The genetic landscape of mitochondrial diseases in the next-generation sequencing era: a Portuguese cohort study

Introduction: Rare disorders that are genetically and clinically heterogeneous, such as mitochondrial diseases (MDs), have a challenging diagnosis. Nuclear genes codify most proteins involved in mitochondrial biogenesis, despite all mitochondria having their own DNA. The development of next-generation sequencing (NGS) technologies has revolutionized the understanding of many genes involved in the pathogenesis of MDs. In this new genetic era, using the NGS approach, we aimed to identify the genetic etiology for a suspected MD in a cohort of 450 Portuguese patients. Methods: We examined 450 patients using a combined NGS strategy, starting with the analysis of a targeted mitochondrial panel of 213 nuclear genes, and then proceeding to analyze the whole mitochondrial DNA. Results and Discussion: In this study, we identified disease-related variants in 134 (30%) analyzed patients, 88 with nuclear DNA (nDNA) and 46 with mitochondrial DNA (mtDNA) variants, most of them being pediatric patients (66%), of which 77% were identified in nDNA and 23% in mtDNA. The molecular analysis of this cohort revealed 72 already described pathogenic and 20 novel, probably pathogenic, variants, as well as 62 variants of unknown significance. For this cohort of patients with suspected MDs, the use of a customized gene panel provided a molecular diagnosis in a timely and cost-effective manner. Patients who cannot be diagnosed after this initial approach will be further selected for whole-exome sequencing. Conclusion: As a national laboratory for the study and research of MDs, we demonstrated the power of NGS to achieve a molecular etiology, expanding the mutational spectrum and proposing accurate genetic counseling in this group of heterogeneous diseases without therapeutic options.

RFT1-CDG: Absence of Epilepsy and Deafness in Two Patients with Novel Pathogenic Variants

This report is on two novel patients with RFT1-CDG. Their phenotype is characterized by mild psychomotor disability, behavioral problems, ataxia, and mild dysmorphism. Neither of them shows signs of epilepsy, which was observed in all RFT1-CDG patients reported to date (n = 14). Also, deafness, which is often associated with this condition, was not observed in our patients. Molecular analysis of RFT1 showed biallelic missense variants including three novel ones: c.827G > A (p.G276D), c.73C > T (p.R25W), and c.208T > C (p.C70R).

Methionine Adenosyltransferase I/III Deficiency in Portugal: High Frequency of a Dominantly Inherited Form in a Small Area of Douro High Lands

Methionine adenosyltransferase deficiency (MAT I/III deficiency) is an inborn error of metabolism resulting in isolated hypermethioninemia, and usually inherited as an autosomal recessive trait, although a dominant form has been reported in several families.During the last 6 years, approximately 520,000 newborns were screened in the Portuguese Newborn Screening Laboratory by MS/MS, and 21 cases of persistent hypermethioninemia were found. One case was confirmed to be a deficiency of cystathionine β-synthase and 20 cases were confirmed by MAT1A gene analysis to have an elevation of methionine due to MAT I/III deficiency, which indicates an incidence for this condition of 1/26,000. Twelve of the MAT I/III deficient newborns, belonging to 11 families, were identified in the northern region of Portugal and sent to the same treatment center, where they are under follow-up. Clinical, biochemical, and genetic characteristics of individuals from these 11 families are presented. Plasma methionine and homocysteine concentrations were found to be moderately increased in all newborns, and molecular analysis revealed that they all were heterozygous for R264H mutation. Normal growth, development, and neurological examination were observed in all cases, and cerebral MRI performed in six cases revealed myelination abnormalities in one case. Plasma methionine concentration for all 12 cases was always below 300 μM, and they are all on a normal diet for their age.

Solitaire stent for endovascular treatment of intracranial aneurysms: immediate and mid-term results in 15 patients with 17 aneurysms

INTRODUCTION

The Solitaire stent is the first fully retractable stent for endovascular treatment (EVT) of intracranial aneurysms. The aim of this study was to evaluate its use in a prospective series with mid-term follow-up.

METHODS

A retrospective review of our prospectively maintained database identified all patients treated with a Solitaire stent. Clinical charts, procedural data, angiographic results were reviewed.

RESULTS

Between June 2008 and September 2009, 15 patients with 17 wide-necked or fusiform aneurysms (16 unruptured/one ruptured) were identified. EVT was successfully performed in all but one patient in whom the stent was removed because it induced flow reduction in the 1.8-mm parent artery. Among 14 treated patients, 13 had an excellent outcome and one had a good outcome. In this latter patient, the first stent could not be delivered and was changed for another one that was successfully deployed. The patient experienced a thrombo-embolic complication 6 hours after EVT and kept a slight hand paresis. In all cases but one, the stent was thus easily navigated and positioned despite a relative poor visibility. Angiographic results included eight complete occlusions, two neck remnants, and six incomplete occlusions. Six-month control in 14 aneurysms showed 13 complete occlusions and one incomplete occlusion.

CONCLUSION

The Solitaire stent is useful for EVT of complex intracranial aneurysms because it is fully retractable, easy to navigate and to precisely place. However, it should be used with caution in arteries less than 2mm in diameter.

Endovascular treatment of anterior choroidal artery aneurysms

INTRODUCTION

Endovascular treatment (EVT) of anterior choroidal artery aneurysms (AChAA) may be challenging because of the close relationship with the parent artery. The aim of this study was to report our experience with EVT of AChAA.

METHODS

A retrospective review of our prospectively maintained database identified all AChAA treated by embolization. The clinical charts, procedural data and angiographic results were reviewed.

RESULTS

From April 2004-August 2008, 11 patients were identified. Five patients presented with a subarachnoid hemorrhage (SAH) and six patients were asymptomatic. Aneurysms size varied from two to 13 mm (mean size=3.6 mm) and nine had an unfavourable neck/sac ratio (>or=0.7). The anterior choroidal artery was arising from the sac (n=6) or from the neck (n=5). Endovascular treatment consisted of balloon-assisted coiling (n=8), coiling (n=2) and stent-assisted coiling (n=1). No procedural complication occurred and all patients had an excellent outcome except one patient who died because of severe vasospasm 8 days after an uneventful EVT. Immediate angiographic control showed six complete occlusions, one neck remnant and four incomplete occlusions. Follow-up controls (mean=18 months) were obtained in eight patients and showed five stable occlusions and three further thrombosis achieving complete occlusion.

CONCLUSION

EVT of AChAA is associated with very good clinical and long-term anatomical results. Because of their small size, unfavourable neck/sac ratio and close relationship with the parent artery, EVT frequently requires the use of adjunctive techniques such as balloon or stent-assisted coiling.

Treatment of vasospasm secondary to subarachnoid hemorrhage using intra-arterial nimodipine in low dosage

SUMMARY

There are currently no consensus criteria to define the dosage of intra-arterial Nimodipine to be used to obtain permanent remission of vasospasm secondary to subarachnoid haemorrhage. The purpose of this paper is to establish a mathematical relation between the number of vessels affected by this kind of vasospasm and the Nimodipine dose that must be used for intra- arterial angioplasty to obtain permanent remission of the vasospasm.

Embryonic thymic epithelium naturally devoid of APCs is acutely rejected in the absence of indirect recognition

Transplants of tissues depleted of passenger leukocytes are upon in vitro culture usually accepted in allogeneic recipients. Accordingly, fully allogeneic embryonic thymic epithelium was suggested to be poorly immunogenic. However, this tissue is capable of inducing donor-specific tolerance to peripheral tissues, when restoring T cell development in nude mice, through the production of regulatory cells. In the present work, adult immunocompetent allogeneic recipients were grafted with embryonic tissues isolated at stages before hemopoietic colonization or even before the establishment of circulation. Allogeneic thymic epithelium of day 10 embryos and heart primordium of day 8 embryonic donors were always rejected. Acute rejection of the thymic anlagen takes place in less than 12 days, with maximal CD4(+) and CD8(+) T cell infiltrates at 10 days post-transplant. In addition, a significant infiltrate of NK1.1(+) cells is observed, although without any essential role in this process. Furthermore, recipients lacking the indirect pathway of Ag presentation to CD4(+) T cells do not reveal any significant delay in rejection, even when CD8(+) T cells are also eliminated. Thus, our experimental approach reveals acute allograft rejection in the absence of all known pathways of naive T cell activation and therefore unveils a novel graft rejection mechanism that should be mediated by direct recognition of parenchymal cells. Given the importance of dendritic cells in naive T cell activation, it is likely that cross-reactive memory T cells may also drive rejection.

On the ontogeny and physiology of regulatory T cells

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.

CD25+ CD4+ T cells regulate the expansion of peripheral CD4 T cells through the production of IL-10

The mechanisms by which the immune system achieves constant T cell numbers throughout life, thereby controlling autoaggressive cell expansions, are to date not completely understood. Here, we show that the CD25(+) subpopulation of naturally activated (CD45RB(low)) CD4 T cells, but not CD25(-) CD45RB(low) CD4 T cells, inhibits the accumulation of cotransferred CD45RB(high) CD4 T cells in lymphocyte-deficient mice. However, both CD25(+) and CD25(-) CD45RB(low) CD4 T cell subpopulations contain regulatory cells, since they can prevent naive CD4 T cell-induced wasting disease. In the absence of a correlation between disease and the number of recovered CD4(+) cells, we conclude that expansion control and disease prevention are largely independent processes. CD25(+) CD45RB(low) CD4 T cells from IL-10-deficient mice do not protect from disease. They accumulate to a higher cell number and cannot prevent the expansion of CD45RB(high) CD4 T cells upon transfer compared with their wild-type counterparts. Although CD25(+) CD45RB(low) CD4 T cells are capable of expanding when transferred in vivo, they reach a homeostatic equilibrium at lower cell numbers than CD25(-) CD45RB(low) or CD45RB(high) CD4 T cells. We conclude that CD25(+) CD45RB(low) CD4 T cells from nonmanipulated mice control the number of peripheral CD4 T cells through a mechanism involving the production of IL-10 by regulatory T cells.

Regulatory CD4 T cells control the size of the peripheral activated/memory CD4 T cell compartment

The mechanisms leading to stable T cell numbers in the periphery of a healthy animal are, to date, not well understood. We followed the expansion of CD45RBhigh (naive) and CD45RBlow (activated/memory) CD4 T cells transferred from normal mice into syngeneic Rag-20/0 recipients and the dynamics of peripheral reconstitution when both populations were coinjected. Naive cells acquired an activated phenotype and showed a high proliferative capacity that was dependent on the environment in which the recipients were kept (specific pathogen-free vs conventional housing conditions), the age of the recipients, and the presence of CD45RBlow T cells in the injected cohort. CD45RBlow CD4 T cells protected the host from CD45RBhigh CD4 T cell-induced inflammatory bowel disease and showed a limited degree of expansion. CD45RBlow CD4 T cells isolated from GF mice also showed the ability to prevent inflammatory bowel disease, indicating that at least part of the natural regulatory T cells are self-reactive. The results indicate that 1) peripheral T cell expansion in lymphocyte-deficient recipients represent classical immune responses, which are mainly promoted by exogenous Ags and 2) natural regulatory T cells control the size of the activated/memory peripheral CD4 T cell compartment.

Naturally activated CD4+ T cells are highly enriched for cytokine-producing cells

Most T cells in a normal non-immunized individual are in a resting state. However, a small proportion of splenic T cells are large activated cells both in specific pathogen-free and antigen-free mice. To further elucidate the effector functions associated with these "naturally" activated CD4+ T cells, we have characterized the expression of various membrane markers, cytokine production and T helper activity by these cells. We show that naturally activated CD4+ T cells express activation markers and contain tenfold higher proportions of cells producing IL-4, IL-10 and IFN-gamma as compared to small CD4+ T cells. Despite the high proportion of IFN-gamma producers, naturally activated CD4+ T cells still induce B cell proliferation and differentiation. These results are discussed in the context of normal physiological autoreactivity.

Neonatal tolerance to alloantigens is induced by enriched antigen-presenting cells

Spleen and bone marrow cells from normal or Rag-1-deficient donors are equally competent in their ability to induce neonatal transplantation tolerance in semi-allogeneic hosts, and the latter are also capable of tolerizing fully allogeneic recipients. Both types of donor cells resulted in comparable levels of haemopoietic chimerism in tolerant animals. Lymphoid hyperactivity, however, was absent in animals tolerized with Rag-1-deficient cells. The authors conclude that neonatal tolerance induced with haemopoietic cells requires no donor lymphocytes, and is thus not the result of deficient antigen presentation. Furthermore, the state of tolerance can be dissociated from the lymphoid hyperactivity that requires donor lymphocytes and is regularly scored in conventionally tolerized animals.

Establishment of tissue-specific tolerance is driven by regulatory T cells selected by thymic epithelium

Grafts of thymic epithelium (TE) rudiments restore T cell development and function in allogeneic athymic mice. These TE chimeras are specifically tolerant to grafts of peripheral tissues (e.g. skin and heart) from the TE donor strain, although they harbor peripheral immunocompetent T cells capable of rejecting those grafts. Initial analysis has shown that TE chimeras also harbor TE-selected CD4 T lymphocytes that inhibit graft rejection by tissue-reactive T cells in immunocompetent recipients. Peripheral tolerance in TE chimeras is thus maintained by dominant mechanisms dependent on regulatory CD4 T lymphocytes. Here we show that TE-selected regulatory T cells recruit nontolerant tissue-reactive CD4 and CD8 T cells to express similar regulatory functions. Only recent thymic emigrants, but not peripheral resident mature T cells are susceptible to this process of functional education, which also requires exposure to specific antigens and occurs entirely in the periphery. We propose that these mechanisms play a major role in establishing and maintaining natural self tolerance to tissue-specific antigens.

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.

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.

Evidence for a thymus-dependent form of tolerance that is not based on elimination or anergy of reactive T cells

The avian embryo has provided an appropriate model to study the ontogeny of the primary lymphoid organs, thymus and bursa of Fabricius. By using the quail-chick marker system the embryonic origin of the highly intricate cell components which form these organs could be traced back to the initial endodermal, mesodermal and ectodermal germ layers. The timing and dynamics of the incoming and outcoming flows of hemopoietic cells which characterize their lymphopoietic activity could be revealed in both quail and chick embryos. This knowledge served as a basis for an investigation on the role of the epithelial component of the thymus (derived from the pharyngeal endoderm) on tolerance to tissue graft and, by extension, tolerance to self. When this work was undertaken, the prevailing view was that exposure of the developing immune system to foreign antigens in the embryo allows them to be assimilated to self components in the mature animal. In fact, this was found to be true for allogeneic grafts between MHC-distinct chickens, of certain tissues, such as for instance wing tissues. However, in heterospecific transplantations, i.e. when a limb bud was grafted from quail to chick embryos, the chick host acutely rejected the foreign limb soon after birth. In contrast, grafts of the quail thymic epithelial (TE) rudiment resulted in the development of a chimeric thymus in which the foreign epithelial component was not only tolerated but able to induce full tolerance of the grafted wing from the same donor. By monitoring the amount of quail TE implanted we showed in addition that only part of the peripheral T-cell population had to differentiate in the context of the quail epithelial cells to induce tolerance to quail tissues. This pointed to the generation in the thymus of regulatory T cells, coexisting with specific anti-quail reactive T cells, but able to inhibit them from reacting against the quail wing antigenic determinants. A mammalian model was then devised to further study this mechanism of tolerance that we have qualified as "dominant" by opposition to the current model based on either clonal elimination or anergy which can be considered as recessive or passive. Nude mice of MHC type A were grafted with TE of E10 type B embryos. They became reconstituted for T-cell function but tolerant for B skin allografts. Spleen cells from such tolerant animals injected to naive A nude mice reconstituted T cell function in the recipient and transferred the tolerance to B skin grafts. Reducing the number of donor cells resulted in the segregation of the two phenomena. For low numbers the recipients were restored but not tolerant, thus showing the coexistence in the tolerant donor of anti-B reactive T cells together with regulatory cells able to abolish their reactivity against B determinants. Other experiments demonstrated that TE-induced tolerance does not rely on clonal deletion or anergy. This was shown on systems where elimination of cells directed toward superantigens was screened. It turned out that tolerance to skin grafts and superantigen T-cell deletion are unrelated phenomena. These observations strongly suggest that tolerance to self results at least in part from the interplay between cells potentially harmful for self component and others which exert a strong control on their reactivity. The latter cell type depends upon interactions of thymocytes with the endodermal component of the thymus.

Regulatory T cells in thymic epithelium-induced tolerance. I. Suppression of mature peripheral non-tolerant T cells

Athymic mice grafted at birth with allogeneic thymic epithelium (TE) display life-long tolerance to tissue grafts of the TE donor strain, in spite of harboring peripheral T cells capable of rejecting those grafts. Tolerance is maintained in these chimeras by TE-specific regulatory CD4 T cells. We presently address the quantification and the mechanisms of this dominant tolerance process. C57BL/6 mice containing variable but defined numbers of peripheral, resident T cells received cell transfers of graded numbers of peripheral T cells from B6(BALB E10) chimeras (C57BL/6 nude mice grafted with TE from 10-day-old BALB/c embryos), resulting in a series of animals containing a wide range of donor (tolerant) versus host (non-tolerant) T cell chimerism. Increasing the relative representation of donor T cells results in a progressive delay in the rejection of BALB/c skin grafts, life-long tolerance being achieved at a ratio of tolerant and non-tolerant T cell populations of 1. In recipients displaying full tolerance, graft-reactive non-tolerant T cells were not deleted, anergized or committed to noninflammatory functions. Thus, sorted host T cells from tolerant recipients readily rejected BALB/c skin grafts upon transfer to immunodeficient animals. Finally, measurements of "helper" and inflammatory activities, as well as interleukin-4 and interferon-gamma production, failed to discriminate between T cell populations from tolerant and non-tolerant animals after specific in vitro stimulation. We conclude that: (a) TE-selected regulatory T cells can suppress, in a quantitative manner, in vivo T cell responses against major and minor histocompatibility antigens expressed by the TE and, (b) this suppressive activity neither inactivates mature non-tolerant T cells, nor does it seem to drive their differentiation along noninflammatory pathways.

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

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

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

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

Developmental shift in the patterns of interleukin production in early post-natal life

In mice, immunological tolerance to self is established in the perinatal period, when tolerance susceptibility to allogenic tissues is higher than in adults. We have now investigated whether this could result from developmental regulation of effector functions of T cells exposed to specific antigens, by studying the "natural" or T cell receptor-induced expression of several interleukin genes. We used qualitative and quantitative polymerase chain reaction methods to study interleukin (IL)-2, IL-4, IL-10 and interferon-gamma mRNA expression by splenic cells at different ages. The results show that newborn peripheral cells (up to day 7), in contrast to the T lymphocytes of adult mice, express high levels of IL-4 and interferon-gamma, and very low levels of IL-2 messenger spontaneously and upon specific T cell activation. This characteristic phenotype depends on intrinsic T cell properties, as it is not due to the newborn environment.

Differential contribution of thymic outputs and peripheral expansion in the development of peripheral T cell pools

The number of peripheral T cells in mice increases up to 100-fold in the first few weeks of life. We have followed the fate of Thy-1 congenic T cells transferred into newborn recipients, to evaluate the relative contribution of thymic output versus peripheral expansion in the constitution of peripheral T cell pools during post-natal development. The results show that in normal animals there is essentially no peripheral expansion of T cells, which show slow turnover rates (1 to 2 months) along that time period. The rates of cell accumulation in the periphery require, therefore, an average of 1 x 10(6)-2 x 10(6) mature thymic emigrants/day for the first 3 weeks of life.

Skewed V beta TCR repertoire of CD8+ T cells in murine Trypanosoma cruzi infection

We have followed CD4 and CD8 TCR V beta repertoires during the acute phase of Trypanosoma cruzi infection in a resistant mouse strain (C57BL/6). No major changes were found in the V beta TCR distributions analyzed (covering roughly 40% of the TCR repertoire) in peripheral CD4 T lymphocytes, confirming the polyclonal nature of CD4 T cell responses. In contrast, in most animals, an over-representation of V beta 5 and V beta 14 TCR families was disclosed in the CD8 T cell compartment, superimposed on a predominantly polyclonal response. The preferential expansion of V beta 5+CD8+ T cells was also observed after infection of sensitive (C3H/HeJ, BALB/c) mouse strains. These observations suggest the existence of CD8 T cell-directed superantigenic activities associated with parasites.

Evidence for a differential avidity model of T cell selection in the thymus

Positive and negative selection of a lymphocytic choriomeningitis virus (LCMV) peptide-specific, H-2Db-restricted T cell clone (P14) was studied using TAP1- and TAP1+ mice transgenic for P14 T cell receptor (TCR) alpha and beta genes. Positive selection of transgenic CD8+ P14 cells was impaired in TAP1- mice. Addition of the LCMV peptide to TAP1- fetal thymic organ cultures (FTOCs) at low and high concentrations induced positive and negative selection of CD8+ P14 cells, respectively, while addition of the same peptide to TAP1+ FTOCs induced negative selection even at low concentrations. Both types of selection were peptide specific. Thus, a critical parameter that controls the fate of a thymocyte seems to be the number of TCRs engaged with complexes of peptide and major histocompatibility complex. When this number is low, positive selection occurs, and when it is high, negative selection takes place. These findings support a differential avidity model of T cell selection.

The role of thymic epithelium in the establishment of transplantation tolerance

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

Peripheral lymphoid hyperplasia and central lymphoid depletion in mice treated with a bacterial B-cell mitogen (F3'EP-Si/p90)

In order to further understand the mechanism mediating the mitogenic and immunosuppressor effects of p90, a protein produced by Streptococcus intermedius, flow cytometric studies were performed on peripheral and central lymphoid organs of mice treated with this protein. p90 induced a strong blastogenic B-cell response in the spleen and lymph nodes, followed by a slight but significant polyclonal T-cell activation. B-cell repertoire analysis indicated that polyclonal B-cell responses affected similarly both CD5+ and conventional (CD5-) B cells in the spleen. Repertoire analysis of T cells failed to reveal any preferential stimulation of the V beta T-cell receptor (V beta-TcR) families studied. Peripheral lymphoid hyperplasia was observed concomitantly with central lymphoid depletion. In the bone marrow, pre-B and B cells were profoundly depleted, with a more pronounced effect on small pre-B cells. In the thymus, double-positive (CD4+CD8+) thymocytes were preferentially eliminated, with a relative enrichment of single positive (either CD4+ or CD8+) and double-negative (CD4-CD8-) thymocytes.

Transplantation tolerance is unrelated to superantigen-dependent deletion and anergy

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

Protective effect of a T-cell-dependent immunosuppressive, B-cell-mitogenic protein (F3'EP-Si, or P90) produced by Streptococcus intermedius

The role of a previously described bacterial protein (F3'EP-Si), now designated P90, in the survival of Streptococcus intermedius in the host was investigated, and the immunosuppressive and B-cell-mitogenic effects of this protein were further characterized. C57BL6 mice treated with P90 were about 50 times more susceptible to infection with this bacterium than untreated mice. One of seven splenocytes of C57BL/6 mice were activated by P90. Marked splenomegaly was observed in mice treated with P90, with increased numbers of splenic mononuclear cells and polyclonal immunoglobulin-secreting plaque-forming cells. Peak responses were seen on day 3 for immunoglobulin M (IgM) and on day 5 for IgG, with an isotypic pattern consisting predominantly of IgG2a and IgG2b. When mice were treated with P90 before being primed with sheep erythrocytes, polyclonal immunoglobulin synthesis was accompanied by an ephemeral stimulation of the specific immune response against sheep erythrocytes that was quickly replaced by a dramatic immunosuppression. In contrast, when mice were treated with P90 after being primed, the polyclonal activation was comparatively much less evident and there was no suppression of the specific immune response. Immunosuppression was considerably reduced in mice thymectomized as adults or depleted of CD8+ cells. Adoptive transfer experiments showed that B cells obtained from P90-treated mice were less able to respond to an antigenic challenge, even in the presence of normal T cells, and that T cells obtained from P90-treated mice could actively suppress the specific immune response of normal B cells.

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

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

Clonal analysis of B lymphocyte responses to Plasmodium chabaudi infection of normal and immunoprotected mice

Parasite infection causes marked perturbations in the host immune system, as shown by hypergammaglobulinemia, autoimmunity and immune depression, but there is little information on the number, specificities and performance of B cell clones activated in the course of infection. We have addressed these questions in a model of murine malaria induced by Plasmodium chabaudi, where primary infection results in very marked B cell responses that shift in Ig isotype pattern in immunoprotected animals, and where immunity can be transferred to naive recipients by injection of serum from late, but not early, infection. We have quantitated B cells responding to infection in two distinct functional compartments, namely blast cells and Ig-secreting cells, and compared normal with immune animals. We have also determined the frequencies of clonal specificities towards several autoantigens (DNA, myosin, transferrin and red cells), non-self protein or polysaccharide antigens (KLH, levan and dextran), and parasite antigens in both compartments, by measuring blast cell reactivities in limiting dilution analyses and Ig secretion in ELISASPOT assays. This experimental design allowed us to assess the specificity of the B cell responses, to compare the clonal composition of these two B cell compartments, and to evaluate putative specific response regulation at the step of terminal differentiation. Our results show that, in this particular experimental system: (i) B cell responses in primary infection are truly non-specific while immune animals show a greater ability to control the massive non-specific response; (ii) parasite specific B cells, particularly those committed to IgG production, are selectively stimulated in immune individuals; (iii) autoreactive B cells are not selectively stimulated, but increased autoantibody production may result from perturbation in the control of terminal differentiation in the respective clones; (iv) clones with specificity to some non-self antigens (e.g. KLH and dextran) are selectively engaged and regulated, which might have implications for the immunosuppression following infection.

Proliferative T cell anergy to MIs-1a does not correlate with in vivo tolerance

Intravenous or intraperitoneal priming of MIs-1b mice with cells from MIs-1a donors drastically reduces secondary in vitro proliferative responses to specific stimulation. We show here that: (i) priming leads to blast transformation of essentially all CD4+ T cells bearing V beta 6 receptors in spleen and lymph nodes, and to their marked clonal expansion; (ii) secondary in vivo (or in vitro) challenges have no effect on the state of activation and numbers of V beta 6 CD4 T cells, which, however, migrate to the site of antigenic exposure; (iii) priming results in the differentiation of specific V beta 6 CD4 T cells to effector helper activities, manifested in vivo by marked increases in the numbers of splenic plasma cells, which include terminally differentiated donor MIs-1a B cells; (iv) primed mice show accelerated 'second set' rejection of antigenic cells; and (v) MIs-1b mice, thymectomized as adults before exposure to MIs-1a cells, show immune responses that are equivalent to those of control animals. We conclude that, in this experimental system, proliferative 'anergy' does not correlate with tolerance but with memory, and relates to the determination of class in immune responses.

Effector T lymphocytes present in demyelinating lesions induced by Theiler's virus

Theiler's virus causes chronic primary demyelination associated with viral persistence in SJL/J mice. We have investigated the effector functions of T lymphocytes isolated from inflammatory brain lesions to detect a local immune dysfunction associated with viral persistence. In vitro, CD4+ T cells induced B-lymphocyte proliferation and antibody secretion; CD8+ T cells had cytolytic activity. Therefore, Theiler's virus persistence does not include local immune unresponsiveness.

V beta 17 gene polymorphism in wild-derived mouse strains: two amino acid substitutions in the V beta 17 region greatly alter T cell receptor specificity

Of 41 wild-derived mouse strains analyzed, 14 contained T cells bearing V beta 17 receptors in spite 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 V beta 17 T cell deletions from those observed with V beta 17 T cells from SJL, implying that the two V beta 17 regions are associated with recognition of distinct superantigens. The structures of the V beta 17 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 V beta 8.2 recognition of Mls-1 superantigen. This demonstrates that the self-superantigen leading to V beta 17 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 V beta domain that is distinct from the conventional combining site.

Localization of gamma/delta T cells to the intestinal epithelium is independent of normal microbial colonization

Using monoclonal antibodies identifying all gamma/delta and alpha/beta T cell receptors in cytofluorometric analysis, we have compared the composition of intestinal intraepithelial lymphocytes (i-IEL) in euthymic and athymic germ-free (GF) and conventional (SPF) mice. The results show a marked influence of microbial colonization in the numbers of single-positive (CD4+ or CD8+) alpha/beta i-IEL, but little effect in the pool size or characteristics of gamma/delta i-IEL. In young athymic mice, virtually no alpha/beta i-IEL are detected, while considerable numbers of gamma/delta i-IEL remain, though reduced in GF animals.

Thymic epithelium tolerizes for histocompatibility antigens

The role of thymic epithelium in the establishment of tissue tolerance was analyzed with a murine chimeric system. All T cells differentiated from birth onward in a thymus comprising allogeneic epithelium and syngeneic hematopoietic cells. Embryonic thymic rudiments that contained no hematopoietic cells from C3H (H-2k) donors were grafted to newborn athymic (nude) BALB/c (H-2d) mice. Chimeras that had normal T cell numbers and function rejected third-party skin grafts, but permanently accepted grafts syngeneic to the thymic epithelium. In vitro functional assays did not always correlate with the state of tolerance in vivo. Thus, pure thymic epithelium induces tolerance to histocompatibility antigens.

Thymic epithelium tolerizes for histocompatibility antigens

The role of thymic epithelium in the establishment of tissue tolerance was analyzed with a murine chimeric system. All T cells differentiated from birth onward in a thymus comprising allogeneic epithelium and syngeneic hematopoietic cells. Embryonic thymic rudiments that contained no hematopoietic cells from C3H (H-2k) donors were grafted to newborn athymic (nude) BALB/c (H-2d) mice. Chimeras that had normal T cell numbers and function rejected third-party skin grafts, but permanently accepted grafts syngeneic to the thymic epithelium. In vitro functional assays did not always correlate with the state of tolerance in vivo. Thus, pure thymic epithelium induces tolerance to histocompatibility antigens.

Different large deletions of T cell receptor V beta genes in natural populations of mice

A panel of geographically separate Mus m. domesticus and Swiss mice from several sources was screened for deletions in the T cell receptor variable (V) beta locus. Four out of forty-three strains tested show a deletion identical to or larger than the deletion previously described in SJL mice. To our knowledge, this is the first time that such important V beta deletions are described in inbred or partially inbred wild-derived strains of mice. On the other hand there seems to be very little polymorphism between the remaining V beta genes. Expression of V beta genes in peripheral and intra-thymic T cells was tested using antibodies specific for different V beta polypeptide chains. Flow cytometry analysis revealed a high expression of V beta 6 and V beta 17 genes in the Copacabana Swiss-derived strain and an absence of V beta 17 expression in the WLA wild-derived strain. The three Mus m. domesticus strains (WLA, DDO and WBG) having deleted two to three additional V beta subfamilies compared to SJL present no apparent immune deficiencies or autoimmune disorders. We conclude that relatively few V beta genes may suffice for unimpaired survival of wild mice and that there is a selective pressure for the structural conservation of the remaining V beta genes.

Characterization of B lymphocytes present in the demyelinating lesions induced by Theiler's virus

Theiler's virus, a murine picornavirus, persists in the central nervous system of SJL/J mice and causes inflammation and demyelination in the white matter of spinal cord. We isolated inflammatory cells from the central nervous system of infected animals and studied their functions in vitro. Flow microfluorimetry analysis showed the presence of all major lymphocyte subsets, namely CD4+ and CD8+ T cells as well as B lymphocytes. B lymphocytes were activated in vitro and the antigenic specificity of secreted Ig was determined by immunoblotting. Secreted Ig reacted strongly with viral capsid proteins VP1 and VP2 and had neutralizing activity. They reacted also with two nonviral white matter components which were present only in infected animals. Therefore, it is likely that Igs secreted at the site of infection play a role in limiting virus spread. It is also possible that virus induced autoreactive antibodies participate in demyelination.

Characterization of B lymphocytes present in the demyelinating lesions induced by Theiler's virus

Theiler's virus, a murine picornavirus, persists in the central nervous system of SJL/J mice and causes inflammation and demyelination in the white matter of spinal cord. We isolated inflammatory cells from the central nervous system of infected animals and studied their functions in vitro. Flow microfluorimetry analysis showed the presence of all major lymphocyte subsets, namely CD4+ and CD8+ T cells as well as B lymphocytes. B lymphocytes were activated in vitro and the antigenic specificity of secreted Ig was determined by immunoblotting. Secreted Ig reacted strongly with viral capsid proteins VP1 and VP2 and had neutralizing activity. They reacted also with two nonviral white matter components which were present only in infected animals. Therefore, it is likely that Igs secreted at the site of infection play a role in limiting virus spread. It is also possible that virus induced autoreactive antibodies participate in demyelination.

Interleukin 2 receptor expression and interleukin 2 production in exponentially growing T cells: major differences between in vivo and in vitro proliferating T lymphocytes

In the present study we have assessed the growth requirements for in vivo proliferating mature T cells. For that purpose we have selected experimental approaches which allow the study of exponential growth in vivo of a major fraction of T cells, and make it possible to obtain large numbers of T cells in cycle. Two types of growing T cell populations were used: peripheral T lymphocytes, proliferating exponentially after transfer into syngeneic athymic nude mice, and activated T cells in lymph nodes of normal mice draining the site of oxazolone administration. The results obtained show that mature T cell growth in vivo is not accompanied by expression of high-affinity interleukin 2 (IL2) receptor in the majority of activated cells, is not abrogated by in vivo administration of anti-IL2 receptor antibodies or enhanced by the in vivo injection of recombinant IL2, and that in vivo growing T cells do not produce detectable amounts of IL2, as evaluated functionally by limiting dilution assays or the presence of IL2 mRNA, detected by Northern blots or in situ hybridization. The presented data thus indicate that the rules known to apply to T cell activation and proliferation in vitro differ from those used by in vivo growing T lymphocytes, at least in the two systems studied.

A model system for the analysis of B-cell activation and effector T-cell functions. T cell-dependent B-cell responses facilitated by anti-I-A antibodies

We have attempted to develop an in vitro system where polyclonal B lymphocyte responses could be induced in 'antigen-like' conditions, that is, where surface immunoglobulin-dependent binding mediates interaction with a mitogen. Monoclonal anti-mu and anti-delta antibodies were covalently bound to lipopolysaccharide (LPS) and these complexes were shown to display mitogenic activity. Polyclonal plaque-forming cell (PFC) responses, however, were diminished in cultures stimulated by anti-mu-LPS (but not by anti-delta-LPS) indicating that 'anti-mu inhibition' of terminal B-cell differentiation also applies to 'specific' antibody responses. Moreover, the analysis of the functional activity of monoclonal antibodies to major histocompatibility complex (MHC) class II molecules revealed a surprising synergy between low, non-stimulatory concentrations of anti-mu-LPS (but not anti-delta-LPS) with anti-I-A antibodies. These responses are T-cell dependent and synergy with anti-mu-LPS conjugates can also be obtained with 'naturally' activated CD4+ cells isolated from normal donors. A model of molecular and cellular interactions was derived, which accounts for the present findings and is applicable in antigen-dependent lymphocyte collaboration.

Transplantation tolerance correlates with high levels of T- and B-lymphocyte activity

Mice tolerized (treated to make them tolerant) at birth to transplantation antigens by injection of semiallogeneic cells contain very high numbers of activated T and B lymphocytes in their spleen. Lymphoid hyperactivity correlates with the tolerant state: it is present only in animals accepting skin allografts. Tolerized mice that reject the allogeneic skin graft have approximately the same numbers of total and activated lymphocytes as normal mice. The high level of lymphocyte activation in tolerant mice persists for up to 1 year of age, although it declines with age, and is markedly increased by a secondary allograft. The magnitudes of both primary and secondary tolerant responses are significantly higher than the immunological response of a normal mouse rejecting the same type of allograft. These observations contradict concepts of clonal deletion or anergy as the basis of neonatally induced transplantation tolerance and may contribute additional approaches to experimentation and control of transplantation reactions.

Preferential expansion of Ly-1 B and CD4- CD8- T cells in the polyclonal lymphocyte responses to murine T. cruzi infection

Acute murine infection with T. cruzi results in polyclonal lymphocyte responses manifested by blast transformation of a large fraction of B, CD4+, and CD8+ cells. We describe here the finding of significant increases in the splenic representation of minor populations, Ly-1 + B cells and CD4-CD8- T cells. These lymphocyte populations might play an important role in the host response, as shown by T. cruzi infection of hosts that had been lethally irradiated and reconstituted with autologous bone marrow. Under these conditions, the splenic polyclonal PFC responses are nearly abrogated, and not restored by the transfer of syngeneic peritoneal cells which, however, reconstitute T15 idiotype production in the same hosts. Control levels of PFC responses, however, are reconstituted by transfer of syngeneic splenic T cells. Since bone marrow-reconstituted animals contain normal numbers of CD4+ and CD8+ T cells which are actually activated by infection, these results suggest the participation of other T cell populations in the host response to infection, as also suggested by the marked increases in T cell receptor gamma and delta messages detected in the spleen of infected animals. The implications of these findings in immunopathology of Chagas' disease are discussed.