KI-S1 AND PCNA EXPRESSION IN ERYTHROID PRECURSORS AND MEGAKARYOCYTES - A COMPARATIVE-STUDY ON PROLIFERATIVE AND ENDOREDUPLICATIVE ACTIVITY IN REACTIVE AND NEOPLASTIC BONE-MARROW LESIONS

The monoclonal antibody Ki-S1 reacts with a cell proliferation-associated nuclear antigen which is expressed in the G1 through G2/M phases of the cell cycle and is resistant to formalin fixation. We have studied Ki-S1 and PCNA (PC10) immunostaining of erythroid precursors (proliferative activity) and megakaryocytes (endoreduplicative activity) in bone marrow trephine biopsies in a variety of reactive and neoplastic lesions using double immunohistochemistry to identify both cell lineages. A significant increase in Ki-S1 labelling compared with PCNA positivity was found in all conditions studied. In particular, specimens derived from secondary polycythaemia (SP), polycythaemia vera (P. vera), and primary osteomyelofibrosis (OMF), and from splenic tissue with myeloid metaplasia (MM), revealed a disproportionally high labelling index of erytkropoiesis, which was not present in chronic myelogenous leukaemia (CML), AIDS, and autoimmune (idiopathic) thrombocytopenia (ITP). Enhancement of Ki-S1 (PCNA) staining in SP and P. vera is in keeping with the relevant increase in erythroid precursor proliferation, but in OMF and MM there is overexpression of both proliferation markers, possibly due to secondary folic acid deficiency, which is known to cause a block in the S-phase of the cell cycle. A significant correlation was observed between the sizes of megakaryocytes and their nuclei with Ki-S1 (and also PCNA) staining. Ki-S1 (and PCNA) labelling of predominantly smaller elements of this lineage supports a hypothesis that the phases of the cell cycle have different durations in the various steps of polyploidization, with a prolongation of G1/G2 at higher ploidy levels. A considerable number of large to giant (hyperpolyploid) megakaryocytes may have reached the end stage of endoreduplicative activity (Ki-S1-/PCNA-negative GO phase). A comparative evaluation of these two proliferation markers adds significantly to our understanding of cell kinetics in erythro- and megakaryo-poiesis, and sheds some light on the biological behaviour of these cell lineages in various disorders.