MyD88 Signaling in B-cell Development and Differentiation
Toll-like receptor (TLR) signaling pathways have been demonstrated to be important in many aspects of innate and adaptive immunity. Binding of TLRs by their respective ligands initiates signaling cascades which promote a number of cellular responses including, but not limited to, pathogen recognition, co-stimulation, cell maturation and activation and initiation of adaptive immune responses. While many roles for TLRs have been rigorously tested, the exact contributions of these receptors to the quality of B-cell immune responses remain unclear. Specifically, a role for intracellular TLR signaling in B-cell development is one area that remains largely uncharacterized. Endogenous TLRs that recognize viral and microbial DNAs and RNAs are also capable of recognizing self-nucleic acids. B cells, which express TLRs, undergo tolerizing mechanisms in the bone marrow (BM) and periphery to eliminate self-reactive clones. I hypothesize that endogenous TLR recognition of self-nucleic acids during B cell development promotes the elimination of these autoreactive cells from the B cell repertoire.
TLRs are also known to be integral in the subsequent triggering of the adaptive immune response. A series of controversial studies has attempted to determine whether TLR signaling is required for antibody responses to thymus-dependent protein antigens. Initial reports indicated that MyD88 was required for antibody responses to a native protein antigen. However, later studies using haptenated protein antigens did not confirm a requirement for TLR signaling in adjuvant-enhanced antibody responses. In an effort to resolve these discordant results, it was suggested that haptenated protein antigens, unlike native proteins, are fundamentally distinct in that they do not require MyD88 signaling. I hypothesize that antibody and germinal center (GC) responses to native and haptenated proteins are independent of MyD88 signaling. I further propose that the unique immunogenicity ascribed to haptenated proteins is simply immunodominance of hapten, a phenomena observed and described almost 100 years ago.
Deficiency in the global adaptor protein for TLR signaling, myeloid differentiation primary response protein 88 (MyD88), through which all TLRs signal (with the exception of TLR 3), effectively silences the TLR pathway. Utilizing mice deficient in MyD88, I show that TLR signaling through MyD88 mediates central tolerance of B cells. Specifically, crossing the MyD88 deficiency onto a 3H9 autoreactive heavy-chain knock-in rescues the immature (imm) and transitional 1(T1) B cell compartments in the BM. This finding demonstrates the requirement for MyD88 signaling in the removal of autoreactive B cell clones at the first tolerance checkpoint.
I also find that MyD88 signaling is not required for antibody, GC or memory responses to native or haptenated proteins. Additionally, affinity maturation, determined by BCR mutation frequency in GC was comparable between MyD88 deficient and sufficient mice. Both MyD88 deficient and sufficient mice were able to elicit secondary immune responses to native and haptenated proteins. Furthermore, I demonstrate that the MyD88 independent immunogenicity attributed to haptenated protein is a misinterpretation of the established concept of immunodominance- haptenated proteins elicited hapten-specific responses that were approx. 20- to 100-fold greater than to the carrier. Regardless of MyD88 signaling, native proteins elicited significantly less serum Ab than their haptenated forms.
I conclude that TLR signaling through MyD88 mediates tolerance during B cell development in the BM, but is not required for B cell immune responses.
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