Natalizumab-Induced Circulating Hematopoietic Stem Cells Have Intact Progenitor Capacity (P1.196)

Objective: To assess the capacity of natalizumab to mobilise functional hematopoietic stem cells (HSC) from the bone marrow (BM) by evaluating the clonogenic potential of circulating HSC in natalizumab-treated multiple sclerosis (MS) patients. Background: Previous studies have observed an increase of peripheral HSC during natalizumab. We have recently shown that there are different HSC mobilization responses among natalizumab-treated MS patients associating with clinical response to the treatment. We also found natalizumab-induced circulating HSC to be predominantly quiescent (Mattoscio et al., submitted). Here we wanted to establish whether natalizumab-induced HSC have intact hematopoietic potential. Design/Methods: Lin-/CD34+ HSC were magnetically sorted from natalizumab (Tysabri®)-treated MS patients; CFU assays were performed to measure the growth of progenitors colonies from HSC cultured in growth factors-enriched methyl cellulose. Both un-manipulated PBMC or purified HSC were seeded in methylcellulose and transferred in 35mm Petri dishes in duplicates; CFUs were enumerated at a transmitted light inverted microscope after 14 days of incubation. The CFUs absolute number and the percentage of CFU derived from the seeded number of HSC or PBMC (CFC %), the CFC number x10^6 per sample and the cloning efficiency (CFUe %) were calculated. We aim to test 20 MS patients’ and 10 healthy donors’ (HD) samples; current analysis refers to 2 MS patients and 3 HD. Results: CFU number, CFC % and CFCx10^6 were higher in the MS patients compared to HD (T test p<0.05). The CFUe % was significantly higher in the MS patients’ samples than in HD9 (T test, p<0.05) suggesting an overall higher clonogenic potential of natalizumab-induced circulating HSC over HD HSC. Conclusions: This preliminary analysis shows natalizumab-induced HSC of MS patients are fully functional and have higher clonogenic potential than HD’ circulating HSC, suggesting the observed natalizumab-induced peripheral HSC increase has functional relevance. Analysis on a larger number of patients is warranted to confirm the current interpretations. Study Supported by: FISM Training Fellowships Ref. 2012/B/8, Project Grant Ref. 2010/R/24. Disclosure: Dr. Mattoscio has nothing to disclose. Dr. Mazzanti has nothing to disclose. Dr. Dal Pozzo has nothing to disclose. Dr. Nicholas has received personal compensation for activities with Biogen Idec and Novartis as a consultant, and with Merck Serono and Teva Neuroscience. Dr. Malik has received personal compensation for activities with Biogen Idec. Dr. Saccardi has received research support from Biosafe. Dr. Muraro has nothing to disclose.