Temporally-regulated genetic access to IL-1β-expressing cellular networks in homeostasis and following peripheral or central immune stimuli

The proinflammatory cytokine Interleukin-1 beta (IL-1β) regulates nearly all aspects of immune function. In the brain, IL-1β is implicated in neural and immune functions under both basal and inflammatory conditions. Under basal conditions, IL-1β is known to alter sleep, memory, and affect. Under inflammatory conditions, IL-1β can induce sickness behaviors, HPA activation, and exacerbate neurological and psychological disorders. Sensitive detection and specific manipulation of IL-1β-expressing cells in the brain is currently not achievable; therefore, we generated the first mouse line to allow both robust visualization and genetic manipulation of the IL-1β-expressing cells.

The IL-1β-TRAP mouse was generated through Crispr9-mediated recombination. IRES-CreER^T2 was inserted following exon 7 after the stop codon of the Il1b gene which yielded an IL-1β-IRES-Cre-ER^T2 mouse (IL-1β-TRAP). To visualize IL-1β-expressing cells, IL-1β-IRES-Cre-ER^T2 line was crossed with ROSA26-lox-stop-lox-tdTomato mouse to generate the IL-1β-IRES-Cre-ER^T2:ROSA26-lox-stop-lox-tdTomato (IL-1β-TRAP-reporter). The IL-1β-TRAP-reporter mice were given intraperitoneal (i.p.) Tamoxifen before peripheral LPS or central IL-1β administration. PBS was used as control. The IL-1β-TRAP-reporter mice were also exposed to repetitive closed head injury (CHI) or i.p. kainic acid. tdTomato was allowed to express for 7d and distribution of IL-1β-expressing cells were observed via immunohistochemistry and/or lightsheet microscopy.

Under basal conditions, IL-1β was found primarily to be expressed in MHCII⁺ and CD206⁺ meningeal and ventricular macrophages with sparse IL-1β-expressing microglia and neurons in brain parenchyma. Following i.p. LPS and i.c.v. IL-1β injections, IL-1β expression was found in meningeal macrophages and parenchymal P2YR12⁺ microglia. Following CHI, IL-1β-expressing macrophages increased in the meninges and IL-1β-expressing microglia were induced in the parenchyma at the injury site and along white matter tracts.

This is the first time active populations of cytokine-expressing cells are visualized, characterized, and genetically accessed using a mouse line with a knockin CreER. With this tool, we identified the choroid plexus as the predominant IL-1β-expressing region in homeostasis. Further, this mouse can be used to identify newly activated IL-1β-expressing cells; they can be targeted for manipulation in physiological and pathological contexts.