Two different progenitors are responsible for establishing a functional T cell compartment: testing a new paradigm of T cell development – Twothyme

Lymphopenia after bone marrow (BM) transplantation severely impairs protection against infection for extended periods of time, resulting in high morbidity and mortality. With aging, the capacity to produce lymphocytes is diminished and increased numbers of memory cells are found in the peripheral lymphoid organs resulting in impaired responses to new antigenic encounters and reduced efficacy of vaccination protocols. To be able to manipulate lymphocyte production and complement deficiencies in the immune system, it is important to define the appropriate progenitor populations that ensure reactive immune responses in the absence of autoimmunity and the mechanisms that modulate lineage commitment.
Fetal liver (FL) is the major hematopoietic organ, in the embryo, while adult hematopoiesis occurs in the BM. Most well defined stages of hematopoietic differentiation are found in FL and BM indicating that the pathways of differentiation are similar in both sites. Hematopoietic progenitors are represented better in the FL than in the BM because of a lower degree of differentiation that in addition progress synchronously, from the newly colonizing multipotent progenitors. We therefore investigated the early events of lymphoid lineage determination in FL. The common lymphoid progenitor (CLP) is a subset of hematopoietic precursors that can differentiate into B and T lymphocytes, innate lymphoid cells (ILC) and some subsets of dendritic cells (DC), while lacking myeloid, erythroid and megakariocytic potential. We found that FL CLP is a highly heterogeneous compartment: 1. acquisition of a4ß7 expression coincides with the loss of B and T cell differentiation potential but maintaining ILC potential 2. modulation of HSA (CD24) in a4ß7- CLP expression defines three CLP subsets: HSAhigh (B cell biased), HSAlow (T cell biased) and HSAintermediate (that comprise multilineage lymphoid progenitors). Genome wide transcriptional analysis of these subsets showed that FL CLP HSAlow express a transcriptional signature found in early thymocytes compatible with their role of thymic settling progenitors (TSP).
T lymphocytes differentiate in the thymus, a specialized organ that provides a unique environment for the acquisition of immune competence. We found that the mouse thymus is colonized by successive waves of progenitors that differ in their function and stage of differentiation. In a first wave, between embryonic day 11 (E11) and E14 precursors (TSP-1), rapidly differentiate to generate ?d- and aß?T cells. They have a phenotype similar to CLP, in that they lack myeloid potential, but lose B cell potential before they enter the thymus. TSPs from a later wave (starting at E16: TSP-2) extensively expand before maturation, thus giving rise to large numbers of double positive (DP) thymocytes. TSP-2 generate large numbers of T cells, in vivo, phenotypically resemble lympho-myeloid primed progenitors (LMPP) and lose B and myeloid potential only in the thymus. Consistent with this phenotype, the differentially expressed transcripts, represented the highest in TSP-1, are tumor suppressor genes, cell cycle regulators and genes from the imprinted gene network. H19, an imprinted long non-coding RNA with a particularly high expression in TSP-1, is a trans-regulator of the imprinted gene network that has tumor suppressor functions.
The project we propose derives from these observations. Our objectives are:
A. Understanding the molecular basis for the initial stages of lymphoid lineage commitment in the FL that give rise to the first TSP.
B. A detailed characterization of the different T cell populations generated by TSP-1 and TSP-2.
C. Determining the molecular cues that modulate the properties of TSP-1 and TSP-2: the role of cell cycle regulators, tumor suppressor genes and the imprinted network.
D. The evaluation of the impact of the cells originated from each wave in establishing the mature immune system.