Recent evidence reveals that immune processes drive Alzheimer’s disease (AD) pathology independently of the pathological protein deposits such as Aβ, thus exacerbating pathology and culminating in a vicious, pathophysiological cycle. Key players of this inflammatory reaction in AD are CNS-resident myeloid cells, i.e. microglia. Since mutations in genes involved in innate immunity such as Trem2 or CD33 – all of which are expressed on microglia and other myeloid cells – have been found to be associated with a higher risk for developing AD, and global Trem2 deficiency in AD-like mouse models resulted in inconsistent data, we will analyze TREM2- and/or CD33 mutations or deficiency in a myeloid cell-/microglia-specific and -restricted fashion (Aim 1). We will therefore utilize a recently established “microglia exchange protocol” in CD11b-HSVTK mice, in which ~ 90% of resident microglia can be replaced by peripherally-derived myeloid cells in vivo (Prokop et al., J Exp Med, 2015), which will either receive intracerebroventricularly injected oligomeric Aβ, or will be crossed to APPPS1 mice. Upon adoptive transfer of (i) wild type bone marrow (BM), of (ii) BM carrying the pathological R47H TREM2 mutation, or of BM (iii) lacking TREM2 or (iv) lacking CD33 into CD11b-HSVTK recipients exhibiting Aβ pathology, resident microglia will be depleted, which is followed by a rapid repopulation of peripherally-derived myeloid cells (harboring the above mentioned TREM2- or CD33-alterations), thus allowing to assess AD pathology in vivo in a CNS environment, in which merely microglia/myeloid cells harbor TREM2 or CD33 alterations.
A second set of experiments aims to dissect the molecular underpinnings of immunotherapy targeting Aβ, thought to be a viable therapeutic option for the treatment of AD, where the precise mechanism of action is however yet not known. While we have recently identified microglia as key effector cells in antibody-triggered Aβ clearance (Eom, Prokop, Heppner, unpublished), we now seek to identify the molecular and operating determinants of microglia effector functions in Aβ-targeting immunotherapy, in order to guide development of more efficient therapeutic approaches for AD. We will characterize the genomic signature (RNA sequencing) and perform a comprehensive microglial surface marker analysis (mass cytometry; collaboration with B. Becher, University of Zurich, along a recently published protocol (Becher at al., Nat Immunol. 2014)) of microglia upon Aβ-immunotherapy in vivo to determine functionally important pathways and molecules that mediate microglia effector functions (Aim 2). Thereafter, will use an in vitro system in organotypic slice cultures and/or primary cell cultures to test the functional impact of pathways identified in the genomic analysis and also probe the microglia dependence of different Aβ-targeting immunotherapy approaches using different antibody subtypes and targeted epitopes within the Aβ-peptide (Aim 3). Functionally important pathways will ultimately be tested for their impact on modulating AD pathology per se as well as upon Aβ-targeting immunotherapy in vivo in mouse models with deficiency in selected pathways (Aim 4).