Lymphoid Gene Upregulation on Circulating Progenitors Participates in Their T-Lineage Commitment.

Extrathymic T cell precursors can be detected in many tissues and represent an immediately competent population for rapid T cell reconstitution in the event of immunodeficiencies. Blood T cell progenitors have been detected, but their source in the bone marrow (BM) remains unclear. Prospective purification of BM-resident and circulating progenitors, together with RT-PCR single-cell analysis, was used to evaluate and compare multipotent progenitors (MPPs) and common lymphoid progenitors (CLPs). Molecular analysis of circulating progenitors in comparison with BM-resident progenitors revealed that CCR9(+) progenitors are more abundant in the blood than CCR7(+) progenitors. Second, although Flt3(-) CLPs are less common in the BM, they are abundant in the blood and have reduced Cd25(+)-expressing cells and downregulated c-Kit and IL-7Rα intensities. Third, in contrast, stage 3 MPP (MPP3) cells, the unique circulating MPP subset, have upregulated Il7r, Gata3, and Notch1 in comparison with BM-resident counterparts. Evaluation of the populations' respective abilities to generate splenic T cell precursors (Lin(-)Thy1.2(+)CD25(+)IL7Rα(+)) after grafting recipient nude mice revealed that MPP3 cells were the most effective subset (relative to CLPs). Although several lymphoid genes are expressed by MPP3 cells and Flt3(-) CLPs, the latter only give rise to B cells in the spleen, and Notch1 expression level is not modulated in the blood, as for MPP3 cells. We conclude that CLPs have reached the point where they cannot be a Notch1 target, a limiting condition on the path to T cell engagement.

Intrathymic SIRPa cDC subsets organization in normal and stress conditions reveal another level of cDCs heterogeneity.

Three major subsets constitute the dendritic cells (DCs) pool in the thymus. They play key roles in self-antigen-specific thymocyte deletion and in the development of immunoregulatory T cells. Resident SIRPa- conventional DCs (cDCs, CD11c+ PDCA1lo ) are derived from intrathymic progenitors, whereas migratory SIRPa+ cDCs and plasmacytoid DCs (pDCs, CD11c+ PDCA1+ ) originate from extrathymic sites. Here, we describe the organization and the shaping of cDC populations at the steady state and under stress conditions in wild-type and mutant mice (CD3eKO, IL7RaKO, and Flt3LKO). In neonates, the thymus is mainly composed of SIRPa- -resident cDCs, whereas both cDC subsets are present in equal proportions in the adult. Upon thymus colonization, migratory SIRPa+ cDCs gain expression of phenotypic markers in a microenvironment dependent way. Here, we show that both processes are deeply impacted by mutations affecting T cell development. Under stress conditions such as sublethal irradiation, intrathymic resident SIRPa- cDCs are the first to regenerate the thymic cDC pool. Upon bone marrow transplantation, migratory SIRPa+ cDCs become the main source of thymic cDCs. These successive waves of regeneration eventually lead to a balance between resident and migratory DCs within the newly colonized thymus. These findings highlight an unrevealed division of labor between resident and migratory subsets for the organization/establishment of the thymic cDC compartment.