Maintaining Tolerance to Nucleic Acids and the Microbiota Early in Life

Throughout life the immune system is faced with the challenging task of appropriately responding to both self- and nonself-derived ligands. The innate immune system utilizes pattern-recognition receptors, such as Toll-like receptors (TLR), to identify specific features of microbes, while the adaptive immune system employs T cells and antibodies (IgA, IgG) to generate robust secondary responses to invasive microorganisms. However, this poses the potential for inappropriate recognition of self-derived ligands or symbiotic bacteria. Therefore, the balance between recognition of non-self while preserving tolerance to self is necessary to prevent autoimmunity and detrimental infections. To better understand this balance, I first examined how the innate immune system prevents recognition of self-nucleic acids. In this study, I explored the consequences of dysregulating compartmentalized activation of a nucleic acid sensing receptor, TLR9. By inducing in vivo expression of a TLR9 mutant (TLR9TM) that bypasses the need for intracellular activation, I discovered that dysregulated TLR9 activation early in life drives a fatal inflammatory disease, driven by type II interferon. In contrast, induced expression of TLR9TM late in life led to a milder, systemic autoinflammatory disease. This study demonstrates that compartmentalized activation of TLR9, especially early in life, is necessary to prevent deleterious recognition of self-nucleic acids./nEarly in life the naïve immune system must be able to differentiate pathogenic microbes from symbiotic, beneficial microbes. The naïve immune system receives some instruction through acquisition of maternal-derived microbiota-reactive antibodies, such as IgG2b and IgG3. These antibodies are anti-inflammatory and help to suppress inappropriate immune responses to commensal bacteria in the neonatal gut. However, the exact mechanism(s) by which maternal-derived IgG2b and IgG3 instruct the naïve immune system remain relatively unknown. To better understand the role of microbiota-reactive IgGs in tuning the immature immune system, I generated mice that produce pro-inflammatory microbiota-reactive IgG2c (termed IgG32c/2c mice). Examination of the IgG32c/2c mice revealed that acquisition of maternal-derived microbiota-reactive IgG2c leads to fatal disease. This disease was characterized by weight loss and intestinal inflammation as measured by an increase in fecal lipocalin-2, an increase in intestinal myeloperoxidase, and an expansion of CD11b+ myeloid cells in the spleen. In contrast, neonates that acquire maternal-derived microbiota-reactive IgG3 do not experience weight loss, inflammation, or fatality. This study establishes that effector functions elicited by maternal-derived microbiota-reactive antibodies help dictate neonatal fitness by fine-tuning the responses generated by the naïve immune system to symbiotic gut bacteria./nThe work in this dissertation establishes how the innate immune system regulates nucleic-acid sensing TLRs to prevent inappropriate recognition of self-derived ligands. In addition, the work in this dissertation reveals how maternal-derived antibodies instruct the naïve immune system in how to respond to beneficial microbes early in life.

Advisor: Barton, Gregory