Differential expression of MHC class II antigens in myelomonocytic leukemia cell lines

Major histocompatibility complex (MHC) class II antigens are discordantly expressed on hematopoietic progenitor cells. Their expression is linked to differential responsiveness of the cells to growth factors and inhibitors. We examined the expression of different MHC class II antigens in a panel of human myelomonocytic cell lines representing different stages of differentiation, by cytofluorographic analysis with monoclonal antibody (MoAb) and Northern blot analysis with specific cDNA probes. These analyses revealed discordant expression of different MHC class II antigens both in basal state and after gamma-IFN induction. Thus KG-1 myeloblast cells express all class II antigens (DR greater than DP greater than DQ) constitutively and their expression increased after gamma-IFN treatment. KG-1a, an immature blast variant of KG-1, does not express class II antigens, even after gamma-IFN treatment. THP-1, a monocytic cell line expresses DR but not DP or DQ under basal conditions. DP and DQ are, however, gamma-IFN inducible. The class II negative HL-60 promyelocytic cell line, expresses DR and DP but not DQ after gamma-IFN induction. In all the above cell lines, surface expression of class II antigens correlated with the levels of mRNA expression as determined with specific cDNA probes. In U-937, a monocytic cell line, no surface expression of class II MHC antigens was observed either with or without gamma-IFN, however, specific mRNA message was observed under basal conditions and was further increased with gamma-IFN, indicating a possible defect in assembly or transport of class II antigens. The patterns of class II MHC antigens in these leukemic cell lines may be a useful model to delineate molecular basis of discordant MHC class II expression during myelomonocytic differentiation.