During transplantation, pancreatic islets release chemokines promoting macrophage attraction, hampering engraftment of islets. The aim of this work was to examine the mechanism of macrophage-pancreatic islets interaction that mediates islet rejection during transplantation. Human macrophages exposed to supernates of human and porcine pancreatic islets for the allogeneic and xenogeneic models, respectively, were evaluated for chemotaxis and expression of chemokine receptors (CCR-5). To modulate migration and identify the signaling pathway of macrophages, we tested pertussis toxin (PTX) to block Gi protein, and staurosporin and wortmannin to inhibit the protein kinase, and phosphoinositol-3 kinase, respectively. The addition of these agents significantly reduced macrophage migration induced by human islet supernates from 3.2 +/- 0.5 to 1.5 +/- 0.2, 0.9 +/- 0.1, and 1 +/- 0.1, respectively (P < .001, n = 3). In a xenotransplantation model, the reduction was less decreased, from 4.1 +/- 0.4 to 2.7 +/- 0.3 (P < .01), to 2.5 +/- 0.3 (P < .01), or to 1 +/- 0.1 (P < .001). Western blot analysis of chemokine receptor expression showed increased CCR-5 expression with human pancreatic islet supernates. Moreover, decreased islet purity increased CCR-5 expression. Pharmacologic study showed that PTX induced an increase in CCR-5 expression in allogeneic transplantation, whereas only staurosporin induced an increased receptor expression in the xenogeneic model, suggesting that chemokines participate in islet rejection even though the chemokine signaling pathways differ between allo- and xenotransplantation. Understanding the molecular mechanisms of islet rejection may improve graft survival.