Leukemic cells often express markers, which are not characteristic of their particular cell lineage. In this study, we identified the "A" subunit of coagulation factor XIII (FXIII-A) in leukemic promyelocytes in de novo AML M3 cases. The cytoplasmic presence of factor XIII-A has previously been shown only in platelets/megakaryocytes and monocytes/macrophages. Furthermore, more recently we described the presence of FXIII-A in leukemic lymphoblasts. We studied 14 patients with this rare type of acute leukemia in a period of 4 years and investigated their bone marrow samples by 3-color flow cytometry upon diagnosis, mainly focusing on FXIII-A expression of leukemic cells. We detected FXIII-A also by ELISA, Western-blot, and confocal laser scanning microscopy. This was a homogenous group of AML M3 patients with translocation t(15;17)(q22;q21) detected by fluorescence in situ hybridization (FISH). In 10 out of 14 samples, FXIII-A was detectable by flow cytometry and was coexpressed with markers characteristic for leukemic promyleocytes (CD45dim/CD13+/CD33+/CD117+/cyMPO+ and HLA-DR-/CD34-/CD14-/CD15-). Staining for the markers GPIIb and GPIX were negative, and FXIII-A was identified in the cytoplasm of the cells by confocal microscopy in a relatively high quantity, as measured by ELISA. By Western blot analysis we could identify FXIII-A in the native 82 kDa form and in cleaved forms corresponding to cleavage products observed when purified FXIII-A was treated by human neutrophil elastase. This novel expression site of FXIII-A in AML M3 can be considered as a leukemia associated immunophenotype and may have pathophysiological significance.
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