Regulation of BCR-mediated Ca2+ mobilization by MIZ1-TMBIM4 safeguards IgG1+ GC B cell–positive selection.

The transition from immunoglobulin M (IgM) to affinity-matured IgG antibodies is vital for effective humoral immunity. This is facilitated by germinal centers (GCs) through affinity maturation and preferential maintenance of IgG⁺ B cells over IgM⁺ B cells. However, it is not known whether the positive selection of the different Ig isotypes within GCs is dependent on specific transcriptional mechanisms. Here, we explored IgG1⁺ GC B cell transcription factor dependency using a CRISPR-Cas9 screen and conditional mouse genetics. We found that MIZ1 was specifically required for IgG1⁺ GC B cell survival during positive selection, whereas IgM⁺ GC B cells were largely independent. Mechanistically, MIZ1 induced TMBIM4, an ancestral anti-apoptotic protein that regulated inositol trisphosphate receptor (IP3R)–mediated calcium (Ca²⁺) mobilization downstream of B cell receptor (BCR) signaling in IgG1⁺ B cells. The MIZ1-TMBIM4 axis prevented mitochondrial dysfunction–induced IgG1⁺ GC cell death caused by excessive Ca²⁺ accumulation. This study uncovers a unique Ig isotype–specific dependency on a hitherto unidentified mechanism in GC-positive selection. Editor's summary: Positive selection of high-affinity immunoglobulin G (IgG)⁺ B cells within germinal centers (GCs) promotes potent humoral immune responses, yet how the positive selection of IgG1⁺ GC B cells is regulated is not well defined. By studying mice with a GC B cell–specific deletion of the transcription factor MIZ1, Zhang et al. identified that MIZ1 was required for affinity maturation of IgG1⁺ GC B cells and high-affinity antibody production. MIZ1 promoted expression of the gene encoding the anti-apoptotic protein TMBIM4, which limited calcium mobilization downstream of BCR activation, and prevented mitochondrial dysfunction and cell death. These findings demonstrate that MIZ1 expression ensures the survival of IgG1⁺ GC B cells during positive selection. —Hannah Isles [ABSTRACT FROM AUTHOR]