Cancer pathogenesis involves the interplay of tumor- and microenvironment-derived stimuli. Here we focused on the influence of an immunomodulatory cell type, myeloid-derived suppressor cells (MDSCs), and their lineage-related subtypes on autologous T lymphocytes. Although MDSCs as a group correlated with an immunosuppressive Th repertoire and worse clinical course, MDSC subtypes (polymorphonuclear, PMN-MDSC, and monocytic, M-MDSCs) were often functionally discordant. In vivo, PMN-MDSCs existed in higher numbers, correlated with different Th-subsets, and more strongly associated with poor clinical course than M-MDSCs. In vitro, PMN-MDSCs were more efficient at blocking T-cell growth and promoted Th17 differentiation. Conversely, in vitro M-MDSCs varied in their ability to suppress T-cell proliferation, due to the action of TNFα, and promoted a more immunostimulatory Th compartment. Ibrutinib therapy impacted MDSCs differentially as well, since after initiating therapy, PMN-MDSC numbers progressively declined, whereas M-MDSC numbers were unaffected, leading to a set of less immunosuppressive Th cells. Consistent with this, clinical improvement based on decreasing CLL-cell numbers correlated with the decrease in PMN-MDSCs. Collectively, the data support a balance between PMN-MDSC and M-MDSC numbers and function influencing CLL disease course.

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In chronic lymphocytic leukemia (CLL), bidirectional interactions of leukemic B cells with components of a complex, yet incompletely defined tumor microenvironment (TME) are critical for leukemic cell survival and proliferation. Ibrutinib, a Bruton's tyrosine kinase (BTK) inhibitor, blocks signals that are crucial for survival of CLL cells which are delivered by the B cell receptor (BCR) and certain other receptors. However since BTK and its family members are expressed by other cell types, ibrutinib can also affect non-leukemic cells, thereby altering their function. Here, we focused on understanding how myeloid-derived suppressor cells (MDSCs), a non-leukemic cell type within the TME, and their main target, T cells, are affected by ibrutinib therapy. Using blood cells from a set of 20 previously untreated patients receiving ibrutinib, we analyzed circulating MDSCs and their subsets 15 days before and 1, 2 and 3 months after treatment initiation. As anticipated, at the first month time point the absolute CLL B-cell count increased significantly (P=0.024), followed by a progressive reduction at consecutive time points (P<0.001). In contrast, absolute MDSC numbers slowly and continuously decreased over time, achieving a significant reduction by the third month (P=0.009). When dividing MDSCs into their cellular subsets, the same pattern was observed for granulocyte-like MDSCs (gMDSCs) (P=0.005) but not for monocyte-like MDSCs (mMDSCs); for the latter an insignificant change was found. Of note, when comparing the changes of MDSC and gMDSC numbers to that of CLL B-cell counts, MDSC and gMDSC neared statistical significance at month one (P<0.1) and achieved significance at the second month (MDSCs: P<0.001; gMDSCs: P=0.033). We also observed differences in T-cell subpopulations shortly after ibrutinib treatment began. T cell counts increased significantly at the second month compared to pre-treatment (P=0.024); this was the case for both CD4+ and CD8+ cells (P = 0.022 and 0.010, respectively). In addition, CD8+ T cells maintained significance through the third month (P = 0.033). When exploring T cell subsets defined by cytokine production, we observed a spike at the second month of CD4+IL17F+ and of CD8+IL17F+, CD8+IL17A+ and CD8+FoxP3+ T cells. However, by the 3rd month, only the IFNγ-producing subset of CD4+ and CD8+ T cells were significantly higher (P = 0.049 and 0.042, respectively). Next we analyzed the function of gMDSCs and mMDSCs in the presence of ibrutinib in vitro, addressing the two main effects of these cells on T lymphocytes: suppression of T-cell proliferation and modulation of T-cell differentiation. Specifically, ibrutinib did not directly reduce T-cell expansion in the absence of MDSCs and did not alter the effect of CLL gMDSCs nor mMDSCs on T-cell proliferation, since the significant reduction induced by gMDSCs (P=0.047) and the insignificant, variable effect of mMDSCs were unchanged by the drug. However when we analyzed the influence of gMDSCs, mMDSCs and monocytes on naïve CD4+ T-cell differentiation in the presence or absence of ibrutinib, to our surprise the only T cell subpopulation directly compromised by ibrutinib was the Th1/IFNγ-producing subset. This was opposite that observed in co-cultures with gMDSCs, mMDSCs or normal monocytes in the presence of ibrutinib where Th1 cells expanded significantly (P= 0.021, 0.010, and 0.005, respectively). In the latter co-cultures not containing ibrutinib, more IL-4-producing (Th2) cells were found. Additionally, ibrutinib had a positive effect on IL-22+ (Th22) and FoxP3+ (Treg) cell numbers in the presence of MDSCs and monocytes. In summary, opposite to what we observed for CLL B and T cells, MDSC counts fell progressively after initiating ibrutinib therapy, possibly due to a direct effect of ibrutinib on BTK in MDSCs or an indirect effect mediated by diminishing signals from CLL B cells that would normally be delivered after BCR engagement. Ibrutinib did not directly alter the T cell suppressive ability of MDSCs, but it did skew T-cell differentiation to Th1 cells when MDSCs were present, in line with our finding higher Th1 cells in the blood after 3 months of treatment. Thus over time, ibrutinib shifts the CLL TME from an immunosuppressive to a more immune effective one. Disclosures Chen: Beigene: Research Funding; Verastem: Research Funding; Pharmacyclics: Research Funding. Barrientos:Pharmacyclics, an AbbVie Company: Consultancy, Research Funding; AbbVie: Consultancy, Research Funding; Gilead: Consultancy, Research Funding; Janssen: Consultancy. Kolitz:Magellan Health: Consultancy, Honoraria. Rai:Pharmacyclics: Membership on an entity's Board of Directors or advisory committees; Cellectis: Membership on an entity's Board of Directors or advisory committees; Roche/Genentech: Membership on an entity's Board of Directors or advisory committees. Chiorazzi:Janssen, Inc: Consultancy; AR Pharma: Equity Ownership.