The expression and the regulatory role of OX40 and 4-1BB heterodimer in activated human T cells

OX40 and 4-1BB are members of the tumor necrosis factor (TNF) family of co-stimulatory receptors whose signaling is important for differential immune responses mediated by CD4⁺ or CD8⁺ T cells. Although activated T cells may acquire OX40/4-IBB double-positive phenotype and signaling from each receptor is expected to influence cell functions, the relevance between OX40 and 4-1BB has never been investigated before. While we were investigating the expression of OX40 and 4-1BB on activated human T cells, we found that they colocalize. The study of receptor gene-transfected cells showed that both receptors coendocytose and the complex of OX40 and 4-1BB was detected by specific ligands or antibodies (Abs). The heterodimer of OX40 and 4-1BB was identified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) under nonreduced conditions and was associated with the tumor receptor-associated factor (TRAF) family proteins in a unique manner. Furthermore, the stimulation of OX40/4-1BB rendered cells sensitive to apoptosis induced by TNF-α that accompanied reduced activation of nuclear factor-κB (NF-κB). Finally, the OX40/4-1BB stimulation repressed the mitogen response in activated CD25⁺CD4⁺ T cells and preactivated CD8⁺ T cells. Thus, the OX40/4-1BB heterodimer appears to represent a unique regulatory receptor in activated T cells.