0x40 is a member of TNF receptor superfamily and is found on activated T cells (Watts and DeBenedette, 1999) but there is a difference in their expression patterns on T cell subsets. While in vitro studies showed the expression peaks at 48 hours following activation on both CD4 f and CD8+ T cells, the vivo experiments, on the other hand, demonstrate a selective expression on CD4+ T cells upon immunizing with adjuvant or under specific clinical conditions (Weinberg, 1998; Weinberg et al., 1998; Higgins et al, 1999). Ox40 is an activation-dependent molecule with a broad tissue distribution, and has been localized on activated T cells (Ваши et al, 1994), В cells (Calderhead et al., 1993), dendritic cells (Ohshima et al., 1997), and vascular endothelial cells (Imura et al., 1996). Data available indicate that both Ox40-Ox40L interactions and anti-Ox40-anti-CD3 combinations provide costimulation to T cells, resulting in elevated cytokine secretion (Weinberg et al, 1998). The authenticity of these findings was further substantiated by the findings that in Ox40-deficient mice the T helper responses were greatly diminished, while the В cell and CTL responses remain unaffected (Kopfet al., 1999). Also, studies with Ox40-Ig fusion protein demonstrate decreased T cell responses under the conditions tested (Weinberg et al., 1999). Ox40 has also been shown to play an important role in various disease conditions (Kaleeba etaL, 1998).