Edinburgh University experts examined the brains of transparent zebrafish using a fluorescent microscope, where they spotted how specialised cells called microglia infiltrate brain tumours and help them to grow.
Microglia usually consume anything that could cause harm to the brain.
The findings could pave the way for new experimental immunotherapy drugs as a treatment for glioblastoma tumours, which affects 265 Scots every year.
Dr Dirk Sieger, from the Cancer Research UK Edinburgh Centre, said: “From the different behaviours of microglia that we have seen in zebrafish, we now know what to look for when testing new immunotherapy drugs for brain tumours.
“If we can make a drug that can convert microglia’s alliance, then we should be able to see these immune cells attacking tumour cells.”
Dr Áine McCarthy, of Cancer Research UK said: “This research provides some exciting new insights into how immune cells in the brain interact with glioblastoma cells.
“The findings could help scientists design and develop new drugs to help the immune system turn its full force on brain tumours.”