Gwennaëlle Bodart

Programming of neuroendocrine self in the thymus and its defect in the development of neuroendocrine autoimmunity.

For centuries after its first description by Galen, the thymus was considered as only a vestigial endocrine organ until the discovery in 1961 by Jacques FAP Miller of its essential role in the development of T (thymo-dependent) lymphocytes. A unique thymus first appeared in cartilaginous fishes some 500 million years ago, at the same time or shortly after the emergence of the adaptive (acquired) immune system. The thymus may be compared to a small brain or a computer highly specialized in the orchestration of central immunological self-tolerance. This was a necessity for the survival of species, given the potent evolutionary pressure imposed by the high risk of autotoxicity inherent in the stochastic generation of the diversity of immune cell receptors that characterize the adaptive immune response. A new paradigm of “neuroendocrine self-peptides” has been proposed, together with the definition of “neuroendocrine self.” Neuroendocrine self-peptides are secreted by thymic epithelial cells (TECs) not according to the classic model of neuroendocrine signaling, but are processed for presentation by, or in association with, the thymic major histocompatibility complex (MHC) proteins. The autoimmune regulator (AIRE) gene/protein controls the transcription of neuroendocrine genes in TECs. The presentation of self-peptides in the thymus is responsible for the clonal deletion of self-reactive T cells, which emerge during the random recombination of gene segments that encode variable parts of the T cell receptor for the antigen (TCR). At the same time, self-antigen presentation in the thymus generates regulatory T (Treg) cells that can inhibit, in the periphery, those self-reactive T cells that escaped negative selection in the thymus. Several arguments indicate that the origin of autoimmunity directed against neuroendocrine glands results primarily from a defect in the intrathymic programming of self-tolerance to neuroendocrine functions. This defect may be genetic or acquired, for example during an enteroviral infection. This novel knowledge of normal and pathologic functions of the thymus constitutes a solid basis for the development of a novel type of tolerogenic/negative self-vaccination against type 1 diabetes (T1D).

Somatotrope GHRH/GH/IGF‐1 axis at the crossroads between immunosenescence and frailty

Immunosenescence, characterized by complex modifications of immunity with age, could be related to frailty syndrome in elderly individuals, leading to an inadequate response to minimal aggression. Functional decline (i.e., the loss of ability to perform activities of daily living) is related to frailty and decreased physiological reserves and is a frequent outcome of hospitalization in older patients. Links between immunosenescence and frailty have been explored and 20 immunological parameters, including insulin‐like growth factor‐1 (IGF‐1), thymopoeisis, and telomere length, were shown to be affected in elderly patients with functional decline. A strong relationship between IGF‐1 and thymic ouput was evidenced. IGF‐1, a mediator of growth hormone (GH), was subsequently shown to induce interleukin‐7 secretion in cultured primary human thymic epithelial cells. We are exploring the stress hypothesis in which an acute stressor is used as the discriminator of frailty susceptibility. GH can counteract the deleterious immunosuppressive effects of stress‐induced steroids. Under nonstress conditions, the immunosenescent system preserves physiological responses, while under stress conditions, the combination of immunosenescence and a defect in the somatotrope axis might lead to functional decline.

The Somatotrope Growth Hormone-Releasing Hormone/Growth Hormone/Insulin-Like Growth Factor-1 Axis in Immunoregulation and Immunosenescence

Most scientific reports debate the thymotropic and immuno-stimulating properties of the somatotrope growth hormone-releasing hormone (GHRH)/growth hormone (GH)/insulin-like growth factor (IGF)-1 axis, but there is still some disagreement about the physiological role of this axis in basal conditions. Moreover, some authors have hypothesized that the physiological role of the somatotrope axis only appears in stressful conditions (such as sepsis or infective and inflammatory diseases). This chapter will provide an extended overview of the expression of the components (signals and receptors) of the somatotrope axis and their properties on cells of the innate and adaptive immune system. It will also summarize some clinical studies suggesting a benefit for a short-term GH treatment in acute immunodeficiencies, and the importance of GH supplementation in adult GH deficiency. A new transgenic mouse model, the hypothalamic GHRH-deficient (Ghrh-/-) mouse, which exhibits a severe deficiency of the somatotrope axis, will be presented since it will be of great help in further deciphering the regulation by the GHRH/GH/IGF-1 axis on both immune development and function. Finally, we will discuss the implication of aging-related somatopause in relation to the general context of Immunosenescence.

The role of the thymus in the integrated evolution of the recombinase-dependent adaptive immune response and the neuroendocrine system.

Before being able to react against infectious non-self-antigens, the immune system has to be educated in recognition and tolerance of neuroendocrine self-proteins. This sophisticated educational process takes place only in the thymus. The development of an autoimmune response directed to neuroendocrine glands has been shown to result from a thymus dysfunction in programming immunological self-tolerance to neuroendocrine-related antigens. This thymus dysfunction leads to a breakdown of immune homeostasis with an enrichment of ‘forbidden’ self-reactive T cells and a deficiency in self-antigen-specific natural regulatory T cells in the peripheral T lymphocyte repertoire. A large number of neuroendocrine self-antigens are expressed by the thymic epithelium, under the control of the autoimmune regulator (AIRE) gene/protein in the medulla. Based on the close homology and cross-tolerance between thymic type 1 diabetes-related self-antigens and peripheral antigens targeted in β-cells by autoimmunity, a novel type of vaccination is currently developed for the prevention and cure of type 1 diabetes. If this approach were found to be effective in reprogramming immunological tolerance that is absent or broken in this disease, it could pave the way for the design of negative/tolerogenic self-vaccines against other endocrine and organ-specific autoimmune disorders.

The severe deficiency of the somatotrope GH-releasing hormone/growth hormone/insulin-like growth factor 1 axis of Ghrh-/- mice is associated with an important splenic atrophy and relative B lymphopenia

A debate is still open about the precise control exerted by the somatotrope GH-releasing hormone (GHRH)/growth hormone (GH)/insulin-like growth factor 1 axis on the immune system. The objective of this study was to directly address this question through the use of Ghrh−/− mice that exhibit a severe deficiency of their somatotrope axis. After control backcross studies and normalization for the reduced global weight of transgenic mice, no difference in weight and cellularity of the thymus was observed in Ghrh−/− mice when compared with C57BL/6 wild-type (WT) control mice. Similarly, no significant change was observed in frequency and number of thymic T cell subsets. In the periphery, Ghrh−/− mice exhibited an increase in T cell proportion associated with a higher frequency of sjTREC and naïve T cells. However, all Ghrh−/− mice displayed an absolute and relative splenic atrophy, in parallel with a decrease in B cell percentage. GH supplementation of transgenic mice for 6 weeks induced a significant increase in their global as well as absolute and relative splenic weight. Interestingly, the classical thymus involution following dexamethasone administration was shown to recover in WT mice more quickly than in mutant mice. Altogether, these data show that the severe somatotrope deficiency of Ghrh−/− mice essentially impacts the spleen and B compartment of the adaptive immune system, while it only marginally affects thymic function and T cell development.

In‐utero coxsackievirus B4 infection of the mouse thymus

Type B coxsackievirus (CV‐B) infections are involved frequently in the triggering of several autoimmune diseases such as myocarditis, dilated cardiomyopathy, pericarditis, pancreatitis, type 1 diabetes, encephalitis, thyroiditis or Sjögrens syndrome. Serological and virological evidence suggests that maternal infections during pregnancy can play a role in the appearance of these diseases in offspring. The current study aims to explore the effect of an in‐utero CV‐B infection on the fetal thymus, the central site for programming immunological self‐tolerance. In this perspective, female Swiss albino mice were inoculated intraperitoneally or orally with the diabetogenic CV‐B4 E2 strain at gestational days 10 or 17. Offspring were killed at different post‐inoculation times, and their thymuses were analysed for evidence of infection and alterations in thymic T cell subsets. In‐utero CV‐B infection of the thymus was demonstrated during the course of vertical transmission, as attested by viral RNA and infectious virus detection in most analysed samples. No histopathological changes were evident. Thymic T cells were not depleted, despite being positive for viral RNA. As evidenced by flow cytometry analysis, CV‐B infection of the fetal thymus induced significant changes of thymic T cell populations, particularly with maternal inoculation at gestational day 10. Altogether, these findings suggest that CV‐B infection of the fetal thymus may play an important role in the genesis of autoimmune diseases.

Growth Hormone (GH) Deficient Mice With GHRH Gene Ablation Are Severely Deficient in Vaccine and Immune Responses Against Streptococcus pneumoniae

The precise impact of the somatotrope axis upon the immune system is still highly debated. We have previously shown that mice with generalized ablation of growth hormone (GH) releasing hormone (GHRH) gene (Ghrh−/−) have normal thymus and T-cell development, but present a marked spleen atrophy and B-cell lymphopenia. Therefore, in this paper we have investigated vaccinal and anti-infectious responses of Ghrh−/− mice against S. pneumoniae, a pathogen carrying T-independent antigens. Ghrh−/− mice were unable to trigger production of specific IgM after vaccination with either native pneumococcal polysaccharides (PPS, PPV23) or protein-PPS conjugate (PCV13). GH supplementation of Ghrh−/− mice restored IgM response to PPV23 vaccine but not to PCV13 suggesting that GH could exert a specific impact on the spleen marginal zone that is strongly implicated in T-independent response against pneumococcal polysaccharides. As expected, after administration of low dose of S. pneumoniae, wild type (WT) completely cleared bacteria after 24 h. In marked contrast, Ghrh−/− mice exhibited a dramatic susceptibility to S. pneumoniae infection with a time-dependent increase in lung bacterial load and a lethal bacteraemia already after 24 h. Lungs of infected Ghrh−/− mice were massively infiltrated by inflammatory macrophages and neutrophils, while lung B cells were markedly decreased. The inflammatory transcripts signature was significantly elevated in Ghrh−/− mice. In this animal model, the somatotrope GHRH/GH/IGF1 axis plays a vital and unsuspected role in vaccine and immunological defense against S. pneumoniae.