Malignant B Cells Skew the Balance between T_reg Cell and T_H17 Cell Differentiation in B-Cell Non-Hodgkin Lymphoma (NHL).

Numerous clinical therapies have attempted to modulate tumor cell immunity, but for the most part, have proven unsuccessful. The inability to produce or augment an effective immune response is due in part to regulatory T (T_reg) cells, which inhibit CD4 and CD8 T cell function. Our group has recently shown that T_reg cell numbers are elevated in NHL tumors and that NHL B cells induce the development of T_reg cells thereby inhibiting anti-tumor responses. The ability of NHL B cells to direct the cellular composition of their microenvironment is critical to our understanding of tumor immunity and we therefore wanted to determine if NHL B cells also directed the expansion or reduction of other T cell populations. IL-17-secreting CD4⁺ T cells (T_H17), a newly characterized CD4⁺ T helper cell lineage, promote inflammation and play an important role in autoimmune disease. IL-17 has been shown to inhibit tumor cell growth suggesting a potential role for T_H17 cells in anti-tumor immunity. We therefore set out to determine if T_H17 cells were present in NHL tumors and whether or not their numbers were regulated by NHL B cells. Using unsorted mononuclear cells from malignant lymph nodes, we were unable to detect IL-17 expression in resting CD4⁺ T cells or CD4⁺ T cells activated with PMA/Ionomycin stimulation (less than 1%). However, IL-17-secreting CD4⁺ T cells could be detected in significant numbers in inflammatory tonsil and normal PBMCs. Interestingly, depletion of CD19⁺ NHL B cells from mononuclear cells obtained from patient biopsies resulted in detection of a clear population of IL-17-secreting CD4⁺ T cells (5%). These results suggest that NHL B cells suppress T_H17 cell differentiation. The frequency of IL-17-secreting CD4⁺ T cells could not be further enhanced by the addition of exogenous TGF-b and IL-6, a cytokine combination favoring for T_H17 differentiation, suggesting a further impairment of T_H17 cell differentiation in the tumor microenvironment. In contrast, Foxp3 expression could be detected in resting CD4⁺ T cells (30%) and could be induced in CD4⁺CD25⁻Foxp3⁻ T cells activated with TCR stimulation (28%). Contrary to the inhibition of TGF-b-mediated T_H17 differentiation, Foxp3 expression could be dramatically upregulated by TGF-b in intratumoral CD4⁺ T cells (35%). In addition, lymphoma B cells strongly enhanced Foxp3 expression in intratumoral CD4⁺CD25⁻Foxp3⁻. Furthermore, when added together, the frequency of Foxp3⁺ T cells and Foxp3-inducible cells reached up to 60% of CD4⁺ T cells in tumor microenvironment of B-cell NHL. These findings suggest that the balance of effector T_H17 cells and inhibitory T_reg cells is disrupted in B-cell NHL and significantly favors the development of inhibitory T_reg cells. Our data indicate that lymphoma B cells are key factor in regulating differentiation of intratumoral CD4⁺ T cells toward inhibitory CD4⁺ T cells.