A new proliferation-associated nuclear antigen detectable in paraffin-embedded tissues by the monoclonal antibody Ki-S1.

A monoclonal antibody (Ki-S1) has been raised that reacts with the nuclei of proliferating cells. The antigen recognized is resistant to formalin fixation and can be detected in frozen tissues as well as in routinely processed specimens. In immunohistochemistry, nuclear staining can be seen in those tissues and cellular compartments known to be actively proliferating. Peripheral blood lymphocytes are negative but show a strong increase in antigen expression after mitogen stimulation. Flow cytometric determination of DNA content and antigen expression revealed negativity of G0 cells and positivity of G1 to G2/M cells. A cytoplasmic co-reactivity, not associated with proliferation, was confined to Langerhans islands of the pancreas. The nuclear localized antigen has a molecular mass of 160 kd and therefore seems to be different from all other known immunohistochemical markers of proliferating cells. We conclude that the monoclonal antibody Ki-S1 might provide a useful tool for studying cell proliferation in situ under normal and pathological circumstances.