Integrating human iPSC-derived macrophage progenitors into retinal organoids to generate a mature retinal microglial niche

In the retina, microglia are resident immune cells that are essential for retinal development and function. Retinal microglia play a central role in mediating pathological degeneration in diseases such as glaucoma, retinitis pigmentosa, age-related neurodegeneration, ischemic retinopathy and diabetic retinopathy. Current models of mature human retinal organoids (ROs) derived from iPS cell (hiPSC) do not contain resident microglia integrated into retinal layers. Increasing cellular diversity in ROs by including resident microglia would more accurately represent the native retina and better model diseases in which microglia play a key role. In this study, we develop a new 3D in vitro tissue model of microglia-containing retinal organoids by co-culturing ROs and hiPSC-derived macrophage precursor cells (MPCs). We optimized the parameters for successful integration of MPCs into retinal organoids. We then reproducibly integrate MPCs into ROs where they develop into mature microglia (iMG) as seen by 1) migration to the appropriate anatomical locations; 2) development of a mature resting morphology; and 3) expression of mature microglial markers. We show that while in the ROs, MPCs migrate to the equivalent of the outer plexiform layer where retinal microglia cells reside in healthy retinal tissue. While there, they develop a mature morphology characterized by small cell bodies and long branching processes which is only observed in vivo. During this maturation process these microglia cycle through an activated phase followed by a stable mature phase characterized by cell-type specific microglia markers Tmem119 and P2ry12. This co-culture system may be useful for understanding the pathogenesis of retinal diseases involving retinal microglia and for drug discovery.