Loss of progranulin leads to dysregulation of innate and adaptive immune cell populations, increased susceptibility to experimental colitis, and brain infiltration of peripheral immune cells

Background Mutations in the progranulin gene (GRN) reduce levels of progranulin (PGRN) and granulins (GRNs) causing frontotemporal dementia (FTD), the most common form of early‐onset dementia. PGRN loss in neurons is associated with lysosome dysfunction resulting in diminished cathepsin protease activity. PGRN is also highly expressed in brain‐resident microglia and peripheral immune cells. Therefore, we asked if PGRN loss impacts immune cell profiles and communication in the gut‐brain axis during chronic systemic inflammation. Methods We measured lysosomal‐associated proteases in microglia and peripheral immune cells using probes for cathepsin D and pan‐cysteine cathepsins; performed multiplexed RNA & digital spatial protein profiling analyses, immunohistochemistry, and deep‐immunophenotyping by flow cytometry on brain, spleen, and peripheral blood of Grn KO and Grn‐suffficient mice aged 3‐30 months. Furthermore, we exposed mice to experimental colitis to interrogate alterations in the gut‐brain axis. Results Highest cathepsin D activity was found amongst Ly6C+, CD11b+ blood monocytes while pan‐cathepsin activity was highest in neutrophils and monocytes. In the spleen of aged Grn KO mice, T cell counts were reduced while Ly6C‐, CD68+ macrophages were increased compared to age‐matched Grn‐sufficient mice. In blood, aberrant T cell and monocyte activation markers in Grn KO mice were evident with minimal increases in inflammatory cytokines in plasma. Aged Grn KO mice exhibited decreased microglia (CD45int, CD11bhi) in the brain relative to Grn‐sufficient mice, with altered MHCII and CD68 expression. Additionally, CD4+ and CD8+ T cells were impacted in the brain of Grn KO mice compared to age‐matched Grn‐sufficient mice. PGRN loss was also associated with increased brain infiltration of activated peripheral monocytes and, interestingly, increased vulnerability to experimental colitis that was mitigated by loss of CCR2 function. Conclusions These novel findings suggest that PGRN plays key regulatory functions in innate and adaptive immune cell populations, in central‐peripheral immune cell crosstalk and traffic to the CNS, and in regulating susceptibility to immune challenges that disrupt the gut‐brain signaling axis. Analysis of peripheral immune cells and colonic tissue from patients with Grn mutations or SNPs is ongoing and may shed light on the mechanisms by which GRN haploinsufficiency causes FTD.