Immunosuppressor T cells (IST),6 capable of inhibiting the rejection of a methylcholanthrene-induced fibrosarcoma (S1509a) in A/Jax mice immune to this tumor, produced soluble factors with similar suppressive activity. The immunosuppressive factor(s) (ISF) has been shown to be immunologically specific, as demonstrated by the complete loss of suppressive activity after absorption with S1509a cells, but not with cells of an unrelated syngeneic tumor. From its behavior on gel filtration, the size of ISF was deduced to be less than 70,000 daltons. The ISF was not removed by passage through a rabbit anti-mouse F(ab′)2 reverse immunosorbent and, hence, it was concluded that ISF was not likely to be an immunoglobulin. The (immunosuppressive) activity of ISF was destroyed by treatment with Pronase, but not with RNase. The ISF was found to share the antigenic determinant(s) of the product(s) of the K end of the major histocompatibility complex of the mouse. Moreover, antibodies to ISF were induced by immunization with ISF-tumor cell complexes. Thus, IST and their factor(s) appear to play an important role in the regulation of the immune response to tumor antigens.


This work was supported by grants from the Medical Research Council of Canada, the National Cancer Institute of Canada and the National Institutes of Health (CA-13192), Bethesda, Maryland.


Abbreviations used in this paper: TBH, tumor-bearing host; FCS, fetal calf serum; s.c., subcutaneous; IST, immunosuppressor T cells; ISF, immunosuppressive factor; MHC, major histocompatibility complex; FCS, fetal calf serum; BBS, borate-buffered saline; FCA, Freund's complete adjuvant.

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