Ouafae Kecha

Characterization of the Insulin-Like Growth Factor Axis in the Human Thymus

The components of the insulin‐like growth factor (IGF) axis have been investigated in the normal human thymus. Using ribonuclease protection assays (RPA), IGF‐II transcripts were detected in the normal human thymus. By reverse transcriptase polymerase chain reaction (RT‐PCR) analyses, promoters P3 and P4 were found to be active in the transcription of IGF2 gene within human thymic epithelial cells (TEC). No IGF‐II mRNA could be detected in human lymphoid Jurkat T cells with 30 cycles of RT‐PCR. By Northern blot analyses, IGFBP‐2 to ‐6 (but not IGFBP‐1) were found to be expressed in TEC with a predominance of IGFBP‐4. Interestingly, Jurkat T cells only express IGFBP‐2 but at high levels. The type 1 IGF receptor was detected in Jurkat T cells but not in human TEC. The identification of the components of the IGF axis within separate compartments of the human thymus adds further evidence for a role of this axis in the control of T‐cell development. The precise influence of thymic IGF axis upon T‐cell differentiation and immunological self‐tolerance however needs to be further investigated.

The Thymic Repertoire of Neuroendocrine-Related Self Antigens: Biological Role in T-Cell Selection and Pharmacological Implications

Thymic epithelium, including nurse cells (TEC/TNC), as well as other thymic stromal cells (macrophages and dentritic cells), express a repertoire of polypeptide belonging to various neuroendocrine protein families (such as the neurophypophysial, tachykinin, neurotensin and insulin families). A hierarchy of dominance exists in the organization of the thymic repertoire of neuroendocrine precursors. Oxytocin (OT) is more expressed in the TEC/TNC than vasopressin (VP); insulin-like growth factor 2 (IGF-2) thymic expression predominates over IGF-1, and much more over (pro)insulin. Thus, OT was proposed to be the self antigen of the neurohypophysial family, and IGF-2 the self antigen precursor of the insulin family. The dual role of the thymus in T-cell life and death is recapitulated at the level of the thymic neuroendocrine protein repertoire. Indeed, thymic polypeptides behave as accessory signals involved in T-cell development and positive selection according to the cryptocrine model of signaling. Moreover, thymic neuroendocrine polypeptides are the source of self antigens presented by thymic MHC molecules to developing pre-T cells. This presentation might induce the negative selection of T cells bearing a randomly rearranged antigen receptor (TCR) oriented against neuroendocrine families. Using an animal model of autoimmune type 1 diabetes (BB rat), we have shown a defect in intrathymic expression of the self antigen of the insulin family (IGF-2) and in IGF-2-mediated T-cell education to recognize and tolerate the insulin family. Altogether these studies have enlighted the crucial role played by the thymus in the induction of the central self tolerance of neuroendocrine families. The tolerogenic properties of thymic self peptides could be used in a novel type of vaccination for the prevention of autoimmune diseases.

THYMIC EXPRESSION OF NEUROENDOCRINE SELF-PEPTIDE PRECURSORS : ROLE IN T CELL SURVIVAL AND SELF-TOLERANCE

For a long time the thymus was considered to be an intrinsic primary steps towards tolerance occur within the thymus. To complete their differentiative programme, immature T cells component of the endocrine system but the endocrine model of cell-to-cell signalling failed to be fully validated in this receive signals from the thymic cellular microenvironment. Such signals may be emitted by thymic stromal cells ( like organ. With the discovery of its primary role in T-lymphopoiesis, the endocrine role of the thymus progresshormones or cytoldnes), or may result from direct interactions between cell adhesion molecules expressed on pre-T cells ively vanished from the literature, although the thymic influence was still believed to be mediated by a humoral (thymocytes) and thymic stromal cells (4, 5). During differentiation, immature T cells randomly rearrange the genes mechanism (1). However, in the past 15 years, the question of a neuroendocrine component in thymic physiology has coding for the segments of their TCR. Many of these random TCR combinations are orientated against self-antigens which resurfaced. From a number of studies, it now appears that the thymus is a crucial site for cross-talk between the neuroare expressed in the thymic microenvironment, then presented by proteins encoded in the major histocompatibility complex endocrine and immune systems, especially during foetal development (2). Thymic epithelial cells (TEC) and nurse cells (MHC). The interaction of self-reactive T cell clones with their cognate self-antigens is thought to lead to their negative (TNC) express a repertoire of neuroendocrine-related genes/ precursors, and thymic polypeptides may serve as signals selection, either by programmed cell death (apoptosis), or by developmental arrest. This process of thymic clonal deletion interacting with receptors on developing pre-T lymphocytes. This cryptocrine form of cell-to-cell signalling could play a was demonstrated with the use of mouse mammary tumour virus (MMTV )-encoded superantigens (6), and with transrole in T cell development and maturation. In addition, there is ample evidence that thymic neuroendocrine-related polygenic mice expressing a TCR specific for the male antigen (H-Y ) (7). Since the thymus does not express all the componpeptides behave as self-antigens which are presented to preT cells, and could induce the negative selection of T cells ents of the self-structure, this organ does not delete all potential autoreactive T cells. Consequently, the existence of bearing a randomly rearranged antigen receptor (TCR) orientated against endogenous neuroendocrine families (selfother mechanisms for developing tolerance (such as T cell anergy) at the periphery was postulated, and they were reactive T cells). The objective of this review is to expose most of the scientific arguments which support the important effectively shown to intervene in the process of immunological self-tolerance. Nevertheless, thymic clonal deletion of selfrole of the thymus in the education of T lymphocytes to recognize and tolerate neuroendocrine functions. reactive T cells is by far the most important mechanism involved in self-education of the immune system (8).

Cryptocrine signaling in the thymus network and T cell education to neuroendocrine self-antigens

Both during phylogeny and ontogeny the thymus appears as a nodal point between the two major systems of cell-to-cell signaling, the neuroendocrine and immune systems. This review presents the experimental observations which support a dual role in T cell selection played by the thymic repertoire of neuroendocrine polypeptide precursors. Through the mode of cryptocrine intercellular signaling thymic neuroendocrine-related precursors synthesized in thymic epithelial cells have been shown to influence the early steps in T cell differentiation. In addition, thymic neuroendocrine-related polypeptides are a source of self-antigens which are presented by the major histocompatibility system of the thymic epithelium. Preliminary data also suggest that the intrathymic T cell education to neuroendocrine self-antigens is not strictly superimposible to the antigen presentation by dedicated presenting cells. Insulin-like growth factor-II (IGF-II) was identified as one dominant member of the insulin family expressed by thymic epithelial and nurse cells. The intrathymic presentation of IGF-II or IGF-II derived self-antigens is under current investigation. If further confirmed, the central tolerogenic properties of IGF-II could be considered in the elaboration of a strategy for an efficient and safe prevention of insulin-dependent diabetes.

Cellular and Molecular Aspects of Thymic T-Cell Education in Neuroendocrine Self Principles: Implications for Autoimmunitya

Abstract: Thymic epithelial and nurse cells from different species express a repertoire of neuroendocrine polypeptide precursors. This repertoire exerts a dual role in T‐lymphocyte selection according to their status either as cryptocrine signals or as neuroendocrine self‐antigens of the peptide sequences that are processed from those precursors then presented to pre‐T cells. Thymic neuroendocrine self‐antigens correspond to peptide sequences highly conserved throughout evolution of their family. Though thymic MHC class I molecules are involved in the processing of thymic neuroendocrine self‐antigens, preliminary data show that their presentation to pre‐T cells is not allelically restricted. Thymic T‐cell education in neuroendocrine families also implies that the structure of a given family may be presented to pre‐T cells. Our studies have evidenced the homology between thymic neuroendocrine‐related self‐antigens and dominant T‐cell epitopes of peripheral neuroendocrine signals (neuroendocrine autoantigens). The biochemical difference between neuroendocrine autoantigens and homologous thymic self‐antigens might explain the opposite immune responses evoked by those two types of antigens (activation and memory induction vs. tolerogenic effect). Altogether, these studies support the therapeutic use of thymic neuroendocrine self‐antigens in reprogramming the immunological self‐tolerance that is broken in autoimmune endocrine diseases like insulin‐dependent diabetes type I. As recently stated by P. M. Allen in an important review, the fate of developing T lymphocytes in the thymus is influenced by the numerous types of peptidic interactions within the thymic cellular environment. 1 To define the precise nature of thymic cells and naturally occurring biochemical peptide signals involved in positive and negative selection of immature T cells has become a prominent objective for the future research efforts in thymic physiology. This paper will try to show how thymic neuroendocrine‐related peptides synthesized and processed within the thymic microenvironment indeed can play a role both in the development of the peripheral T‐cell repertoire and in the death of randomly rearranged, self‐reactive T cells.

Phosphorylation of Proteins Induced in a Murine Pre-T Cell Line by Neurohypophysial Peptides

The thymus is the primary lymphoid organ responsible for the positive and negative selection of the peripheral T-cell repertoire. Specific neuroendocrine-immune interactions were shown to take place in the thymic microenvironnement. Two distinct types of cell-tocell signaling are mediated by thymic neuroendocrine-related peptides and might partially explain the dual physiological role of the thymus in T-cell differentiation (1–3). Thymic epithelial cells (TEC) from different species express a repertoire of proteins belonging to various neuroendocrine families. Prooxyphysin is the dominant thymic precursor of the neurohypophysial family, and proinsulin-like growth factor-II is the dominant thymic insulin-related polypeptide.

Developmental and evolutionary aspects of thymic T-cell education to neuroendocrine self

Thymic epithelial cells, including nurse cells (TECs/TNCs), from various species synthesize neuroendocrine-related precursors belonging to neurohypophysial, tachykinin and insulin hormone families. The thymic repertoire of neuroendocrine-related polypeptides illustrates at the molecular level the paradoxical role of the thymus in both T cell positive and negative selection. On the one hand, these precursors are a source of signals which interact with neuroendocrine-type receptors expressed by target pre-T cells according to the cryptocrine type of cell-to-cell signaling. On the other hand, the same precursors constitute a source of self-antigens which are presented to pre-T cells by the thymic major histocompatibility complex system. Basically, the model of thymic T cell education to neuroendocrine self was established by the identification in TECs/TNCs of immunoreactive (ir) oxytocin as the self-antigen of the neurohypophysial family. Nevertheless, through the expression in TECs/TNCs of ir-neurokinin A and ir-insulin-like growth factor-II, the model also applies to the tachykinin and insulin hormone families.

Thymic neuroendocrine self peptides and T cell selection.

Our previous studies have shown that the thymic epithelial cells (TEC) of different animal species were the site for synthesis of polypeptide precursors belonging to the neurohypophysial (NHP), tachykinin (TK), and insulin neuroendocrine families1,2,3,4,5. However, at least in basal conditions, cultured human TEC do not secrete NHP-related peptides, neurokinin A (NKA) nor insulin-like growth factor 2 (IGF2); the existence of a classical secretory pathway in the thymic epithelium may thus be questioned. We also failed to detect immunoreactive (ir) thymic NHP-related peptides in classical secretory granules and a very elegant recent study has demonstrated that ir oxytocin (OT), the dominant thymic NHP-related peptide, was located diffusely in the cytosol and in clear vacuoles of murine TEC6. The term cryptocrine has been introduced in the word-list of Endocrinology to describe this particular type of cell-to-cell signaling in specialized microenvironments constituted by large “nursing” epithelial cells (like TEC/TNC in the thymus, or Sertoli cells in the testis) enclosing cell populations that migrate and differentiate at their very close contact (respectively, T cells and spermatids)7. In the general evolution of cell-to-cell communication, the cryptocrine type of signaling is located at a rather primitive step, between intercellular adhesion and paracrine exchanges of soluble signals. Moreover, in the thymus, the cryptocrine stage is closely associated with the presentation of the self molecular structure to the developing T cell system. Therefore, the thymus appears as one crucial meeting point for the two major systems of intercellular communication: therein, the endocrine system may influence the early steps of the immune response, whereas the immune system is educated in self neuroendocrine principles8. We would like to present here our experimental arguments that permit to transpose at the level of the thymic repertoire of neuroendocrine-related peptides the dual physiological role of this primary lymphoid organ in T cell positive and negative selection.

Central self-tolerance by thymic presentation of self-antigens and autoimmunity

Before reacting against non-self infectious agents, the immune system is educated to tolerate the host molecular structure (self). The induction of self-tolerance is a multistep process that begins in the thymus during fetal ontogeny (central tolerance) and also involves inactivating mechanisms outside the thymus (peripheral tolerance). The thymus is the primary lymphoid organ implicated in the development of competent and self-tolerant T cells. During ontogeny, T cell progenitors originating from hemopoietic tissues (yolk sac, fetal liver, and then bone marrow) enter the thymus and undergo a program of proliferation, T cell receptor (TCR) gene rearrangement, maturation and selection. Close interactions between thymocytes (pre-T cells) and the thymic cellular environment are crucial both for T cell development and induction of central self-tolerance. Thymic epithelial and stromal cells synthesize polypeptides belonging to various neuroendocrine families. The thymic repertoire of neuroendocrine-related precursors transposes at the molecular level the dual role of the thymus in T cell negative and positive selection. Thymic precursors not only constitute a source of growth peptides for cryptocrine signaling between thymic stromal cells and pre-T cells, but are also processed in a way that leads to the presentation of self-antigens by thymic major histocompatibility complex (MHC) proteins. Thymic neuroendocrine self-antigens often correspond to peptide sequences highly conserved during the evolution of their corresponding family. The thymic presentation of some neuroendocrine self-antigens is not restricted by MHC alleles. Following the presentation of neuroendocrine self-antigens by thymic MHC proteins, the T cell system might be educated to tolerate main hormone families. Recent experiments argue that a defect in the thymic essential tolerogenic function is implicated as an important factor in the pathophysiology of many autoimmune diseases.