Immunosuppressive factor from liver induces apoptosis in thymoma EL-4 cells but not normal MHC class II-specific T lymphocytes

Abstract

An endogenously produced immunosuppressive factor (ISFnp, immunosuppressive factor-neutral protein), inducing a decrease in viability of thymoma EL-4 cells in vitro, was isolated from murine liver using ion exchange, gel filtration and hydrogen-bonding chromatography. Polyclonal rabbit antibodies against this factor were developed and attached to periodate-activated Sepharose CL-6B. The immunoaffine sorbent obtained significantly depleted the biological activity of ISFnp from tested fractions. The factor shows liver-specific location, an M_r of about 70–80 kDa and consists of 2 subunits (40 and 42 kDa) as determined by SDS-PAGE and Western blotting. ISFnp induced DNA degradation in EL-4 cells similar to the cleavage of DNA onto olygonucleosomal fragments in dexamethasone-treated thymocytes. This DNA degradation preceded lysis of thymoma cells, suggesting an induction of apoptosis in ISFnp-treated EL-4 cells. Addition of the factor into primary mixed lymphocyte culture (MLC) strongly inhibited proliferative response but failed to induce any decrease in the ability of normal MHC class II-specific alloreactive cells to respond in the secondary MLC. Moreover, addition of ISFnp into primary MLC on the peak of proliferative response resulted in aug-mentation of secondary responses of primed cells as compared with the same quantities of primed cells from untreated cultures. These results suggest a possible role of liver both in deletion of transformed clones of T lymphocytes and formation of allospecific memory T cells.