by injecting b4 IgG. The IgG was injected either in a soluble form or emulsified in adjuvant. Injection of soluble IgG elicited only a low response, if any, revealing no differences between the various groups. All rabbits responded upon immunization with IgG in adjuvant. We have not found any good evidence for natural tolerance to a non-inherited allotype, although progeny of a1/a3 mothers had slightly decreased responses to a1. On the contrary, progeny of b4/b6 mothers responded even better than offspring of b6/b6 mothers, upon such immunization with b4. To induce tolerance experimentally, we injected newborn rabbits, either from heterozygous a1/a3 or from homozygous a3/a3 mothers, with a1 serum or IgG. Newborn of heterozygous b4/b6 or of homozygous b6/b6 mothers were injected with b4 serum or IgG in the same way. Such treatment resulted in partial tolerance to each allotype. In an attempt to amplify the tolerizing effect of the maternal a1 Ig, we injected newborn rabbits of a1/a3 mothers with the serum of their mother. The response upon subsequent immunization with a1 allotype of another individual did not differ significantly from the response of control rabbits. The response was much poorer when rabbits were injected with nonmaternal tolerogen at birth, and when the same Ig preparation was used as immunogen. In a control experiment, neonatal injection of xenogeneic proteins, human IgG or bovine serum albumin, clearly resulted in tolerance. We speculate that tolerance to allotypes is established in the T cell repertoire only but bypassed by recognition of idiotypic determinants on antigen molecules by helper T cells, which trigger anti-allotype antibody formation by allotype-specific B cells. The end result of it is a lack of natural tolerance.

T and B cell tolerance and responses to viral antigens in transgenic mice: implications for the pathogenesis of autoimmune versus immunopathological disease

Experiments with transgenic mice illustrate clonal elimination of T cells specific for antigens expressed appropriately in the thymus, but presence of inducible T cells when the antigen presented on class I MHC antigens is expressed exclusively on non-lymphohemopoietic cells in the periphery (pancreatic beta islet cells). TCR-transgenic LCMV-carrier mice expressing LCMV in the thymus exhibit clonal elimination at the early CD4+8+ thymocyte stage, causing CTL unresponsiveness in these mice. In contrast, studies with RIP LCMV-GP-transgenic mice (expressing GP in pancreatic beta cells) and with TCR-RIP LCMV-GP double-transgenic mice show that CTL reactivity is normal. These experiments argue against so-called peripheral anergy of class I MHC antigen-restricted cytotoxic T cells as a general mechanism of peripheral immunological tolerance to self. They reveal that self epitopes that are genetically self and presented by class I antigens may not be considered immunologically self if expressed solely extrathymically, despite the fact that they are antigenic and can be recognized by induced effector T cells. Genetic self that is presented on cells which can induce neither tolerance nor an immune response is immunologically dealt with as foreign and therefore may be called nonimmunological self. Appropriate presentation of the same epitope on antigen-presenting cells promptly induces effector T cells and causes disease; such disease should not be called autoimmune because it is an immunopathological T-cell mediated disease, comparable to an unfavorably balanced immunopathological T-cell response to a virus. Mechanisms that control autoantibody responses were studied in mice expressing a viral transgene. Such mice generate neutralizing antiviral autoantibody responses only when the transgenic viral antigen is linked to a foreign T-helper determinant. These findings, therefore, document differences in levels of T- vs B-cell tolerance (so-called split tolerance) under a given expression level of a "self" antigen. They illustrate how unresponsiveness of B cells to produce T-independent IgM is dose-dependent and that IgG autoantibodies are triggered by introducing foreign T-helper determinants that can be recognised in a linked fashion. This model suggests that, while T-cell tolerance to tolerogenic self in the thymus is solid, B-cell tolerance in general is not. From the point of view of autoantibody responses these T-helper cells may also be called immunopathological; i.e., these T-helper cells are specific for foreign epitopes that, via linked recognition, trigger truly autoimmune B cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Induction of self-tolerance in T cells but not B cells of transgenic mice expressing little self antigen

Self-tolerance to a transgene-encoded protein, hen egg lysozyme, was examined in the T and B cell repertoires of a series of lines of transgenic mice that expressed different serum concentrations of soluble lysozyme. T cells were tolerant in all lines in which lysozyme was expressed irrespective of the antigen concentration, whereas B cell tolerance did not occur when the serum lysozyme concentration was less than 1.5 nanograms per milliliter (0.1 nM). Induction of elevated transgene expression could restore B cell tolerance. These findings support the hypothesis that autoimmune disease may in some instances arise through a bypass of T cell tolerance.

Antigen presentation and self-nonself discrimination

Self-nonself discrimination is primarily carried out by T cells. Since the ligand recognized by T cells is a complex formed by antigenic peptides bound to MHC molecules, positive and negative selection of T lymphocytes must be based on the recognition of complexes formed by self-peptides bound to MHC molecules. This requires that self-antigens are continuously processed, bound by MHC molecules, and presented to T cells under conditions inducing both positive selection of T cells potentially able to recognize foreign antigens and negative selection, either by physical deletion or functional inactivation, of potentially autoreactive T cells. Self-nonself discrimination is not confined to intrathymic development of T lymphocytes, but it is a continuing process among peripheral T cells. Accordingly, autoimmunity is induced when self-antigens, or foreign antigens cross-reactive with self antigens, bound to MHC molecules, are presented under conditions able to activate self-reactive T cells. Based on these premises, a way of interfering with the induction of autoimmune diseases could rely on blocking the MHC binding site presenting the autoantigen.

Virus-induced autoantibody response to a transgenic viral antigen

The induction of autoantibodies and their possible role in the pathogenesis of autoimmune disease are poorly understood. Involvement of infectious agents has been suspected, but direct evidence is sparse. Whether immunological unresponsiveness to self by antibody-forming B cells is maintained by clonal abortion, clonal anergy or suppression, or how the scenario of interactions between helper T cells, B cells and antigen-presenting cells is distorted in autoantibody responses, is being analysed and widely debated. To evaluate tolerance of neutralizing B-cell responses we used transgenic mice expressing the cell membrane associated glycoprotein (G) of vesicular stomatitis virus (VSV) as self-antigen. We show that autoantibodies to VSV-G cannot be induced by VSV-G in adjuvant or by recombinant vaccinia virus expressing VSV-G, but are triggered by infection with wild-type VSV. The data show that helper T-cell tolerance is crucial in maintenance of B-cell non-reactivity and that cognate T-B recognition is necessary to break tolerance of self-reactive B cells. These results may help to understand mechanisms of virus-induced autoimmunity.

Clonal deletion of B lymphocytes in a transgenic mouse bearing anti-MHC class I antibody genes

B lymphocytes can be rendered specifically unresponsive to antigen by experimental manipulation in vivo and in vitro, but it remains unclear whether or not natural tolerance involves B-cell tolerance because B cells are controlled by T lymphocytes, and in their absence respond poorly to antigen (reviewed in ref. 7). In addition, autoantibody-producing cells can be found in normal mice and their formation is enhanced by B-cell mitogens such as lipopolysaccharides. We have studied B-cell tolerance in transgenic mice using genes for IgM anti-H-2k MHC class I antibody. In H-2d transgenic mice about 25-50% of the splenic B cells bear membrane immunoglobulin of this specificity, and abundant serum IgM encoded by the transgenes is produced. In contrast, H-2k x H-2d (H-2-d/k) transgenic mice lack B cells bearing the anti-H-2k idiotype and contain no detectable serum anti-H-2k antibody, suggesting that very large numbers of autospecific B cells can be controlled by clonal deletion.

The autoantigenic response to rabbit cytochrome c

In this report we describe the production and characterization of autoantibody responses to rabbit cytochrome c (cyt c) in rabbits immunized with either the native monomeric or polymerized form of rabbit cyt c. Fine-specificity analyses of the response indicated that the majority of the response was to the evolutionarily conserved amino-terminal region of the molecule. The relative affinity of the autoantibody interaction with rabbit cyt c was assessed by a solid-phase assay and was found to be lower than that observed for rabbit anti-horse cyt c antibody populations. These findings are consistent with the prediction that low-affinity self-reactive B cells may escape tolerance induction.

T cell immunity or tolerance as a consequence of self antigen presentation

In this study we investigated the basis for immunity or tolerance to a mouse serum protein, the fifth component of complement (C5). In C5-deficient mice this protein is absent from serum and therefore they are not tolerized. Immunization of C5-deficient mice with C5-sufficient serum generates CD4+ T cells, which recognize C5 presented in the context of class II. No C5-specific responses were observed in T cells from C5-sufficient mice. We show that this self protein is processed and presented with class II by cells from C5-sufficient tolerant mice and can be recognized by C5-specific T cell clones and hybrids in the absence of exogenously added antigen. The stimulation of C5-specific T cells by C5-sufficient antigen-presenting cells is not a consequence of C5 secretion and subsequent processing in vitro but rather employs C5 peptide/class II complexes generated in vivo. We conclude that this self antigen is presented in normal mice in a form recognizable by T cells to induce and maintain immunological tolerance.

Role of the thymus in natural tolerance to an autologous protein antigen

C5-deficient mice grafted with thymus from C5-sufficient donors and immunized with C5 failed to make humoral antibody to C5, suggesting that the transfer of thymus had induced tolerance. Irradiated C5-deficient hosts repopulated with lymphoid cells from thymectomized C5-deficient mice grafted with C5-sufficient thymus also failed to respond to immunization with C5, thus showing that the state of tolerance can be adoptively transferred. These results demonstrate that natural tolerance to self-protein antigen is "learned" in the thymus.

Purification of the fifth component of murine complement from ascites fluid

The murine complement component C5 was purified on an affinity column using a monoclonal anti-mouse C5 antibody. We describe in this paper that ascites fluid from normal (C5-sufficient) mice contains almost as much C5 protein as mouse serum. Since ascites fluid is much easier to obtain in large quantities it is a convenient source for the purification of this mouse serum protein.

Role of alveolar macrophages in lipopolysaccharide-induced neutrophil accumulation

B6D2F1/TRU mice were exposed to a lipopolysaccharide (LPS) aerosol that resulted in a 1-h-postexposure lung burden of about 290 ng of LPS. This exposure caused an accumulation of neutrophils in the lung that peaked between 6 and 12 h after exposure. To determine the potential role of alveolar macrophages (AM) in the induction of neutrophil accumulation by LPS, 10(6) AM from normal or LPS-exposed mice were transferred to the lungs of groups of naive recipient mice. A third group of mice was instilled intratracheally with vehicle only. After 5 h, the lungs of the mice were lavaged and the numbers of neutrophils in the lavage fluids were determined. The instillation of AM from unexposed mice did not cause significantly more neutrophils to accumulate than did the instillation of vehicle only, whereas the instillation of AM from LPS-exposed mice caused nearly a threefold increase in the numbers of neutrophils in lavage fluids. Transfer of AM from LPS-exposed mice into cutaneous air pouches of naive mice also caused greater local neutrophil accumulation (10-fold) than did the transfer of AM from normal mice. Repeated freeze-thawing of the suspensions of AM before transfer to recipients significantly reduced the ability of the suspensions to induce neutrophil accumulation. This indicated that AM viability is necessary to cause a maximal neutrophil infiltration upon transfer of the AM. To determine the extent to which LPS-induced neutrophil accumulation depends on the presence of AM, the ability of LPS to elicit neutrophil accumulation when injected alone or together with AM into air pouches was determined. The injection of either AM or LPS alone caused few neutrophils to accumulate, whereas the injection of LPS and AM together caused a large number of neutrophils to accumulate. The results of this study indicate that LPS deposition in the lung can stimulate AM to induce neutrophil accumulation and that this may be the major mechanism by which LPS causes neutrophil accumulation.

Immunological responsiveness of neonatal A/J mice to isotypic determinants of syngeneic IgE

We have previously shown that adult A/J mice produce high titers of anti-IgE with isotypic or idiotypic specificities in response to challenge with a conjugate of KLH with syngeneic monoclonal IgE. Thus, B cells that can synthesize anti-IgE are present in the mice. Adult mice are unresponsive to unconjugated IgE in CFA, suggesting that tolerance exists at the level of T cells. The present study shows that neonatal mice produce anti-IgE antibodies in response to unconjugated IgE in CFA, but that this capacity is lost after the age of 2-3 wk. The loss of responsiveness corresponds closely with the appearance of detectable IgE in serum, suggesting that the IgE may induce tolerance. The affinities of anti-IgE antibodies produced by neonatal mice fall in the range of values obtained with KLH-IgE in adult mice. Tolerance to unconjugated IgE in CFA can be induced in neonatal mice by administration of IgE in saline. In addition, the tolerant state can be induced by adoptive transfer of spleen cells from adult mice. The time-dependent acquisition of tolerance provides a useful model for studying mechanisms of tolerance and autoimmunity.

Alloreactive immune responses of transgenic mice expressing a foreign transplantation antigen in a soluble form

Transfection of cells with the H-2Kk gene lacking the transmembrane and cytoplasmic segments resulted in secretion of the H-2Kk protein, as determined by immunoprecipitation with monoclonal anti-H-2Kk antibodies. Transgenic (H-2b X H-2d)F1 mice were established carrying integrated copies of the modified H-2Kk gene. Expression of the soluble H-2Kk antigen in the transgenic mice was demonstrated in cell supernatants of biosynthetically labeled splenic and thymic Con A blasts as well as bone marrow-derived macrophages. Soluble H-2Kk molecules were also present in the sera of the transgenic animals. No cell-surface expression of the H-2Kk antigen could be observed. In spite of the presence of the soluble H-2Kk molecules in the transgenic mice, the animals were able to generate H-2Kk-specific cytolytic T cells as well as antibody responses when stimulated with cell-surface-bound H-2Kk antigens. These responses were indistinguishable from those of the nontransgenic littermates. Possible explanations for the observed lack of tolerance are discussed.

Induction of an immune response to a self antigen

The question has been addressed whether the endogenous B cell population of a mouse can be induced to secrete antibodies specific for a self antigen present in serum. The antigen studied was the fifth component of mouse complement (C5). Nude BALB/c mice which are C5 sufficient were used as a source of potentially C5-reactive B cells and endogenous serum C5 provided the antigenic stimulus. We purposely avoided immunization with C5 in adjuvant. T cells from C5-deficient mice which lack this component in serum and are therefore not tolerant of C5 were injected into nude mice as a source of T cell help for anti-C5 reactive B cells. Control groups received T cells from C5-sufficient euthymic donors, which are tolerant of C5. Initiation of a response to C5 was monitored by testing the hemolytic function of serum. Reduction of C5-dependent hemolysis was observed in sera of mice which had received T cells from C5-deficient donors. Recipients of T cells from C5-sufficient donors maintained normal hemolytic complement levels throughout the test period of 45 days. Reduction of functional complement levels correlated with the presence of immune complexes of anti-C5/C5. C5-specific antibodies were mainly IgG1 and carried the IgG1 allotype of BALB/c providing unequivocal evidence that they were derived from the endogenous B cell population of the C5-sufficient host.

Generation of a monoclonal antibody to mouse C5 application in an ELISA assay for detection of anti-C5 antibodies

We have generated a monoclonal antibody with specificity for the fifth component of mouse complement (C5). This antibody precipitates the two chains of C5 from normal mouse serum and inhibits C5-dependent hemolysis in a functional complement test. In this study we describe its application in an enzyme-linked immunoadsorbent assay (ELISA assay) for the detection of anti-C5 antibodies in serum. Monoclonal anti-C5 coupled to wells of an ELISA plate specifically binds C5 from unfractionated normal mouse serum. This subsequently serves as antigen to bind anti-C5 serum antibodies. By this approach we have circumvented the need for extensive purification of C5 from serum which would be required if C5 was directly coupled to ELISA plates as antigen. Serum antibodies from C5-immunized mice bound with high avidity to wells containing normal serum as antigen source in amounts representing 1 microgram to 250 ng C5. There was no antibody binding to wells containing C5-deficient serum as antigen source. The immune reaction was detected by development with enzyme-coupled goat-anti mouse Ig antibodies specific for various mouse Ig subclasses. This method allows the qualitative characterization of immune responses to mouse C5 which is an ideal model for a natural self antigen in studies of immunological tolerance.

A study on location of synthetic site which mainly synthesizes and delivers fifth component of complement system in vivo

Tissue sites for synthesis of the fifth component of complement (C5) in vivo have been investigated by using allogeneic bone marrow chimeras and bone marrow chimeras which were transplanted in addition with hepatocytes. Our prior studies have demonstrated that bone marrow chimeras which had been prepared by transplanting marrow cells from C5-sufficient donor mice into irradiated C5-deficient recipients lacked detectable levels of C5 in the sera. However, when such potentially C5-deficient [C5(+)---C5(-)] chimeras were introduced into their spleens by means of injections of fully dispersed single cell suspensions of hepatocytes isolated from the C5-sufficient donor strain, they accepted the transplantation of hepatocytes for prolonged periods and developed a measurable amount of C5 in the sera. These results indicate that C5 protein in sera is not synthesized in significant amount by cells that are descendants of bone marrow cells but rather that this complement component is synthesized and delivered to the blood in vivo by somatic cells including liver cells that are not derivatives of the bone marrow.

Mice naturally tolerant to C5 have T cells that suppress the response to this antigen

We examined whether C5-sufficient mice which are naturally tolerant to this antigen have suppressor T cells to C5 humoral immune response. Two congenic strains of mice B10.D2 (NSN) and B10.D2 (OSN) differing only in the presence or absence of C5 were used. Irradiated (760 rds) sufficient hosts were reconstituted with a nonadherent spleen cell suspension from either sufficient or deficient mice or a mixture of both. Hemolytic C5 levels were assayed. Sufficient spleen cells appeared to prevent the drop of C5 level caused by anti-C5 antibody made by deficient spleen cells. Spleen cell suspensions from sufficient mice primed with deficient spleen cells exhibited better anti-C5 activity than normal sufficient spleen cell suspensions. This anti-C5 activity is abrogated by treatment of the NSN spleen cell suspensions obtained from NSN primed with OSN spleen cells with anti-Thy-1.2 antiserum and complement. Suppression of the humoral response to C5 failed to affect the anti-sheep red blood cell immune response. Suppressor T cells are resistant to low-dose irradiation, cortisone treatment and adult thymectomy. In contrast, they are sensitive to high doses of irradiation and both high and low doses of cyclophosphamide treatment. Thus, C5-sufficient mice, in contrast to C5-deficient mice, appear to have antigen-specific suppressor T cells which downregulate the humoral immune response to C5. In addition, we examined the relationship of these suppressor T cells to the state of tolerance in helper T cells of C5-sufficient mice. This was done in irradiated deficient mice which were repopulated with spleen cell suspensions selectively depleted of either Lyt-1+ or Lyt-2+ T cell subsets. These chimeras were challenged with murine C5 and both the primary and secondary immune response was measured by inhibition of the C5 hemolytic activity. It was found that only spleen cell suspensions of the deficient mice selectively depleted from the Lyt-2+ subset of T cells responded to the antigen both in the primary and secondary response. In contrast, either subset of T cells from the sufficient mice failed to respond. Thus, it appears that in sufficient mice helper T cells to C5 are intrinsically tolerant or physically and/or functionally deleted. In conclusion, the data suggest that both T cell compartments are unresponsive and play a role in the mechanism of tolerance to a physiologic antigen.

An in vitro model for clonal anergy in continuously growing antigen-specific B-cell lines

Two continuously growing nonmalignant B-cell lines specific for the hapten DNP have been used to study tolerance in developing B cells. These cell lines have previously been shown to consist of small cells without sIgM but with cytoplasmic mu chains, and mature sIgM- and sIgD-bearing cells. When the sIgM-negative cells are placed in culture, mature DNP-specific B cells begin to appear. The studies reported here have shown that when these cell lines were propagated in the presence of either 200 micrograms/ml or 1 mg/ml of the tolerogen DNP-MGG there was no inhibition of cell line growth as measured by thymidine incorporation, and no inhibition of receptor expression by maturing B cells. The cell line lymphocytes propagated in the presence of 200 micrograms/ml DNP-MGG for 7, 30, 45, or 60 days became tolerant and the tolerance persisted for at least 6 days after removal of DNP-MGG. However, tolerance was lost between 6 and 10 days after removal of DNP-MGG. Propagation of the cell lines for 30 days in either DNP-KLH or DNP-Ficoll produced the same results. Limiting dilution cultures of cell line lymphocytes made tolerant by growing them for 30 days in the presence of DNP-MGG demonstrated that there was a marked decrease in precursor frequency compared to controls. However, cell line lymphocytes made tolerant by a 48-hr incubation with DNP-MGG did not have a significant decrease in precursor frequency. These data suggest that tolerance induced by growing these cell lines in the presence of DNP-MGG is a valid in vitro model of tolerance in developing antigen-specific B cells. Tolerance induced in this model is consistent with the clonal anergy hypothesis, but requires the continued presence of DNP-MGG to maintain unresponsiveness. This suggests that clonal anergy can occur in B cells but may not be the sole mechanism of self tolerance for those antigens which are sequestered from the immune system.

Analysis of synthetic sites of fourth and fifth components of serum complement system in allogeneic bone marrow chimaeras

Synthetic tissue sites for the fourth and fifth components of complement (C4 and C5) have been investigated using allogeneic bone marrow chimaeras in mice. One group of chimaeric mice was prepared by transplanting bone marrow cells from C5-sufficient donor mice into irradiated C5-deficient recipients or vice versa, and another group was prepared by transplanting marrow cells from mice that produce high levels of C4 into irradiated recipients that are characterized by having low levels of C4 or vice versa. In such chimaeras, lymphoid cells and serum immunoglobulin allotypes were shown to be exclusively of donor origin. However, haemolytic activities of sera from the chimaeras were consistently identical with those of normal mice of the recipient strain. Similar results were obtained when the complement component levels of the sera were evaluated by double diffusion assays. C4 or C5 antigens were detected in sera of the chimaeras only when recipients were strains that are characterized by having high C4 level or were C5-sufficient mice, respectively. These findings indicate that circulating C4 or C5 complement components present in the blood are not synthesized primarily by cells that are descendants of bone marrow cells in these chimaeric mice.