VGL101 Rescues CSF1R Dysfunction in Human Microglia And Macrophages: Evaluation of In Vitro Trem2 Agonism in Models Of A CSF1R‐Dependent Leukodystrophy

Abstract

AbstractBackgroundAdult‐onset leukoencephalopathy with axonal spheroids and pigmented glia (ALSP) is an autosomal dominant primary microgliopathy caused by kinase loss‐of‐function mutations in the CSF1 receptor (CSF1R). The cardinal features of ALSP include demyelination, axonopathy, and symptoms ranging from frontal temporal dementia‐like behavior, movement disorders, epilepsy, and psychiatric manifestations. Many of these hallmarks further resemble Nasu‐Hakola disease, another rare leukoencephalopathy associated with homozygous mutations in triggering receptor expressed on myeloid cells 2 (TREM2) or its tyrosine kinase binding adaptor (TYROBP) which result in complete loss‐of‐function of TREM2 or its signaling partner DAP12. Following binding to its cognate ligands, IL‐34 or CSF‐1, CSF1R dimerization initiates a signaling cascade both through its kinase domain and through DAP12, which together activate SYK to promote myeloid cell survival, proliferation, and morphological changes. While analogous to CSF1R in cellular expression and interaction with DAP12, TREM2 lacks intrinsic signaling capacity and therefore requires DAP12 to translate extracellular damage‐associated cues and orchestrate a constellation of intracellular and functional responses which phenocopy CSF1R activation. Accordingly, we hypothesize TREM2 activation may rescue CSF1R loss‐of‐function and ameliorate ALSP‐associated pathology.MethodsTo test whether TREM2 gain‐of‐function could compensate for CSF1R loss‐of‐function, we developed multiple approaches to model ALSP in human myeloid cells by CSF‐1/IL‐34 depletion or CSF1R kinase inhibition using PLX‐5622 and explored functional intervention via a human TREM2 agonistic antibody, VGL101, currently in Phase‐1 clinical development. Rescue for CSF1R was quantified using a combination of live‐cell morphology captured via automated longitudinal microscopy, viability monitored by ATP Cell‐Titer Glo, and apoptosis quantified by Caspase‐3/7 fluorogenic‐imaging.ResultsIn vitro phospho‐SYK profiling of VGL101 in iPSC‐derived microglia is consistent with its agonist profile (EC50: 0.77 ± 0.08 nM), while the absence of pSYK induction in TREM2‐knockout microglia demonstrates exquisite on‐target selectively. In both CSF‐1/IL‐34 depletion and CSF1R kinase inhibition paradigms of ALSP, VGL101 demonstrated full rescue of microglial and monocyte‐derived macrophage viability, inhibited apoptosis, and promoted morphological changes resembling intact CSF1R activation.ConclusionCollectively, our studies support the hypothesis that TREM2 agonism can compensate for CSF1R dysfunction and highlight the therapeutic potential for treating ALSP with TREM2 agonism via VGL101 passive immunotherapy.