Permanent neuroglial remodeling of the retina following infiltration of CSF1R-inhibition resistant peripheral monocytes

Abstract

Previous studies have demonstrated that ocular injury can lead to prompt infiltration of bone marrow-derived peripheral monocytes into the retina. However, the ability of these cells to integrate into the tissue and become microglia has not been studied. Here we show that such peripheral monocytes not only infiltrate into the retina after ocular injury, but that they engraft permanently, migrate to the three distinct microglia strata, and adopt a microglia-like morphology. However, contrary to the original microglia, after injury the engrafted peripheral monocytes are resistant to depletion by colony stimulating factor 1 receptor (CSF1R) inhibitor and remain pro-inflammatory, expressing high levels of major histocompatibility complex II (MHC-II) for long-term. In the absence of ocular injury, on the other hand, the peripheral monocytes that repopulate the retina after CSF1R inhibition remain sensitive to CSF1R inhibition and can be re-depleted. The observed permanent neuroglia remodeling after injury constitutes a major immunological deviation that may contribute to progressive retinal degeneration. These findings may be relevant also to other degenerative conditions of the retina and central nervous system.Significance statement: Ocular injury causes permanent neuroglia remodeling that promotes neuroinflammation.

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

“…Here we show that, contrary to the accepted notion (2,3), PLX5622, a commonly used small molecule CSF1R inhibitor for microglia depletion (3,10,12), does not affect only microglia but also leads to long-term changes in the myeloid and lymphoid compartments of the bone marrow by suppressing CCR2 + monocyte progenitor cells, CX3CR1 + bone marrow-derived macrophages (BMDM), CD117 + (C-KIT +) hematopoietic progenitor cells, and CD34 + hematopoietic stem cells Cessation of CSF1R inhibition causes rebound of some, but not all, lymphoid cells ( fig. 1 G).…”

“…Microglia depletion was performed by chow administration for 3 weeks of PLX5622 (Plexxikon Inc., Berkeley, CA). Flow cytometry, and ex vivo BMDM evaluation was performed as previously described(10,12).…”

“…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).…”

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).…”