Homeostatic proliferation plus regulatory T-cell depletion promotes potent rejection of B16 melanoma

Purpose: To investigate the antitumor efficacy of T-cell anergy reversal through homeostatic proliferation and regulatory T-cell (Treg) depletion in a clinically relevant murine adoptive immunotherapy model. Experimental Design: B16 melanoma cells were engineered to express the model SIYRYYGL (SIY) antigen to enable immune monitoring. Tumor-specificTcells expanded in tumor-challenged wild-type hosts but became hyporesponsive.To examine whether lymphopenia-induced homeostatic proliferation could reverse tumor-induced T-cell anergy, total splenicTcells were transferred into lymphopenic RAG2^-/- mice or control P14/RAG2^-/- mice. Tumor growth was measured, and SlY-specific immune responses were monitored using ELISPOT and SIY/K^b tetramers. To determine whether Treg depletion could synergize with homeostatic proliferation, RAG2^-/- mice received total or CD25-depleted T cells, followed or preceded by B16.SIY challenge. This approach was further investigated in wild-type mice lymphodepleted with sublethal total body irradiation. Results: Adoptive transfer of total splenic T cells into RAG2 ^-/- mice moderately affected the growth rate of B16.SIY. As Treg expansion occurred in tumor-bearing mice, CD25⁺ T cells were depleted from total Tcells before adoptive transfer. Interestingly, transfer of CD25-depleted T cells into RAG 2^-/- mice resulted in potent rejection of B16 melanoma in both prophylactic and short-term preimplanted tumor settings and was associated with maintained T-cell effector function. Using a clinically applicable approach, wild-type mice were lymphodepleted using sublethal total body irradiation, which similarly supported tumor rejection upon transfer of CD25-depleted Tcells. Conclusions: Our results indicate that combined CD25 depletion and homeostatic proliferation support a potent antitumor immune response - an approach with potential for clinical translation.