RECOMBINANT SOLUBLE TRIMERIC CD40 LIGAND IS BIOLOGICALLY-ACTIVE

CD40 ligand (CD40L) is expressed on the surface of activated CD4(+) T cells, basophils, and mast cells. Binding of C40L to its receptor, CD4 0, on the surface of B cells stimulates B cell proliferation, adhesion and differentiation. A preparation of soluble, recombinant CD40L (Tyr -45 to Leu-261), containing the full-length 29-kDa protein and two sma ller fragments of 18 and 14 kDa, has been shown to induce differentiat ion of B cells derived either from normal donors or from patients with X-linked hyper-IgM syndrome (Durandy, A., Schiff, C., Bonnefoy, J.-Y. , Forveille, M., Rousset, F., Mazzei, G., Milili, M., and Fischer, A. (1993) fur. J. Immunol. 23, 2294-2299). We have now purified each of t hese fragments to homogeneity and show that only the 18-kDa fragment ( identified as Glu 108 to Leu-261) is biologically active. When express ed in recombinant form, the 18-kDa protein exhibited full activity in B cell proliferation and differentiation assays, was able to rescue of B cells from apoptosis, and bound soluble CD40. Sucrose gradient sedi mentation shows that the 18-kDa protein sediments as an apparent homot rimer, a result consistent with the proposed trimeric structure of CD4 0L. This demonstrates that a soluble CD40L can stimulate CD40 in a man ner indistinguishable from the membrane-bound form of the protein.