CN is a rare autosomal dominant disease characterized by predictable cycling of patients’ neutrophils from normal levels to a nadir of ≤100 cells / μL. While heterozygous mutations in the neutrophil elastase gene (ELA2) occur in the majority of patients and altered subcellular trafficking of neutrophil elastase (NE) has also been demonstrated, it remains unknown how these defects lead to the CN phenotype. Similarly, mutations in the gene for the beta subunit of adapter protein 3 (AP3), which binds NE and shuttles it to endosomal vesicles, are present in severely neutropenic patients with Hermansky-Pudlak syndrome (type 2). Interestingly, separate single base-pair mutations in the zinc finger domains of Gfi-1, a gene that maintains the long-term potential of hematopoeitic stem cells and that negatively regulates ELA2 expression, were reported in two different patients with severe congenital neutropenia (SCN). Decreased immunity against recall antigens, and an increase in the proportion of T-cells bearing a mature memory phenotype have also been described in patients with Gfi-1 mutations. As yet, there is no unifying hypothesis linking these various abnormalities. We hypothesized that aberrantly expressed NE, resulting from mutations in ELA2 or associated regulatory genes, could predispose autologous T-cells to target myeloid precursors that aberrantly express NE. Such a reversal of immune tolerance could contribute to neutropenia in CN and SCN patients. We report in a 25 year-old man with CN from birth the simultaneous occurrence of heterozygous mutations in Gfi-1 (N382S and K403R), which were previously identified in separate patients with SCN. Neither mutation was found in this patient’s parents however, and sequencing of mRNA encoding PRTN3, ELA2 and AP3 showed no mutations. Because Gfi-1 serves as a transcriptional repressor of ELA2, we hypothesized that NE may be overexpressed and / or mis-trafficked in neutrophils. To address this hypothesis, we analyzed granulocytes by FACS staining and confocal microscopy. NE was disproportionately expressed in the plasma membrane of 75% of the patient’s granulocytes compared to only 3% of granulocytes from healthy donors (HD). We have previously demonstrated that cytotoxic T-lymphocytes (CTLs) that recognize proteinase 3 (P3) and NE-derived peptides, such as the HLA-A2 restricted peptide PR1 (VLQELNVTV), can kill leukemia but not normal myeloid cells due to aberrant expression of P3 and NE in the leukemia and that these PR1 specific CTLs (PR1-CTL) are not detected in healthy donors. Because this patient expressed the HLA-A2 allele, we used fluorescently labeled PR1/HLA-A2 tetramers to study whether PR1-CTL were present. Strikingly, 0.5% to 1% of total CD8+ cells from the patient were specific for PR1, as measured over 2 years, whereas no PR1-CTL were present in control HD or the patient’s mother, who is also HLA-A2+. The PR1-CTLs co-expressed CCR7 and CD45, consistent with a naïve phenotype, and cytokine secretion after crosslinking the T-cell receptors was similar to controls. These data suggest that aberrant NE expression can lead to a reversal of immune tolerance and a corresponding increase in PR1-CTL. The mechanism of neutrophil cycling in CN patients is not known; however, an autoimmune response against peptides derived from aberrantly expressed granule proteins in myeloid precursors could lead to a CTL-mediated feedback loop that would also be consistent with the chalone hypothesis of feedback regulation already proposed for this disease.
Disclosure: No relevant conflicts of interest to declare.