Regulation of germinal center B cell apoptosis and pyroptosis by linear ubiquitin chain assembly complex (LUBAC) in antibody and autoantibody responses 

GC B cell survival is critical for positive selection and depends on Tfh cells, which produce CD154 and IL-21. LUBAC (complex of HOIP catalytic subunit, SHARPIN structural subunit and HOIL-1) catalyzes linear ubiquitination at methionine (M1-Ub) and promotes GC B cell survival in vivo and suppresses IL-21-induced death of CD154-activated B cells in vitro. As shown by our transcriptome and proteome analyses, IL-21 skewed BCL2 family protein expression towards apoptosis and induced hallmark pyroptosis factors, such as Gasdermin D, prompting us to analyze GC B cell apoptosis and pyroptosis and address the role of LUBAC. Mice with B cell-specific deficiency in Sharpin displayed reduced specific Abs elicited by NP-CGG immunization and virtually abrogated anti-dsDNA IgG autoantibodies induced by pristane. Likewise, treatment with LUBAC-specific inhibitor HOIPIN-8 dampened Ab responses, in association with elevated apoptosis and pyroptosis of GC B cells, which, but not IgD⁺ B cells, has both death pathways operational in immunized mice and lupus-prone Sle1.2.3. mice. Mechanistically, genetic ablation of SHARPIN compounded pharmacological inhibition of HOIP to exacerbate IL-21-triggered loss of M1-Ub.  These led to Gasdermin D cleavage for pyroptosis during the early phase of B cell activation and Caspase 3 activation for apoptosis during the late phase of B cell differentiation. Thus, LUBAC protects GC B cells from two cell death pathways to mediate Ab and autoantibody responses.