The Toll like receptors (TLRs) are a family of innate immune pattern recognition receptors that have been implicated in the pathogenesis of the autoimmune diseases rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE). Activation of these receptors by exogenous or endogenous ligands stimulates production of cytokines such as TNF and IL-6. Elevated serum cytokine levels correlate with disease activity and are targeted by modern therapeutics.
Peripheral blood monocytes contribute to disease through extravasation into inflamed tissues and secretion of pro-inflammatory cytokines. This study sought to identify the expression of TLRs 2, 3, 4, 5, 7, 8 and 9 on monocytes from RA and SLE donors and healthy controls (HC). TLR-induced TNF, IL-1β, IL-6, IL-10 and IP-10 secretion from these cells was also assessed, as was the basal expression of signalling inhibitors which regulate the cascade downstream of TLRs. These results were correlated with each other and to the clinical data of the donors.
Significant differences in the expression of TLRs relative to HC were determined for RA and SLE peripheral blood monocytes. However, none of the observed differences correlated with a functional effect in terms of cytokine secretion in response to stimulation. Instead, dysregulation of downstream TLR signalling inhibitors was identified in the disease groups that contributed to these responses.
Firstly, in agreement with other reports expression of TNF-induced protein 3 (TNFAIP3, A20) was decreased in the majority RA and SLE monocytes. However, asubpopulation of donors existed with higher A20 expression. Secretion of TNF following TLR8 stimulation of RA and SLE monocytes was elevated in those donors with low A20 expression relative to both HC and RA/SLE donors that exhibited higher A20 expression.
Secondly, expression of sterile-α and HEAT/Armadillo motifs containing protein (SARM) was decreased in RA monocytes. Decreased SARM correlated with disease activity and secretion of IL-1β in response to TLR2 stimulation. The mechanism of action was independent of SARMs documented role in the inhibition of the TLR signalling adaptor TIR domain containing adaptor inducing interferon-β (TRIF).
Finally, Interleukin-receptor associated kinase-M (IRAK-M), another TLR signalling inhibitor, was observed to correlate with IL-6 production by SLE monocytes following TLR5 activation, which was elevated relative to HC. Following TLR5 stimulation, IRAK-M induction by SLE monocytes was lower than HC. Lower IRAK-M expression correlated with elevated serum IL-6 levels, potentially indicating a clinically significant role.
These data suggest that TLR-induced cytokine secretion by monocytes is regulated at the level of signal transduction as opposed to the TLR expression level in RA and SLE. Failure to control aspects of the cascade downstream of TLRs may allow for elevated cytokine production, which could influence disease activity.
|Date of Award||Feb 2016|