Myeloid-derived suppressor cells are expanded in the antiphospholipid antibody syndrome Free

Introduction: Myeloid derived suppressor cells (MDSCs) constitute a heterogeneous population of immature myeloid cells that arise in response to pathologic stressors such as cancer or autoimmunity. MDSCs are broadly classified according to immunophenotypic and functional assays into polymorphonuclear (PMN), monocytic (M) or early stage/uncommitted (UC) subtypes. Functionally, they are defined by their immunosuppressive capabilities and ability to inhibit T cell function. They are not generally present in healthy states and arise in response to pathologic stimuli. MDSCs have not been described in the antiphospholipid antibody syndrome (APS) and their presence and potential role in disease pathogenesis is unknown.

Methods: To characterize MDSCs in APS, we performed immunophenotyping on APS patients and healthy controls (HC). We also analyzed patients with antiphospholipid antibodies (aPL) without a history of thrombosis. Patients were recruited from the Cleveland Clinic and consented to participate in an IRB-approved biorepository. Peripheral blood mononuclear cells (PBMCs) were prepared from sodium citrate-anticoagulated whole blood by density gradient. PBMCs were analyzed by flow cytometry and characterized as CD11b⁺CD33⁺HLA-Dr^- (total MDSC population), CD11b⁺CD33⁺CD15⁺CD14^-HLA-Dr^- (PMN/granulocytic-MDSCs), CD11b⁺CD33⁺CD14⁺CD15^-HLA-Dr^low/- (M-MDSC), and CD11b⁺CD33⁺CD14^-CD15^-HLA-Dr^- (UC-MDSC). Differences in APS vs HC were analyzed by unpaired T test with Welch's correction. We computed a Pearson correlation matrix to assess pairwise linear relationships between continuous variables, including aPL level and type (beta 2 glycoprotein-1 (β2GP1) IgG, IgM, anticardiolipin (aCL) IgG, IgM) and MDSC subtypes. A p-value of less than 0.05 was considered significant.

Results: We evaluated 21 patients who met clinical and laboratory criteria for primary APS, 6 patients with positive aPL without history of thrombosis or other criteria, and 4 HC. Patients with APS were median (IQR) 51(36-56.5) years old, 62% female. 12/21 had a history of venous thrombotic events, 4/21 had isolated arterial events, and 5/21 had VTE and ATE including 3 patients with a history of catastrophic APS. Regarding laboratory features, 18/21 APS patients were triple positive for aPL, two were lupus anticoagulant (LAC) positive only, and one was double positive for β2GP1 and aCL IgG with indeterminate LAC. The aPL carriers were all female, 62(52-64) years old, 3/6 triple positive, one LAC positive only, one double positive for β2GP1 and aCL IgM at high titers with negative LAC, and one double positive for β2GP1 and aCL IgG at high titers with indeterminate LAC.

Total MDSCs (CD11b⁺CD33⁺) were significantly expanded in APS vs HC (mean APS 1.128 vs HC 0.308, p=0.0001). Of MDSC subtypes, PMN-MDSC (0.788 vs 0.226, p=0.003) and M-MDSC (0.237 vs 0.023, p=0.0006) were significantly increased in APS vs HC. Early stage/UC-MDSCs did not differ significantly in APS vs HC (0.087 vs 0.055, p=0.190).

Patients with aPL without a history of thrombosis or other clinical manifestations were found to have an intermediate expansion of MDSCs. Total MDSCs were higher in aPL patients than HC (0.657 vs 0.307, p=0.039) but lower than APS (0.657 vs 1.128, p=0.021). PMN-MDSC were increased in APS vs aPL carriers (0.788 vs 0.360, p=0.017) while M-MDSC were increased in aPL carriers over HC (0.180 vs 0.023, p=0.019).

In a preliminary Pearson correlation matrix, B2GP1 IgG level positively correlated with total MDSC population (r 0.389, CI 0.01 to 0.67, p=0.045) and PMN-MDSC (r 0.432, CI 0.06 to 0.70, p=0.024).

Conclusions: In this preliminary characterization of MDSCs in APS we observed a population of MDSCs, with distinct PMN and monocytic subtypes, significantly increased in APS. Intriguingly, patients with aPL positivity without thrombosis history demonstrated an intermediate expansion of MDSCs over HC, to a more modest degree than APS patients. β2GP1 IgG level positively correlated with MDSC expansion and specifically PMN-MDSC subtypes. To our knowledge this is the first description of MDSCs in APS. These findings are hypothesis generating and support further studies to validate the T cell–suppressive function of MDSCs in APS, assess the role of MDSCs in the elevated levels of circulating neutrophil extracellular traps described in APS, explore their relationship to thrombosis risk, and identify additional clinical or immunologic drivers of MDSC expansion.