B cell regulation in autoimmune disease

Abstract

Our immune system protects us from both foreign antigens and self-derived autoantigens by sophisticated regulation. During an immune response, the innate and adaptive parts of the immune system closely collaborate at every stage of the response. However, abnormal and unbalanced responses can lead to harmful reactions, such as in autoimmune diseases. Systemic Lupus Erythematosus (SLE) is one of many autoimmune disorders affecting multiple tissues and organs, such as the skin, kidney, and cardiovascular system. And the majority of the damages observed in SLE are the direct results of autoantibodies produced by autoreactive B cells. These B cells escape from the immune tolerance and secrete self-targeted antibodies, either attacking the healthy tissues or forming antigen-antibody complex deposition that drives inflammation and tissue damage. Although the pathogenesis is not fully understood, both the innate and adaptive immune systems are involved. The aim of this thesis is to explore different mechanisms in B cell regulation, thus providing potential new clinical treatment options.

In paper I we established a self-reactive mouse model by using repeated injection of inflammatory cytokine IL-18 combined with one dose of α-GalCer to activate iNKT cells. We found that there is a significant increase in anti-DNA antibodies and GC B cells. With the addition of α-GalCer, iNKT cells abolished their suppressive role and adopted an iNKTfh phenotype, which supported the production of autoreactive antibodies. Moreover, we observed increased GFP+ PC and hCD2+ GC B cells in AID reporter mice, in which the AID expression turns on GFP and also expresses hCD2. Together, our study found that there is a dominant B cell helper role of iNKT cells under inflammatory/autoimmune conditions.

To further investigate the ability of iNKT cells to modulate B cells in autoimmune disease, in paper II we investigated a balance between conventional Tfh cells and unconventional iNKTfh cells in governing autoreactive B cells. With weekly AC injections, we were able to establish a SLE-like mouse model, and the co-administration of AC with α-GalCer could drive iNKTfh cells formation, which contributed to the generation of short-lived GC B cells and IgG1 autoantibodies while also affecting the generation of Tfh cells. Additionally, we observed that CD1d depletion on B cells restricted the iNKTfh cell development, B cell activation, and autoantibodies production. Also, signaling through endosomal TLRs was involved in the activation of iNKT cells and GC response after injection of self-antigens.

In paper III we unveiled a novel interplay between neutrophils and iNKT cells in modulating autoreactive B cells in a CD1d-dependent manner. To address this, we performed weekly AC injections in a neutropenic mouse model and mixed bone marrow chimera mice, which allowed us to compare the cellular responses without neutrophils or neutrophils without CD1d. We found that mice deficient in neutrophils or neutrophil expression of CD1d generated more autoantibodies in an SLE mouse model. Furthermore, iNKT cells showed remodeling with altered iNKT/iNKT2 ratio and lower GATA3 expression. These findings demonstrate that neutrophils play a negative regulatory role through iNKT cells in this SLE mouse model.

In paper IV we discovered a novel role of scavenger receptor CD36 in B cell regulation. CD36 is a transmembrane receptor expressed on various cells including both innate and adaptive cells and even some non-immune cells. As a pattern recognition receptor, CD36 can recognize and bind to multiple ligands. We demonstrate that in T cell-dependent immune response, CD36-deficient B cells exhibited a significant reduction in PC development, Ig class-switching, and metabolic changes. Furthermore, we found compromised autophagosome formation, which was also related to the co-localization of CD36 with autophagosome membrane protein LC3B-II. These findings prove new information on the capacity of CD36 to function as a B cell regulator and give a new possibility to generate targeted treatment for B cell malignancies.