IMMUNOSUPPRESSIVE FACTOR FROM LIVER INDUCES APOPTOSIS IN THYMOMA EL-4CELLS BUT NOT NORMAL MHC CLASS II-SPECIFIC T-LYMPHOCYTES

An endogenously produced immunosuppressive factor (ISFnp, immunosuppre ssive factor-neutral protein), inducing a decrease in viability of thy moma EL-4 cells in vitro, was isolated from murine liver using ion exc hange, gel filtration and hydrogen-bonding chromatography. Polyclonal rabbit antibodies against this factor were developed and attached to p eriodate-activated Sepharose CL-6B. The immunoaffine sorbent obtained significantly depleted the biological activity of ISFnp from tested fr actions. 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 SD S-PAGE and Western blotting. ISFnp induced DNA degradation in EL-4 cel ls similar to the cleavage of DNA onto olygonucleosomal fragments in d examethasone-treated thymocytes. This DNA degradation preceded lysis o f thymoma cells, suggesting an induction of apoptosis in ISFnp-treated EL-4 cells. Addition of the factor into primary mixed lymphocyte cult ure (MLC) strongly inhibited proliferative response but failed to indu ce any decrease in the ability of normal MHC class II-specific allorea ctive cells to respond in the secondary MLC. Moreover, addition of ISF np into primary MLC on the peak of proliferative response resulted in augmentation of secondary responses of primed cells as compared with t he same quantities of primed cells from untreated cultures. These resu lts suggest a possible role of liver both in deletion of transformed c lones of T lymphocytes and formation of allospecific memory T cells.