Increased activity of T follicular helper (Tfh) cells plays a major pathogenic role in systemic lupus erythematosus (SLE). However, the mechanisms that cause aberrant Tfh cell responses in SLE remain elusive. Here we showed the OX40 ligand (OX40L)-OX40 axis contributes to the aberrant Tfh response in SLE. OX40L was expressed by myeloid antigen-presenting cells (APCs), but not B cells, in blood and in inflamed tissues in adult and pediatric SLE patients. The frequency of circulating OX40L-expressing myeloid APCs positively correlated with disease activity and the frequency of ICOS+ blood Tfh cells in SLE. OX40 signals promoted naive and memory CD4+ T cells to express multiple Tfh cell molecules and were sufficient to induce them to become functional B cell helpers. Immune complexes containing RNA induced OX40L expression on myeloid APCs via TLR7 activation. Our study provides a rationale to target the OX40L-OX40 axis as a therapeutic modality for SLE. Display omitted •OX40L is expressed by myeloid antigen-presenting cells in patients with active SLE•OX40 signals promote the differentiation of human Th cells toward the Tfh lineage•Strong TCR signals promote the expression of Tfh molecules by human Th cells•RNP-Anti-RNP immune complexes induce monocytes to express OX40L via TLR7 Although increased activity of T follicular helper (Tfh) cells plays a pathogenic role in systemic lupus erythematosus (SLE), the mechanism has been unclear. Ueno and colleagues show that exaggerated OX40 signals promote the generation of Tfh cells in SLE.