DISCOORDINATE SURFACE EXPRESSION OF IFN-GAMMA-INDUCED HLA CLASS-II PROTEINS IN NONPROFESSIONAL ANTIGEN-PRESENTING CELLS WITH ABSENCE OF DM AND CLASS-II COLOCALIZATION

We compared HLA class II expression ia a human melanoma line (a nonpro fessional APC), induced by IFN-gamma or by stable transfection with. C IITA, with constitutive class II expression in an EBV-transformed B ly mphoblastoid cell line (a professional APC) from the same donor. IFN-g amma-induced and CIITA-transfected melanoma cells expressed DR, DP, an d DQ at levels similar to those expressed by the professional APC; how ever, DP and DQ proteins and DM-dependent DR epitopes were delayed in appearing on the cell surface when induced by IFN-gamma. The delay in cell surface expression of some IFN-gamma-induced class II epitopes wa s observed even though Northern blots demonstrated class II and DM gen es to be coordinately transcribed and their mRNA levels to be equivale nt to that in B lymphoblastoid cells. Confocal microscopy suggests tha t discoordinate cell surface expression of class II results from diffe rent intracellular trafficking for IFN-gamma-induced class II proteins in the melanoma line compared with that in professional APCs. Specifi cally, although DR and DM proteins were present 2 days after IFN-gamma induction, colocalization of DR and DM proteins intracellularly was n ot apparent in cells at any time after induction. Failure of DR and DM proteins to colocalize suggests that IFN-gamma-induced cells lack an intracellular MIIC-like compartment. The absence of a compartment cont aining DR and DM to facilitate interaction between the two proteins ma y account for the delayed surface expression of class II epitopes whos e formation requires both class II and DM.