Distributed apoptotic checkpoints in B cell selection, tolerance, and autoimmunity.

Generating a diverse B cell repertoire is essential for protective immunity, yet the required genetic alterations inevitably produce self-reactive, damaged, and non-competitive cells. To maintain immune homeostasis and ensure effective humoral immune responses, recent evidence supports the concept that a distributed network of apoptotic checkpoints operates at sequential stages of B cell development and activation. We discuss how these spatiotemporally distinct checkpoints collaborate with other tolerance mechanisms to regulate B cell selection and prevent systemic autoimmunity. Central tolerance mainly uses receptor editing to resolve high-avidity autoreactivity, while peripheral transitional B cell selection relies on B-cell activating factor and tonic B-cell receptor positive-selection signals, with cells failing to secure adequate signaling eliminated through apoptosis. Within germinal centers, apoptosis serves distinct functions in the dark and light zones, eliminating B cells damaged by somatic hypermutation and culling clones that fail to successfully acquire antigen and secure adequate T follicular helper cell help, respectively. We further discuss how extrafollicular responses provide a parallel, GC-independent pathway for autoreactive B-cell activation that is distinct from the stringent selection processes operating within germinal centers. Finally, we highlight evidence for an apoptosis-dependent process after activation that restricts the persistence of diversified autoreactive clones entering the memory B-cell and plasma-cell pools. Viewing B cell tolerance through the emerging conceptual framework of distributed apoptotic checkpoints may help explain how disruptions at distinct stages contribute to heterogeneous autoimmune pathologies and identify potential targets for therapeutic intervention.