Expression of FcγRIII defines distinct subpopulations of fetal liver B cell and myeloid precursors

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Abstract
We have isolated four distinct fetal liver (FL) populations based on the expression of AA4.1 and the low-affinity Fcγ receptors type II and III (FcγRII/III), and characterized them with respect to B cell, T cell, and myeloid precursor content. Polymerase chain reaction analysis revealed that the prevalent FcγR isoform at this stage of FL development (day 12 of gestation) was FcγRIII. Two of the four populations, one which expressed AA4.1 but little if any FcγRII/III (AA4.1+), and one which expressed abundant levels of both markers (AA4.1+/ FcR+), contained B cell precursors that grew and differentiated to generate VHDJH-rearranged B-lineage cells on S-17 stromal cells in the presence of IL-7. When cultured on FLST2 stromal cells only the AA4.1+ cells generated VHDJH-rearranged B-lineage cells. T cell precursors as assayed by their ability to repopulate fetal thymi in organ culture were found only in the AA4.1+ fraction. In contrast to the lymphoid precursors, myeloid precursors able to generate colonies in methyl cellulose cultures were found in all four fractions including the one which expressed FcγRII/III but no AA4.1 (FcR+) and the one which expressed neither marker (AA4.1−/FcR−). The AA4.1+ population which contained both B cell and T cell precursors was enriched for precursors from many myeloid lineages including the most immature ones which generated multilineage colonies. In contrast, the AA4.1+/FcR+ population, which also contained B cell precursors, was almost devoid of myeloid precursors and the few that were detected were committed to the macrophage lineage. The population defined as FcR+ was also enriched for precursors; however, the majority of these were committed to the erythroid, the macrophage and the mast cell lineage. The fourth population which expressed neither marker (AA4.1−/FcR−) was enriched for relatively mature erythroid precursors which were not present in any of the other fractions. Together, these findings demonstrate that fractionation of FL cells on the basis of AA4.1 and FcγRII/III expression distinguishes subpopulations of B cell and myeloid precursors and suggests that the low-affinity FcγRIII could play a role in the development of early hematopoietic cells at this stage of ontogeny.