Polysialic acid attenuates alternative complement activation, inhibits microglial reactivity and reduces vascular leakage after retinal laser-damage

Abstract

Purpose: Unbalanced activation of the complement system and radical production by activated microglia/macrophages are key features of age-related macular degeneration (AMD). Polysialic acids constitute the outermost part of the neuronal glycocalyx and serve as ligands for the inhibitory human-specific Siglec-11 receptor which regulates microglial activation. Sialic acid loss induces complement activation and complement receptor mediated superoxide production by microglia. Here, we hypothesized that low molecular polysialic acid (PSA) may interfere with complement activation and could reduce microglial reactivity via interaction with the Siglec-11 receptor and thus may attenuate immune mechanisms during AMD pathology.

Methods: Retinal polysialic acid expression was studied by immunohistochemistry and Siglec-11 receptor transcripts were analyzed by RT-PCR. To determine the immune modulatory potential of PSA on human induced pluripotent stem (iPS)-cell derived microglia, we analyzed pro-inflammatory cytokine production, phagocytosis of RPE cell debris and superoxide release after PSA treatment. We further studied the effect of PSA on membrane attack complex formation and subsequent cell lysis by immunohistochemistry. To study the in vivo effects of PSA, we applied PSA intravitreally in humanized Siglec-11 transgenic mice directly after retinal laser-coagulation. Fourty eight hours later microglial reactivity was analyzed in the retina and the RPE/choroid and vessel leakage was assessed by fluorescein angiography.

Results: We found that polysialic acids and the Siglec-11 receptor are expressed in the human retina. Application of PSA on human microglial cells inhibited production of the pro-inflammatory cytokine tumor necrosis factor-alpha, reduced inflammatory phagocytosis and blocked superoxide release after cell debris stimulation. PSA inhibited formation of the membrane attack complex and complement mediated lysis. Intravitreally injected PSA led to decreased accumulation of reactive microglia/macrophages in the laser spots of Siglec-11 transgenic mice and significantly reduced retinal vessel leakage.

Conclusions: We conclude that PSA attenuates neurotoxicity of human microglia in vitro and that intravitreal application of PSA decreases pathological features of AMD. Our findings suggest that PSA may provide a novel therapeutic strategy for AMD.