T-CELLS OF STAPHYLOCOCCAL-ENTEROTOXIN B-TOLERIZED AUTOIMMUNE MRL-LPR LPR MICE REQUIRE CO-STIMULATION THROUGH THE B7-CD28/CTLA-4 PATHWAY FORACTIVATION AND CAN BE REANERGIZED IN-VIVO BY STIMULATION OF THE T-CELL RECEPTOR IN THE ABSENCE OF THIS COSTIMULATORY SIGNAL/

The CD28/CTLA-4 receptors on T cells interact with the B7 molecule on antigen-presenting cells (APC) to produce a co-stimulatory signal that determines the outcome of activation. The role of this co-stimulatory signal in T cell activation and loss of tolerance in autoimmune MRL-l pr/lpr mice has not been investigated previously. The present study ex amines the contribution of the CD28/CTLA-4 co-stimulatory pathway to t he loss of T cell tolerance in V beta 8 transgenic MRL-lpr/lpr and -+/ + mice in which neonatal tolerance has been induced by the superantige n staphylococcal enterotoxin B (SEB). An artificial APC transfected wi th the murine B7 gene, and a CTLA-4-Ig fusion protein were used to ana lyze the significance of the CD28/CTLA-4 pathway in vitro. The CTLA-4- Ig fusion protein was also used to inhibit the pathway in vivo. Our re sults demonstrate that CD28 and CTLA-4 mRNA was overexpressed in the l ymph nodes of lprllpr mice (MRL, C57BL/6, C3H and AKR), but not in +/ mice of the same background strain. Lymph node T cells and thymocytes from SEB neonatally tolerized MRL-Ipr/lpr mice that had undergone tol erance loss, proliferated when cultured with SEB and B7(+) fibroblasts lit vitro, but did not proliferate when the SEB was presented in the context of B7(-) fibroblasts. This in vitro tolerance loss could be pr evented by blocking of B7 signaling by CTLA-4-Ig. This loss of toleran ce did not occur in lymph node T cells from thymectomized MRL-lpr/lpr mice. SEB challenge of tolerized MRL-lpr/lpr mice in vivo led to weigh t loss, increased serum cytokine levels and depletion of V beta 8(+) T cells. These effects were blocked by blocking of the co-stimulatory p athway by treatment with the CTLA-4-Ig fusion protein prior to and dur ing challenge with SEB. T cells from thymus and lymph nodes of these m ice did not proliferate later in response to stimulation in vitro with SEB even in the presence of B7(+) APC. Nonresponsiveness was not due to deletion of V beta 8(+) CD28(+) T cells, as the number of these cel ls was increased after treatment with SEB and the CTLA-4-Ig fusion pro tein. These results suggest that the response of autoreactive T cells in the thymus and lymph nodes depends on signaling by B7 in vivo and i n vitro and that SEB-reactive T cells can be reanergized in vivo by st imulation of the T cell receptor in the absence of signaling through t he CD28/CTLA-4 co-stimulatory pathway.