In CLL, Myeloid-Derived Suppressor Cells and Their Monocytic and Granulocytic Varieties Differ in T-Cell Subset Association and Polarization Induction

In chronic lymphocytic leukemia (CLL) monoclonal B cells expand and progressively accumulate in the bone marrow, eventually migrating to secondary lymphoid organs for even greater proliferation. At both sites, CLL cells engage in complex, incompletely defined cellular and molecular interactions involving multiple cell types such as T cells, myeloid cells, mesenchymal stromal cells, and matrix, collectively referred to as the "tumor microenvironment". This microenvironment is critical for the survival and proliferation of CLL cells, and data indicate that T cells and myeloid cells have an important role in these processes. In this study, we focus on two cells types: CD4⁺ T lymphocytes and myeloid-derived suppressor cells (MDSCs). In CLL patients, these populations are altered and impact on clinical outcome. CD4⁺ T cells comprise several subtypes, and CLL patients often have expanded Th2 and Tregs populations, consistent with the immunosuppressive status of these patients. Moreover, patients with higher numbers of another CD4⁺ subset, Th17 cells that produce IL-17 and other pro-inflammatory cytokines, can have longer survival times. Although studied minimally in CLL, MDSCs are known suppressors of T cell proliferation in vitro, and expand along with malignant cells in several cancers. However, no information is available about their effects on CD4⁺ T cell differentiation or on B-cell biology in CLL.

In a cohort of 56 untreated CLL patients, we first explored correlation of the numbers of MDSCs and autologous T cells, using flow cytometry. CD3⁺ cell numbers significantly paralleled total MDSCs and monocyte-like MDSCs (mMDSCs) (P = 0.002, Spearman r = 0.44; P = 0.004, Spearman r = 0.41, respectively). Interestingly, MDSCs correlated with CD4⁺ and CD8⁺ T-cells (P < 0.001, Spearman r = 0.646; P < 0.001, Spearman r = 0.61, respectively). However, the correlation of MDSC subpopulations with CD4⁺ and CD8⁺ cells differed: mMDSCs associated significantly with CD4⁺ cells (P < 0.001, Spearman r = 0.73) and granulocyte-like MDSCs (gMDSCs) with CD8⁺ cells (P= 0.008; Spearman r = 0.45). Furthermore, although gMDSCs did not correlate with the numbers of CD4⁺ T-cells, we observed that they positively paralleled Tregs defined as CD3⁺/CD4⁺/CD25⁺/CD127^-/FoxP3⁺ cells (P = 0.020, Spearman r = 0.44). Other subpopulations are currently under study.

To address the effect of MDSCs on CD4⁺ cell differentiation, we FACS sorted CD3⁺/CD45RO^- naïve CD4⁺ lymphocytes and stimulated them in vitro with anti-CD3/CD28 beads and IL2 in the presence or absence of mMDSCs (HLA-DR^lo/CD11b⁺/CD33⁺/CD14⁺), gMDSCs (HLA-DR^lo/CD11b⁺/CD33⁺/CD15⁺) or monocytes (HLA-DR^hi/CD11b⁺/CD14⁺); these studies involved samples from 3 CLLs and 3 healthy controls (HCs). On day 7, cells were harvested and cytokine production was quantified by intracellular flow cytometry as the percentages of the following populations: Th1 (INFγ), Th2 (IL-4), Tregs (FoxP3), Th17 (IL-17A and IL-17F), Th9 (IL-9) and Th22 (IL-22). Culturing CLL or HC T cells in the absence of MDSCs revealed a lower percentage of cytokine-producing cells (24% vs. 55%; P = 0.017) in CLL, which was mainly due to a reduction in IL-4⁺ cells (P = 0.066). However, when analyzing the effects of MDSC subsets on the polarization of CLL or HC T cell, gMDSCs promoted significantly more FoxP3⁺ and less IL-22⁺ cells in CLL than in HC (P = 0.025 and P = 0.048, respectively). When analyzing only CLL T cells, supplementation with mMDSCs induced a reduction in IL-22⁺ cells (P = 0.027) and an insignificant increase of IL-4⁺ and IL-17⁺ cells. Conversely monocytes supported an expansion of INFγ⁺ T-cells (P=0.066), and gMDSCS promoted an increase of IL-9⁺ cells (P = 0.046) and a reduction of FoxP3⁺ cells (P = 0.019).

In summary, in CLL the absolute numbers of total MDSCs and T cells are tightly linked. There is a significant correlation between CD4⁺ T cells and mMDSCs, and between CD8⁺ T cells and gMDSCs. Additionally, in CLL naïve CD4⁺ differentiation appears reduced compared to HC, in concordance with lower T-cell responses previously reported. Moreover, the preliminary aspects of the study suggest that CLL mMDSCs promote an expansion of Th2, Th17 cells and a reduction of Th22 cells, and monocytes enhance Th1s. Unexpectedly, since we observed a significant positive correlation in the PBMCs, gMDSCs may reduce Tregs and augment Th9. These findings depict differential consequences of CLL T cell - MDSC / mMDSC / gMDSC interactions.