SAR650984 directly induces multiple myeloma cell death via lysosomal-associated and apoptotic pathways, which is further enhanced by pomalidomide.

The anti-CD38 monoclonal antibody SAR650984 (SAR) is showing promising clinical activity in treatment of relapsed and refractory multiple myeloma (MM). Besides effector-mediated antibody-dependent cellular cytotoxicity and complement-mediated cytotoxicity, we here define molecular mechanisms of SAR-directed MM cell death and enhanced anti-MM activity triggered by SAR with Pomalidomide (Pom). Without Fc-cross-linking agents or effector cells, SAR specifically induces homotypic aggregation (HA)-associated cell death in MM cells dependent on the level of cell surface CD38 expression, actin cytoskeleton and membrane lipid raft. SAR and its F(ab)'2 fragments trigger caspase 3/7-dependent apoptosis in MM cells highly expressing CD38, even with p53 mutation. Importantly, SAR specifically induces lysosome-dependent cell death (LCD) by enlarging lysosomes and increasing lysosomal membrane permeabilization associated with leakage of cathepsin B and LAMP-1, regardless of the presence of interleukin-6 or bone marrow stromal cells. Conversely, the lysosomal vacuolar H+-ATPase inhibitor blocks SAR-induced LCD. SAR further upregulates reactive oxygen species. Pom enhances SAR-induced direct and indirect killing even in MM cells resistant to Pom/Len. Taken together, SAR is the first therapeutic monoclonal antibody mediating direct cytotoxicity against MM cells via multiple mechanisms of action. Our data show that Pom augments both direct and effector cell-mediated MM cytotoxicity of SAR, providing the framework for combination clinical trials.

Expression and regulation of CD30 ligand and CD30 in human leukemia-lymphoma cell lines.

The CD30 antigen was originally described as a specific surface marker for Hodgkin's lymphoma. Recent work established CD30 as a member of the tumor necrosis factor/nerve growth factor receptor superfamily whose ligand (CD30L) has also been cloned and expressed; CD30L is active as membrane-bound type II glycoprotein. Here, CD30L mRNA expression was studied in a panel of 102 continuous human leukemia-lymphoma cell lines and was found only in four Burkitt lymphoma, one Burkit-type acute lymphoblastic leukemia and one non-Hodgkin's lymphoma (NHL) cell line. The product of CD30L mRNA is expressed as a membrane protein on the surface of these malignant B-cell lines. Treatment of these cell lines with soluble CD27L, phorbol ester or staphylococcus aureus Cowan antigen resulted in the enhancement of cell surface CD30L protein expression. CD30L mRNA was not detected in normal unstimulated peripheral blood (PB) monocytes, monocyte-derived macrophages, or T-cells, but was detected in primary granulocytes; exposure to activating reagents induced and upregulated CD30L transcription in these different PB populations. While CD40 and CD30L surface protein expression on PB monocytes could be enhanced or induced by treatment with gamma-interferon, these cells remained negative for CD30, both at the mRNA and at the protein level. Similarly, PB monocyte-derived macrophages and granulocytes remained negative for CD30 mRNA and protein expression, regardless of stimulation. Only activated T-cells expressed CD30 mRNA and surface protein. CD30L-transfected cells and cells constitutively expressing CD30L delivered a similar stimulus for proliferation of the CD30+ Hodgkin's disease (HD)-derived cell line HDLM-2, but inhibited proliferation of the CD30+ large cell anaplastic lymphoma cell line KARPAS-299. These data provide strong evidence for the involvement in growth regulation of recombinant and natural CD30L through its interaction with the CD30 receptor. Collectively, these data suggest that the CD30L-CD30 interaction has potent biological activity and might play a critical role in the immune response and pathogenesis of HD and some NHL, in particular Burkitt lymphomas.

Expression of the FLT3 gene in human leukemia-lymphoma cell lines.

The FLT3 gene encodes a protein that appears to function as a receptor for a hematopoietic growth factor; together with the KIT and FMS receptors, FLT3 belongs to the superfamily of receptors with tyrosine kinase activity. We examined the expression of FLT3 mRNA in 36 human leukemia-lymphoma cell lines using Northern blot analysis. FLT3 transcripts were found in seven of seven pre B-ALL cell lines (derived from cases with pre B-acute lymphoblastic leukemia or chronic myeloid leukemia in lymphoid blast crisis), and in one of six B-cell lines (namely in a cell line established from a hairy cell leukemia). FLT3 message was not detected in five T-cell, five myeloid, four monocytic, four erythroid and five megakaryocytic cell lines. Two major mRNA species were expressed differentially by positive cell lines. KIT mRNA expression was also investigated in the same panel of cell lines, but was found only in cell lines with erythroid and megakaryocytic features (and not in any of the FLT3-positive cell lines). The pattern of expression of FLT3 contrasts with the transcription of FMS and KIT and suggests that the FLT3 product may play a role primary in immature lymphoid cells.