sign in sign up
<<
Home , Red blood cell

THE DIAGNOSIS OF PAROXYSMAL NOCTURNAL : ROLE OF FLOW CYTOMETRY

Alberto Orfao

Department of Medicine, Cancer Research Centre (IBMCC-CSIC/USAL), University of Salamanca, Salamanca, Spain

Paroxysmal nocturnal hemoglobinuria (PNH) on one or multiple populations of peripheral is an acquired clonal disorder typically affecting red cells and/or leucocytes of PNH patients, this young adults, which involves the phosphatidylino- translating into a new diagnostic test. Because sitol anchor biosynthesis class A gene of this flow cytometry became an essential tool (PIGA) coded in X, in virtually every in the diagnosis of PNH. In turn, it could be also case. The altered gene encodes for a defective pro- demonstrated that investigation of the presence of tein product, the pig-a enzyme which is involved GPI-deficient cells among circulating mature neu- in the early steps of the synthesis of glycosylphos- trophils and , increases the sensitivity phatidylinositol (GPI). This later molecule (GPI) of red blood screening because of the shorter acts as an anchor of a wide range of to lifetime of both populations of leucocytes. Despite the cell surface. At present a large number of GPI- all the above, routine assessment of CD55 and anchor associated proteins have been described CD59 is also associated with several limitations which are expressed by one or multiple distinct due to distinct patterns of expression among dif- compartments of hematopoietic cells. The altered ferent cell populations, dim and heterogeneous PIGA gene leads to an altered GPI anchor which expression among normal individuals and the lack leads to defective expression of GPI-AP on the of experience in many labs as regards the normal cytoplasmic membrane, on the cell surface. Among patterns of expression of both markers in distinct other GPI-AP, expression of the CD55 and CD59 populations of hematopoietic cells. In recent years complement regulatory molecules is down regu- this has led to the investigation of the potential lated; such defective expression of the CD55 and clinical utility of other GPI-associated markers CD59 molecules, leads to an increased sensitivity together with the development of a unique fluores- of blood cells to complement-induced cell ; cent reagent based on a bacterial –aerolysin-. this contributes to a large extent to the clinical This reagent, FLAER, is a fluorochrome-conjugated manifestations and complications of the disease. modified bacterial toxin which has the property of Such increased susceptibility of blood cells to specifically binding to GPI domains on the surface complement-induced cell lysis has long been used of white blood cells and , in the absence as the basis for classical diagnostic tests based of cytotoxic effects. Thus, this single reagent on the degree of achieved after comple- allows simultaneous evaluation of the presence of ment activation, e.g. the Ham test. Despite this, GPI-deficient cells in multiple populations of leu- such methods are associated with a relatively low cocytes coexisting in a sample, including mature sensitivity and specificity which has limited their and monocytes. However, it can not diagnostic utility due to previous transfusions, the be reliably used to identify GPI-deficient red blood relatively low sensitivity of detection of hemolysis cells due to unspecific binding (most probably to and to other causes of hemolysis, among other A) on the surface of erythrocytes. In reasons. Increased knowledge about the physiopa- turn, the most informative CD markers for the thology of the disease, leaded to the demonstration detection of GPI-deficient neutrophils and mono- of defective expression of both CD59 and CD55 cytes, include CD24, CD16 and/or CD66b and

66 XXXVII. Ulusal Hematoloji Kongresi CD14, respectively. In order to avoid false positive to be highly informative as regards the detection results due to inappropriate flow cytometric gat- of type II cells among mature neutrophils. Despite ing of these cell populations, additional markers this it should be noted that the clinical significance are typically used for the selection of neutrophils of the presence of type II cells still remains to be (e.g. CD15 and CD45) and monocytes (CD64 and/ fully established. or CD33 and CD45). Altogether, the combination In summary, combined flow cytometric assess- of FLAER and the above cited markers allows unequivocal identification of GPI-deficient cells ment of FLAER and GPI-associated markers on in patients suspected of carrying a GPI-deficiency mature peripheral blood neutrophils and mono- in multicentric settings, with virtually no false- cytes and/or red cells is currently recommended positive and false negative cases. For this purpose, for the diagnostic screening of subjects suspected screening of peripheral blood over marrow of PNH. In the near future, refinement of the samples is preferred. Despite all these advances, flow cytometric data analysis strategies used for few markers are still available for the assessment the identification of the distinct subpopulations of the degree of involvement of red blood cells and of white blood cells, together with more efficient CD59 remains as the most relevant marker to dis- algorithms to select for patients who should be tinguish between normal and GPI-deficient eryth- screened for the presence of GPI-deficient cells, rocytes; in addition, usage of appropriate sensitive will shorten time from symptoms to diagnosis and clones/fluorochrome-conjugated reagents, allows increase the efficiency of PNH diagnosis particu- discrimination between type II and type III GPI- larly among patients presenting with non-classical deficient red cells. Noteworthy, CD55 also appears forms of the disease.

19-22 Ekim 2011, Ankara 67