Increased Frequency of Paroxysmal Nocturmal Hemoglobinuria Type Cells in Patients with Chronic Idiopathic Neutropenia Correlates with the Severity of the Disease and the Presence of Skewed T-Cell Expansions

Introduction: Chronic idiopathic neutropenia (CIN) of adults is an acquired disorder of granulopoiesis characterized by an unexplained, prolonged reduction in the number of peripheral blood (PB) neutrophils. We have previously shown that CIN pathophysiology is related to T-cell activation and increased apoptosis of granulocytic progenitor cells. Notably, in our cohort of patients we exclude cases with antineutrophil antibody activity, i.e. autoimmune neutropenia cases.

Aim of the study: Preliminary data have shown the presence of minor populations of paroxysmal nocturmal hemoglobinuria (PNH) type cells in the PB of CIN patients. The present study aims to investigate further the presence of PNH type cells in the PB of a large cohort of patients fulfilling the diagnostic criteria of CIN and probe the possible association with the severity of the disease and the skewed T-cell profile.

Patients - Methods: We have studied 91 adults with CIN and 55 hematologically healthy subjects, age- and sex-matched with the patients. We have used flow cytometry for the detection of PNH type cells according to the International Clinical Cytometry Society guidelines and for the identification of T-cell expansions based on the analysis of T-cell receptor beta variable (TRBV) gene repertoire.

Results: CIN patients displayed increased proportion of PB PNH type FLAER^-/CD24^- neutrophils (0.05% ± 0.18%) and FLAER^-/CD14^- monocytes (1.33% ± 1.38%) compared to healthy individuals (0.005% ± 0.01% and 0.64% ± 0.66%, respectively; P=0.0044 and P=0.011, respectively). Similarly, the patients displayed increased proportion of both, type II CD235⁺/CD59^dim and type III CD235⁺/CD59^- (0.059% ± 0.29% and 0.065% ± 0.28%, respectively) PB PNH red blood cells, compared to healthy controls (0.01% ± 0.03% and 0.01% ± 0.03%, respectively; P<0.0001 and P< 0.0001, respectively). Interestingly, an inverse correlation was found between the proportion of PB PNH type neutrophils and the number of absolutely neutrophil counts in CIN patients (r=-0.2525, P=0.0157) whereas no correlation was found between the proportion of PNH red blood cells or monocytes and absolutely neutrophil counts. In accordance with our previously reported data, increased frequency of skewed T-cell expansions were identified in CIN patients (71.4%) compared to healthy subjects (29.9%; P<0.0001). PNH type neutrophils (FLAER^-/CD24^-) were identified at a higher frequency in patients with skewed TRBV repertoire (63.08%) compared to those with normal TRBV distribution (34.61%) (P = 0.0194) suggesting a possible pathogenetic/pathophysiologic association between T-cell activation and emergence of PNH neutrophils in CIN.

Conclusion: CIN patients display minor populations of PNH type PB neutrophils, monocytes and red blood cells. The frequency of PNH type neutrophils is associated with the severity of neutropenia and it is higher among patients with skewed TRBV repertoire. These data support further the hypothesis that CIN displays common pathophysiologic features with immune-mediated bone marrow failure syndromes and should be included in this spectrum of disease entities.