Monoclonal antibodies to human tumor nucleolar antigens: probes for studying biological function and determining clinical significance

Using novel strategies, monoclonal antibodies (MAbs) were developed to two proliferation-associated nucleolar antigens. These two new antigens, termed P145 and P120, according to their molecular weights, were found in a broad range of tumor tissues. Overall, P120 was more restricted in its distribution than P145 because it was absent from many benign tumors and some malignant tumors. Using the MAbs as probes, antigens P145 and P120 were localized to separate nucleolar components. P145 was mainly associated with high molecular weight nucleolar RNPs. Antigen P120 was localized to a novel beaded microfibril found in a nucleolar residue fraction. DNAse and high salt treatment of this fraction provided optimal extraction of the P120 antigen.

P145 and P120 differed from other proliferation-associated nuclear/nucleolar antigens identified by autoimmune sera and by other MAbs. In PHA stimulated lymphocytes, both antigens were expressed in early G₁ prior to or concurrent with increased RNA Pol I transcription. Microinjection of tumor cells with the P120 MAb inhibited cell proliferation and blocked the appearance of nucleolar pleomorphism that is characteristic of tumor cells.

Clinical studies showed that P145 was not detected in normal bone marrow but was expressed in leukemic marrows (AML); the percentage of P145 positive cells correlated with percentage of malignant blasts (Raza et al., Am. Soc. Hematology, 1990). Antigen P145 was detected in 10 of 39 marrows from patients in complete remission. This result suggested that these cells have a higher proliferative potential than normal stem cells. P120 was expressed in 54% of breast cancers examined; 92% of these patients died, whereas only 28% of the survivors had P120 positive tumors, suggesting that expression of P120 correlates with poorer patient prognosis. The P120 MAb was used to isolate cDNA clones to P120. Using the P120 cDNA as a probe it was found that the level of P120 mRNA in tumor cells was increased as much as 60–80-fold by comparison to normal resting cells. These studies indicate that the presence of P145 and P120 relates to the hyperactivity of nucleolar function in rapidly proliferating cells. MAbs to these antigens are being used as tools to further assess the biological and clinical significance of these antigens.