Although activated T lymphocytes can migrate through unstimulated neural endothelium to perform immune surveillance or initiate inflammation, the precise mechanism by which this occurs is not clear. In this study, we have used intravital scanning laser ophthalmoscopy to show that circulating, activated T cells induce early changes in the retinal venules that enable T cell diapedesis in the absence of cell rolling, and without any reduction in shear stress within the venules. Concanavalin A (Con A)-activated T cells, but not naive T cells, were able to penetrate the normal blood-retinal barrier (BRB) 8-16 h after adoptive transfer. A minimum number (> or =1 x 10(5) cells/mouse) of Con A-activated T cells needed to be transferred before lymphocytes crossed the normal BRB. Cell rolling and reduction of shear stress did not occur in normal retinal venules and post-capillary venules. In contrast, in mice with experimental autoimmune uveoretinitis (EAU), in which the BRB has broken down, 45% of blast cells were rolling in retinal venules. Cell rolling correlated with significantly reduced shear stress. Both naive and Con A-activated T cells could cross the disabled barrier, with Con A-activated T cells migrating faster and in greater numbers than naive cells. Adoptive transfer of Con A-activated cells into normal recipient mice induced limited and transient breakdown of the BRB and up-regulation of ICAM-1 but not P-selectin. Pretreatment of Con A-activated cells with anti-LFA-1 significantly suppressed T cell infiltration in normal recipient mice. Our data indicate that critical to immune surveillance in the central nervous system (CNS) is the ability of activated T cells to interact with the endothelium, up-regulating ICAM-1 and inducing transient breakdown of the barrier.