B cells isolated from the B cell receptor (BCR) transgenic model AM 14 recognize a prototypic autoantigen, lgG2a, with relatively low affinity, and are relatively unresponsive to most lgG2a-containing immune complexes (IC). However, these rheumatoid factor producing B cells proliferate vigorously in response to IC consisting of lgG2a bound to DNA- or RNA-associated autoantigens via a mechanism that involves sequential engagement of the BCR located on the plasma membrane and Toll-like receptor 9 (TLR9) or TLR7, located in a cytoplasmic compartment. This experimental system has clearly implicated TLR7 and TLR9 in the activation of autoreactive B cells and shown that these receptors are specific for CG-rich dsDNA - potent endogenous DNA ligands include CpG islands. Importantly, we have shown that the functions elicited by physiologically relevant ICs cannot be reproduced with artificial ligands. However the exact role played by TLR7 and TLR9 in disease onset and progression is still unclear, and very little is known about the distinct functions elicited by TLR9 compared to TLR7 triggered pathways. The overall goal of the current application is to gain a better understanding of exactly how TLR9 and TLR7-expressing cell types contribute to SLE pathogenesis. Specific questions that will be addressed in this project include: (1) what are the natural sources of endogenous TLR ligands and when do they become available to the immune system; (2) what structural features of RNA determine TLR7 reactivity; (3) can TLR7 and TLR9 form functional heterodimers that detect DNA/RNA autoantigens; (4) which cellular compartments are involved in the detection of TLR7 and TLR9 ligands and how does the specific compartment regulate function; (5) how do type 1 interferons modulate the autoantibody repertoire and autoreactive B cell function; and (6) what functional outcomes distinguish BCR/TLR9 from BCR/TLR7 activation. The results of the studies will provide important information regarding the development of novel therapies for the treatment of systemic diseases such as SLE. RELEVANCE (See instructions): SLE is a chronic life threatening autoimmune disorder that afflicts up to 2 million individuals within the United States. Current therapeutic options can moderate disease severity but often have deleterious side effects that limit their extended use. Insights gained from this proposal should facilitate the development of drugs that specifically target the relevant immune effector mechanisms without the debilitating side effects of now associated with standard treatments