Program: Oral and Poster Abstracts
Type: Oral
Session: 641. CLL: Biology and Pathophysiology, excluding Therapy: Characterizing and Targeting the Microenvironment in CLL
Using flow cytometry on cryopreserved PBMCs, we found that the absolute numbers of MDSCs (HLA-DRlo/CD11b+/CD33+) in 49 untreated CLL patients were significantly higher than 15 healthy controls (HCs) (966 446 vs. 163 578 cells/ml, P<0.001). Moreover, we observed that the absolute numbers of MDSCs significantly correlated with CLL B-cell counts in the blood (P=0.005, Spearman r=0.423). Of note, the distribution between m-MDSCs (CD14+) and g-MDSCs (CD15+) was dramatically different, with CLL patients exhibiting significantly higher numbers and percentages of g-MDSCs than HCs (702 296 vs. 26 818 cells/ml, P<0.001; 50.89 vs. 16.98%, P<0.001).In line with these results, when we explored the MDSC populations (CD11b+/GR1+) in Eμ-TCL1 mice of 5-16 months of age with leukemia cell blood counts ranging from 0.1 to 100 x 106cell/ml. This analysis indicated a positive correlation between MDSCs and leukemic CD19+CD5+ cells (P=0.003; Spearman r=0.328). Furthermore, the dot-plot analysis of GR1 and CD11b showed three well defined cell populations: one monocytic (Ly6-C+) and two granulocytic (Ly6-G+CD11blo and Ly6-G+CD11bhi). As in patients, the g-MDSC population was larger than the m-MDSC population (884 100 vs. 454 700, P=0.016). However in this case, the m-MDSCs correlated with the numbers of circulating leukemic cells (P<0.001; Spearman r=0.463) and the g-MDSCs did not. The latter was the case even when they were subdivided into both CD11blo and CD11bhisubgroups. A similar pattern was observed when analyzing single cell suspensions from murine spleens.
When we evaluated the ability of MDSCs to inhibit autologous T-cell proliferation in CLL patients (n=7), we observed a consistent reduction of proliferation only when co-culturing with g-MDSCs (P=0.034). In contrast, the effects of m-MDSCs on T-cell expansion were varied and insignificant statistically. In 5 CLL samples, we induced m-MDSCs (im-MDSCs) from purified CD33+ cells in vitro with GM-CSF, IL10, and IL6; the im-MDSCs effectively suppressed T-cell proliferation in 4 of 5 cases at an average inhibition of 33% (range: 10-79%). Thus, dysfunctional m-MDSC suppression was not inherent and functional suppression could be achieved by stimulation of CLL precursor cells. Similarly in 3 independent experiments performed with MDSCs from Eμ-TCL1 mice (12-14 months of age), we observed a reduction of in vitro proliferation with g-MDSCs (P=0.049) and not with m-MDSCs. In addition, for those Eμ-TCL1 animals for which sufficient sample was available, we subdivided the g-MDSC population into the two subpopulations based on CD11b density; the CD11blosubset present less nuclear segmentation and higher suppressive activity.
In summary, absolute numbers of MDSCs in the blood of CLL patients and Eμ-TCL1 mice are elevated and correlate with the levels of expansion of the leukemia. The major subtype in both situations was g-MDSCs. These g-MDSCs were functionally competent suppressors, whereas m-MDSCs were impaired in this function. In CLL patients, this m-MDSC suppressor defect could be corrected by in vitro stimulation with growth factors that support monocyte differentiation. The high similarity between CLL patients and Eμ-TCL1mice in relation to MDSC number and function suggest that an imbalance in g-MDC vs. m-MDSC function may affect CLL development and expansion, altering interactions with members of the microenvironment such as T cells.
Disclosures: No relevant conflicts of interest to declare.
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