Intratumoral T_reg Cells Completely Inhibit the Induction and Function of Tumor-Infiltrating CD8+ T-Cells in B-Cell NHL.

Background: Numerous studies have shown that lymphoma B-cells are resistant to CTL-mediated death, however the underlying mechanism for this resistance is not clear. In previous work, we have identified a subset of CD4+CD25+ T-cells overrepresented in the tumor sites of B-cell NHL that display a phenotype compatible with regulatory T cells (T_reg cells). These cells express high levels of Foxp3 and CTLA-4, and are capable of suppressing the proliferation and cytokine production of autologous infiltrating CD4+CD25- T cells in B-cell NHL.

Goal: To explore whether T_reg cells exert a suppressive effect on the induction and function of autologous tumor-infiltrating CD8+ T-cells in B-cell NHL.

Results: Using fresh specimens obtained from patients with B-cell lymphoma, we found that intratumoral T_reg cells had a significantly suppressive effect on autologous tumor-infiltrating CD8+ T-cells in B-cell NHL. When activated CD8+ T-cells were cocultured with infiltrating CD4+CD25- T-cells, they displayed less proliferation than CD8+ T-cells cultured alone. However, we found that with a 1:4 ratio of stimulating cells (T_reg cells) to responding cells (CD8+ T-cells), intratumoral T_reg cells completely inhibited the proliferation of autologous tumor-infiltrating CD8+ T-cells activated with PHA. Furthermore, we have observed that 20% of infiltrating CD8+ T-cells in fresh isolated samples from B-cell NHL coexpress perforin and granzyme B. When intratumoral T_reg cells were cocultured with CD8+ T-cells, the T_reg cells inhibited the activation-induced production of perforin and granzyme B of autologous tumor-infiltrating CD8+ T-cells in a dose-dependent fashion. We also found that cytokine treatment (IL-2 and IL-12) or PHA activation induced the capability of tumor-infiltrating CD8+ T-cells to kill lymphoma B-cells, which was accompanied by upregulation of expression of CD107a on the surface of cytotoxic CD8+ T-cells. Intratumoral T_reg cells significantly inhibit cytotoxicity of activated infiltrating CD8+ T-cells to lymphoma B-cells. Finally, we found that there was an inverse correlation of cell frequencies between intratumoral T_reg cells and tumor-infiltrating CD8+ T cells in patients with B-cell NHL, suggesting that T_reg cells may inhibit the migration of cytotoxic T-cells to the sites of B-cell lymphoma.

Conclusion: Intratumoral T_reg cells dramatically suppress the proliferation and activation-induced production of granules in infiltrating CD8+ T-cells in B-cell NHL. These T_reg cells also inhibit the ability of activated tumor-infiltrating CD8+ T-cells to kill lymphoma B-cells in vitro, and may prevent the migration of CD8+ cells to the sites of lymphoma. Taken together, these data indicate an important role for intratumoral T_reg cells in CTL-mediated anti-tumor immunity in B-cell NHL.