Autoantibodies make a U-turn: the toll hypothesis for autoantibody specificity

Efficient T Cell Activation via a Toll-Interleukin 1 Receptor-Independent Pathway

Here, we describe a previously unrecognized pathway for activation of antigen-specific adaptive immune responses that was independent of Toll-Interleukin 1 Receptor signaling and directed toward detection of antigens expressed by apoptotic cells. This pathway is represented within Flt-3 Ligand-derived dendritic cells (DCs) that represent immature lymphoid DCs, but not within GM-CSF-treated bone marrow-derived dendritic cells. Exposure of these DCs to apoptotic cells resulted in production of type I interferon and favored the development of cytotoxic T cell responses. The N-Ethyl-N-Nitrosourea-induced germline mutation 3d (Unc3b1(3d/3d)) abolished both MHC class I and II responses elicited by this pathway, whereas a null allele of Cd36 selectively abolished class II responses. We propose that this mode of adaptive immune activation evolved to permit the sensitive detection of intracellular microbial infections, particularly viral infections, which frequently induce apoptotic cell death, but may also be important in transplantation, autoimmunity, and vaccine development.

TLR4, TLR9 and MyD88 are not requiredfor the positive selection of autoreactive B cells intothe primary repertoire

Toll-like receptors (TLR) have been shown to play an essential role in the generation of autoantibodies in mouse models of autoimmunity, but the timing and context of these effects are poorly understood. One hypothesis is that TLR ligands assist in the positive selection of self-reactive B cells into the primary repertoire and, in this way, distinguish between immunogenic and tolerogenic forms of self-antigen. To explore this idea we generated hen egg lysozyme-specific immunoglobulin (Ig(HEL)) and isotype class-switching anti-HEL mice deficient in MyD88, TLR4 or TLR9 signalling and studied B cell development and autoantibody secretion in the presence or absence of an intracellular form of self-antigen HEL that positively selects B1 cells. Our findings show that TLR4, TLR9 and MyD88 are not required for the positive selection of autoreactive B cells in the primary B cell repertoire, nor is MyD88 required for the generation of isotype-switched antibodies in the absence of antigen. These results suggest that the significant effects of TLR on autoimmunity occur in the established repertoire and not during B cell development.