IgG autoantibodies mediate pathology in systemic lupus patients and lupus-prone mice. Here we showed that the class-switched IgG autoantibody response in MRL/Faslpr/lpr and C57/Sle1Sle2Sle2 mice was blocked by the CID 1067700 compound, which specifically targeted Rab7, an endosome-localized small GTPase that was upregulated in activated human and mouse lupus B cells, leading to prevention of disease development and extension of life-span. These were associated with decreased IgG-expressing B cells and plasma cells, but unchanged numbers and functions of myeloid cells and T cells. The Rab7 inhibitor suppressed T cell-dependent and T cell-independent antibody responses, but did not affect T cell-mediated clearance of Chlamydia infection, consistent with a B cell-specific role of Rab7. Indeed, B cells and plasma cells were inherently sensitive to Rab7 gene knockout or Rab7 activity inhibition in class-switching and survival, respectively, while proliferation/survival of B cells and generation of plasma cells were not affected. Impairment of NF-κB activation upon Rab7 inhibition, together with the rescue of B cell class-switching and plasma cell survival by enforced NF-κB activation, indicated that Rab7 mediates these processes by promoting NF-κB activation, likely through signal transduction on intracellular membrane structures. Thus, a single Rab7-inhibiting small molecule can target two stages of B cell differentiation to dampen the pathogenic autoantibody response in lupus.

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.

Maturation of antibody responses entails somatic hypermutation (SHM), class-switch DNA recombination (CSR), plasma cell differentiation, and generation of memory B cells, and it is thought to require T cell help. We showed that B cell Toll-like receptor 4 (TLR4)–B cell receptor (BCR) (receptor for antigen) coengagement by 4-hydroxy-3-nitrophenyl acetyl (NP)–lipopolysaccharide (LPS) ( Escherichia coli lipid A polysaccharide O-antigen) or TLR5-BCR coengagement by Salmonella flagellin induces mature antibody responses to NP and flagellin in Tcr β −/− Tcr δ −/− and NSG/B mice. TLR-BCR coengagement required linkage of TLR and BCR ligands, “linked coengagement.” This induced B cell CSR/SHM, germinal center–like differentiation, clonal expansion, intraconal diversification, plasma cell differentiation, and an anamnestic antibody response. In Tcr β −/− Tcr δ −/− mice, linked coengagement of TLR4-BCR by LPS or TLR5-BCR by flagellin induced protective antibodies against E. coli or Salmonella Typhimurium. Our findings unveiled a critical role of B cell TLRs in inducing neutralizing antibody responses, including those to microbial pathogens, without T cell help.

During the antibody response, B cells are exposed to innate and adaptive T cell stimuli, although it is unclear whether and how ensuing receptor signals are functionally integrated. We have identified IL-27 as the cytokine specifically secreted by B cells upon sequential stimulation with TLR ligands and CD154 plus IL-21, the hallmark factors of T follicular helper cells. B-cell IL-27 production is concomitant with increased Il27p28 locus accessibility and depends on newly induced transcription factor BATF3. IL-27-producing B cells are inefficient in antibody secretion, but cooperate with IFNγ to promote proliferation, survival, class-switching and plasma cell differentiation of their target B cells, leading to IgG responses to a conjugated hapten and virus infection. Thus, IL-27-producing B cells function as “helper” B cells that integrate the innate and adaptive stages to optimize antibody responses.One-sentence summaryB cells integrate innate TLR and adaptive CD40 signals to induce BATF3 transcription factor for production of IL-27, which together with INFγ optimizes antibody responses.