Functional and genetic identification of lineage committed intrathymic progenitors in adult mice

• Main Author: Belyaev, Nikolai Nikolaevich
• Published: University College London (University of London) 2007
• Subjects: 571.96
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.497494

T-lymphocytes are an essential component of the adaptive immune system and require a distinct anatomical location for their development, namely the thymus. This study focuses on early events during development of these cells in the adult mouse. The most immature compartment of murine thymocytes is subdivided by the cell surface expression of CD44 and CD25. The most primitive progenitor population resides in the CD44+CD25-double negative 1 (DN1) fraction and can be further subdivided according to expression of the receptor tyrosine kinase c-kit (CD 117). The DN1 CD117+ fraction is homogeneous, unlike the CD117-fraction, which constitutes a preponderant CD45R+CD127- population and a smaller CD45R-CD127+ population. Analysis of gene expression was employed to understand the relationship between the DN1 CD117+ (DN1 CD117) and the DN1 CD45R+ (DN1 CD45R) populations. Expression of genes associated with the T-cell lineage, such as Notch-1, Runx-1 and Rag-1 was observed in both populations. Furthermore, substantial expression levels of genes involved in T-cell commitment, such as pre-Ta, in the DN1 CD45R population could reflect the initiation of T-cell fate specification. In parallel, a Cre recombinase based lineage tracing approach was utilised to further dissect the earliest thymocyte pools. Upon expression of the Cre recombinase, all cells and their progeny are permanently marked by the expression of a fluorescent reporter protein. In this study, the human CD2 (hCD2) promoter and locus control regions drove expression of the Cre recombinase. Analysis of the DN1 population revealed that nearly all the DN1 CD117 fraction was reporter negative, whereas the DN1 CD45R subset was completely reporter positive. Since expression of hCD2 has so far been reported only in the lymphoid lineage, this observation further underlines the fundamental differences between the two progenitor fractions and indicates their alternate developmental origins. To address the developmental potential of these populations, the DN1 CD117 and the DN1 CD45R pools were functionally assessed in vitro and in vivo. The DN1 CD117 population showed a robust T-, B- and NK-cell potential in vitro and in vivo, in addition to a residual myeloid activity in vivo, whereas the DN1 CD45R fraction was strongly biased towards a CD8 NK-T cell lineage in vivo. In summary, these data indicate that developmentally distinct progenitor populations seed the adult murine thymus and can potentially contribute to discrete subsets of mature haematopoietic cells.