Synthesis and expression of MHC class II molecules in the absence of attached invariant chains by recombinant-interferon-gamma-activated bone-marrow-derived macrophages.

Pure populations of in vitro propagated bone marrow-derived macrophages are constitutively Ia negative. Co-culturing of these cells with recombinant interferon-gamma (rIFN-gamma) resulted in the appearance of high amounts of Ia antigens at the cell surface of essentially all cells. The continuous presence of the stimulus was a prerequisite for sustained Ia expression because removal of the stimulus resulted in rapid decline of surface Ia. Two-dimensional (2D) gel analysis (1D isoelectric focusing, 2D sodium dodecyl sulfate-polyacrylamide gel electrophoresis) of class II molecules synthesized by rIFN-gamma-stimulated bone marrow macrophages (BMM phi) revealed that, in contrast to class II complexes hitherto described, BMM phi-derived I-A and I-E subregion-encoded subunits are synthesized without invariant chains. The invariant chain-deficient alpha,beta heterodimers are expressed at the cell surface in high proportions demonstrating that their correct assembly and transport to the cell surface is accomplished in the absence of invariant chains. The lack of invariant chains appears not to be due to a failure of rIFN-gamma to induce transcription of the gamma-chain gene because rIFN-gamma-induced, in contrast to uninduced, BMM phi accumulate high levels of invariant chain-specific transcripts as evidenced by Northern blot analysis. These findings suggest that translation of gamma-chain-specific mRNA is blocked in BMM phi for as yet unknown reasons. Alternatively, newly synthesized gamma chains might have escaped their regular intracellular maturation pathway as a result of unidentified modifications mediated by altered post-translational processing mechanisms.