Complement-regulated homeostatic proliferation controls memory B cell longevity and repertoire composition.

Memory B cell (Bmem) survival is essential for guarding against reinfection, yet processes ensuring their longevity remain unclear. As decay-accelerating factor (DAF, CD55), a negative regulator of complement activation, is requisitely downregulated on germinal center B cells and is reexpressed on Bmem, we investigated the effects of deleting DAF on murine (B1-8hi) Bmem in competitive settings. Kinetic analysis showed a progressive reduction in DAF-/- Bmem numbers over 6 wk, without affecting Bmem production, pool size, or their ability to respond to rechallenge. Following transfer into unimmunized hosts, wild-type Bmem proliferated to maintain stable Bmem pool sizes, outcompeting DAF-/- Bmem, reflecting homeostatic proliferation. Reduced proliferation and increased cell death in DAF-/- Bmem associated with transcriptional differences in metabolism and migration pathways. Wild-type Bmem proliferation increased in C3-/- hosts, and vaccination with a heterologous antigen, which induces local complement activation, locally inhibited bystander B1-8hi Bmem proliferation. Thus, complement-dependent regulation of Bmem homeostatic proliferation influences Bmem longevity and repertoire composition in mice.