tokens: To investigate the role of costimulation of OX40 in human immunodeficiency virus type 1 ( HIV-1 ) infection by its natural ligand , gp34 , the OX40-transfected ACH-2 cell line , ACH-2/OX40 , chronically infected with HIV-1 , was cocultured with paraformaldehyde ( PFA ) -fixed gp34-transfected mouse cell line , SV-T2/gp34 .