PLX5622 Reduces Disease Severity in Lethal CNS Infection by Off-Target Inhibition of Peripheral Inflammatory Monocyte Production

Abstract

PLX5622 is a CSF-1R inhibitor and microglia-depleting reagent, widely used to investigate the biology of this central nervous system (CNS)-resident myeloid population, but the indirect or off-target effects of this agent remain largely unexplored. In a murine model of severe neuroinflammation induced by West Nile virus encephalitis (WNE), we showed PLX5622 efficiently depleted both microglia and a sub-population of border-associated macrophages in the CNS. However, PLX5622 also significantly depleted mature Ly6Chi monocytes in the bone marrow (BM), inhibiting their proliferation and lethal recruitment into the infected brain, reducing neuroinflammation and clinical disease scores. Notably, in addition, BM dendritic cell subsets, plasmacytoid DC and classical DC, were depleted differentially in infected and uninfected mice. Confirming its protective effect in WNE, cessation of PLX5622 treatment exacerbated disease scores and was associated with robust repopulation of microglia, rebound BM monopoiesis and markedly increased inflammatory monocyte infiltration into the CNS. Monoclonal anti-CSF-1R antibody blockade late in WNE also impeded BM monocyte proliferation and recruitment to the brain, suggesting that the protective effect of PLX5622 is via the inhibition of CSF-1R, rather than other kinase targets. Importantly, BrdU incorporation in PLX5622-treated mice, suggest remaining microglia proliferate independently of CSF-1 in WNE. Our study uncovers significantly broader effects of PLX5622 on the myeloid lineage beyond microglia depletion, advising caution in the interpretation of PLX5622 data as microglia-specific. However, this work also strikingly demonstrates the unexpected therapeutic potential of this molecule in CNS viral infection, as well as other monocyte-mediated diseases.

Distinguished Functions of Microglia in the Two Stages of Oxygen-Induced Retinopathy: A Novel Target in the Treatment of Ischemic Retinopathy

“…As reported, the reduction of antigen-presenting cells (APC) in the blood could be observed in the peripheral blood [ 41 ]. In our study, the CSF1R inhibitor had no lethal effect on immature mice and had no significant impact on the weight growth of mice, but it had a substantial effect on the proportion of monocytes and macrophages in the bone marrow, consistent with a previous study [ 42 , 43 ]. After inhibiting microglia by gene knockout, minocycline, and other methods, changes in cytokines could also be observed, and the changes in some cytokines and the degeneration of retinal cells were consistent with what we observed.…”

“…A large number of studies have shown that inhibiting the over-activation of immune cells might decelerate or alleviate the progress of the diseases. In a model of neuroinflammation induced by the West Nile virus encephalitis, PLX5622 inhibited the proliferation and lethal recruitment of immune cells into the infected brain, reducing neuroinflammation, and proved that PLX5622 was a potential target in immune cell-mediated diseases [ 43 ]. In our study, the inhibition of activated microglia reduced pathological angiogenesis in the early stage and then rescued the over-apoptosis of photoreceptors.…”

Early microglial response, myelin deterioration and lethality in mice deficient for very long chain ceramide synthesis in oligodendrocytes

“…8B-C). These include the previously reported CD86- and CD86+ microglial subsets (Spiteri et al, 2022; Spiteri et al, 2021), as well as a distinct population that we have termed myelin-responsive microglia (MRM), prominent in CerS2 ΔO/ΔO but not CerS2 fl/fl mice (Fig. 8B-C).…”

“…Brains were processed into single cell suspensions for spectral cytometry, as previously described (Spiteri et al, 2022; Spiteri et al, 2021). Briefly, brain cells were isolated using a 30%/80% Percoll gradient after homogenizing tissue on a gentleMACS dissociator (Miltenyi Biotec).…”

Microglia protect fungi against copper starvation and promote brain infection

“…Future studies should examine microglia subsets and migration patterns during fungal infection to identify the different ways in which these cells contribute towards pathology. Some of the microglia depletion methods used in our study, including the CSF1R inhibitor PLX5622 (Kerkhofs et al, 2020;Spiteri et al, 2022), are reported to have off target effects on border macrophages within the CNS. The role of non-microglia CNS border macrophages in the control and dissemination of C. neoformans is not fully understood, but they have been shown to become heavily infected with the fungus (Kaufman-Francis et al, 2018).…”