Abstract
The immune system is essential for maintaining homeostasis, as well as promoting growth and healing throughout the brain and body. Considering that immune cells respond rapidly to changes in their microenvironment, they are very difficult to study without affecting their structure and function. The advancement of non-invasive imaging methods greatly contributed to elucidating the physiological roles performed by immune cells in the brain across stages of the lifespan and contexts of health and disease. For instance, techniques like two-photon in vivo microscopy were pivotal for studying microglial functional dynamics in the healthy brain. Through these observations, their interactions with neurons, astrocytes, blood vessels and synapses were uncovered. High-resolution electron microscopy with immunostaining and 3D-reconstruction, as well as super-resolution fluorescence microscopy, provided complementary insights by revealing microglial interventions at synapses (phagocytosis, trogocytosis, synaptic stripping, etc.). In addition, serial block-face scanning electron microscopy has provided the first 3D reconstruction of a microglial cell at nanoscale resolution. This review will discuss the technical toolbox that currently allows to study microglia and other immune cells in the brain, as well as introduce emerging methods that were developed and could be used to increase the spatial and temporal resolution of neuroimmune imaging. A special attention will also be placed on positron emission tomography and the development of selective functional radiotracers for microglia and peripheral macrophages, considering their strong potential for research translation between animals and humans, notably when paired with other imaging modalities such as magnetic resonance imaging.
Highlights
The immune system of the central nervous system (CNS) is essential to maintain its homeostasis [reviewed in Tay et al (2017)]
Light-sheet fluorescence microscopy achieves a resolution of ∼26 μm (x-y) (Colombelli and Lorenzo, 2014) with 3D optical sectioning and high-speed of imaging (2.7 × 104 μm3s−1) (Lu C.-H. et al, 2019) that limits the effects of photobleaching compared to confocal microscopy (Santi, 2011; Power and Huisken, 2017)
focused ion beam scanning electron microscopy (FIB-SEM) produces a higher 3D resolution reaching 3–5 nm (x, y, and z) (Briggman and Bock, 2012), which is required to study the fine geometry of organelles, phagosomes and autophagosomes and cytoskeletal elements at the expense of having a reduced field of view and slower acquisition speed compared with SBFSEM (Heymann et al, 2006; Knott and Genoud, 2013; Peddie and Collinson, 2014; Savage et al, 2019b). With this level of resolution, FIB-SEM imaging of immune cells allows to discern between cellular elements that are partially surrounded by a microglial process, for instance during synaptic stripping, from the ones that are fully engulfed and internalized
Summary
The immune system of the central nervous system (CNS) is essential to maintain its homeostasis [reviewed in Tay et al (2017)]. The role of microglia and other immune cells in the brain was uncovered using innovative methods allowing to image them in situ (Savage et al, 2019a) and in vivo (Davalos et al, 2005; Herz et al, 2012).
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