Increased hypothalamic microglial activation after viral-induced pneumococcal lung infection is associated with excess serum amyloid A production.
BACKGROUND
It is well established that lung pathology and inflammation are more severe during respiratory infections complicated by the presence of both bacteria and viruses. Whilst co-infection can result in invasive pneumococcal disease and systemic inflammation, the neuroinflammatory consequences of co-infection are poorly characterised.


METHODS
In this study, we utilised a mouse co-infection model involving Streptococcus pneumoniae (S. pneumoniae) and influenza A virus (IAV) lung infection, and we also isolated microglia for ex vivo stimulation with pneumococcus or serum amyloid A (SAA).


RESULTS
Co-infection but not S. pneumoniae or IAV alone significantly increased the number of amoeboid-shaped microglia and expression of pro-inflammatory cytokines including tumour necrosis factor α (TNFα), interleukin-1β (IL-1β), interleukin-6 (IL-6), and C-C motif chemokine ligand-2 (CCL-2) in the hypothalamus. Pneumococcus was only detected in the hypothalamus of co-infected mice. In addition, the systemic inflammatory cytokines TNFα, IL-1β and IL-6 were not elevated in co-infected mice relative to IAV-infected mice, whereas SAA levels were markedly increased in co-infected mice (p < 0.05). SAA and its functional receptor termed formyl peptide receptor 2 (Fpr2) transcript expression were also increased in the hypothalamus. In mouse primary microglia, recombinant SAA but not S. pneumoniae stimulated TNFα, IL-1β, IL-6 and CCL-2 expression, and this response was completely blocked by the pro-resolving Fpr2 agonist aspirin-triggered resolvin D1 (AT-RvD1).


CONCLUSIONS
In summary, lung co-infection increased the number of 'activated' amoeboid-shaped microglia and inflammatory cytokine expression in the hypothalamus. Whilst persistent pneumococcal brain infection was observed, SAA proved to be a much more potent stimulus of microglia than pneumococci, and this response was potently suppressed by the anti-inflammatory AT-RvD1. Targeting Fpr2 with pro-resolving eicosanoids such as AT-RvD1 may restore microglial homeostasis during severe respiratory infections.
['Wang|Hao|H|', 'Blackall|Melissa|M|', 'Sominsky|Luba|L|', 'Spencer|Sarah J|SJ|', 'Vlahos|Ross|R|', 'Churchill|Melissa|M|', 'Bozinovski|Steven|S|']
D016229:Amyloid beta-Peptides D000818:Animals D002135:Calcium-Binding Proteins D016207:Cytokines D004195:Disease Models, Animal D005904:Glial Fibrillary Acidic Protein D006801:Humans D007031:Hypothalamus D009980:Influenza A virus D007251:Influenza, Human D008297:Male D051379:Mice D008810:Mice, Inbred C57BL D008840:Microfilament Proteins D017628:Microglia D011018:Pneumonia, Pneumococcal D012333:RNA, Messenger D013296:Streptococcus pneumoniae D014409:Tumor Necrosis Factor-alpha
Serum amyloid A Pneumonia Microglia Neuroinflammation Resolvin-D1