Circulating Myeloid-Derived Suppressor Cells Reflect Mycosis Fungoides/Sezary Syndrome Disease Stage and Response to Treatment

Background: Despite mycosis fungoides (MF) and Sezary syndrome (SS) comprising the most common forms of cutaneous T cell lymphoma, the pathophysiology underlying these disorders remains poorly understood. Consequently, current prognostic guidelines based on disease spread exhibit wide variations in clinical outcome within each stage, underscoring an urgent need for novel approaches to MF/SS disease evaluation. A growing body of research suggests that systemic immune dysregulation represents an early, cardinal feature of MF/SS. We hypothesized that tracking this immune dysfunction in conjunction with disease spread may generate important pathophysiologic and prognostic information for patients. We focused on myeloid-derived suppressor cells (MDSCs), a recently discovered population of immunosuppressive innate immune cells related to neutrophils and monocytes, because their expansion in numerous solid tumor settings have correlated reliably with poor patient outcomes. Whether MF/SS augments circulating MDSC abundance remains unexplored, prompting us to evaluate whether this could serve as a marker for disease progression and treatment response.

Methods: We used multiparametric flow cytometry to analyze the frequency and immunophenotype of MDSCs from the peripheral blood of 15 healthy donors and 30 patients with MF/SS. Patients at varying stages of MF/SS disease progression and treatment were included in the study. We defined granulocytic MDSCs (G-MDSCs) as cells positive for CD15, CD11b, and the recently discovered surface marker LOX-1, and negative for CD14. Monocytic MDSCs (M-MDSCs) were defined as cells positive for CD14 and CD11b, negative for CD15, and low/negative for HLA-DR. Each patient sample also underwent flow cytometry evaluating for circulating neoplastic T cells. These results were correlated with each participant's other hematologic parameters and clinical information through manual chart review.

Results: We found that healthy donors harbored no quantifiable circulating MDSCs of either monocytic or granulocytic lineage, a result in keeping with previous studies. In contrast, MF/SS patients exhibited robust, statistically significant increases in the frequencies of both G-MDSCs and, to a lesser extent, M-MDSCs. G-MDSCs exceeded 20% of all CD15-positive cells in some patients.

When patients were stratified by MF/SS clinical stage, those with more advanced disease displayed significantly higher G-MDSC abundance than early-stage patients. G-MDSC frequency was positively correlated with circulating CD4+ CD26- T cell counts often used in evaluating Sezary syndrome (R² = 0.498; p < 0.0001). However, patients with early, skin-restricted disease also showed statistically significant increases in circulating G-MDSCs compared to healthy controls. This suggested that G-MDSC expansion may serve as a sensitive, blood-based disease marker even in the absence of systemic involvement by neoplasia.

Patients who underwent recent treatment exhibited variable G-MDSC counts in the peripheral blood that were lower than in similar untreated patients on average. Serial measurements for two patients enrolled in a clinical trial for dual phosphoinositide 3-kinase and histone deacetylase inhibition revealed that G-MDSC frequencies markedly decreased over the course of treatment, mirroring the decrements of aberrant T cells circulating in the blood.

Conclusion: These findings provide clear evidence of G-MDSC expansion in the peripheral blood of MF/SS patients that begins in early/locally restricted disease, grows with disease progression, and responds to systemic therapy. Such immunometric assays may illuminate a novel source of staging and prognostic information and may permit less invasive disease monitoring than current methods require.