According to the current view, an efficient T-lymphocyte activation requires not only a specific signal delivered through the engagement of a T-cell receptor (TCR) by antigenic peptide-major histocompatibility complex (MHC), but also signals provided by costimulatory molecules. Accumulating evidence indicate that one of the possible reasons for poor immunogenicity of human tumors underlies in the lack of expression of costimulatory molecules (1 ). Recent studies reveal that, in many cases, antigens alone, especially weak tumor antigen, are insufficient to stimulate immune responses, that is, they are ignored by immune system unless second accessory signals are provided (2 ).
