Mesenchymal Stromal Cells Derived From Bone Marrow of Chronic Lymphocytic Leukemia Patients Promote Th17 Cell Differentiation

Mesenchymal stromal cells (MSC) derived from normal subjects are known to have immunosuppressive capacity by virtue of inhibiting T- and B-cell activation. A novel subset of T cells, Th17, plays an important role in inflammation and autoimmunity. A recent report demonstrated that normal MSC ameliorates experimental autoimmune encephalomyelitis by inhibiting CD4+ Th17 cells in a chemokine ligand 2-dependent manner (J Immunol. 2009, 182: 5994). It remains unknown if MSC derived from leukemic or cancer patients play a role in Th17 cell differentiation. In particular this would be of interest to study in B-Chronic Lymphocytic Leukemia (CLL) where immunosuppression is evident even in early stage disease.

MSC derived from bone marrow of CLL patients or normal subjects were expanded in vitro as previously described by us (Br J Haematol. 2009, 147:471). CD4+ cells positively selected from normal peripheral blood mononuclear cells were co-cultured with either CLL MSC or normal MSC at a ratio of 50:1 for 3 days with stimulation via CD3/CD28 beads, as well as interleukin-1β (IL-1β; 50 ng/ml). Then phorbol 12-myristate 13-acetate (50 ng/ml) and ionomycin (500 ng/ml) were introduced into the co-culture for 5 hrs in the presence of brefeldin A. Subsequently, cells were stained with CD4-phycoerythrin (PE) and IL-17-Alexa647 using intracellular flow to analyze the percent expression of IL-17 in CD4 + cells. Cytokine production from both CLL MSC and normal MSC as secreted into culture medium (CM) were tested using a commercial multiplex cytokine array (Invitrogen, CA). This array measures the level of 30 different cytokines.

Positively selected CD4+ cells from peripheral blood of normal donors contain minimal percentages of Th17 cells (range: 0.48–0.71%). IL-1β stimulation induced increased IL-17 expression (range: 1.05–1.12%). Co-culture of CLL MSC with CD4+ cells induced significantly increased IL-17 expression in the CD4+ T cells (range: 1.16–1.32%). The promoting effect of CLL MSC on these Th17 cells appeared to be mediated by direct contact since the CM of CLL MSC was not able to induce increased IL-17 expression (mean = 0.66%) to a similar level as direct co-culture. When IL-1b was used to stimulate Th17 cell differentiation from CD4+ cells, CLL MSC were able to further promote the level of Th17 cell differentiation (range: 2.01–2.63%), indicating synergistic function for CLL MSC with IL-1β. This latter finding again appeared to be more pronounced for CLL MSC as normal MSC with IL-1β was not able to promote Th17 cell differentiation to a similar degree. To further investigate the mechanism of CLL MSC on Th17 cell differentiation, we assessed the cytokine production for resting CLL MSC and normal MSC compared to cytokine production of CLL and normal MSC stimulated with IL-17. The data from multiplex cytokine arrays revealed that the cytokine profiles were not different between resting CLL and normal MSC; however, when MSC were stimulated with IL-17, there were significant differences between CLL and normal MSC in terms of IL-6 and MCP-1 production (IL-6, CLL vs. normal, 957.9 ± 98 vs. 554.2 ± 92.3 pg/ml, p = 0.01; MCP-1, CLL vs. normal, 787.7 ± 166.9 vs. 330.2 ± 116.5 pg/ml, p = 0.04, n = 7). Since both IL-6 and MCP-1 have been demonstrated to play important roles in Th17 differentiation, we are conducting further studies to dissect the mechanism of CLL MSC in the promotion of Th17 cell differentiation.

These results indicate that MSC derived from CLL patients promotes Th17 cell differentiation in vitro, which is in contrast to the previous published suppressive role of normal MSC on Th17 cell differentiation. Recent findings have indeed demonstrated that CLL patients do have high percentage of Th17 cells (Cancer Res. 2009. 69: 5922) when compared to other lymphoproliferative diseases. Given this data we believe that CLL MSC are intrinsically different from normal MSC in terms of immune regulation and cytokine production. This may occur as a result of the bi-directional activation that we found to be present between MSC and CLL leukemic cells (Br. J Haematol. 2009. 147:471). In total, our findings demonstrated that the dynamic interactions between the CLL leukemic cells and MSC appear to influence the Th 17 cell levels. This is of biological and clinical interest in that Th17 cells have the potential to regulate the immune environment to favor tumor proliferation and progression.

No relevant conflicts of interest to declare.