Background Donor lungs are particularly susceptible to the haemodynamic instability and systemic inflammation which occurs following induction of brain death (BD). Increased donor lung inflammation with IL-6 or IL-8 is associated with poor post-transplant outcomes. Donor LPS pre-conditioning significantly ameliorates the lung inflammation after BD in a rat model suggesting that TLR-4 ligation following BD contributes to donor lung inflammation, even in the absence of LPS. We hypothesised that HMGB1, an alarmin released from damaged tissue and immune cells and known TLR-4 ligand, may act as an inflammatory mediator in BD induced lung injury.
Methods BD was induced in a rat model using rapid intra-cranial balloon inflation and bronchoalveolar lavage (BAL), serum and lung tissue were collected and compared with sham operated controls rats. The HMGB1 concentration in the rat serum and BAL was measured by ELISA. Real-time PCR was used to assess HMGB1 mRNA expression in lung tissue following BD. HMGB1 immunolocalisation studies were performed on BD and sham rat lung tissue. Finally HMGB1 staining was assessed in lung tissue from human BD donors and normal controls.
Results The BAL HMGB1 concentration was significantly higher in the BD group (965±302 ng/ml) than in the sham group (655 ± 274 ng/ml) (p=0.0172). There was however no difference in HMGB1 gene expression between the two groups. HMGB1 positive staining was visualised in a nuclear and extra-nuclear location and was dispersed throughout the rat and human lung with a greater density around the bronchioles. There was a significantly higher area of positive staining in BD rat lung tissue than sham tissue (p=0.0345). No difference was seen in the human BD lungs compared to controls (Abstract P254 Figure 1).
Conclusions HMGB1 is likely to be released passively from lung cells suffering sustained damage during BD. As HMGB1 gene transcription is unchanged up to 5 h following BD, the lung tissue is most likely releasing presynthesised cellular HMGB1. This study provides evidence of the presence of an alarmin which is likely to potentiate inflammation in the donor lung via the TLR-4 pathway. If release occurs early following BD, HMGB1 could be an important initiating mediator in donor lung inflammation.