Impaired Efferocytosis by Monocytes in Multiple Myeloma

Efficient clearance of apoptotic cells by efferocytosis is important for tissue homeostasis. Impaired efferocytosis leads to the accumulation of cell debris, which is regarded as a trigger in chronic inflammation and autoimmune diseases. Patients with hematological neoplastic disorders such as multiple myeloma (MM) exhibit high blood levels of apoptotic microparticles. The present study investigated whether these high levels of apoptotic microparticles are associated with insufficient dead cell clearance. Blood samples were collected from patients with MM immediately prior to and 3, 7 and 10 days after the initial cycle of bortezomib-based therapy. In addition, bone marrow aspirates (BMA) were collected prior to and following therapy. Prior to therapy, a 52% reduction in efferocytosis by blood monocytes was observed compared with the healthy controls (P<0.017). This was associated with an elevated number of 7-AAD dead cell remnants in the blood flow as well as in BMA. A portion of the blood samples contained active caspase 3. The subsequent bortezomib-based therapy had no effect on efferocytosis, although the quantity of dead cell remnants decreased. This reduction was associated with a decline in cluster of differentiation 8 (CD8) and CD4 T cells and an increase in the number of monocytes. However, of 28 distinct soluble immune-modulating molecules (i.e. chemokines, cytokines and soluble co-stimulators) only C-C motif chemokine ligand 2 (CCL2), CCL24 and sCD27 were affected by bortezomib-based therapy. The levels of all other molecules remained unchanged or were below the detection threshold in all samples. The present study results revealed that the presence of dead cell remnants in the blood and bone morrow of patients with MM is associated with impaired efferocytosis by monocytes; however, its contribution to inflammatory events during MM remains unclear.