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A T-dependent factor in mouse serum suppressing lymphocytic anti-tumor cytotoxicity.

In a microcytotoxicity assay composed of pooled, urethane-induced lung adenoma cells from BALB/c mice and syngeneic lymphocytes sensitized in vivo to these tumour cells, the addition of serum either from adenoma-bearing or normal BALB/c mice resulted in a depression of the cytotoxic response. This suppressive factor was stable to heating at 56° for 30 min and was unaffected by freezing and thawing. Pre-treatment of tumour targets with serum prior to the addition of lymphocytes was not protective, but pretreatment of sensitized lymphocytes with serum inhibited their cytotoxic ability to the same extent as did treatment of the whole culture with serum. Suppressive activity could be removed from serum by absorption with sensitized, but not with unsensitized lymphocytes. While sera from syngeneic or adenoma-bearing nude mice were free of suppressive activity, the activity appeared in nude animals following thymus implantation. 
 
When whole BALB/c serum was fractionated on a Sephadex G-200 column, suppressive activity equivalent to that found in the whole serum was observed consistently in fractions eluting in the region of 14,000 dalton proteins. No activity could be found in this region when sera lacking suppressive ability were separated. Sodium dodecyl sulphate—polyacrylamide gel electrophoresis (SDS-PAGE) of Sephadex fractions representing the activity zone revealed a protein with a molecular weight of 14,000 daltons which was present in fractions from active sera, but absent from fractions of inactive sera. SDS-PAGE of Sephadex fractions without suppressive activity, i.e. not representing the activity zone, did not produce this band. Both the activity zone in the Sephadex profile and the SDS-PAGE band could be demonstrated with sera from thymus-restored nude mice. 
 
Our observations indicate the existence of a low-molecular weight protein in the circulation of normal BALB/c mice which is capable of in vitro suppression of cell-mediated anti-tumour cytotoxicity. The presence of this factor appears to be thymus-dependent, and it seems to exert its effect by binding to sensitized killer lymphocytes.