A proliferation-inducing ligand (APRIL) from the tumor necrosis factor (TNF) superfamily co-stimulates B-cell activation. When overexpressed in mice, APRIL induces B-cell neoplasia. 1 The promoting activity of APRIL for B-cell derived tumors has been recently reported in vitro for human Hodgkin’s lymphoma (HL) cell lines. 2 In this work, we sought to ascertain whether APRIL plays a role in the in situ development of HL with a particular emphasis on the pathway (s) involved.

We first confirmed the report from Chiu et al. 2 that soluble oligomerized APRIL, ACRP30-APRILA88, 3 provides a survival signal to the HL cell lines L428, L591 and KMH2 when placed under serum starvation. Molecularly, HL cell lines bound soluble APRIL (Figure 1a). APRIL binding to HL cell lines was inhibited in part by heparin, a soluble heparan sulfate proteoglycan (HSPG) characterized as a APRIL-binding partner. 4, 5 APRIL binding was also observed, although to a reduced level, with another soluble form of APRIL, APRILH98, lacking the membrane proximal domain interacting with HSPG. Hence, the transmembrane activator and calcium modulator and cyclophilin interactor (TACI) and/or the B-cell maturation antigen (BCMA) were also involved in APRIL binding in addition to HSPG. An anti-syndecan-1 and an anti-BCMA stained specifically the surface of the L428 HL cell line (Figure 1b). The L591 and KMH2 HL cell lines were also stained with the anti-BCMA but not with the anti-syndecan-1 (data not shown). As binding of APRILA88 on L591 and KMH2 was also inhibited by heparin (data not shown), this indicates that another proteoglycan proteic core carrying heparan sulfate side chains served as APRIL-binding partner at the surface of L591 and KMH2. In contrast to BCMA, the second APRIL receptor belonging to the TNF receptor (TNF-R) superfamily, TACI was not detected at the surface of the HL cell line. Absence of staining was observed with seven different anti-TACI monoclonal antibodies, all validated for flow cytometry on the IM9 B-cell line. BAFF-R, another member of the TNF-R family that may also bind to APRIL in some circumstances, 6 was also not expressed on the HL cell lines. Absence of TACI expression was confirmed by sensitive (40 cycles) reverse transcription-PCR (RT-PCR) analysis in L428 and KMH2, whereas L591 produced a detectable band for TACI mRNA (data not shown). The discrepancy between mRNA and surface protein expression for TACI in L591 warrants