Ectopic MHC II expression drives B-cell development. (A) Competitive in vitro differentiation assay between CD45.1⁺ WT HSCs transduced with green fluorescent protein (GFP), CD45.2⁺H2^dlAb1-Ea HSCs transduced with GFP, and CD45.2⁺H2^dlAb1-Ea HSCs transduced with MHC II Ab1 and Aa chains. MHC II expression is shown 2 days after transduction of the respective HSCs, distinguished by GFP and CD45 allotype expression. (B) Expression of B-cell lineage markers B220, CD24, and PB-1 and percentage of lineage-negative (Lin⁻) and Fr A-C cells in each of the HSC donors described in panel A, following growth in B-cell media (n = 2). FCS-A, forward scatter area.