A novel T cell activation antigen identified by monoclonal IMN3.1 antibody and expressed preferentially on human T cells susceptible to apoptotic cell death.

When cultured without appropriate growth factors, most of activated (CD45RO+) T cells expanded in acute EBV-induced infectious mononucleosis (IM) easily die via an apoptotic cell death mechanism, indicating one Ag-driven selection in the periphery. In this work, we attempted to raise the mAb against cell surface molecules, preferentially expressed on T cells entering apoptosis, by immunizing PBMC from an acute IM patient. We obtained one mAb, termed IMN3.1, that could define clearly the expansion of activated (CD45RO+) T cells in the blood of acute IM patients. In contrast to its intense expression on IM T cells, the IMN3.1-reactive Ag was only dimly expressed on CD45RO+ (memory or previously activated) populations of T cells from normal individuals. Although naive (CD45RO-) populations of T cells usually lacked IMN3.1 Ag, this Ag was inducible relatively late after in vitro activation of naive T cells. The cellular distribution and molecular characterization of IMN3.1-reactive Ag suggested that IMN3.1 mAb appeared to recognize a novel activation-associated cell surface determinant of about 120 kDa m.w., which might be predominantly expressed on apoptosis-prone T cell lineage cells, such as IM T cells, thymocytes, cytokine-dependent T cell lines, and anti-Fas-sensitive T cell lines.