CD80 and cancer: B71-transduced tumor cells are expected to present both the antigen (T-cell receptor) and the costimulatory (CD28-mediated) signals to CD8+ cytotoxic T lymphocytes simultaneously, leading to efficient activation of cytotoxic T lymphocytes without requiring the assistance of CD4+ helper T cells.19 The potential of immune-based cancer treatments has been limited by negative immune-regulatory mechanisms, including tumor-derived factors that support cellular immunity and also host factors that suppress/inhibit cellular immune responses.