As the resident macrophages of the brain and spinal cord, microglia are crucial for the phagocytosis of infectious agents, apoptotic cells and synapses. During brain injury or infection, bone-marrow derived macrophages invade neural tissue, making it difficult to distinguish between invading macrophages and resident microglia. In addition to circulation-derived monocytes, other non-microglial central nervous system (CNS) macrophage subtypes include border-associated meningeal, perivascular and choroid plexus macrophages. Using immunofluorescent labeling, flow cytometry and Cre-dependent ribosomal immunoprecipitations, we describe P2ry12-CreER, a new tool for the genetic targeting of microglia. We use this new tool to track microglia during embryonic development and in the context of ischemic injury and neuro-inflammation. Because of the specificity and robustness of microglial recombination with P2ry12-CreER, we believe that this new mouse line will be particularly useful for future studies of microglial function in development and disease.

As the resident macrophages of the brain and spinal cord, microglia are crucial for the phagocytosis of infectious agents, apoptotic cells and synapses. Developmentally, microglia originate from the embryonic yolk sac and serve important roles in the sculpting of neonatal neural circuits. During brain injury or infection, bone-marrow derived macrophages invade neural tissue, making it difficult to distinguish between invading macrophages and resident microglia. In addition to circulation-derived monocytes, other non-microglial central nervous system (CNS) macrophage subtypes include borderzone (meningeal and perivascular) and choroid plexus macrophages. To distinguish between resident microglia and these other CNS macrophage subtypes, we generated a P2ry12-CreER mouse line. P2RY12 is a microglialspecific nucleotide sensing GPCR that is important for microglial response to tissue damage. Using immunofluorescent labeling and flow cytometry experiments, we show that P2ry12-CreER recombination is exceptionally specific to parenchymal microglia. We also perform ribosome immunoprecipitations and transcriptional profiling of P2ry12-CreER recombined cells, using a Cre-dependent Rpl22-HA mouse line. By identifying genes enriched in this dataset that are not correspondingly enriched in a broader Cx3CR1-CreER; Rpl22 dataset, we isolate a number of borderzone macrophage-specific transcripts, including the gene PF4. Using a PF4-Cre mouse line, we show that PF4 expression robustly marks borderzone macrophages. Together, we demonstrate two new methods to genetically target distinct CNS macrophage subtypes.

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Here, we explore whether PEGylation of antibodies can modulate their biodistribution to the eye, an organ once thought to be immune privileged but has recently been shown to be accessible to IV-administered large molecules, such as antibodies. We chose to PEGylate an anti-MerTK antibody, a target with known potential for ocular toxicity, to minimize biodistribution to retinal pigment epithelial cells (RPEs) in the eye by increasing the hydrodynamic volume of the antibody. We used site-specific conjugation to an engineered cysteine on anti-MerTK antibody to chemically attach 40-kDa branched or linear PEG polymers. Despite reduced binding to MerTK on cells, site-specifically PEGylated anti-MerTK retained similar potency in inhibiting MerTK-mediated macrophage efferocytosis of apoptotic cells. Importantly, we found that PEGylation of anti-MerTK significantly reduced MerTK receptor occupancy in RPE cells in both naïve mice and MC-38 tumor-bearing mice, with the branched PEG exhibiting a greater effect than linear PEG. Furthermore, similar to unconjugated anti-MerTK, PEGylated anti-MerTK antibody triggered type I IFN response and exhibited antitumor effect in syngeneic mouse tumor studies. Our results demonstrate the potential of PEGylation to control ocular biodistribution of antibodies.