The antigen-mediated activation of a number of T cell clones by bone marrow (BM) cells cultivated in the presence of various colony-stimulating factor (CSF) preparations was investigated. BM macrophages (BMM phi) grown in L929 cell supernatant as a crude source of macrophage colony-stimulating factor (M-CSF) as well as BM cells propagated in the presence of recombinant M-CSF exhibited transient antigen presentation potential to some T cell clones, being maximal on day 7 and having declined to a low level by day 19 of in vitro culture. Treatment of these long-term-cultivated BMM phi populations with recombinant interferon-gamma (IFN-gamma) resulted in predominant antigen presentation capacity. In contrast, BM cells differentiated in the presence of recombinant granulocyte (G)M-CSF developed highly efficient accessory cell function to all T cell clones examined. This function became apparent earlier, was retained during the time period tested (up to day 19 of continuous culture) and did not require prior stimulation by IFN-gamma. The functionally competent cells were shown to belong to the monocyte/macrophage lineage. These findings are consistent with the demonstration of substantial levels of major histocompatibility complex (MHC) class II molecules synthesized by GM-CSF-cultured BM cells in the absence of exogenous IFN-gamma. In contrast, M-CSF grown BM cells synthesized only minute amounts of Ia antigens unless they were stimulated by IFN-gamma. Because GM-CSF-cultivated BM cells proved clearly superior to M-CSF-grown and IFN-gamma-activated BM cells with respect to antigen-presenting capacity but exhibited lower levels of MHC class II molecules, other properties acting in addition to surface Ia antigens might be responsible for their pronounced T cell accessory function.

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.