- Abstract CD40, a TNF receptor (TNFR) family member, is composed of four cysteine rich domains (CRDs) followed by a transmembrane domain and a signaling intracellular C-terminus. CD154 ligation to CD40 regulates major inflammatory and immune processes. A natural soluble form of CD40 was detected in uremic patient’s serum which might be alternative splicing product. Our aim was to identify CD40 isoforms produced by primary human cells and to evaluate their biological activity. By RT-PCR, we isolated from primary human cells three major products differing from the expected wild type (WT) transcript which lacks exon 5 (CD40-5), exon 6 (CD40-6) and both exons 5 and 6 (CD40-5 + 6). The first two were predicted computationally to be soluble decoy receptors with various CRDs numbers. Recombinant soluble CD40 (sCD40) isoforms containing either three or four CRDs are able to bind coated surfaces or membranous CD154 while the sCD40 isoform containing 2 CRDs did not. sCD40 proteins inhibited RANTES secretion, but did not block B cell proliferation induced by soluble CD154. These data suggest that sCD40 natural isoforms encompassing either three or four CRDs might exert different antagonistic effects from known CD154 antibodies by recognition of only membranous CD154.