Función tolerogénica, origen y diferenciación de las células dendríticas plasmacitoides residentes en el timo humano

Resumen

The generation of CD4+ CD25+ Foxp3+ ¿natural¿ regulatory T cells (nTregs) in the thymus is fundamental for the establishment of immunological self-tolerance and the prevention of autoimmunity. Thymic dendritic cells (DCs) have been proposed to be involved in this process, although the particular contribution of the two main subtypes of thymic DCs; i.e., conventional (cDCs) and plasmacytoid DCs (pDCs), is still unclear. To approach this issue we have developed in this study an in vitro experimental system that allowed us to confirm the tolerogenic potential of both cDCs and pDCs resident in the human thymus, as they were able to induce the generation of autologous nTregs upon activation in response to different stimuli: either the cytokine Thymic Stroma Lymphopoietin or the combination of CD40 ligand (CD40L) plus IL-3, respectively. Interestingly, the progenitors of nTregs are identified within the subset of CD4+ CD8+ double positive (DP) thymocytes that have accomplished positive selection, as judged by their CD69hi TCRhi phenotype. Supporting the involvement of the CD40-CD40L pathway in pDC-induced nTreg generation, positively-selected DP progenitors specifically transcribe CD40L in vivo and up-regulate CD40L protein expression upon TCR engagement, thereby promoting the activation of pDCs. The physiological relevance of these findings is supported by the identification of Treg-pDC interactions in vivo in the steady-state human thymus. Also, evidence is provided that nTregs primed by either pDCs or cDCs are both dependent on the CD28-CD86/CD80 signalling pathway for their development, although they display a reciprocal IL-10/TGFß cytokine expression profile that supports a non-redundant tolerogenic role for thymic pDCs and cDCs in the human thymus. Also, the functional relevance of intrathymic DCs prompted us to investigate the mechanisms that govern their development in the thymus. We found that thymic cDCs and pDCs differentiate from thymic multipotent progenitors (TLMPs) through the generation of intermediate progenitors that display phenotypic and genetic features of myeloid cells. Such myeloid progenitors, identified as CD123+ CD5lo, express DC-related genes and still display NK potential, but have lost the capability to generate T cells. Since T-cell commitment is induced during thymopoiesis at the expense of non-T cell lineages through the Notch signalling pathway, we next analysed the impact of Notch-ligand interactions on the generation of the myeloid DC/NK progenitor subset. We show that signalling induced by the Notch ligand Delta-like-1 (DLL1) impairs the development of CD123+ CD5lo myeloid progenitors from TLMPs, while Jagged-1 (JAG1) is permissive. Once generated, both DLL1 and JAG1 ligands promote the survival of these progenitors, but block their differentiation into NK cells and differentially regulate their development into pDCs. In fact, while DLL1 promotes cDCs but not pDCs differentiation, both DC subtypes are induced by JAG1- mediated signalling. Moreover, JAG1, but not DLL1, induces the expression of genes selectively linked to the pDC lineage, such as SPIB and AIOLOS, in the myeloid immediate progenitors of DCs. These results provide evidence that Notch signalling controls the development of intrathymic DCs in a stage-specific manner and suggest that the spatial regulation of JAG1 vs DLL1 expression in the human thymic microenvironment could provide specific niches for inducing the final fate of pDC/cDC/NK progenitors.