Regulation of Germinal Center Antibody Responses by TLR Signaling in Dendritic Cells and B Cells

The ultimate goal of the immune system is to eliminate pathogens and establish an immunological memory for rapid response upon reinfection. It can be broadly divided into innate and adaptive compartments. Immediately or shortly after infection occurs, the innate immune system uses pattern recognition receptors (PRRs) to identify ancient conserved features of pathogens, for example, DNA, RNA and carbohydrate, termed pathogen associated molecular patters (PAMPs). Stimulation of PRRs by PAMPs triggers signaling events that both limit the spread of infection as well as inform the adaptive immune system to generate the appropriate type of immunological memory. For example, the adaptive system uses cellular immunity to eliminate pathogens such as Mycobacterium tuberculosis that reside within cells while it generates antibodies (humoral immunity) to neutralize and remove extracellular pathogens such as the bacteria Pseudomonas aeruginosa that causes pneumonia. Thus, innate responses provide the first wave of defense by containing infection while the slower acting adaptive response eliminates pathogens and programs the ability to respond more efficiently upon subsequent exposures./nHow innate signals sculpt adaptive responses is not well understood, and therefore, this thesis focuses on the ability of innate signaling by a family of PRRs known as toll-like receptors (TLRs) to program antibody responses. Ten and twelve TLRs have been identified in humans and mice, respectively, and they recognize a variety of PAMPs expressed by bacteria, viruses, fungi, and parasites including lipopolysaccharide (TLR4), flagellin (TLR5 and TLR11), and unmethylated CpG motifs in DNA (TLR9). Briefly, antibody responses may be broadly divided into those that require T cell help (T-dependent) and those that are independent of T cell help (T-independent). T-dependent antibody responses can occur either outside the B cell follicle (extrafollicular) or within the follicle in specialized transient structures termed germinal centers (GCs) that generate high affinity, long-lived humoral memory. To dissect how TLR signaling impacts GC antibody responses, I immunized mice with an oligovalent T-dependent protein antigen that was linked to either oligonucleotides containing (CpG) or lacking (nonCpG) a TLR9 ligand. In chapter two, I focus on the cellular contribution of TLR signaling to GC magnitude and quality by using a Cre recombinase system to selectively delete the TLR signaling adaptor MyD88 in either dendritic cells or B cells. This series of experiments revealed a division of labor for TLR signaling in DCs and B cells that controls GC magnitude and quality, respectively. In chapter three, I address the role of costimulation by the inducible costimulator (ICOS) on T cells and ICOS ligand (ICOSL) on B cells to direct the GC response against antigen linked to a TLR9 ligand. These experiments revealed that ICOSL acted B cell-extrinsically to impact GC quality and also unveiled a surprising B cell-intrinsic role for TLR9 signaling in affinity maturation. Collectively, these studies suggest that innate signals imprint the quality of T cell help that subsequently defines the antibody response and that they also act on B cells receiving TLR9 stimuli to directly enhance their affinity maturation. These results have implications in both human disease and rational vaccine design.

Advisor: DeFranco, Anthony L
Barton, Greg M