A T-cell immunostimulatory factor derived from the soluble form of lymphocyte-activation gene 3 (LAG-3) protein with potential antineoplastic activity. Upon administration, alone or in combination with tumor antigens, IMP321 binds, with high affinity, to MHC class II molecules expressed by dendritic cells (DC), which may result in DC maturation, DC migration to lymph nodes, enhanced DC cross-presentation of antigens to T cells, and antitumor cytotoxic T cell responses.