Antonio Bandeira

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.

Vβ17 gene polymorphism in wild-derived mouse strains: Two amino acid substitutions in the Vβ17 region greatly alter T cell receptor specificity

Abstract Of 41 wild-derived mouse strains analyzed, 14 contained T cells bearing Vβ17 receptors in splte of the concomitant expression of I-E antigens. Reciprocal F1 and F2 hybrids of one of these strains, PWK, with laboratory strains revealed different patterns of Vβ17 T cell deletions from those observed with Vβ17 T cells from SJL, implying that the two Vβ17 regions are associated with recognition of distinct superantigens. The structures of the Vβ17 alleles differ by two amino acid substitutions, which lie together in an area distant from the predicted site of T cell receptor interaction with peptide-MHC complexes but overlapping with that implicated in Vβ8.2 recognition of Mls-1 superantigen. This demonstrates that the self-superantigen leading to Vβ17 T cell deletion varies with the allele of the receptor gene and confirms that T cell deletions by such ligands involve interactions with a region of the Vβ domain that is distinct from the conventional combining site.

On the ontogeny and physiology of regulatory T cells

Summary: Lymphocytes can interfere with the activity of other lymphocytes in a thousand and one ways. A particular subset of so‐called regulatory CD4+ T cells is capable of controlling the activity of other lymphocytes in yet another way. Their function is primarily defined by the ability to protect the integrity of tissues and organs in vivo. This was demonstrated in experimental models of natural tolerance to peripheral tissues, transplantation tolerance and the regulation of immune responses promoted by exogenous antigens at the level of the intestinal mucosa. Moreover, regulatory T cells also play a major role in the systemic homeostatic mechanisms that control total lymphocyte numbers. There is good evidence to support the contention that a significant fraction of the naturally occurring regulatory T cells is generated in the thymus following selection mediated by high avidity T‐cell receptor/ligand interactions. Symbolically, self‐reactive regulatory T cells do represent the breakthrough of concepts challenging the long‐lasting Burnetian dogma that all autoreactive cells should be eliminated or inactivated. Although clonal deletion of self‐reactive cells is a fundamental process in T‐cell development, controlled autoreactivity is part of the physiology of the immune system. Thus, autoreactive regulatory T cells also protect immunologists from the desperate hunting for the evil of horror autotoxicus.

Th17 Cells Are the Dominant T Cell Subtype Primed by Shigella flexneri Mediating Protective Immunity

The T cell response to Shigella, the causative agent of bacillary dysentery, remains poorly understood. Using a murine model of infection, we report that Shigella flexneri primes predominately IL-17A– and IL-22–producing Th17 cells. Shigella-specific Th1 cells are only significantly induced on secondary infection, whereas specific Th2 and CD8+ T cells are undetectable. Apart from Th17 cells that are primed in a MHC class II- and IL-6–dependent, but IL12/23p40-independent manner, we identified γδ T cells as an additional but minor source of IL-17A. Priming of IL-17A+ γδ T cells is dependent on IL12/23p40, but independent of MHC-class II and IL-6. Th17 cells have emerged as important players in inflammatory, autoimmune, and infectious diseases. Among the yet unresolved questions is their role in long-term immunity to pathogens. In this study, we show that the elicited S. flexneri-specific Th17 pool gives rise to an enhanced recall response up to 12 mo after priming, suggesting the presence of a long-term memory state. The clearance of primary infection is impaired in the absence of T cells, but independently of IL-17A. However, after reinfection, IL-17A produced by S. flexneri-specific Th17 cells becomes important to ultimately restrict bacterial growth. These findings bring new insights into the adaptive immune response to Shigella infection and highlight the importance of pathogen-specific Th17 cell immunity for secondary immune protection.

IL-2 coordinates IL-2–producing and regulatory T cell interplay

Regulation of IL-2–producing CD4+ T cell numbers is controlled by a quorum-sensing feedback loop as regulatory T cells sense the IL-2 produced.

The Participation of B Cells and Antibodies in the Selection and Maintenance of T Cell Repertoires

The imtnune system (IS) of vertebrates is endowed with obvious cognitive properties: it learns, remembers, and makes discriminatory inferences about molecular profiles. Typically, properties of this nature cannot be ascribed to individual components in the system (in our case, to the presence or activity of given clones), but they emerge as global behaviors from its general organization. Immune self/ nonself discrimination is, thus, a characteristically dispersed property of the IS (Varela et al. 1987). It follows that, to understand self/nonself discrimination, we have to be concerned at least as much v̂ fith such global behaviors and distributed properties as with individual clonal specificities. Distributed properties require connectivity between the elements of the system, and of these with other self components. Therefore, we believe, the understanding of self/nonself discrimination must start from the consideration of self-directed reactivities, in clear contrast with the classical notion that immune self/nonself discrimination is based upon the elimination of self-reactive clones (Vaz et al. 1984. Coutinho et al. 1984). Clearly, and this is also the case for conventional points of view, the behavior of an IS at any point in life can only be understood in the context of its own history in the ontogenic development of the individual. Snapshots of adult immune systems based on the study of lymphocyte repertoires, while necessary to realize the structure of the systems components and to construct the anatomy of interactions, will always tell us little about history and overall organization. We must therefore make explicit here the limitations of an approach (clonal

Origin, trafficking, and intraepithelial fate of gut-tropic T cells

Tropism to the small intestinal epithelium is a general property of unconventional and conventional recent thymic emigrants, but for both cell types only GALT-related cycling thoracic duct lymphocytes are the precursors of cytotoxic intraepithelial lymphocytes.

Limiting Dilution Analysis of Interleukin 2‐Producing Mature T Cells

Evaluation of lymphokine production by individual activated T cells is necessary to characterize their growth requirements. We have studied interleukin 2 (IL‐2) secretion by mature T lymphocytes using a high resolution assay system with the following characteristics: (a) a threshold of IL‐2 detection 25 times lower than classic IL‐2 titration: (b) the ability to discriminate between IL‐2 and IL‐4 activities: (c) absence of ‘in situ’ IL‐2 absorption; (d) IL‐2 production revealed al the single cell level By this method an average of 10% of spleen cells, and 75% of L3T4 cells were detected as producers in a concanavalin A (Con A)‐dependent T cell activation system. Our results also suggest the complete restriction of II ‐2 secretion to cells with this phenotype. Therefore, factors other than IL‐2 must play a major role in Lyt 2+ mitogen‐driven, helper‐independent T cell proliferation.

Some reasons why deletion and anergy do not satisfactorily account for natural tolerance

We would feel quite foreign to this Forum’s discussion on “deletion and anergy”, were it not for expressing our conviction that either “models or reality” of those phenomena are not the key to understanding the physiopathology of autoimmunity. Our position is not easy to define, for we do share the notion that V-region-dependent deletion of lymphocytes operates extensively in the normal immune system. Were we not the first, after all, to describe massive bone marrow pre-B/B cell deletion in normal mice (reviewed in Coutinho et al., 1992)? And are not our theoretical models constructed around the definition of lymphocyte physiology by “bellshaped” responses to receptor occupancy, with highdose inactivation (Coutinho, 1974; Varela and Coutinho, 1991)? We strongly object, however, to simplistic views that attempt to reduce natural tolerance to deletion and/or inactivation of self-reactivities, because we have also seen many examples of positively selected functional autoreactivities in normal animals, and quite a few cases of tolerance without deletion and/or anergy of the corresponding lymphocytes. From lymphocyte physiology, we also understand that if a particular ligand concentration presented to the continuous affinity distribution of an unselected repertoire leads to deletion or inactivation of some cells, it will necessarily activate some others. If negative and positive selection are the two sides of the same coin (repertoire selection by the molecular environment of lymphocytes that respond with “bell-shaped” profiles), it is a mistake to consider only the former in the context of natural tolerance (the physiological outcome of that selection). Beyond the molecular and cellular bases of repertoire selection, self-nonself discrimination has to deal with the “origin of the ligand”, and thus with its history in the organism. To us, “single cell solutions” of a problem that concerns organisms are bound to be incomplete, or wrong. In brief, natural tolerance is a developmental question, but one of organisms and not of single lymphocytes.

Murine Acariasis. II. Immunological Dysfunction and Evidence for Chronic Activation of Th-2 Lymphocytes

The authors describe the immunological profile of BALB/c mice with Mite‐Associated Ulcerative Dermatitis (MAUD)‐like disease, due to Myocoptes musculinus (Koch 1844) infestation. The disease probably involves allergic mechanisms and is characterized by erythematous and pruritic skin lesions, widespread hair loss, lymphadenopathy, lymphocytopenia, granulocytosis and wasting. Affected individuals had much reduced numbers of pre‐B and B cells in bone marrow and B cells in blood; decreased T‐cell numbers in peripheral lymphoid organs and blood; hypergammaglobulinaemia with selective increases of IgG1, IgE and IgA, and depletion on IgM and IgG3, the same isotype distribution being detected in splenic plasmocytes; qualitative modifications of the serum antibody reactivity pattern; and increased production of IL‐4 with decreased IL‐2 production after in vitro polyclonal stimulation of T cells. Taken together, these results suggest that infestation by M. musculinus in BALB/c mice leads to a significant immunological disorder resulting in a T‐helper‐2 (Th‐2) type response, with marked systemic consequences. This pathological condition may thus provide a useful model system for the immunobiological perturbation associated with chronic allergic disease.