Interactions Between BAFF and BAFF Receptors Are Involved in Macrophage-Induced Bortezomib Resistance in Myeloma

Background:B-cell-activating factor (BAFF) is a member of the TNF family that critical for maintenance of B-cell development and homeostasis. BAFF receptor (BAFF-R), B-cell maturation antigen (BCMA) and transmembrane activator and CAML interactor (TACI) are three BAFF receptors. It has been reported that BAFF is expressed by neutrophils, monocytes, dentritic cells and macrophages and modulates the proliferation, survival and drug resistance of multiple myeloma (MM) cells. Our previous study showed that, macrophages protect MM cells from drug-induced apoptosis by direct interaction with MM cells. We hypothesized that BAFF/BAFF receptors play a role in macrophage-induced bortezomib resistance in myeloma.

Methods: First, the expression levels of BAFF and its three receptors in primary MM cells, MM cell lines and peripheral blood monocyte(PBMC)-induced macrophages were detected by semiquantitative real time-polymerase chain reaction (qPCR),Western blot and flow-cytometry. Also the concentration of BAFF in the supernatants of MM patients' bone marrow, MM cell lines and macrophages were determined by ELISA. Second, Primary MM cells and MM cell lines were cocultured with macrophages for the indicated time (usually 4-6h and 24h), for some experiments, we added bortezomib to the coculture system. Cell viability and apoptosis of MM cells were verified by Cell Counting Kit-8(CCK8) after treated with recombinant human (rh) BAFF, BAFF neutralizing antibody and BAFF siRNA. The interactions between BAFF and its receptors are unveiled by flow-cytometry. Then, cell survival signaling activations that may confer MM drug resistance were examined by Western blot.

Results: Two receptors of BAFF, TACI and BCMA were highly expressed in various MM cell lines. The expressions of BAFF in PBMC-induced macrophages were heterogeneous. Functional studies showed that rhBAFF promoted RPMI8226 and ARP1 myeloma cells growth (P<0.05) and protected them from bortezomib-induced apoptosis (P<0.05). Then we verified macrophage-mediated MM drug resistance by directly coculturing MM cells (ARP-1, RPMI8226) with PBMC-derived macrophages from healthy donors. The macrophage-induced bortezomib resistance was attenuated by neutralizing antibodies(P<0.05) and siRNA of BAFF(P<0.01) . Next we found that in MM cells cocultured with macrophages, bortezomib-induced PARP and caspase-3 cleavages were highly repressed and phosphorylated Src ,AKT and Erk1/2 were upregulated which indicated that BAFF-mediated MM drug resistance may be through ERK1/2 and Src pathway .In addition, BAFF induced activation of NF-κB2,a pathway critical for the growth and survival of these cells.

Conclusions: Our data show that macrophage might induce drug resistance of MM cells by the interaction of BAFF and BAFF receptors, leading to a reduction in caspase proteins and subsequent activation of Src and Erk1/2 kinases and NF-κB2 pathways .These studies reveal a promising unknown role for BAFF/BAFF receptors, suggesting that targeting macrophage-MM interactions may represent a promising therapeutic modality.