Early events in TNFa - p55 receptor interactions-experiments with TNF dimers

The first essential step in TNF signal transduction is believed to be clustering of the membrane bound receptors around the trimeric TNF molecule. To check if one receptor binding site would be enough to trigger the signal, we tried to prepare several types of TNF dimer. For this purpose, two TNF analogs bearing different cysteine mutations at the inner subunit binding surfaces were designed, expressed in E. coli and prepared in pure form. By mixing equimolar quantities of these analogs under appropriate conditions, two different types of dimer were prepared. The first, Dim/S2, proved to be composed mainly of a disulfide-linked dimer, which was still capable of trapping the third subunit of either of the precursor analogs, thus showing relatively high residual cytotoxicity. To avoid trimeric structures, Dim/S2 was further transformed into Dim/Iaa2 by alkylation of -SH groups of the newly introduced cysteines, allowing binding of only two TNF subunits through native contact surfaces. These dimers showed substantially reduced cytotoxicity on the L929 cell line. In addition, it appears that Dim/Iaa2 is able to competitively inhibit cytotoxicity of native TNF, as assessed on the L-M cell line.