Neutralizing agents to tumor necrosis factor (TNF) are the most successful means to ameliorate systemic autoimmune inflammation. Neutralization of TNF, however, is often associated with the development of autoantibodies, in particular to nuclear antigens, the mechanisms of which are unknown. Here, we analyzed the effect of TNF and its neutralization on MHC class II expression and function of antigen-presenting myeloid cells in rheumatoid arthritis (RA). Monocytes were isolated from the peripheral blood of RA patients before and after anti-TNF mAb treatment and from controls by negative selection, differentiated in vitro into macrophages and analyzed by flow cytometry for HLA-DR expression. T-cell responses to activation by myeloid cells were assessed in proliferation assays, and mRNA levels of the class II transactivator (CIITA) were determined by semi-quantitative RT-PCR. HLA-DR expression was significantly reduced on myeloid cells from RA patients with active disease, but was increased to normal levels after TNF mAb treatment. Concordantly, in vitro application of TNF to monocytes from healthy individuals reduced their ability to upregulate HLA-DR during differentiation to macrophages and, importantly, inhibited their ability to stimulate T cells in mixed lymphocyte reactions. Molecular analysis revealed that the effect of TNF on HLA-DR expression was mediated via suppression of the transcription factor CIITA. The data indicate that TNF decreases HLA-DR expression by reducing CIITA mRNA levels in myeloid cells, functionally resulting in a decreased stimulatory capacity of myeloid cells for T cells. Concordantly, ameliorating disease activity in chronic inflammatory diseases by neutralizing TNF restores HLA-DR expression of myeloid cells and their ability to stimulate T cells. Thus, anti-TNF treatment might lead to augmented T-cell activation by myeloid cells, thereby promoting immune responses to (auto)antigens and the development of anti-nuclear antibodies that are frequently associated with anti-TNF therapy.