Immunology and Cell Biology (1996) 74, 57-64 Characterization of IgG Fc receptors on CD34 antigen- expressing cell lines (KG-1 and KG-la) ZHANHE WU, BOBAN MARKOVIC, COLIN N CHESTERMAN and BENG H CHONG Centre for Thrombosis and Va.scular Research. University of New South Wales. Kensington. New South Wales. Australia Summary Although Fc^ receptors (Fc^R) on mature blood cells have been extensively studied, there are only limited data on Fc^R expression in the early haematopoietic progenitor cells. In this study, we used the stem cell antigen (CD34)-exprcssing cell line (KG-I) and its less differentiated subline (KG-la) as a model for the study of Fc-^R in the early haematopoietic progenitors. Flow cytomctr\ and immunoprecipitation studies on KG-1 and KG-1 a cells with anti-Fc^R mAb showed that Fc^RIl is the only Fc^R expressed on the cell surface. Analysis of the steady-state levels of Fc^R mRNA in KG-1 and KG-la cells using a quantitative /// silu hybridization assay revealed the presence of only Fc^RlI mRN.A. On further analysis Fc-^RH.\ mRNA but no Fc^RIIBor Fc.j,RIIC transcripts were found in these cells: Fc.j,RlIA transcripts with and without the transmem- brane exon were present in approximately equal amounts. These findings are surprisingly similar to those observed previously with Fc^R in platelets and megakaryocytic cells but dilferent from those found w iih Fc.^R in cells of other lineages. These data suggest that the Fc^R transcript distribution pattern observed in the early haematopoietic progenitors (KG-l cells) is retained in later stages of haematopoietic differentiation only in cells of megakaryocytic lineage. Key words: early haematopoietic progenitors. Fc receptors, megakaryocytes. Introduction Receptors for the Fc region of IgG (Fc^R) are expressed on a variety of human haematopoietic cells and play an important role in immunological processes such as clear- ance of immune complexes, phagocytosis of opsonized paniculate antigens and release of inflammatory medi- ators. '••' Three classes of Fc^R have been defined, both at protein and DNA levels: Fc^RI, Fc^RlI and Fc^RIIl. Each class of FCyR is encoded by dilferent genes: Fc^RI A. B and C, Fc^RlI A, B and C, and Fc.RIIl A and B. '-* cDNA cloning studies revealed multiple transcripts for each class of Fc^R gene. For instance, several Fc.^Rll transcripts have been identified including Fc^RIIal, Fc.^RIIa2. Fc^RIIbl. Fc,RlIb2, Fc,RiIb3 and Fc^RIIc.'' Recent data suggest that Fc^R may have previously unrecognized roles in haematopoietic cell ontogeny.^ However, data on Fc^R in early haematopoietic progeni- tor cells are limited even though Fc^R in mature blood cells have been extensively studied.'- The reason for this is that early haematopoietic cells are present in very small numbers in the bone marrow and it is technically difficult to isolate sufficient cells to study Fc^R expression. To circumvent this difficulty, we have used the CD34- expressing cell line, KG-1 and its less mature subline KG-la as a model for the investigation of Fc^R expression Correspondence: Associate Professor BH Chong. Department of Haematology, Prince of Wales Hospital, High Strccl. Rnnd- wick, NSW 2031, Australia. Received 17 July 1995; accepted 5 October 1995. in the early haematopoietic progenitor cells.''^ KG-1 and KG-la cells express not only the stem cell antigen (CD34) but also possess early myeloid and immature T cell char- acteristics. These features suggest that KG-l cells may be derived from a very early haematopoietic progenitor, just before ditfcrentiation into cells of myeloid and lymphoid lineages.*' In this study, we investigated Fc^R protein expression and measured Fc^R mRNA levels in KG-1 and KG-la cells under steady-state conditions and during induction by the differentiating agent I 2-0-tctradecanoyl-phorbol- 13-acetatc (TPA: Sigma, St Louis. USA). Material and methods Antibodies Anti-Fc^Rl mAb 32.2 and 197. iititi-Fc,RII MAb, IV.3 and anti- Fc^RIIl mAh. 3G8 (Mcdarcx. NH. USA). anli-Fc,RIlI mAb, Lcu-1 Ib and anti-CD34 niAbs. anti-HPCA-l and anti-HPCA-2 (Becton Dickinson. CA, USA), anti-macrophage mAb (anti- CD68. Dako. Denmark), murine igG,. M0PC2I and murine IgG,,,. MOPC141 (Sigma. St Louis, MO. USA), were purchased as indicated. Anti-Fc^Rll mAb, KB6I was a generous gift from Dr DY Mason. Oxford. UK." Cells KG-1, KG-la and U937 cells oblamed from the American Type Collection Culture (ATCC) were grown in Iscove's medium with 10% FCS at 37°C."' Meg-01 cells (a human megakaryoblastic