Toll-like receptor activation in the pathogenesis of systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is characterized by the production of pathogenic autoantibodies to nucleoproteins and DNA. The level of anti-DNA antibodies correlates with disease severity, and the deposition of these immune complexes (ICs) in the kidneys is thought to contribute to disease pathogenesis. Recent evidence suggests that the DNA component of ICs purified from SLE patients (SLE DNA-ICs) contributes to the development of SLE pathology. SLE DNA-ICs induce proliferation of self-reactive B cells and cytokine production by plasmacytoid dendritic cells (PDCs) in a TLR9-dependent manner. One of the cytokines induced by DNA-containing ICs is interferon alpha (IFN-alpha). Elevated serum levels of IFN-alpha and overexpression of interferon-induced genes have been observed in SLE patient blood and shown to correlate with disease severity. We have recently found that the mechanism of IFN-alpha production by PDCs depends on TLR9 and FcgammaRIIa (CD32), and CD32 delivers SLE DNA-ICs to intracellular lysosomes containing TLR9. This CD32-TLR9 pathway, which is operative in PDCs, is distinct from the BCR/TLR9 pathway in B cells and may prove to be a novel target for future SLE therapies. In this article, the role of toll-like receptors, cytokines, and Fc receptors expressed by PDCs in the pathogenesis of SLE is summarized.