The innate immune response is essential for combating infectious disease. Macrophages and other cells respond to infection by releasing cytokines, such as interleukin-1 beta (IL-1 beta), which in turn activate a well-described, myeloid-differentiation factor 88 (MYD88)-mediated, nuclear factor-kappa B (NF-kappa B)-dependent transcriptional pathway that results in inflammatory-cell activation and recruitment(1-4). Endothelial cells, which usually serve as a barrier to the movement of inflammatory cells out of the blood and into tissue, are also critical mediators of the inflammatory response(5,6). Paradoxically, the cytokines vital to a successful immune defence also have disruptive effects on endothelial cell-cell interactions and can trigger degradation of barrier function and dissociation of tissue architecture(7-9). The mechanism of this barrier dissolution and its relationship to the canonical NF-kappa B pathway remain poorly defined. Here we show that the direct, immediate and disruptive effects of IL-1 beta on endothelial stability in a human in vitro cell model are NF-kappa B independent and are instead the result of signalling through the small GTPase ADP-ribosylation factor 6 (ARF6) and its activator ARF nucleotide binding site opener (ARNO; also known as CYTH2). Moreover, we show that ARNO binds directly to the adaptor protein MYD88, and thus propose MYD88-ARNO-ARF6 as a proximal IL-1 beta signalling pathway distinct from that mediated by NF-kappa B. Finally, we show that SecinH3, an inhibitor of ARF guanine nucleotide-exchange factors such as ARNO, enhances vascular stability and significantly improves outcomes in animal models of inflammatory arthritis and acute inflammation.