Single-Cell Analysis of FVIII-Specific Memory B Cells in Murine Hemophilia A.
Authors: Hausl Christina , Ahmad Rafi U. , Baumgartner Bernhard , Schwarz Hans Peter , Ehrlich Hartmut , Reipert Birgit M. 
Published: October 2019

Abstract
The elimination of FVIII-specific memory B cells is an essential step in the design of new therapeutic strategies for the induction of immune tolerance in hemophilia A with FVIII inhibitors. Using a mouse model of hemophilia A we recently reported that low dose FVIII stimulates the differentiation of FVIII-specific memory B cells into antibody-secreting plasma cells whereas high dose FVIII inhibits this process. The inhibition of memory-B-cell re-stimulation is irreversible and seems to be due to an induction of apoptosis. Further understanding of the complex interactions that lead to either re-stimulation and differentiation of memory B cells or inhibition and eradication of these cells requires appropriate technologies for single-cell analysis and functional studies. We established a new technology for single-cell analysis and cell sorting of FVIII-specific murine memory B cells. A combination of magnetic bead separation and multi-color flow cytometry enabled us to analyze and purify FVIII-specific memory B cells obtained from hemophilic mice treated with FVIII. In a first step, we depleted undesirable cell populations (IgM+, IgD+, CD11c+, F4/80+, Gr1+ and CD49b+ cells) from total spleen cells by magnetic bead separation. In a second step, we used multicolor flow cytometry to exclude CD4+ T cells and analyze the FVIII-specific memory B cell compartment. This compartment was specified by staining the specific B-cell receptor with FVIII and anti-IgG antibodies. Frequencies of cells in this compartment ranged from 0.1–0.5% of total spleen cells in animals treated with 4 intravenous doses of FVIII, given at weekly intervals. We could not detect any FVIII-specific memory B cells in naïve mice. By means of single cell sorting we isolated FVIII-specific memory B cells for further functional studies. We were able to cultivate FVIII-specific memory B cells in microwell cultures in vitro and differentiate them into antibody-secreting plasma cells. The re-stimulation and differentiation of single-cell sorted memory B cells was strictly dependent on the presence of activated CD4+ T cells. CD4+ T cells obtained from naïve mice did not support the memory response. Furthermore, the re-stimulation and differentiation of memory B cells in the presence of activated CD4+ T cells did not require additional dendritic cells for antigen presentation. Obviously, memory B cells provide sufficient antigen presentation to CD4+ T cells to enable them to trigger the memory response. Our approach for single-cell analysis and purification of FVIII-specific memory B cells provides a new tool for tracking memory B cell populations in vivo and for directly analyzing the regulation of memory B cell function. It opens the field for future studies which should elucidate signals and molecules involved in activation or inhibition and eradication of FVIII-specific memory B cells. These activities will eventually lead to the identification of targets for the design of new treatment strategies for patients with FVIII inhibitors.