T-cells are recruited from the blood into extra-vascular tissues during acute inflammation. However, in chronic inflammatory diseases, an inappropriate accumulation of T-cells in the diseased tissue contributes to pathogenesis. Very little is known about the mechanisms by which T-cell trafficking is regulated during inflammation. Here we describe a unique immune regulatory peptide that imposes a tonic inhibition of T-cell trafficking during inflammation. PEPtide Inhibitor of Trans-Endothelial Migration (PEPITEM) introduces a new paradigm into the pathways that regulate the inflammatory response. We present evidence that this new pathway is compromised in chronic inflammatory diseases. We propose that loss of this regulatory pathway makes the immune system ‘leaky’, allowing inappropriate access of T-cells to vulnerable tissues. Lymphocyte trafficking was assessed in vitro using videomicroscopy on TNF-α/IFN-γ activated endothelial cells (EC) and lymphocytes isolated from healthy donors or patients with chronic inflammatory diseases. In vivo, lymphocyte recruitment was assessed in a model of zymosan-driven peritoneal inflammation. PEPITEM was identified using mass spectrometry. Our studies began with an interest in adiponectin, an anti-inflammatory adipose tissue-derived cytokine. Using an in vitro migration assay, we observed that the migration of human lymphocytes was dose-dependently blocked by adiponectin with an EC50=37.2nM. Adiponectin achieves its effects on T-cell migration by the induction of a novel mediator released from B-cells. Thus, the effect of adiponectin was lost when B cells are absent, but could be regained by the addition of supernatants from adiponectin stimulated B-cells. Interestingly, the B-cell derived product did not act directly on T-cells; rather, it stimulated EC to release the lipid mediator sphingosine-1-phosphate, which in turn inhibited the migration of T-cells. We used mass spectrometry to isolate a B-cell derived peptide, corresponding uniquely in the human genome to a proteolytic excision product of the 14.3.3ζδ protein. Synthetic PEPITEM could also effectively inhibit T-cell migration with an EC50=18.6pM. In zymosan-induced peritonitis in the mouse, T-cell recruitment was significantly increased in BABL/C mouse strain lacking B cells (Jh-/-) when compared to wild-type animals. This excess of T-cell recruitment was ameliorated by intravenous treatment with PEPITEM. Importantly, lymphocytes isolated from patients with chronic inflammatory disease (type-1-diabetes) were released from the inhibitory effects of adiponectin, but this regulatory pathway could be re-established by the addition of exogenous PEPITEM. We believe that PEPITEM and its associated pathway may have therapeutic efficacy in a number of disease scenarios.