Breakdown of the blood-retinal barrier induced by activated T cells of nonneural specificity

The cellular and microvascular responses of JC Lewis rats to an intravenous injection of activated T cells specific for ovalbumin were examined with the retinal whole mount technique. The retina was examined at various times post-injection (pi) with the use of antibodies to the alpha beta T cell receptor (TCR) or to major histocompatibility complex class IT (MHC II), the monoclonal antibody ED1, and intravascular tracers. By 12 hours pi, small numbers of TCR+, ED1(+), and MHC II+ cells were present within the lumen of retinal vessels, and minor breakdown of the blood-retinal barrier (BRB) and microglial activation were evident. The intensity of these responses had increased by I day pi, when small numbers of TCR+ cells had also undergone extravasation, By 2 to 5 days pi, the numbers of TCR+, ED1(+), and MHC II+ cells in the retinal parenchyma had increased, but the BRB breakdown and microglial activation had subsided. Thus, in the absence of target antigen, activated T cells induced limited and transient breakdown of the BRB, microglial activation, and the extravasation of ED1(+), MHC II+ monocytes, In contrast, the retina of rats that received an intraocular injection of ovalbumin in addition to the intravascular injection of T tells showed massive cellular recruitment and breakdown of the BRB, These results indicate that an increase in the number of activated T cells in the circulation, such as that which occurs during viral or bacterial infection, has the potential to result in transient breakdown of the BRB and a mild local. microglial response.