Multiple myeloma (MM) patients often suffer from hematopoietic impairment already at the time of diagnosis with anemia as the prevailing symptom. Given the overt affection of the bone marrow in MM patients by the invasion of malignant plasma cells, we hypothesized that hematopoietic insufficiency in these patients may originate from a functional impairment of hematopoietic stem and progenitor cells.
Quantitative analysis of BM CD34+ HSPC cell subsets from MM patients and age-matched healthy donors showed a significant decline of all HSPC subsets including hematopoietic stem cells, common myeloid and lymphoid progenitors, granulocyte-macrophage progenitors and megakaryocyte-erythrocyte progenitors in MM patients. The greatest diminution was observed in megakaryocyte-erythrocyte progenitors (MEP) which were 4.9-fold reduced in comparison to healthy donors.
Transcriptional analyses of CD34+ HSPC subsets revealed a significant deregulation of signaling pathways that was particularly striking for TGF beta signaling and suggested increased activation of this signaling pathway. Immunhistochemical staining of phosphorylated smad2, the downstream mediator of TGF receptor I kinase activation, in bone marrow sections and immunoblotting of purified CD34+ HSPC of MM patients confirmed the overactivation of TGF beta signaling.
On a functional level, we observed significantly reduced long-term self-renewal and clonogenic growth, particularly of the erythroid precursors BFU-E and CFU-E, in CD34+ HSPC of MM patients which could be restored by inhibition of TGF beta signaling. Proliferation and cell cycle analyses revealed a significantly decreased proliferation activity in CD34+ HSPC and, particularly, MEP. Again, this was reversible after inhibition of TGF beta signaling.
In addition, the transcriptional analyses showed disturbance of pathways involved in the adhesion and migration of HSPC and the gene encoding for the principal hyaluronan receptor CD44 throughout the HSPC subsets. This was corroborated by immunofluorescence imaging of CD44 on HSPC subsets showing a marked downregulation in the patients' cells. In line, the adhesion of CD34+ HSPC subsets to hyaluronan and their migration towards SDF-1 was significantly inhibited.
Subsequent xenotransplantation of CD34+ HSPC from MM patients and healthy donors into myeloma-free recipients revealed even increased long-term engraftment of CD34+ HSPC obtained from MM patients and normal differentiation capacities suggesting that the observed functional alterations in fact depend on the MM-related bone marrow microenvironment.
Our data show that hematopoietic impairment in patients with multiple myeloma originates, at least in part, from functional alterations of hematopoietic stem and progenitor cells. These alterations seem to depend on the disease-related changes of the bone marrow microenvironment. Currently, experiments are underway to elucidate in more detail the role of the microenvironment and the responsible structures for the impairment of HSPC in MM patients. These data will be presented.
Kobbe:Celgene: Consultancy, Research Funding; Ortho Biotec: Consultancy.
Asterisk with author names denotes non-ASH members.