Characterisation of the immune response and investigation of autophagy as an immunotolerance mechanism in flying foxes infected by Australian bat lyssavirus

Abstract

Australian bat lyssavirus (ABLV) is harboured by flying foxes and can result in zoonotic disease. It is known to cause a nonsuppurative meningoencephalitis, and intensity of inflammation is highly variable across individuals. Recent in vitro studies suggest that autophagy has the potential to clear ABLV infection in flying fox cell lines and is expressed at a higher level in these flying foxes. This study aims to characterise the inflammatory response observed in ABLV-infected flying foxes and explores autophagy as an anti-viral mechanism that could account for the variability in lesions across individuals.

Histological changes were scored to determine the overall severity of inflammation, and immunohistochemistry (IHC) was used to label the associated inflammatory cells. Meningitis, gliosis, and perivascular cuffing were the most frequent lesions. In ABLV qPCR-positive flying foxes, macrophages were the most abundant, with a 3-fold increase compared to negative controls. T-lymphocytes and B-lymphocytes exhibited a 10-fold increase in ABLV-positive animals and were significantly more numerous in the meninges. Lymphocyte counts correlated with greater lesion severity, and T-lymphocytes were significantly associated with reduced viral load. No significant variation in macrophage distribution was observed between anatomical regions.

Autophagy was assessed via IHC of autophagy marker LC3 and immunolabelling was compared between ABLV-positive and ABLV-negative flying foxes, to ascertain whether autophagy levels are elevated in association with ABLV infection. ABLV-positive flying foxes had a lower immunolabelling of the LC3 autophagy marker compared to ABLV-negative flying foxes. Additionally, the percentage of immunolabelling cells did not appear to correlate with type/severity of inflammation or viral load. Hence, the underlying cause for the reduced LC3 immunolabelling in infected animals remains to be unravelled and would be an opportunity for further investigation.