Aceruloplasminemia and Paroxysmal Nocturnal Hemoglobinuria Uncover Differential Expressions of Ceruloplasmin and Ferroportin in Immune Cells

Ceruloplasmin (CP) is a multicopper ferroxidase that oxidizes ferrous iron promoting the binding of ferric iron to transferrin. The secreted form of CP (sCP) produced mainly by the liver is essentially absent in patients with aceruloplasminemia, a rare type of hereditary iron overload with brain, liver and pancreatic siderosis. Alternative RNA splicing generates a form of CP that is anchored by glycosylphosphatidylinositol (GPI-CP) to the membrane of astrocytes and immune cells. GPI-CP has been reported to help stabilize ferroportin, the only known iron exporter in mammal cells, so we aimed to investigate whether ferroportin expression is abnormal in circulating blood cells in aceruloplasminemia and in paroxysmal nocturnal hemoglobinuria (PNH), a naturally-occurring human model of acquired deficiency of GPI-anchored proteins. Peripheral blood samples were collected from two patients with aceruloplasminemia with different mutations on the CP gene: CP c.2879-1 G>T (splice site mutation) and CP c.2756 T>C (missense mutation), both with undetectable levels of sCP (<0.02g/L), one patient with a large PNH clone (89.9% type III), and a healthy control. Immunophenotype was determined by incubation with fluorescent antibodies against GPI-CP, ferroportin, and known lineage surface markers (CD45, CD14, CD19, and HLA-DR), data acquisition on a FACS Canto equipment, and analysis with software FACS Diva. GPI-CP and ferroportin were only detectable in CD19⁺ lymphocytes and monocytes in all samples. We found no significant differences across subjects regarding lymphocytic expression of GPI-CP or ferroportin. In monocytes, the expressions of both proteins in aceruloplasminemia with CP c.2879-1 G>T were similar to those seen in the control. Nevertheless, monocytic expression of GPI-CP and ferroportin were significantly reduced in CP c.2756 T>C and PNH, when compared to the control. These data confirm previous observations that B lymphocytes and monocytes express GPI-CP and ferroportin, and concomitant reduction of both expressions in PNH and in CP c.2756 T>C support that GPI-CP fosters ferroportin stability on the cell membrane. We also show that, while germline mutations of the CP gene generally cause undetectable sCP, there is heterogeneity in GPI-CP expression, which may remain preserved, as observed in CP c.2879-1 G>T. Further studies are necessary to clarify why this splice site mutation would still allow GPI-anchoring, while the CP c.2756 T>C point mutation abrogates the ability to anchor GPI-CP. While the preservation of lymphocytic GPI-CP was not surprising in an essentially myeloid PNH clone, normal GPI-CP in B lymphocytes in aceruloplasminemia suggests there are lineage-specific differences in physiological expression of ceruloplasmin forms between B lymphocytes and monocytes, with possible implications to the importance of iron metabolism in immune responses. We also noticed that the CP c.2756 T>C patient with monocytic reduction ferroportin presented with slightly more intense anemia and microcytosis. This could result from lower expression of ferroportin in bone marrow macrophages, with impaired iron delivery to erythroblasts, in analogy to monocytes. Finally, acquired ferroportin deficiency in PNH monocytes implies that loss of GPI-anchored protein not only exposes these cells to lysis by complement, but also to intracellular iron retention, generation of reactive oxygen species and may be involved in the pathophysiology of PNH. In summary, our data show that heterogeneity in GPI-CP expression in B lymphocytes and monocytes results in differential expression of ferroportin in aceruloplasminemia and PNH, and future studies should aim at investigating the implications of dysregulated iron metabolism in immune cells.