CENTRAL FUNCTION OF THE THYMUS IN THE REC OGNITION OF NEUROENDOCRINE FUNCTIONS BY T-LYMPHOCYTES DURING THEIR DEVELOPMENT

The dual physiological role of the thymus in T cell positive and negat ive selection appears prominent in the establishment of appropriate ho st immune defenses. However, the cellular and molecular mechanisms und erlying those thymic functions begin only to be understood. On the bas is of our previous investigations about the thymic expression of diffe rent neuroendocrine-related signals, we have advanced a model which tr ansposes at the peptide level the intervention of this primary lymphoi d organ in both T cell positive and negative selective processes. Ther e is now ample evidence that the thymic subcapsular and medullary epit helium is the site for synthesis of neurohypophysial (NHP)-related pep tides (reviewed in GEENEN et al., 1992a). We have also demonstrated th at the epithelial component of thymic ''nurse'' cells (TNC) synthetize s NHP-related peptides and expresses a neuroendocrine-like phenotype ( GEENEN et al., 1988a). This observation was a remarkable example of th e intimate neuroendocrine-immune interactions that take place during T cell ontogeny. Further immunocytochemical analyses have confirmed tha t one dominant NHP-related epitope belongs to the oxytocin (OT) lineag e of the NHP peptide superfamily (ROBERT et al., 1991, 1992). The intr athymic coexpression of this OT-like epitope with a neurophysin protei n domain is a strong argument for a local synthetic process similar to the hypothalamo-NHP one. However, the absence of ir-OT in secretory g ranules of thymic epithelial cells (TEC), as well as of NHP-related pe ptides in the supernatant of TEC cultures questioned the application t o the thymus of the classical neurosecretory model established for hyp othalamic magnocellular neurons (SCHARRER & SCHARRER, 1944). The model of cell-to-cell cryptocrine signaling was recently introduced by FUND ER (1990) to characterize the exchange of chemical informations betwee n large stromal cells (such as TEC/TNC) and migratory developing cells . The expression of functionaL NHP receptors by immature T celLs stron gly support the existence of an effective in vivo cryptocrine signalin g mediated by NHP-peptides and receptors between TEC/TNC and pre-T cel ls (GEENEN et al., 1988b; MARTENs et al., 1992). Murine RL12-NP pre-T cells express an OT-type receptor. This latter finding suggests a matu ration of the NHP receptor molecule expressed by T cells according to thier stage of differentiation. Different NHP-related signals were als o shown to exert mitogenic activities upon human and murine freshly is olated thymocytes (pre-T-cells) that were cultured in serum-free condi tons (MARTENs et al., 1992). Altogether, these observations strongly s upport the active involvement of thymic NHP-related peptides as access ory cryptocrine signals in the process of T cell positive selection an d activation. The pharmacological manipulation of this type of signali ng with novel OT receptor antagonists was recently shown to significan tly inhibit the productions of some cytokines (IL1beta, IL6 and TNFalp ha) elicited by anti-CD3 in human whole blood cell cultures (GEENEN et al., 1992b). Such compounds could lead to selective immunotherapy in cases of immune desequilibrium determined by a general activation of t he oxytocinergic system such as the one observed in the post-partum pe riod. By contrast with other tissues where similar cell-to-cell signal ing might occur (for example, in the testis and in the bone marrow), t hymic cryptocrine signaling is closely associated with which appears a s an obligatory step in embryology : the recognition of the ''self'' m olecular identity by differentiating T cells. Through the use of sever al polyclonal and monoclonal Abs to NHP-related peptides, we have iden tified the dominant epitopes able to represent the NHP peptide superfa mily in face of the developing T cell system. One is located within th e central ''constant'' part of the neurophysin domain, which exhibits a high degree of conservation throughout evolution. The other NHP self peptide is located within the cyclic part of the OT molecule delineat ed by the Cys residues at positions 1 and 6. The amino acid sequence C ys-Tyr-Ile-Rrr-Asn-Cys characterizes nearly all members of the OT line age, as well as vasotocin (VT), an ancestral peptide of the NHP family present in non-mammalian vertebrates (GEENEN et al., 1991, 1993). Thi s latter NHP self epitope also possesses some characteristics which re nder feasible its presentation by MHC class I molecules, in particular the hydrophobic Tyr residue at position 2 (FALK et al., 1991; MARYANS KI et al., 1991). This specific point is under current active investig ation in our laboratory. Since vasopressin (VP) differs by one residue in the corresponding sequence, it might be less ''tolerated'' by the immune system. Indeed, autoimmune ''hypothalamitis'' leading to idiopa thic diabetes insipidus has been previously reported (SCHERBAUM & BOTT AZZO, 1983). Given the primary implication of OT-related peptides in t he control of reproductive functions at different levels, their higher tolerance by the immune system is highly significant for the preserva tion of one given species. Another indirect argument for the higher im mune tolerance of OT-related peptides resides in the frequence and the titers of Abs induced by active immunization against NHP peptides (VP > OT > VT). The thymic repertoire of neuroendocrine self antigens als o applies to tachikinin as well as insulin super-families (GEENEN et a l., 1992a), and neurokinin A was identified as the dominant thymic pep tide of the tachykinin family (ERICSSON et al., 1990). Even uncomplete , our model presents however some advantages by comparison with other previous working hypotheses. These latter were based on what appears a s an important paradox in Immunology, i.e. that both T cells positive and negative selection could be mediated by the same trimolecular comp lex (T cell receptor for antigen/TCR on one side, and MHC + self antig enic peptide on the other side). We propose another explanation which is based on two distinct types of interactions mediated by separate se ts of molecules between TEC/TNC and pre-T cells. The positive selectio n is mediated by the novel type of cryptocrine signaling and follows t he interaction of various thymic-derived neuroendocrine signals with t heir cognate receptors expressed by target pre-T cells. The negative s election results from the presentation by thymic MHC class I molecules of neuroendocrine sellf epitopes and a subsequent high affi