Purification of immunoprotective tumor antigens by preparative isotachophoresis

Tumor rejection antigens of chemically induced sarcomas of inbred mice

Chemically induced sarcomas of inbred mice are immunogenic in syngeneic hosts, and preimmunization with tumor cells leads to resistance to subsequent tumor transplants. The tumor rejection antigens (TRAs) that mediate this reaction are highly specific for each tumor; cross-protection between different syngeneic sarcomas is rare. Isolated membrane and cytosol fractions from two antigenically distinct BALB/c sarcomas, Meth A and CMS5, have TRA activity, and biochemical characterization of the active components from the cytosol and plasma membranes of these two tumors identified a glycoprotein of Mr 96,000. Immunization with unfractionated Meth A cytosol frequently leads to tumor enhancement, but the tumor-enhancing activity (TEA) is lost on fractionation and TRA activity becomes demonstrable. As Meth A and CMS5 lack expression of murine leukemia virus (MuLV) antigens or transcripts, MuLV-related antigens cannot be involved in the TEA or TRA activities of these tumors. In contrast to the lack of cross-reactivity between Meth A and CMS5 TRAs in transplantation tests, rabbit antiserum prepared against the Meth A Mr 96,000 antigen reacted with the CMS5 Mr 96,000 antigen. In view of the biochemical and antigenic similarities of Meth A and CMS5 TRAs, we propose that structurally related but distinct Mr 96,000 glycoproteins are expressed in chemically induced sarcomas and that these molecules are responsible for the individually specific immunogenicity of these tumors.

The effect of purification on the immunogenicity of tumor-specific transplantation antigens

Immunization with the tumor-specific transplantation antigens (TSTA) of experimental, chemically induced sarcomas engenders specific host resistance to challenge with viable, homotypic neoplastic cells. The strength of tumor resistance depends upon the physical state of the TSTA used for immunization. Treatment with 10(5)-10(6) irradiated tumor cells, a 2-log dose range, induces complete rejection of neoplastic challenges, while immunization within a 1-log dose range with crude 3 M KCl or with 2.5% butanol extracts containing TSTA evokes a weak state of resistance characterized by decreased outgrowth of tumor challenges, but not neoplastic regression. The reduced immunogenicity may be due to either contamination with substances that antagonize host resistance, for example by induction of suppressor cells, or an intrinsic limitation by virtue of the molecular properties of extracted compared with cell-surface TSTA. MCA-F and MCA-D, two noncross-reactive fibrosarcomas induced in C3H/HeJ mice with 3-methylcholanthrene, were employed to compare the relative immunogenic activity of intact tumor cells, 2.5% butanol extracts, and materials sequentially purified by preparative isoelectric focusing (pIEF), preparative isotachophoresis (pITP), and high performance gel permeation chromatography (HPGPC). Immunoprotective TSTA activity purified 50,000-fold by this protocol extended the effective dose range by four to five logs: 15 pg to 1.5 micrograms MCA-F or 1 pg to 10 ng MCA-D antigen-induced specific host resistance. However, despite the appreciable purification of TSTA, immunization with extracted materials only delayed neoplastic outgrowth. They induced neither immediate rejection nor only temporary progression of transplanted tumor cells. Thus, purified TSTA preparations by themselves lack the immunogenic properties of intact cells that result in maximal induction of tumor resistance.