HUMAN NATIVE SOLUBLE CD40L IS A BIOLOGICALLY-ACTIVE TRIMER, PROCESSEDINSIDE MICROSOMES

CD40 ligand (CD40L) is a glycoprotein expressed on the surface of acti vated helper T cells, basophils, mast cells, and eosinophils. Binding of CD40L to its receptor CD40 on the B cell surface induces B cell pro liferation, adhesion, and immunoglobulin class switching. We have iden tified soluble cleavage products of human CD40L in the supernatant of a stimulated human T cell clone. Subcellular fractionation experiments have shown that the transmembrane CD40L is processed inside the micro somes and that its cleavage is stimulation-dependent. The native human soluble CD40L is trimeric and, when used in conjunction with interleu kin-4, induces B cell proliferation.