Combined treatment with anti-HER2/neu and anti-4-1BB monoclonal antibodies induces a synergistic antitumor effect but requires dose optimization to maintain immune memory for protection from lethal rechallenge.

Human epidermal growth factor receptor type 2 (HER2)-positive breast cancer that is treated with anti-HER2/neu monoclonal antibody (mAb) is not free from late recurrences. Addition of anti-4-1BB mAb to anti-HER2/neu mAb has been demonstrated to strengthen the cytotoxic antitumor response. Our study expands on this by revealing the influence of anti-4-1BB mAb addition on the immune memory of anti-HER2/neu mAb. We designed murine breast cancer models by implanting TUBO and TUBO-P2J cell lines in mice, which were then treated with anti-HER2/neu and/or anti-4-1BB mAb. After complete surgical and/or chemical regression of the tumor, the mice were rechallenged with a second injection of cancer cells. Notably, anti-HER2/neu and anti-4-1BB mAb combination therapy had a synergistic antitumor effect at the initial treatment. However, the combination therapy did not evoke immune memory, allowing the tumors to thrive at rechallenge with reduced CD44+ expression in CD8+ T cells. Immune memory was also impaired when anti-4-1BB mAb was administered to naive CD8+ T cells but was sustained when this was administered to activated CD8+ T cells. In an attempt to resist the loss of immune memory, we controlled the dose of anti-4-1BB mAb to optimize the stimulation of activated CD8+ T cells. Immune memory was achieved with the dose regulation of anti-4-1BB mAb to 1 mg/kg in our model. Our study demonstrates the importance in understanding the adaptive immune mechanism of anti-HER2/neu and anti-4-1BB mAb combination therapy and suggests a dose optimization strategy is necessary to ensure development of successful immune memory.