Profiling the impact of anti-human CD20 monoclonal antibodies on lymphocyte B cell subsets and their precursors in the bone marrow and in lymphoid tissues in an immunocompromised mouse engrafted with human cells

Ofatumumab (OFA) and ocrelizumab (OCRE) are two anti-CD20 monoclonal antibodies approved for the treatment of relapsing forms of multiple sclerosis due to their ability to deplete B lymphocytes. The aim of this study was to investigate the impact of these anti-hCD20 antibodies on B lymphocyte subsets in the circulation and in primary and secondary lymphoid organs in an immune system humanized mouse model (immunocompromised Rag2^-/-Il2rg^-/-CD47^-/-) engrafted with human CD34+ hematopoietic stem cells. Three months after humanization, mice, which present adaptive immune cells only of human origin, were treated with OFA (0.3 mg/Kg; day 1, 3 and 5), or OCRE (10 mg/kg; day 1) or saline. Seven days after the last injection a robust (>90 %) decrease of circulating human CD20+ B lymphocytes was observed in both OFA- and OCRE-treated mice. A partial replenishment of B lymphocytes was detectable in blood 36 days from the last injection in OFA-treated mice, while no B lymphocytes could be detected in OCRE-treated mice up to 65 days post injection. Bone marrow profiling showed that during hCD20+ B cell depletion and replenishment, OCRE-treated mice preserved only preB-I cells in the bone marrow, while the bone marrow of OFA-treated mice presented both preB-I as well as preB-II cells, with the latter subset being the one closest to differentiate into immature B cells. These data together with changes in B cell distribution in other tissues suggest that ofatumumab preserve BM niches, critical for B lymphocyte replenishment, limiting potential side effects of the treatment associated with the increased risk of infection.