Injury induced changes in host innate immunity : dysregulation of Toll like receptor responses
Major injury leads to host immune dysregulation and increased susceptibility to infectious challenge. Toll-like receptors (TLRs) are archetypical pattern-recognition receptors that, in addition to a role in mediating innate immune responses to components of Gram-positive and Gram-negative pathogens, have been implicated in the recognition of endogenous mediators, released during host tissue injury. A murine model of thermal injury was employed to examine the impact of injury on TLR-mediated immune cell responses. Lymph node and spleen cell suspensions were prepared from wild type, TLR4-/- and IL-1RI-/- mice at 24 hours or 7 days after injury/sham injury, cultured for 48 hours with lipid A (LA), lipopolysaccharide (LPS), lipoteichoic acid (LTA) or peptidoglycan (PGN) and production of IL-1b, IL-6, IL-10 and TNFa measured by ELISA. Cell subset localisation of cytokine production was assessed by intracellular cytokine detection and immunomagnetic bead T-cell depletion. TLR4/MD2 cell surface expression was measured by flow cytometry and TLR gene induction by Real Time RT-PCR. Injury caused augmented wild type splenocyte production of IL-1b and TNFa at 24 hours and of IL-1b and TNFa at 7 days in response to all stimuli. Cytokine production was localised to macrophages and dendritic cells and the injury-augmented reactivity was independent of T-cells. Responses to LA, LPS and LTA required TLR4 whereas PGN responses were TLR4-independent. TLR4 was not required for the in vivo establishment of injury-augmented proinflammatory responses. Injury did not substantially change TLR gene expression assessed by Real Time RT-PCR or TLR4/MD2 cell surface expression. IL-1 signalling was not essential for the injury-augmented proinflammatory response but was required for injury-augmented production of IL-6 and IL-10 and therefore may be important for the development of anti-inflammatory responses.