Preserved B Cell Function in CD23-Cre Transgenic Mice Allows Reliable Flox-Only Controls in Mouse Models

B cells are pivotal for adaptive immunity by mounting antigen-specific humoral protection. They form unique B cell-receptors, enabling them to differentiate into effector cells upon antigen activation. T-independent B cell activation results in rapid IgM production, whereas T-dependent activation initiates a germinal center reaction in which memory B cells and plasma cells that produce high-affinity antibodies are generated. However, dysregulated B cells lead to diseases like autoimmunity or immunodeficiency, highlighting the importance of understanding B cell mechanisms. As immune responses are complex, in vivo models are essential. Conditional knockout-mouse models using the Cre-loxP system allow cell-type-specific gene deletion and subsequent analysis of protein functions. Hereby, CD23-Cre transgenic mice are used to study mature B cell responses. Unfortunately, the CD23-Cre transgene was inserted randomly, and Cre-associated toxicity has been reported in other mouse strains. Therefore, off-target effects cannot be excluded, so most studies use Cre-only mice as controls. Unfortunately, breeding Cre-only mice and knockout littermates yields significant numbers of non-experimental genotypes and contradicts the 3R-rule for animal welfare. Here, we demonstrate that CD23-Cre-only mice show no signs of Cre-mediated toxicity and establish a reliable mature B cell compartment. T-independent activation of CD23-Cre-only B cells results in normal plasmablast differentiation and IgM production. Furthermore, CD23-Cre-only mice demonstrate robust vaccination-induced T-dependent germinal center responses and stable antigen-specific humoral immunity. Thus, the CD23-Cre transgene leaves the B cell compartment fully functional and enables the use of flox-only controls in immunological studies. This reduces the number of surplus animals generated and supports the 3Rs principle in animal experiments.