Crucial Role of Linear Ubiquitin Chain Assembly Complex-Mediated Inhibition of Programmed Cell Death in TLR4-Mediated B Cell Responses and B1b Cell Development.

Linear ubiquitin chain assembly complex (LUBAC)-mediated linear polyubiquitin plays crucial roles in thymus-dependent and -independent type II Ab responses and B1 cell development. In this study, we analyzed the role of LUBAC in TLR-mediated B cell responses. A mouse strain in which LUBAC activity was ablated specifically in B cells (B-HOIP^Δlinear mice) showed defective Ab responses to a type I thymus-independent Ag, NP-LPS. B cells from B-HOIP^Δlinear mice (HOIP^Δlinear B cells) underwent massive cell death in response to stimulation of TLR4, but not TLR9. TLR4 stimulation induced caspase-8 activation in HOIP^Δlinear B cells; this phenomenon, as well as TLR4-induced cell death, was suppressed by ablation of TRIF, a signal inducer specific for TLR4. In addition, LPS-induced survival, proliferation, and differentiation into Ab-producing cells of HOIP^Δlinear B cells were substantially restored by inhibition of caspases together with RIP3 deletion, but not by RIP3 deletion alone, suggesting that LPS stimulation kills HOIP^Δlinear B cells by apoptosis elicited via the TRIF pathway. Further examination of the roles of cell death pathways in B-HOIP^Δlinear mice revealed that deletion of RIP3 increased the number of B1 cells, particularly B1b cells, in B-HOIP^Δlinear mice, indicating that B1b cell homeostasis is controlled via LUBAC-mediated suppression of necroptosis. Taken together, the data show that LUBAC regulates TLR4-mediated B cell responses and B1b cell development and/or maintenance by inhibiting programmed cell death.

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