Mesenchymal Stromal Cells Support Follicular Lymphoma-Associated Follicular Helper T-Cells, in Part, Through an IL-6 and IL-21-Dependent Mechanism.

Abstract 2707

The biology of follicular lymphoma (FL) is largely dictated by the immune-effector and stromal cells that comprise its tumor microenvironment. Recent studies have demonstrated that various infiltrating T-cell populations can be predictive of clinical outcome, with two such populations, follicular helper T-cells (T_FH) and regulatory T-cells (T_reg) likely playing critical roles in the biology of this disease. T_FH are normally responsible for the activation and maintenance of germinal center B-cells while T_regs suppress effector T-cell priming and function as well as suppress normal B-cell proliferation and differentiation. There is also data to support the notion that both of these cells have direct and/or indirect effects on FL B-cells. These cells and other immune-effector cells within the FL microenvironment have dynamic interactions with mesenchymal stromal cells (MSC), multipotent cells residing in most adult tissues, which can be recruited by FL B-cells into the tumor microenvironment where they inhibit anti-lymphoma T-cell responses and FL B-cell apoptosis. We hypothesized that such MSC play a role in modulating T_FH viability, similar to what has been previously reported for T_reg, and herein we show such a novel role for MSC, that being the selective support of FL-derived T_FH populations.

To determine the effect of MSC on the viability of discrete T-cell populations infiltrating FL lymph nodes (FLN), un-separated single cell suspensions (SCS) from FLN (n=13) were cultured for 48 hours alone or with normal tonsillar-derived MSC. The fold change in the proportion of CD4 T-cells that are T_FH (CXCR5⁺PD-1⁺Bcl-6⁺), T_regs (CD25⁺FoxP3⁺), T_H1 (T-bet⁺), T_H2 (GATA-3⁺) or T_H17 (ROR-gamma-t⁺) cells in SCS incubated with MSC was compared to that of SCS incubated without MSC. A 2.3-fold increase in the percentage of T_FH (p=0.006) cells and a 2-fold increase in the percentage of Tregs (p=0.007) were observed in the SCS cultured with MSC compared to those cultured without MSC. The effect of MSC in maintaining T_FH and T_regs was selective, as in contrast, the proportion of T-cells expressing the canonical T_H1, T_H2 and T_H17 transcription factors was not increased after culture with MSC. B-cells provide survival signals to T_FH and stromal cells provide survival signals to B-cells, therefore we next determined whether the stromal support of T_FH was mediated via B-cells. Both purified FL T-cells and B-cell-depleted FL SCS cultured on MSC showed a similar increase in T_FH (and T_reg) populations as seen with un-separated FL-B SCS, indicating that MSC support of T_FH and T_regs was independent of B-cells. MSC only partially supported T_FH and T_reg populations in transwell experiments suggesting a role for both cell-cell contact and soluble factors. In this regard, IL-6 and IL-21 are known to be secreted by MSC and to signal through gp130 receptors on T_FH to induce the transcription of Bcl-6, which is required for T_FH differentiation. Blocking IL-6 and IL-21 decreased MSC support of T_FH by 42% (p=0.021), while reducing MSC support of T_regs by only 17% (p=0.018) suggesting that IL-6 and IL-21 mediate, in part, the protective effect that MSC have on T_FH but that other cytokines are likely to play a role in supporting T_regs. Finally, we have shown that FLN-derived MSC support both FL T-cells and normal lymph node T-cells to a similar extent as the tonsillar MSC used in these experiments. This provides support for the use of tonsillar MSC in these experiments as we were able to generate and expand these more consistently than MSC from FLN, which allowed us to use a consistent MSC product for all experiments.

These findings therefore demonstrate a new role for MSC in FL, that being to support FL T_FH in addition to T_reg populations in the tumor microenvironment. MSCs have been shown to support FL B-cell viability and suppress anti-lymphoma T-cell responses. This finding that MSCs support T_FH cells, a population that may provide survival signals to FL B-cells, further supports the potential of targeting MSC as a novel therapeutic strategy for patients with FL.