LUBAC Suppresses IL-21-Induced Apoptosis in CD40-Activated Murine B Cells and Promotes Germinal Center B Cell Survival and the T-Dependent Antibody Response

Abstract

B cell activation by Tfh cells, i.e., through CD154 engagement of CD40 and IL-21, and survival within GCs are crucial for the T-dependent Ab response. LUBAC, composed of HOIP, SHARPIN, and HOIL-1, catalyzes linear ubiquitination (Linear M1-Ub) to mediate NF-κB activation and cell survival induced by TNF receptor superfamily members, which include CD40. As shown in this study, B cells expressing the Sharpin null mutation cpdm (Sharpincpdm) could undergo proliferation, CSR, and SHM in response to immunization by a T-dependent Ag, but were defective in survival within GCs, enrichment of a mutation enhancing the BCR affinity, and production of specific Abs. Sharpincpdm B cells stimulated in vitro with CD154 displayed normal proliferation and differentiation, marginally impaired NF-κB activation and survival, but markedly exacerbated death triggered by IL-21. While activating the mitochondria-dependent apoptosis pathway in both Sharpin+/+ and Sharpincpdm B cells, IL-21 induced Sharpincpdm B cells to undergo sustained activation of caspase 9 and caspase 8 of the mitochondria-dependent and independent pathway, respectively, and ultimately caspase 3 in effecting apoptosis. These were associated with loss of the caspase 8 inhibitor cFLIP and reduction in cFLIP Linear M1-Ub, which interferes with cFLIP poly-ubiquitination at Lys48 and degradation. Finally, the viability of Sharpincpdm B cells was rescued by caspase inhibitors but virtually abrogated – together with Linear M1-Ub and cFLIP levels – by a small molecule HOIP inhibitor. Thus, LUBAC controls the cFLIP expression and inhibits the effects of caspase 8 and IL-21-activated caspase 9, thereby suppressing apoptosis of CD40 and IL-21-activated B cells and promoting GC B cell survival.

Human LUBAC deficiency leads to autoinflammation and immunodeficiency by dysregulation in TNF-mediated cell death

“…5b and Extended Data Fig. 6b), consistent with a histological finding observed in B cellspecific Sharpin knock out mice 28 . These data suggest that the LUBAC deficiency results in a dysregulated secondary lymphoid organ structure irrespective of the degree of clinical immunodeficiency.…”

A Retrospective and Multicenter Study on COVID-19 in Inner Mongolia: Evaluating the Influence of Sampling Locations on Nucleic Acid Test and the Dynamics of Clinical and Prognostic Indexes

“…This may have been because imported patients were younger and thus recovered faster or because the drug treatment used for these patients was more effective than for domestic patients. The median incubation period was only 4 days (interquartile range, 2–7) in earlier COVID-19 patients in China ( 5 , 23 ). In contrast, the median incubation period was 8 days (interquartile range, 4–11), and the most prolonged incubation period was 18 days for native COVID-19 patients in Inner Mongolia.…”

Impaired B Cell Apoptosis Results in Autoimmunity That Is Alleviated by Ablation of Btk

“…Together, autoAb array, ELISA and ELISpot data demonstrate that SLE/SS characteristic autoAbs were elevated much more frequently among BBim fl/fl mice relative to WT controls. Dysregulation of B cells and autoAb production in SLE-like autoimmune disease are influenced by both innate (TLR) and adaptive (CD40) pathways and Tfh secreted IL-21 which regulates B cell differentiation into plasma cells, memory B cells and CD11c hi ABC B cells (50,54,55). IL-21 can regulate B cell proliferation, differentiation or apoptosis in a contextdependent manner (56,57).…”

“…IL-21 can regulate B cell proliferation, differentiation or apoptosis in a contextdependent manner (56,57). For example, IL-21 can promote both proliferation and differentiation as well as apoptosis in B cells costimulated with anti-CD40, whereas this combination can rescue BCR and TLR9 induced cell death (23,(55)(56)(57). Mechanistically, IL-21 inhibits TLR4 and TLR9 induced proliferation by downregulating anti-apoptosis members of the Bcl-2 family and inducing Bim-dependent apoptosis (23,(55)(56)(57).…”

“…For example, IL-21 can promote both proliferation and differentiation as well as apoptosis in B cells costimulated with anti-CD40, whereas this combination can rescue BCR and TLR9 induced cell death (23,(55)(56)(57). Mechanistically, IL-21 inhibits TLR4 and TLR9 induced proliferation by downregulating anti-apoptosis members of the Bcl-2 family and inducing Bim-dependent apoptosis (23,(55)(56)(57). Due to relevance of both adaptive and innate pathways in autoAb production (58, 59), we determined whether B cells in the BBim fl/fl mouse model are protected from IL-21 induced apoptosis under TLR or CD40 costimulatory conditions (Figures 7A-D and Supplementary Figures 7A-C).…”