To determine the levels of thymic stromal lymphopoietin (TSLP) and the numbers of TSLP receptor (TSLPR)-expressing CD1c+ (blood dendritic cell antigen 1-positive) myeloid dendritic cells (MDCs) in the joints as compared with the peripheral blood (PB) of patients with rheumatoid arthritis (RA), as well as to determine the capacity of TSLP to induce MDC-dependent T cell activation. TSLP levels were measured in synovial fluid (SF) samples from patients with RA and those with osteoarthritis (OA). MDC numbers in PB and SF samples from RA patients and TSLPR expression on these cells were assessed by fluorescence-activated cell sorter analysis. PB and SF MDCs from RA patients were stimulated with TSLP, and cytokine production was measured by multiplex immunoassay. TSLP-primed MDCs were cocultured with autologous CD4+ T cells in the absence of additional stimuli, and subsequently, cell proliferation and cytokine production were measured. TSLP levels were significantly increased in SF samples from RA versus OA patients. The numbers of TSLPR-expressing MDCs in the SF of RA patients were significantly increased as compared to those in the PB, and SF MDCs displayed increased levels of TSLPR. TSLP selectively stimulated the production of thymus and activation-regulated chemokine and macrophage inflammatory protein 1α by CD1c+ MDCs. TSLP-primed MDCs from PB and SF potently stimulated the proliferation of autologous CD4+ T cells as compared to unstimulated MDCs. Enhanced proliferation was associated with increased production of interferon-γ, interleukin-17 (IL-17), and IL-4. These data support an inflammatory mechanism by which increased intraarticular TSLP in RA potently activates TSLPR-expressing CD1c+ MDCs in the joints to secrete chemokines, causing chemotaxis and subsequent activation of CD4+ T cells. In addition to the demonstrated inflammatory potential of TSLP in experimental arthritis, this suggests that TSLP and TSLPR-expressing MDCs could both play a pivotal role in the immunopathology of RA.