Contradictory studies report either pro-or anti-inflammatory endothelial cell (EC) responses to human cytomegalovirus (hCMV) infection, hindering the validation of a potential link between this virus and associated inflammatory pathologies. Clarifying this issue, we report that hCMV induces a biphasic response. Early after inoculation, hCMV promoted lymphocyte and, to a lesser extent, neutrophil capture under in vivo relevant shear stresses. In contrast, later stages of infection rendered EC refractory to basal, or cytokine-induced, leukocyte recruitment. E ndothelial cells (EC), the master regulators of leukocyte extravasation, are critical sites of human cytomegalovirus (hCMV) infection in vivo (for a review, see reference 1). hCMV antigens are found in EC in inflammatory pathologies, such as atherosclerosis (2) and inflammatory bowel disease (3), but the role of the virus in their development remains highly controversial. Studies report that hCMV induces production of the EC proinflammatory mediators interleukin 8 (IL-8) (4, 5) and IL-6 (6) and expression of the leukocyte adhesion receptor ICAM-1 (7, 8). In addition, reports have shown that hCMV-infected EC (hCMV-EC) supported enhanced leukocyte recruitment (5, 9-11). These heavily cited studies, particularly those demonstrating a role for hCMV-induced IL-8 in neutrophil transmigration (4, 5), support the idea that hCMV contributes to the development or exacerbation of inflammatory pathologies. However, these same studies used static adhesion assays involving prolonged coculture of leukocytes with hCMV-EC or hCMV-EC supernatants before quantification of recruitment. Thus, these systems circumvent components of the leukocyte recruitment paradigm that are critical in vivo, i.e., initial capture from flowing blood and rolling on the EC surface (for a review, see reference 12). Therefore, these results do not determine if hCMV-EC can induce leukocyte recruitment under vascular shear forces. Nevertheless, investigators have shown that hCMV can induce EC VCAM-1 and E-selectin expression, important capture receptors involved in leukocyte tethering from blood flow (11), implying that hCMV-mediated recruitment could occur under shear forces. These results, however, contradict an earlier study that claimed hCMV induces no such increase (8). Furthermore, other data indicate that hCMV renders EC refractory to cytokine-mediated adhesion molecule induction (8) and IL-8 secretion (13), suggesting that hCMV can induce an antiinflammatory phenotype, although there are no reports of the subsequent effects on leukocyte recruitment. Here, we addressed these contradictions in the literature by examining the ability of hCMV to induce a pro-or anti-inflammatory EC phenotype. Importantly, we examined the functional relevance of infection by studying leukocyte recruitment under shear stress in an in vivo relevant flow-based recruitment assay. We found that hCMV induced a biphasic inflammation response, which may explain many of the inconsistencies in the current literature and also h...