Monoclonal antibody specific for histone H1 phosphorylated by cyclin-dependent kinases: A novel immunohistochemical probe of proliferation and neoplasia

Monoclonal antibody 12Dll (MAb 12Dll) has been shown to bind histone H1 isolated from human placenta and other tissues but not histone H1 that has been digested with bacterial alkaline phosphatase. We show here that phosphorylation of phosphatase-treated histone H1 with cyclin dependent-kinase (CDK) restores binding by MAb 12Dll. We conclude that MAb 12Dll selectively binds histone H1 that has been phosphorylated by CDKs, and we have investigated the use of MAb 12Dll as an immunohistochemical probe of CDK activity in situ. Previous immunofluorescence studies have revealed strong nuclear staining by MAb 12Dll in proliferating cultured cells and the absence of staining in terminally differentiated cells. Immunohistochemical staining of frozen and formalin-fixed, paraffin-embedded sections of benign tissues with MAb 12Dll was nuclear and confined to recognized foci of cell proliferation. In lymphoid germinal centers, MAb 12Dll preferentially stained large lymphoid cells with a relative lack of staining in small cleaved cells, contrasting with a lack of cell size discrimination observed with the monoclonal antibody proliferation probe, MIB-1. Tumor tissues displayed strong albeit heterogeneous staining of malignant cells by MAb 12Dll, with little or no staining observed in surrounding nonneoplastic stromal cells. Differential staining by MAb 12Dll of invasive and in situ carcinoma suggest applications in prognostication. MAb 12Dll may also be useful in identification of tumors more likely to respond to therapeutic CDK inhibitors.