SAN DIEGO—Preclinical research has revealed a mechanism of rituximab resistance in chronic lymphocytic leukemia (CLL), as well as a possible new treatment approach.
The study suggests the TNF-family member BAFF contributes to CLL cells’ resistance to direct and rituximab-induced natural killer (NK) cell reactivity.
However, the BAFF-specific antibody belimumab can overcome this resistance and restore CLL cells’ sensitivity to chemotherapy.
Julia Wild, a PhD student at Eberhard Karls University in Tuebingen, Germany, and her colleagues presented these findings at the AACR Annual Meeting 2014 as abstract 148.
“It is widely known that NK cells’ ability to react against lymphoblastic leukemia is impaired, as compared to myeloid leukemia,” Wild said. “And we now think we’ve identified a factor involved in this—a member of the tumor necrosis factor family called BAFF.”
Previous research showed that NK cells’ ability to target malignant cells and mediate antibody-dependent cellular cytotoxicity (ADCC) is compromised in CLL, but the underlying mechanisms were not clear. Wild and her colleagues therefore speculated that BAFF, which regulates B-cell proliferation and survival, plays a role.
The researchers’ initial experiments revealed that NK cells express BAFF at the mRNA level and release the protein in a soluble form, with levels depending on the state of activation.
The team then assessed BAFF expression in primary CLL cells. They collected cells from 17 CLL patients and analyzed surface expression of the 3 BAFF receptors. All 17 samples were positive for BAFF-R and TACI, and about 53% (9/17) were positive for BCMA.
Additional experiments revealed that BAFF enhances the metabolic activity of primary CLL cells. To uncover this result, the researchers incubated the cells with increasing concentrations of BAFF. And they compared untreated cells to cells treated with belimumab, isotype control, BAFF, BAFF plus belimumab, and BAFF plus isotype control.
The team then compared untreated CLL cells to cells treated with fludarabine and cyclophosphamide (Flu/Cy); Flu/Cy plus BAFF; and Flu/Cy, BAFF, and belimumab.
They found that BAFF protects CLL cells from chemotherapy-induced death. However, belimumab inhibits the protective effects of BAFF and restores CLL cells’ sensitivity to chemotherapy.
The researchers then performed cytotoxicity assays in primary CLL cells, comparing untreated cells to cells exposed to BAFF alone; rituximab alone; BAFF and rituximab; BAFF and belimumab; and rituximab, BAFF, and belimumab.
They found that lysis was highest among cells treated with rituximab alone or rituximab, BAFF, and belimumab, with comparable results in these 2 groups.
The team also evaluated CLL cell lysis due to ADCC induction. They analyzed peripheral blood mononuclear cells from CLL patients, comparing untreated cells to cells exposed to rituximab alone; rituximab and BAFF; rituximab, belimumab, and BAFF; and rituximab, bevacizumab, and BAFF.
Results showed that CLL cell survival was lowest (and comparable) in cells treated with rituximab alone or rituximab, belimumab, and BAFF.
“When CLL cells are cultured in the presence of BAFF, cytotoxicity is decreased as compared to untreated cells, and this is not only for direct NK-cell activity, but also for ADCC-mediated activity,” Wild summarized.
“But we can block this effect with belimumab, which suggests we could use this antibody for CLL treatment.”
