Autoimmunity and the microbiome: T‐cell receptor mimicry of “self” and microbial antigens mediates self tolerance in holobionts

I propose a T‐cell receptor (TcR)‐based mechanism by which immunity mediates both “genetic self” and “microbial self” thereby, connecting microbiome disease with autoimmunity. The hypothesis is based on simple principles. First, TcR are selected to avoid strong cross‐reactivity with “self,” resulting in selection for a TcR repertoire mimicking “genetic self.” Second, evolution has selected for a “microbial self” that mimics “genetic self” so as to share tolerance. In consequence, our TcR repertoire also mimics microbiome antigenicity, providing a novel mechanism for modulating tolerance to it. Also, the microbiome mimics the TcR repertoire, acting as a secondary immune system. I call this TcR‐microbiome mimicry “holoimmunity” to denote immune tolerance to the “holobiont self.” Logically, microbiome‐host mimicry means that autoimmunity directed at host antigens will also attack components of the microbiome, and conversely, an immunological attack on the microbiome may cross‐react with host antigens producing “holoautoimmunity.”

     

Mechanisms in immune tolerance. I. A specific block of immunological memory in HSA-tolerant mice

Evidence is provided indicating that the transient immune response detected during the induction of tolerance to HSA in mice, results in immunological memory which persists in an immunosuppressed state in the tolerant animals. The mechanism which blocks this memory is antigen-specific and can be selectively inactivated by total body irradiation in the range of 650–900 R. Consequently tolerant mice, irradiated within this range and restored with normal syngeneic spleen cells, respond better to the tolerizing antigens than do similarly treated normal mice. The tolerizing block can be transmitted from “tolerant” to normal spleen cells when a mixture of both is transferred to normal irradiated recipients. Since similar inhibitory activity is also demonstrated by fresh sera of tolerant mice, it is suggested that the blocking mechanism depends on inhibitory cells which act via a humoral inhibitor.     

Serological Comparison of Antigens Related to Local Virus and Bacterial Infections and Aspecific Stresses in Tobacco Leaves

Two “new” precipitin bands (antigens) detected by the immunodiffusion test were demon strated in leaf extracts of tobacco inoculated with tobacco mosaic virus (TMV), Pseudomonas tabaci or treated with mercuric chloride, sodium azide or sodium hypochlorite. One of the precipitin bands was stronger, than the other, These antigens were also detected in the upper, non-infected leaves of tobacco plants when the lower leaves were locally stressed (necrotized) either by TMV or by chemical injury. The “new” antigens formed in the upper leaves were detected even if the TMV-inoculated lower leaves were removed one day after inoculation. The “new” antigens were identical both in the lower and upper leaves and their induction was independent from the stress whether pathogenic or chemical. A coincidence exists between the appearance of “new” antigens and acquired resistance, but this does not mean necessarily a cause-and-effect relationship between the two phenomena. Our experiments indicate that the induction of the synthesis of “new” stress proteins in tobacco is aspecific and the proteins formed are related to the aspecific stress itself rather than to pathogenesis.     

Autophagy and T-cell education in the thymus: Eat yourself to know yourself

During intrathymic generation of the T cell repertoire, a series of selection processes ensure that only self-MHC (Major Histocompatibility Complex) restricted and self-tolerant T cells are allowed to survive. Interactions with MHC ligands on the surface of thymic epithelial cells (TECs) play a pivotal role in the decision-making of developing thymocytes. A number of distinct cell-biological features of TECs have emerged that may predispose them to serve non-redundant functions in thymocyte “education”. Thus, cortical TECs express a rather unique set of proteolytic enzymes for antigen processing in the context of positive selection, whereas medullary TECs "ectopically" express a plethora of otherwise strictly tissue-restricted antigens (TRAs), a property that obviously has evolved to make these self-antigens "visible" to developing thymocytes for negative selection. One of the latest additions to this growing list of functional adaptations of TECs is their constitutively high rate of autophagy. Recently, we have provided evidence that autophagy in TECs shuttles cytoplasmic self-antigens into the MHC class II loading pathway for positive selection of T cells and tolerance induction.     

Further implications of a theory of immunity.

A theory of immunity presented previously showed that many immune phenomena could be explained in terms of a simple model involving interactions between (i) the receptors on immunologically competent cells, (ii) antigenic determinants, (iii) natural antibody and (iv) complement. Several features of the theory subsequently gained experimental support. The present paper extends the analysis and predicts that two mechanisms operate in the induction of immunological tolerance to self antigenic determinants. High specificity anti-self cells are destroyed by a mechanism involving complement. In contrast, low specificity anti-self cells are stimulated by self determinants and increase in numbers. This increases the concentration of the natural antibody secreted by these cells which acts as a “blocking” antibody preventing their continued stimulation by self determinants.The expanded clones of low specificity anti-self cells, which may be of high specificity for “near-self” determinants, (i) are responsible for the greater immunological responsiveness between non-identical members of the same species than between members of different species (“alloaggression”), and (ii) provide a barrier opposing the progressive evolution of the surface determinants of a pathogen into forms identical with the surface determinants of its host.Following exposure to a given self or not-self antigenic determinant, the distribution curve for cells of varying specificities for the determinant shows a sharp cut-off point between high and low specificity cells. The position of this cut-off point is critical in determining the subsequent response of the organism to the determinant. Variables affecting the cut-off point include, antibody present prior to the exposure of cells to the determinant, complement, complement inhibitors, the cell membrane and certain drugs. Autoimmune diseases are improved by drugs (e.g. chloroquine) which move the position of the cut-off point towards cells of low specificities for self determinants.     

Activation of Thymus Cells by Histocompatibility Antigens

“Educated” or “activated” thymus cells have been shown, by the use of “strong” histocompatibility antigens, to be immunocompetent only against the antigens by which they were originally activated.     

On the control between cell-mediated, IgM and IgG immunity

An hypothesis is proposed here describing some of the conditions that determine the type of response an antigen will induce, and explaining how the induction of one type of immunity affects the induction of other types of immunity. In more detail, the hypothesis attempts to account for the following observations: Some antigens induce only cell-mediated immunity, whereas others can, under different conditions, induce either cell-mediated or humoral immunity. The humoral response to most antigens consists of an initial period of IgM antibody synthesis, followed by a period of IgG synthesis. Some polymeric antigens induce the synthesis of only IgM antibody. There is a tendency for the immune response to an antigen, at a particular time, to be exclusively of the cell-mediated, IgM or IgG type.The hypothesis may also be relevant to some observations that, I believe, have been incorrectly interpreted to mean that “tolerance” to some antigens requires the presence of T (thymus-derived) cells specific for these antigens. The hypothesis suggests teleological reasons for the existence of the different types of immunity. It also suggests ways of controlling the type of response an antigen induces.     

Role of self carriers in the immune response and tolerance. III. B cell tolerance induced by hapten-modified-self involves both active T cell-mediated suppression and direct blockade.

The induction of B cell unresponsiveness with hapten-modified syngeneic murine lymphoid cells (hapten-modified self, HMS) can be achieved in vivo and in vitro. Tolerance in vivo in mice required a latent period of 3 to 4 days. Moreover, B cell unresponsiveness could not be induced by HMS in athymic nude mice, although their nu/+ littermates were rendered hyporesponsive by HMS. Pretreatment of normal mice with cyclophosphamide (cyclo) prevented their susceptibility to tolerance induction by haptenated lymphoid cells. Nude mice became sensitive to HMS-induced suppression if they were first reconstituted with spleen cells from normal (but not cyclo-treated) donors.Interestingly, labeling of H-2 antigens was not necessary for tolerance induction by HMS since haptenated teratoma cells (lacking H-2) were tolerogenic in normal recipients.In contrast, suppression of the in vitro response to haptenated flagellin occurred equally well with nude, nu/+ and anti-Ly 2 + C-treated spleen cells. These data suggest that cyclo-sensitive modified self-reactive (T) cells may regulate the immune response and mediate tolerance to HMS in vivo. However, the in vitro “blockade” of B cell reactivity may be directly mediated on hapten-specific PFC precursors.     

The virtuous self‐tolerance of virtual memory T cells

“Virtual” memory CD8⁺ T cells are a subset of immune cells produced by homeostatic mechanisms involving response to self‐antigens, raising the possibility that these cells could mediate autoimmunity. New work by Drobek et al demonstrates that virtual memory T cells are indeed favored by stronger T‐cell receptor signals but exhibit minimal autoreactivity while maintaining self‐tolerance.     

Extrathymic tolerance of mature T cells: clonal elimination as a consequence of immunity

The mechanism by which T lymphocytes are tolerized to self or foreign antigens is still controversial. Clonal deletion is the major mechanism of tolerance for immature thymocytes; for mature T cells, tolerance is considered to reflect anergy rather than deletion, and to be a consequence of defective presentation of antigen. This paper documents a novel form of tolerance resulting when mature T cells encounter antigen in immunogenic form. Evidence is presented that exposure of mature T cells to Mlsa antigens in vivo leads to specific tolerance and disappearance of Mlsa-reactive V beta 6+ T cells. Surprisingly, the clonal elimination of V beta 6+ cells is preceded by marked expansion of these cells. Thus, tolerance induction can be the end result of a powerful immune response. These data raise important questions concerning the relationship of tolerance and memory.     

Enhancement of anti-tumor CD8 immunity by IgG1-mediated targeting of Fc receptors

Dendritic cells (DCs) function as professional antigen presenting cells and are critical for linking innate immune responses to the induction of adaptive immunity. Many current cancer DC vaccine strategies rely on differentiating DCs, feeding them tumor antigens ex vivo, and infusing them into patients. Importantly, this strategy relies on prior knowledge of suitable “tumor-specific” antigens to prime an effective anti-tumor response. DCs express a variety of receptors specific for the Fc region of immunoglobulins, and antigen uptake via Fc receptors is highly efficient and facilitates antigen presentation to T cells. Therefore, we hypothesized that expression of the mouse IgG1 Fc region on the surface of tumors would enhance tumor cell uptake by DCs and other myeloid cells and promote the induction of anti-tumor T cell responses. To test this, we engineered a murine lymphoma cell line expressing surface IgG1 Fc and discovered that such tumor cells were taken up rapidly by DCs, leading to enhanced cross-presentation of tumor-derived antigen to CD8+ T cells. IgG1-Fc tumors failed to grow in vivo and prophylactic vaccination of mice with IgG1-Fc tumors resulted in rejection of unmanipulated tumor cells. Furthermore, IgG1-Fc tumor cells were able to slow the growth of an unmanipulated primary tumor when used as a therapeutic tumor vaccine. Our data demonstrate that engagement of Fc receptors by tumors expressing the Fc region of IgG1 is a viable strategy to induce efficient and protective anti-tumor CD8+ T cell responses without prior knowledge of tumor-specific antigens.     

Modification of cell surface antigenicity as a function of culture conditions

Adaptation of monolayer cultures of a clonal line of rat glial cells to suspension culture resulted in the nearly complete loss of certain surface antigens. This change in surface antigenicity was paralleled by the loss of the ability of the cells to accumulate in vitro a protein specific to the nervous system (“S100-protein”). In contrast, when glial cells were co-cultivated in monolayer culture with another cell line apparently lacking these surface antigens, the number of these antigens was markedly increased. The possibility of a causal relationship between the changes in the surface antigenicity and the expression of differentiated function is considered.     

Studies on the mechanism of transplantation tolerance: I. Do suppressor cells play a role in the induction and maintenance of neonatal transplantation tolerance?

Neonatal transplantation tolerance was induced in CBA (H-2^k) mice to A (H-2^a) mice by injection of (CBA × A)F₁ spleen cells. Animals carrying an A-skin test allograft for more than 4 months without any visible sign of rejection were considered to be permanently tolerant. Permanently tolerant CBA mice were given normal syngeneic spleen cells to abrogate the state of tolerance. Abrogation of tolerance was greatly facilitated by antithymocyte serum (ATS) treatment of tolerant mice prior to the normal syngeneic cell transfer. Survival of A allografts on normal, adult, ATS-treated CBA mice was significantly prolonged (and in many cases “adult” tolerance was achieved) by transfer of spleen cells of syngeneic mice made permanently tolerant at neonatal age. The possible role of the F₁-cell “contamination” in the tolerance-inducing effect of the transferred “tolerant” spleen cells was excluded. The results indicate that ATS-sensitive suppressor cells play a definite role in the induction, maintenance, and transfer of neonatally induced transplantation tolerance.     

组织特异抗原异位表达与免疫耐受

诱导和维持T细胞耐受是免疫系统区分自我和非我的关键。自身免疫调节因子(autoimmune regulator,AIRE)作为转录因子,在胸腺髓质上皮细胞中可驱动一系列组织特异抗原基因的表达,从而在诱导中枢免疫耐受的过程中发挥重要作用。外周免疫耐受的机制复杂一些,清除耐受是其重要机制之一。外周淋巴结的基质细胞可表达部分组织特异抗原,递呈给T淋巴细胞,激活并最终清除它。中枢免疫耐受和外周免疫耐受机制可清除潜在的自身反应性T淋巴细胞,维持对自身组织耐受。一旦免疫耐受被打破,将发生自身免疫反应和自身免疫疾病。     

Collateral presentation of antigens as physiological prototype for lymph node metastases

The formation of lymphogenic metastases remains enigmatic. In particular, the much more pronounced predilection of carcinomas than of sarcomas to metastasizing into regional lymph nodes is an unsolved problem. We suggest that this difference could be due to the ability of epitheliocytes for a hypothetical process termed by us “collateral presentation of antigens”. Under conditions of infection of epithelium with intracellular pathogens or during inflammation, epithelial cells acquire a special receptor phenotype, undergo the epithelial-mesenchymal transition, and migrate along lymphatic vessels into lymph nodes where they present antigen to immunocytes. The collateral presentation of antigens can be of significant biological importance in the case of insufficient classical pathway of antigen presentation (by dendritic cells) or on disturbance in the death mechanisms of the infected cells. Depending on conditions of induction of the epithelial-mesenchymal transition and on possible ability of epitheliocytes to express MHC II with co-stimulating molecules, two pathways, “container-mediated” and “MHC II-dependent”, of antigen presentation in lymph nodes resulting in development of immunogenesis or anergy of immunocytes are supposed to exist. All pathways of delivery of the epithelial cells into lymph nodes and of antigen presentation by epitheliocytes terminate by death of these cells. The lymphogenic metastasizing realizes the same mechanism under conditions of tumor disease; however, this is not associated with cell death, but they actively colonize the lymph node. The proposed hypothesis allows us to explain the metastasizing of sarcomas into lymph nodes. The main prerequisite for lymphogenic metastasizing seems to be related with the mesenchymal-epithelial transition of sarcoma cells promoting their involvement in the presentation of antigens.     

Antigen binding to lymphoid cells from unimmunized mice: IV. Shedding and reappearance of multiple antigen binding Ig receptors of T- and B-lymphocytes

Patterned antigen-binding cells (ABC) can bind two antigens and show “islands” of Ig receptor with mixed specificity. However, these cells, when unfixed, lose most of their bound fluorescent antigen within minutes upon warming above 0 °C. Residual antigen moves to one pole of the cell forming a “cap” within 5 min at room temperature. If such patterned ABC are capped with a single antigen, receptors to a second antigen can be detected on a portion of the capped cells, but only in the cap. The frequency of capped, “double” ABC approximated the frequency of patterned “double” ABC originally present.If lymphoid cells are mixed with fluorescent antigens at 0 °C and then incubated for 4 hr at 37 °, no ABC are found. When the cells are then fixed and the fluorescent antigens readded, new antigen-binding Ig receptors can be shown to have reappeared on the cell surface during the 4-hr incubation. The reappearance of antigen receptor could be inhibited by prior addition of either 10^?2M sodium azide or 50 μg/ml cycloheximide, implying that the receptors were actively synthesized by the cell. These inhibitors did not prevent shedding, but azide did inhibit the capping process. Both B-cells (bone marrow or spleen cells) and T-cells (splenic T-cells or 99.5% pure cortisone-resistant T-cells) were shown to regenerate multispecific ABC to the frequency found prior to incubation.     

Peripheral tolerance induced in lymph nodes by syngeneic spleen cells inhibits generation of CTLs to hapten-altered self antigens but not to alloantigens.

The immunological tolerance that is induced in lymph nodes that have been exposed to syngeneic spleen cells has been examined. Development of cytotoxic T lymphocytes was used to assess the immunological status of the lymph node cells. The tolerance was studied from the viewpoint of its induction, its activation, and its specificity. We had already reported that injecting either T or B cells of splenic origin into a regional lymph node environment a week prior to immunization for CTL to hapten-altered self antigens prevents development of the CTL. Here, we confirm that syngeneic splenic cells but not lymph node cells will induce the suppression provided that spleen cells are not coupled with hapten. We now report that splenic cells that cannot replicate or synthesize and secrete protein are capable of inducing the suppression. The data suggest a preformed surface marker peculiar to spleen cells and perhaps on cells that traverse the thymus induces local tolerance that is mediated by suppressor cells. Triggering the induced suppressor T cells (previously identified as CD8-) was achieved by syngeneic spleen cells as well as by H-2-compatible, Mls-disparate spleen cells but not by syngeneic lymph node cells or apparently by allogeneic spleen cells. Furthermore, triggering suppression was achieved by hapten-coupled syngeneic spleen cells whereas such cells would not induce the suppression. Thus, activating the suppressor cells requires reexposure to splenic cells of the proper MHC haplotype, unaltered or coupled with either TNP or FITC. Once triggered, the suppression was manifested toward CTL generation against hapten-coupled syngeneic antigens on either spleen or lymph node cells but not against allogeneic antigens. Thus, the specificity of the tolerance was directed to altered self antigens despite its induction by unaltered spleen antigen. Furthermore, for suppression to be seen the spleen antigen was not required to be on the hapten-coupled syngeneic cells used for the CTL immunization. The relationship of the splenic cell "antigen" to hapten-altered self antigens and to other surface markers and its site of acquisition within the body and its significance for cell homing have become intriguing questions of importance. This information has been discussed from the viewpoint of its applicability to autoimmune diseases as well as to cessation of inflammatory reactions that may be mediated by lymph node cells.     

Induction of “Low Dose” Tolerance to a Bacterial Somatic Antigen in Neonatal Mice

THE concept of “low dose” tolerance rests on the observation that specific immunological unresponsiveness can be induced by extremely small doses of antigen, sometimes in quantities too small to provoke antibody formation as such^1–3. The initial observations were made with soluble serum proteins, which are generally not considered “strong” antigens^4,5. Low dose tolerance can be induced also to the highly immunogenic protein antigen derived from Salmonella flagella, as assessed at the level of serum antibody^6,7. Furthermore, recent studies at the level of antibody forming cells indicate that immunological tolerance to complex antigens such as sheep erythrocytes and bacterial extracts, as well as to serum proteins or chemical haptens, can be induced in neonatal or adult animals^9–14. In this regard, relatively large doses of antigen are reportedly necessary to induce tolerance in adult rodents to a bacterial antigen such as Escherichia coli lipopolysaccharide (LPS). For example, 10–15 mg of the LPS is necessary to induce tolerance, as measured by an indirect haemolytic plaque assay with antigen coated sheep erythrocytes^15,16. Lower doses of LPS reportedly induce only immunity.     

Dendritic cells in oral tolerance in the gut

Oral tolerance is a process that allows generation of systemic unresponsiveness to food antigens. Hence if the same antigen is introduced systemically even under immunogenic conditions it does not induce immune responsiveness. Dendritic cells (DCs) have been identified as essential players in this process. DCs in the gut are located in a strategic position as they can interact directly with luminal antigens or indirectly after their transcytosis across epithelial cells. DCs can then migrate to associated lymphoid tissues to induce tolerance. Antigen presenting cells in the gut are specialized in function and have divided their labour so that there are cells capable to migrate to the draining mesenteric lymph node for induction of T regulatory cells, while other subsets are resident and are required to enforce tolerance locally in the gut after food antigen exposure. In this review, I shall summarize the characteristics of antigen presenting cells in the gut and their involvement in oral tolerance induction. In addition, I will also emphasize that tolerance to food allergens may be contributed by plasmacytoid DCs in the liver that participate to the elimination or anergy of allergen-specific CD8 T cells. Hence specialized functions are associated to different subsets of antigen presenting cells and different organs.     

Active Suppression of the Allogeneic Histocompatibility Reactions During the Metamorphosis of the Clawed Toad Xenopus

Metamorphosis is a privileged period for the induction of tolerance to allografts. Transfers of lymphocytes from metamorphosing Xenopus into isogenic adults prevented the rejection of a skin graft differing from the adult host by minor histocompatibility antigens. This implies that active suppression is involved at one step of the induction of tolerance to the self-antigens that differentiate at the time of metamorphosis. The reciprocal experiments of preventing tolerance induction by transfer of normal adult cells into metamorphosing animals failed. However, passive transfer of anti-graft immunity in tolerant animals was partially observed, provided that a transfer of primed cells was done simultaneously with the challenging graft. Thus, memory cells are not as sensitive to the suppression as are the cells that respond in a first set reaction.