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

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.

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.

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.

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.