Non-Hodgkin Lymphoma B-Cells Induce Intratumoral CD4⁺CD25⁻ T Cells To Express Foxp3 and Gain Regulatory Function.

We have previously shown that CD4⁺CD25⁺Foxp3⁺ regulatory T cells from NHL tumors suppress the function of infiltrating CD4⁺ T cells and cytolytic CD8⁺ T cells. Expression of Foxp3 has been demonstrated to be crucial to the development and function of CD4⁺CD25⁺ regulatory T cells. However, the mechanistic details that drive development of Foxp3 expression in T cells, in both the normal and malignant scenario, remains to be fully elucidated. Previous studies suggest that Foxp3 expression in CD4⁺CD25⁻ T cells can be upregulated by tolerizing stimuli such as activation through TCR, corticosteroids, estrogen, and TGF-beta. Because lymphoma B cells have been shown to induce T-cell tolerance, we postulated that lymphoma B cells may play a role in the generation of regulatory T cells by inducing Foxp3 expression in CD4⁺CD25⁻ T cells.

FoxP3 expression was initially thought to be restricted to CD4⁺CD25⁺ regulatory T cell population. However, recent literature suggests that Foxp3 may also be expressed in CD4⁺CD25⁻ T cells. Using biopsy specimens from patients with B-cell NHL, we found that a subset, 15%, of infiltrating CD4⁺CD25⁻ T cells express Foxp3 and are capable of suppressing the proliferation and granule production of infiltrating cytotoxic CD8⁺ T cells. These initial studies suggest that CD4⁺CD25⁻Foxp3⁺ T cells have regulatory function. To explore the underlying mechanism by which Foxp3 expression is regulated, we determined the effect of costimulatory signals on Foxp3 expression in CD4⁺CD25⁻Foxp3⁻ T cells. Activation with OKT3/anti-CD28 Ab as well as DC-mediated activation induced Foxp3 expression in a subset of CD4⁺CD25⁻ T cells. We also found that the presence of lymphoma B cells during activation augmented the induction of Foxp3 expression in CD4⁺CD25⁻ T cells and that NHL B cell-mediated Foxp3 expression was cell contact-dependent. To better understand the contribution of NHL B cells in Foxp3 expression, we explored the possibility that CD27-CD70 interaction may be involved in Foxp3 expression. Lymphoma B cells express CD70, but not B7-1 and B7-2, which have been shown to be important in protecting tumor cells from lysis and contributing to cancer pathogenesis. Ligation of CD27 by receptor cross-linking enhanced Foxp3 expression in infiltrating CD4⁺CD25⁻ T cells in B-cell NHL.

Taken together these studies reveal a novel role for NHL B cells in development of regulatory T cells. Our data show that lymphoma B cells induce expression of Foxp3 in infiltrating CD4⁺CD25⁻ T cells and may result in development of T cells with regulatory function within the tumor microenvironment. Our results also suggest a potential role for CD27-CD70 interactions in this process. The ability of malignant B cells to drive development of regulatory T cells may be one mechanism by which lymphoma B cells protect themselves from anti-tumor immunity. (Supported in part by the Iowa/Mayo Lymphoma SPORE CA97274).