Monocyte infiltration rather than microglia proliferation dominates the early immune response to rapid photoreceptor degeneration

Abstract

Background: Activation of resident microglia accompanies every known form of neurodegeneration, but the involvement of peripheral monocytes that extravasate and rapidly transform into microglia-like macrophages within the central nervous system during degeneration is far less clear. Methods: Using a combination of in vivo ocular imaging, flow cytometry, and immunohistochemistry, we investigated the response of infiltrating cells in a light-inducible mouse model of photoreceptor degeneration. Results: Within 24 h, resident microglia became activated and began migrating to the site of degeneration. Retinal expression of CCL2 increased just prior to a transient period of CCR2 + cell extravasation from the retinal vasculature. Proliferation of microglia and monocytes occurred concurrently; however, there was no indication of proliferation in either population until 72-96 h after neurodegeneration began. Eliminating CCL2-CCR2 signaling blocked monocyte recruitment, but did not alter the extent of retinal degeneration. Conclusions: These results demonstrate that the immune response to photoreceptor degeneration includes both resident microglia and monocytes, even at very early times. Surprisingly, preventing monocyte infiltration did not block neurodegeneration, suggesting that in this model, degeneration is limited by cell clearance from other phagocytes or by the timing of intrinsic cell death programs. These results show monocyte involvement is not limited to disease states that overwhelm or deplete the resident microglial population and that interventions focused on modulating the peripheral immune system are not universally beneficial for staving off degeneration.

When is a control not a control? Reactive microglia occur throughout the control contralateral visual pathway in experimental glaucoma

“…Extravasation and increased monocyte infiltration has been demonstrated as an early and persistent feature of glaucoma in animal models (35) and in post-mortem human tissue (3), and CD45 hi monocytes are present in the retina in low numbers under normal physiological conditions (42,43). We observed Isolectin B4+ ameboid cells in the majority of retinas in all experimental groups but no significant change in average monocyte density across experimental groups (Figure 6C).…”

Macrophage Plasticity and Function in the Eye and Heart

“…By contrast, others suggest a lesser role of monocyte-derived macrophages in photoreceptor degeneration, noting the absence of recruited monocytes to the subretinal space or ONL, as well as a lack of phagocytic activity [35,37]. In a separate arrestin 1 (S-antigen visual arrestin, Sag) knockout mouse model of rapid light-induced photoreceptor degeneration, in vivo imaging and scRNA-Seq showed early infiltration of monocytes from retinal vessels and from the optic nerve head relative to baseline, followed by microglial migration to the photoreceptor layer after light exposure [36,41]. However, elimination of monocyte recruitment by intravitreal injection of neutralizing antibody to C-C motif chemokine ligand 2 (CCL2), or genetic ablation of Müller glia Ccl2, did not alleviate photoreceptor degeneration in this model, suggesting that infiltrating monocytes might not exacerbate photoreceptor degeneration [41].…”

CSF1R inhibition by a small molecule inhibitor affects hematopoiesis and the function of macrophages

“…Rather, they persist long after cassation of CSF1R inhibitor. Considering the role of peripheral monocytes in CNS disease, (9,10) this work suggests that future microglia depletion studies need to consider the effect of CSF1R inhibition on peripheral immune cells.…”

“…However, this argument has been solely based on cell count measurements of blood monocytes and evaluation of blood brain barrier rather than direct assessment of cellular subtypes and their function. Given that peripheral monocytes are known to participate in CNS disease (6)(7)(8)(9)(10)(11)(12) and in the repopulation of microglia following their depletion, (10,12) it is important to definitively determine if CSF1R inhibition affects the function of peripheral immune cells (3,5,6,(12)(13)(14)(15).…”

“…In conclusion, this study provides compelling evidence that CSF1R inhibition by PLX5622 does not simply deplete microglial cells, but also affects the myeloid and lymphoid compartments and causes functional impairment in BMDMs. Considering the role of peripheral monocytes in CNS disease, (9,10) this work suggests that microglia depletion studies with small molecule inhibitors are not specific to microglia but additionally affect the number and function of peripheral macrophages, and these effects perdure long after cessation of the treatment and may have significant implications on the interpretation of relevant experimental data. PLX5622 was kindly provided by Plexxikon Inc.…”