Cytochemically unreactive neutrophils from subjects with myeloperoxidase (MPO) deficiency show a complex pattern of immunoreactivity with anti-MPO monoclonal antibodies: A flow cytometric and immunocytochemical study

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Abstract
Neutrophil granulocytes from 12 subjects with primary myeloperoxidase (MPO) deficiency (six totally deficient) and 16 patients with secondary partial MPO deficiency were tested using two different anti-MPO antibodies, in combination with either a flow-cytometric technique or an immunoalkaline phosphatase staining method. Results demonstrated three different cytofluorimetric patterns of immunoreactivity with the MPO protein: (a) a bright MPO antigenic expression, typical of patients with secondary MPO deficiency (comparable to that observed in the control group); (b) a medium MPO antigenic expression, typical of subjects with primary partial MPO deficiency; and (c) a dim MPO antigenic expression, characteristic of individuals with hereditary total MPO deficiency. No significant differences in granulocyte MPO reactivity were demonstrated for the two antibodies. Furthermore, in two individuals with complete primary enzyme deficiency, the single histogram analysis of MPO fluorescence determined by flow cytometry seemed to show that only 38% (case 1) and 44% (case 2) of neutrophils were reactive with the anti-MPO antibodies: the use of multiple histogram analysis in combination with Kolmogorov-Smirnov statistics allowed us to demonstrate that all the cells express a low density of MPO antigen. These data were more or less confirmed by the APAAP labeling method, which showed a reduced staining only in subjects with primary deficiency, while all patients with secondary deficiency had scores similar to those observed in controls (healthy subjects). Compared with the immunoenzymatic technique, the flow-cytometric procedure showed a higher sensitivity to MPO, being able to estimate even minor decreases in neutrophil MPO antigenic expression, as previously postulated by other authors. This work suggests that patients with primary MPO deficiency have different amounts of MPO antigens in the neutrophil granulocytes, and the levels of MPO fluorescence seem to decline concurrently with enzyme activity, thereby suggesting the presence of a diminished MPO production. In contrast, the normal antigenic reactivity of neutrophils from patients with acquired MPO deficiency indicates the presence of a functionally inactive form of the enzyme.

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