Dendritic cells play a key role in the adaptive immune system by influencing T‐cell differentiation. Annexin‐1 (Anx‐Al) has recently been shown to modulate the adaptive immune response by regulating T‐cell activation and differentiation. Here we investigated the role of endogenous Anx‐A1 in dendritic cells as major cellular counterpart of T‐cell‐driven immune response. We found that Anx‐Al^–/– bone marrow derived dendritic cells show an increased number of CD11c⁺ cells expressing high levels of some maturation markers, such as CD40, CD54, and CD80, coupled to a decreased capacity to take up antigen compared to control Anx‐A1^+/+ cells. However, analysis of LPS‐treated dendritic cells from Anx‐A1^–/– mice demonstrated a diminished up‐regulation of maturation markers, a decreased migratory activity in vivo, and an attenuated production of the inflammatory cytokines interleukin (IL)‐1β, tumor necrosis factor (TNF)‐α, and IL‐12. This defect was also accompanied by impaired nuclear factor (NF)‐κB/DNA‐binding activity and lack of Anx‐A1 signaling, as demonstrated by the reduced activation of extracellular‐signal regulated kinase (ERK)1/2 and Akt compared to cells from control littermates. As a consequence of this phenotype, Anx A1^–/– dendritic cells showed an impaired capacity to stimulate T‐cell proliferation and differentiation in mixed leukocyte reaction. Together, these findings suggest that inhibition of Anx‐A1 expression or function in dendritic cells might represent a useful way to modulate the adaptive immune response and pathogen induced T‐cell‐driven immune diseases.—Huggins, A., Paschalidis, N., Flower, R. J., Perretti, M., D'Acquisto, F. Annexin‐1‐deficient dendritic cells acquire a mature phenotype during differentiation. FASEB J. 23, 985–996 (2009)