TIM-family molecules in embryonic hematopoiesis: Fetal liver TIM-4lo cells have myeloid potential

Trans-membrane (or T cell) immunoglobulin and mucin (TIM) molecules are known regulators of immune response whose function in hematopoiesis is unknown. Earlier, we found that tim-1 and tim-4 are expressed by CD45+ cells in the para-aortic region of chicken embryo. Because the para-aortic region is a known site for hematopoietic stem cell (HSC) and hematopoietic progenitor cell (HPC) differentiation and expansion, we hypothesize that TIM molecules have a role in hematopoiesis. To study this role further, we analyzed TIM expression more precisely in chicken para-aortic region and mouse fetal liver hematopoietic cells. Additionally, we examined the hematopoietic potential of TIM-4+ mouse fetal liver cells with a colony-forming assay. tim-1 gene expression was detected in chicken and mouse embryos in the aorta-gonads-mesonephros-region at the time of HSC emergence, whereas tim-3 mRNA was widely expressed in different tissues. tim-4 expression was restricted to fetal liver CD45+F4/80+ cells. Moreover, two TIM-4+ populations were distinguished: F4/80hiTIM-4hi and F4/80loTIM-4lo. F4/80hiTIM-4hi cells had no hematopoietic potential and were morphologically similar to mature macrophages, suggesting that they are yolk sac–derived macrophages. Instead, many of the F4/80loTIM-4lo cells were c-kit+ and Sca-1+ and had primitive morphology and multilineage colony-forming ability. In addition, F4/80loTIM-4lo cells included a considerable population expressing ER-MP12, a known marker for macrophage colony–forming cells and other myeloid progenitors. We conclude that TIM molecules are expressed in embryonic hematopoietic tissues in chicken and mouse and that in fetal liver, TIM-4 is expressed by myeloid progenitor cells.