We describe here the biochemical properties of suppressor T cell factor (TsF2) released from an inducible anti-idiotypic T cell hybridoma (C57BL/6 T cell × BW5147) which mediates antigen (keyhole limpet hemocyanin) specific and genetically (H-2b) restricted suppression of IgG plaque-forming cell responses. We examined the suppressive activity by in vitro functional assay in fractions of chromatography and in the materials eluted from gels of sodium dodecyl sulfate polyacrylamide and isoelectric focusing and determined the molecular weight(s) (22–37 kD) and the isoelectric point(s) (pH 6.0–6.1) of this secreted factor. Messenger RNA products of the hybridoma translated in the rabbit reticulocyte lysate system were similarly examined, and the functionally active molecule was seen to migrate to almost the similar molecular weight(s) (23–40 kD), and isoelectric point(s) (pH 5.5–6.2) range as those of secreted TsF. Moreover, the TsF activity was recovered from gel slice corresponding to the similar molecular weight range analyzed in sodium dodecyl sulfate polyacrylamide gel electrophoresis under reducing and nonreducing conditions. Thus, we speculate that this molecule is composed of a single chain, biologically active. Comparison of autoradiograms on in vitro translation products between activated and resting hybridomas by two-dimensional gel electrophoresis showed that the molecular weights and isoelectric points of two spots (28 kD, 5.7; 25 kD, 5.5) newly appearing or markedly enhanced after activation in the area with suppressor activity were concordant with the data on secreted TsF, suggesting that one of these two spots represents the functional molecule which causes antigen-specific and genetically restricted suppression of IgG r
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