A Cytotoxic Monoclonal Anti-Leukemia Antibody Binds to Histone H1

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Abstract
Monoclonal antibody (MAb) AP64 is a mouse IgM MAb raised against human acute non-lymphocytic leukemia (ANLL) cells. It has been shown to bind to a wide variety of cell lines and is capable of initiating complement (C) dependent cytotoxicity. Other studies indicated that MAb AP64 can effect long term cure in a leukemia minimal residual disease model. By using various techniques we have determined the identity of a protein which is bound by this MAb. Immunofluorescent studies have shown that MAb AP64 stains the nuclei of fixed cells as well as metaphase chromosomes, indicating that this MAb binds to a component of chromatin. Biochemical characterization revealed that MAb AP64 western blots a 31 and 32 kilodalton doublet from NP-40 extracts from both rat and human leukemia cells. The mobility of this doublet is identical under reducing and non-reducing conditions. Further studies have shown that the bands detected by western blot analysis using MAb AP64 as a probe have a similar migration to those of bovine histone H1. Also, 1 nanogram of bovine histone H1 can be detected by MAb AP64 when spotted onto nitrocellulose. These data demonstrate that MAb AP64 binds to a conserved epitope present on molecules coded for by the histone H1 gene family.