Hematopoietic Stem/Progenitor Cells (HSPC) Mobilization Parameters in Patients Chronically Treated with the CD49d Blocking Antibody Natalizumab.

Blockade of CD49d-mediated lymphocyte trafficking has been used therapeutically for certain autoimmune diseases. In addition to effects on mature lymphocytes, in mice and monkeys CD49d blockade also mobilizes immature hematopoietic cells from bone marrow (BM), capable of complete long-term engraftment despite a partial BM seeding defect. The aim of these studies was to ascertain effects on the biology of HSPC mobilization of single or chronic CD49d blockade in Multiple Sclerosis (MS) patients receiving disease-modifying treatment with the anti-functional anti-CD49d antibody, Natalizumab. These studies represent the first observations on HSPC mobilization by anti-CD49d antibodies in humans and on chronic exposure to a mobilizing agent. Circulating CD34+ cells (1.8±0.4/μL) and CFU-C (638±128/mL) were normal in MS patients (n=9) prior to the first Natalizumab infusion (normal controls: 1.3±0.1/μL CD34+ cells, 608±129/mL CFU-C). In chronically treated patients (i.e patients who had received ≥5 prior doses of Natalizumab) HSPC numbers were significantly elevated 30 days after the last infusion (CD34+ cells 9.0±1.2/μL, CFU-C 3243±332/mL, n=10, p<0.005). The blood of chronically Natalizumab-treated subjects also contained SCID-repopulating cells (n=3). After the first Natalizumab infusion, peripheral blood CD34+ cells and CFU-C were significantly increased over pre-treatment values (CD34+ cells 1.6±0.2/μL vs. 8.0±2.1/μL, CFU-C 414±161/mL vs. 2560±726/mL, n=7, p<0.005), indicating that a single dose of Natalizumab was sufficient to elicit similar mobilization levels as in chronic recipients. Renewed Natalizumab infusion in “chronic” Natalizumab-recipients did not result in additional mobilization, compared to values from just before that infusion (CD34+ cells 7.9±1.7/μL vs. 7.9±0.9/μL, CFU-C 3133±335/mL vs. 3525±305/mL, n=4). These data indicate that in the absence of CD49d-mediated BM retention a new equilibrium is established between HSPC in BM and in circulation. In long-term Natalizumab recipients a total of ca. 50x10E6 CD34+ cells (8 CD34+ cells/μL * 6L of blood) are in circulation at any given time. Considering the documented short half-life of circulating HSPC, (100 min in the mouse) this implies that approximately 700x10E6 CD34+ cells/day are trafficking in the average Natalizumab recipient. Whether the elevated numbers of circulating HSPC play a clinically relevant role for Natalizumab-treated MS patients is currently unclear. Given reports of MS flares under G-CSF, we propose that “steady-state” apheresis without additional mobilizing agents may be a safe and effective alternative to G-CSF if MS patients on Natalizumab are considered for autologous HSPC transplantation, an emerging treatment for refractory MS. The usefulness of Natalizumab as a mobilizing agent is likely restricted to the autologous setting, given the risk of immunization against Natalizumab and its prolonged effects. Instead, short-acting small-molecule inhibitors of CD49d could be developed as a mobilizing strategy for patient groups intolerant or unresponsive to G-CSF, alone or in combination with other novel mobilizing agents.