Human Thymus Contains Hematopoietic Stem and Lymphoid Progenitor Populations That Can Be Identified by Differential Expression of CD7.
Authors: Hao Qian-Lin , George Ann A. , Zhu Yuhua , Barsky Lora W. , Zielinska Ewa , Cormano Julia , Crooks Gay M. 
Published: October 2019

Abstract
It is widely accepted that T cell production from the thymus originates from a progenitor population derived continuously during postnatal life from the bone marrow. However, the exact identity of the T cell precursors that seed the thymus remains controversial. Competing theories are based largely on murine immunophenotypes that are not useful for human studies. The goal of these studies was to determine the immunophenotype and lineage potential of the earliest progenitors in human thymus. Analysis of 38 human thymus samples showed that although the vast majority of CD34+ thymocytes express high levels of CD7, a continuum of CD7 expression exists, with a rare CD7− subpopulation comprising 1.6 ± 0.23% (mean±SEM) of all CD34+lin-cells. Cells within the CD34+lin-population were fractionated by FACS based on CD7 expression into three subsets (CD7++, CD7dim and CD7−) and tested for lineage potential by culturing in bulk in B lymphoid (OP9 stroma), T lymphoid (OP9-DL1 stroma) and myeloid-erythroid (CFU-C) conditions. Progressive restriction in lineage potential correlated with CD7 expression, i.e. the CD7++ fraction produced T and NK cells but lacked B and myelo-erythroid potential, the CD7dim (CD10+) fraction produced B, T and NK cells but lacked myelo-erythroid potential. The CD7− fraction produced CD7+ T and NK cells, CD19+ B cells and myelo-erythroid progenitors (3.7% ± 0.7 CFU, n=20). Clonal analysis of the three CD34+lin-populations confirmed that T/NK progenitors were found in CD7++ subset, B/NK cells in the CD7dim subset and lympho-myeloid progenitors in the CD7− subset. FACS analysis showed that the CD34+lin-CD7− population represented only a small fraction (<5%) of the CD34+CD1a- thymocyte population [Weerkamp F, et al. Blood.2006; 107:3131] and was enriched >10 fold for CD34+CD38− cells relative to the CD34+CD7++ population. Despite their primitive phenotype and multi-lineage potential, the engraftment capacity of CD34+lin-CD7− cells was dramatically depleted compared to an immunophenotypically similar bone marrow population. RT-PCR and FACS analysis demonstrated key differences between CD34+lin-CD7− cells in the thymus, bone marrow and peripheral blood: only CD34+lin-CD7− thymocytes expressed Tdt, preTa, IL-7Ra, and (in contrast to DN1 murine thymocytes) IL-2Ra. The expression of these early T lymphoid genes in multipotent thymocytes suggests that signals directing T cell commitment and corresponding loss of HSC function are rapidly initiated upon arrival of HSC in the human thymus. Thus, the differential expression of CD7 in CD34+lin- thymocytes can be used to identify stages in lineage commitment in the human thymus, from lympho-myeloid progenitors with stem cell characteristics to multi-lymphoid progenitors and T/NK restricted progenitors. The identification of these subpopulations reveals critical differences in immunophenotype, and possibly regulation, of the earliest stages of murine and human thymopoiesis.