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Monoclonal antibodies coupled to drugs and toxic agents (immunotoxins) or radionuclides (radioimmunoconjugates) represent new tools for immunotherapy of haematological malignancies. Immunotoxins constructed with toxins of either plant or bacterial origin have shown a powerful antitumor activity both in vitro and in mice with severe combined immunodeficiency bearing various kinds of leukaemias and lymphomas. Preliminary clinical trials have shown an activity of these compounds at least in a proportion of patients. However, tumour responses have generally been partial and transient. The main problems with immunotoxin therapy remain the inability of immunotoxins to target tumour cells in the presence of a high burden of disease, the host immune response against both the antibody and the toxin moieties, which precludes repeated administration of immunotoxins, and the vascular leak syndrome. Targeting of tumour cells with specific antibodies armed with radionuclides (usually iodine-131 or yttrium-90) appears to be an even more attractive approach. Preliminary clinical studies have clearly demonstrated the ability of radioimmunoconjugates, especially when administered at high dose followed by bone marrow rescue, to induce durable complete remission in patients with non-Hodgkin's lymphomas refractory to conventional therapies. Radioimmunotherapy also overcomes the antigenic heterogeneity of the tumour cell population, since antigen negative tumour cells will be irradiated by the nearby targeted antigen-positive cells. Efforts should now be focused on defining more precisely the optimal clinical setting for administration of immunotoxin and radioimmunoconjugates (e.g. minimal residual disease), to reduce the immunogenicity of these compounds and solve the problem of vascular leak syndrome.

The anti-CD30 immunotoxin (IT) Ber-H2/saporin is effective in patients with refractory Hodgkin's disease. However, responses are short and partial, one of the main reasons being the inability to repeat IT doses because of formation of human antibodies against the murine antibody and/or the toxin. To overcome this problem, we constructed two new anti-CD30 ITs by covalently linking the mouse monoclonal antibody Ber-H2 to the type 1 ribosome-inactivating proteins (RIPs) momordin (MOM) and pokeweed antiviral protein from seeds (PAP-S), which do not cross-react with each other or with saporin. Both ITs inhibited protein synthesis by Hodgkin's disease and anaplastic large-cell lymphoma (ALCL)-derived CD30+ target cell lines with a very high efficiency (IC50 ranging from5 x 10(-13) M to 2.75 x 10(-11) M, as RIP). In a SCID mouse model of xenografted CD30+ human ALCL, a 3d treatment with non-toxin doses of Ber-H2/MOM (50%LD50), started 24 h after transplantation, prevented tumour development in about 40% of the animals and significantly delayed tumour growth rate in the others. Main toxicity signs in mice and rabbits were dose-related increase of serum transaminases (AST and ALT) and creatine phosphokinase (CPK). LD50 (as RIP) in Swiss mice was 7 mg/kg for Ber-H2/MOM and 0.45 mg/kg for Ber-H2/PAP-S. Sequential administration of two anti-CD30 ITs (Ber-H2/MOM and Ber-H2/saporin) was well tolerated and did not result in formation of antibodies cross-reacting and with the two plant toxins. The results presented in this paper suggest that in the future, sequential administration of anti-CD30 humanized antibodies linked to antigenically distinct type 1 RIPs (saporin, MOM, PAP-S) should be feasible.

PG-M3 is a new monoclonal antibody (MoAb) specifically directed against a peptide sequence located in the aminoterminal region of the human PML protein. PML gene fuses with the retinoic acid receptor alpha (RAR alpha) gene during the t(15; 17) chromosomal translocation of acute promyelocytic leukemia (APL). The epitope recognized by PG-M3 is species-specific and fixative-resistant and is shared by most PML isoforms and PML/RAR alpha fusion proteins. PML is consistently located within the nucleus, although a minority of cells (about 20%), both in vitro and in vivo, show positivity for PML also in the cytoplasm. The nuclear staining pattern of PG-M3 varies from speckled (cells other than APL) to micropunctate (APL cells). Although two physiologically expressed PML isoforms are detectable by immunocytochemistry only or predominantly in the cytoplasm of transfected cells, the cytoplasmic localization of PML is a property also shared by the PML isoforms that predominantly localize to the nuclei. Immunohistologic analysis of normal human tissues with the PG-M3 MoAb showed variable PML expression, with the highest levels of the protein in postmitotic, differentiated cell types, such as endothelial cells, epithelia, and tissue macrophages, especially activated ones. In keeping with this in vivo finding, PML appears strongly upregulated in the U937 promonocyte cell line after exposure to agents that induce monocyte/macrophage activation (interferon gamma) or maturation (vitamin D3 and transforming growth factor beta 1).

The RING-finger promyelocytic leukemia (PML) protein is the product of the PML gene that fuses with the retinoic acid receptor-alpha gene in the t(15; 17) translocation of acute promyelocytic leukemia. Wild-type PML localizes in the nucleus with a typical speckled pattern that is a consequence of the concentration of the protein within discrete subnuclear domains known as nuclear bodies. Delocalization of PML from nuclear bodies has been documented in acute promyelocytic leukemia cells and suggested to contribute to leukemogenesis. In an attempt to get new insights into the function of the wild-type PML protein and to investigate whether it displays an altered expression pattern in neoplasms other than acute promyelocytic leukemia, we stained a large number of normal and neoplastic human tissues with a new murine monoclonal antibody (PG-M3) directed against the amino-terminal region of PML. As the PG-M3 epitope is partially resistant to fixatives, only cells that overexpress PML are detected by the antibody in microwave-heated paraffin sections. Among normal tissues, PML was characteristically up-regulated in activated epithelioid histiocytes and fibroblasts in a variety of pathological conditions, columnar epithelium in small active thyroid follicles, well differentiated foamy cells in the center of sebaceous glands, and hypersecretory endometria (Arias-Stella). Interferons, the PML of which is a primary target gene, and estrogens are likely to represent some of the cytokines and/or hormones that may be involved in the up-regulation of PML under these circumstances. In keeping with this concept, we found that PML is frequently overexpressed in Hodgkin and Reed-Sternberg cells of Hodgkin's disease, a tumor of cytokine-producing cells. Among solid tumors, overexpression of PML was frequently found in carcinomas of larynx and thyroid (papillary), epithelial thymomas, and Kaposi's sarcoma, whereas carcinomas of the lung, thyroid (follicular), breast, and colon were frequently negative or weakly PML+. We did not observe any changes in the levels of PML expression as the lesion progressed from benign dysplasia to carcinoma. Our immunohistological data are consistent with the hypothesized growth suppressor function of PML and strongly suggest that PML expression levels are likely to be modulated by a variety of stimuli, including cytokines and hormones.