The roll of Toll-like receptors in the antiphospholipid syndrome.

The antiphospholipid syndrome (APS) is an autoimmune disease characterized by thrombosis, recurrent fetal loss, and the presence of antiphospholipid antibodies (aPL). Recent data support the idea that the thrombotic activity in APS patients is attributed to enhanced cytokine release via activation of certain Toll-like receptors. To investigate these mechanisms more precisely, different experimental approaches were used to investigate this connection in detail. IgG fractions and/or monoclonal aPL, either generated from murine or human B cells were intensely used for stimulation experiments of monocytes, endothelial cells, or dendritic cells. All these stimuli induced an enhanced expression and secretion of cytokines, especially tumor necrosis factor (TNF)-alpha, caused by specific regulation or activation of Toll-like receptors. Using specific agonists or inhibitors could confirm the causal connection of these stimulatory effects. This review focuses on these recent developments, connecting the binding of aPL with the activity of Toll-like receptors, especially in monocytes, endothelial cells, and dendritic cells.

Human antiphospholipid antibodies induce TNFalpha in monocytes via Toll-like receptor 8.

The antiphospholipid syndrome (APS) is characterized by recurrent arterial and/or venous thromboses, pregnancy loss and the presence of antiphospholipid antibodies (aPL). One of the discussed mechanisms of this thrombotic activity in APS patients is attributed to TNFalpha secretion in monocytes after aPL stimulation. To investigate this mechanism in detail, we employed a monoclonal aPL and IgG fractions of APS patients for stimulation of human peripheral monocytes. Stimulation with this monoclonal aPL resulted in an increased expression and secretion of TNFalpha, caused by specific upregulation of TLR8 mRNA and protein expression levels. To confirm the specificity of this finding we could demonstrate that the TNFalpha enhancement could be neutralized by a TLR8-specific inhibitory DNA-oligonucleotide and could be further increased by adding the specific ligands for TLR8. Using APS patients IgG fractions for stimulation of peripheral monocytes revealed a similar TLR8 mRNA elevation and increase in TNFalpha-production. Furthermore the TLR8 expression level in PBMC's of APS patients was as well significantly elevated. It could be demonstrated that the TNFalpha release in monocytes resulting from aPL stimulation was exclusively induced by TLR8 engagement. This could be confirmed in PBMC's of APS patients, hinting that endogenous stimulation of TLR8 in APS patients and consecutive elevation of TNFalpha promotes a proinflammatory environment.

TLR7 and TLR8 ligands and antiphospholipid antibodies show synergistic effects on the induction of IL-1beta and caspase-1 in monocytes and dendritic cells.

TLRs represent the first line of defense against invading pathogens in the innate immune system. Certain cytokines are important mediators and essentially necessary to assure an appropriately regulated immune response. Recent data gave initial evidence that IL-1beta is one of the most relevant members of these regulating cytokines. We investigated the induction of IL-1beta production in monocytes and pDCs stimulated with ligands for TLR7 and TLR8 and with antiphospholipid antibodies (aPL). Using human monocytes and pDCs for stimulation with specific TLR7 and TLR8 ligands such as resiquimod (R848) and single stranded RNA (RNA42) as well as with a human monoclonal aPL HL5B resulted in a specific upregulation of IL-1beta mRNA and protein in these cells. Determination of expression-levels using real-time RT-PCR showed significantly augmented TLR-dependent IL-1beta and caspase-1 expression. This increase could be substantially enhanced by adding the monoclonal aPL HL5B. To demonstrate the direct dependency between TLR stimulation and IL-1beta production, specific TLR inhibitors were applied and the IL-1beta and caspase-1 secretion could be explicitly decreased. The respective protein levels were determined using Western Blot, FACS analysis or ELISA assays. In conclusion we demonstrated that the downstream signaling pathway of TLR7 and TLR8 in monocytes and pDCs after stimulation with specific ligands included not only the secretion of cytokines such as TNFalpha and IL-1beta but as well the activation of necessary regulating proteins like caspase-1. APL seem to enforce this process hinting that endogenous stimulation of TLRs in the Antiphospholipid Syndrome (APS) patients resulted in enhanced secretion of proinflammatory cytokines.

Toll-like receptors and autoimmunity.

The understanding of autoimmune diseases experienced an impressive boost since the Toll-like receptors (TLRs) have been identified as possible key players in autoimmune pathophysiology. Although these receptors recognize a variety of structures derived from viruses, bacteria and fungi leading to subsequent initiation of the relevant immune responses recent data support the idea that TLRs are crucial in the induction and perpetuation of certain autoimmune diseases, especially the systemic lupus erythematosus (SLE). In this review we will summarize recent data on involvement of TLRs in the development of autoimmune diseases. This review will focus on TLRs 7, 8 and 9 which were originally identified as receptors specific for bacterial and viral RNA/DNA, but more recent in vitro and in vivo studies have linked these receptors to the detection of host RNA, DNA, and RNA- or DNA-associated proteins in the context of autoimmunity.