Adoptive Therapy for Lymphoma with CD4 Memory T Cells Depleted of CD137-Expressing Tregs.

Abstract 1693

Poster Board I-719

A central goal of cancer immunotherapy is an adaptive immune response against tumors. The memory T cell is a critical mediator of this response as it can give rise to effector cells as well as self-renew. Regulatory T cells (T_regs) present a barrier to effective cancer immunotherapy. Indeed, cancer patients have increased numbers of CD4⁺CD25⁺ T_regs and cancer vaccination strategies in some cases expand this cell population. Here, we demonstrate that (1) CD4⁺CD44^hi memory T cells are effective in mediating anti-tumor immunity and (2) that expression of CD137 can be used to exclude tumor-reactive T_regs from the CD4⁺CD44^hi population.

We have established a model for adoptive therapy of mantle cell lymphoma in which CD4 T cells mediate anti-tumor immunity. Specifically, we use a whole tumor-cell vaccine to induce anti-tumor immune cells in vivo. These cells are isolated and adoptively transferred into lethally irradiated, syngeneic, recipient mice. We show that CD4 but not CD8 T cells from vaccinated donor mice can prevent tumor growth when adoptively transferred into irradiated recipient mice.

We observed that a majority of anti-tumor T cells display a memory phenotype. 10 days after transfer, T cells from recipient mice were co-cultured with irradiated lymphoma cells for 24 hours. Only CD4 T cells produced IFN-γ in response to co-culture and greater than 95% of IFN-γ⁺ CD4 T cells expressed the memory marker CD44. Finally, we observed that transferred CD4⁺CD44^hi T cells persisted for over 100 days suggesting that this CD4 subset is important for lasting anti-tumor immunity.

Contaminating T_regs may limit the effectiveness of our CD4 T cell adoptive therapy. In order to functionally deplete these T_regs from the CD4 population, we sought to identify a surface marker that could uniquely distinguish tumor-reactive T_regs from other CD4 T cells. T cells from vaccinated donor mice were co-cultured with irradiated lymphoma cells for 24 hours. We evaluated a panel of activation markers and observed that CD137 expression defined a distinct population of T_regs. Based on these results, we used flow cytometry to isolate a sub-population of CD4⁺CD44^hiCD137^- T cells from vaccinated donor mice. Adoptive transfer of less than 100,000 CD44^hiCD137^- but not other sub-populations of CD4 T cells provided significant protection from tumor challenge. These results suggest that CD137 defines a novel population of tumor-reactive T_regs. This marker can facilitate the enrichment of anti-tumor CD4 memory T cells within the CD44^hiCD137^- population.

In conclusion, these findings highlight the potential use of CD4 T cells in adoptive therapy for mantle cell lymphoma.