Functional analysis of membrane TNF-α

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The membrane TNF-α expressed on the cell surface od CD4+ T cells is a novel candidate that transmit signals from T cells to B cells and vice versa. The 26-kDa membrane TNF-α was induced on activated human CD4+ lymphocytes. As the biological functions of the membrane TNF-α is still not well understood, we studied the reverse signal transmitted by membrane TNF-α. Activation by anti-TNF-α antibody (Ab) against membrane TNF-αresulted in the induction of an adhesion molecule, E-selectin (CD62E), on the CD4+ T cells with the peak of 12 to 24 h, which was completely disappeared at 48 h. When wild-type or mutant membrane TNF-α (R78T/S79T) resistant to proteolytic cleavage was introduced into Jurkat or HeLa ceils, E-selectin was introduced upon activation of membrane TNF-α with the similar kinetics. These results not only indicates that membrane TNF-αtransmits reverse signals into the cells expressing this molecule on the surface, but also presented for the first time that E-selectin was inducible in cell types different from endothelial cells. We then studied the functionally critical motif in the cytoplasmic domain of the membrane TNF-α for its reverse signaling. As cytoplasmic serine residues have been shown to be phosphorylated in several monocyte cell lines, the serine residues were supposed to be important for reverse signaling. We constructed a series of mutant membrane TNF-α that carries serine to alanine replacement by sitedirected mutagenesis and transfected into Jurkat cells. Then the membrane TNF-α on the s transfectants were stimulated by anti-TNF-α Ab, however E-selectin expression was not altered in these transfectants. It is thus concluded that none of the three serine residues in the cytoplasmic domain were responsible for the reverse signaling. We are now trying to find the binding protein(s) to the cytoplasmic domain of membrane TNF-α by using yeast two-hybrid system.