Required for Their Activation Survival of Anergic Anti-DNA B Cells , Yet Is TLR 9 Promotes Tolerance by Restricting Shlomchik

Nucleic acid–reactive B cells frequently arise in the bone marrow but are tolerized by mechanisms including receptor editing, functional anergy, and/or deletion. TLR9, a sensor of endosomal dsDNA, both promotes and regulates systemic autoimmunity in vivo, but the precise nature of its apparently contradictory roles in autoimmunity remained unclear. In this study, using the 3H9 anti-DNA BCR transgene in the autoimmune-prone MRL.Fas lpr mouse model of systemic lupus erythematosus, we identify the stages at which TLR9 contributes to establishing and breaking B cell tolerance. Although TLR9 is dispensable for L chain editing during B cell development in the bone marrow, TLR9 limits anti-DNA B cell life span in the periphery and is thus tolerogenic. In the absence of TLR9, anti-DNA B cells have much longer life spans and accumulate in the follicle, neither activated nor deleted. These cells retain some characteristics of anergic cells, in that they have elevated basal BCR signaling but impaired induced responses and downregulate their cell-surface BCR expression. In contrast, whereas TLR9-intact anergic B cells accumulate near the T/B border, TLR9-deficient anti-DNA B cells are somewhat more dispersed throughout the follicle. Nonetheless, in older autoimmune-prone animals, TLR9 expression specifically within the B cell compartment is required for spontaneous peripheral activation of anti-DNA B cells and their differentiation into Ab-forming cells via an extrafollicular pathway. Thus, TLR9 has paradoxical roles in regulating anti-DNA B cells: it helps purge the peripheral repertoire of autoreactive cells, yet is also required for their activation. A utoreactive BCRs arise as a result of V(D)J recombi-nation. As many as 55–75% of developing B cells display BCRs with measurable affinity for self-epitopes (1). Several self-tolerance mechanisms efficiently eliminate the majority of self-reactive BCR specificities before or shortly after entry into the mature B cell repertoire. These include editing of autoreactive BCRs through additional rounds of recombination at the L chain loci, deletion of autoreactive B cells, or the acquisition of a functionally unresponsive phenotype termed anergy (2, 3). Recently, we and others have shown that TLR9, an endosomal innate immune sensor of dsDNA (4), is required for formation of spontaneous anti-DNA autoantibodies in several in vivo mouse models of systemic lupus erythematosus (SLE) (5–10). These findings are consistent with a model in which autoreactive B cells in SLE break tolerance due to the unique ability of nucleic acid– containing self-Ags to coengage the BCR and one or more innate immune sensors of nucleic …