Deletion of TLR2+ erythro-myeloid progenitors leads to embryonic lethality in mice.

Early embryonic hematopoiesis in mammals is defined by three successive waves of hematopoietic progenitors which exhibit a distinct hematopoietic potential and provide continuous support for the development of the embryo and adult organism. Although the functional importance of each of these waves has been analyzed, their spatio-temporal overlap and the lack of wave-specific markers hinders the accurate separation and assessment of their functional roles during early embryogenesis. We have recently shown that TLR2, in combination with c-kit, represents the earliest signature of emerging precursors of the second hematopoietic wave, erythro-myeloid precursors (EMPs). Since the onset of Tlr2 expression distinguishes EMPs from primitive progenitors which coexist in the yolk sac from E7.5, we generated a novel transgenic "knock in" mouse model, Tlr2Dtr , suitable for inducible targeted depletion of TLR2+ EMPs. In this model, the red fluorescent protein and diphtheria toxin receptor sequences are linked via a P2A sequence and inserted into the Tlr2 locus before its stop codon. We show that a timely controlled deletion of TLR2+ EMPs in Tlr2Dtr embryos results in a marked decrease in both erythroid as well as myeloid lineages and, consequently, in embryonic lethality peaking before E13.5. These findings validate the importance of EMPs in embryonic development.