Human Retinal Organoid Model of Ocular Toxoplasmosis

The health burden of ocular toxoplasmosis is substantial, and there is an unmet need for safe and curative anti-microbial drugs. One major barrier to research on new therapeutics is the lack of in vitro human-based models beyond two-dimensional cultured cells and tissue explants. We aimed to address this research gap by establishing a human retinal organoid model of ocular toxoplasmosis. Retinal organoids, generated from human induced pluripotent stem cells and grown to two stages of organization, were incubated with a suspension of live or heat-killed GT-1 strain <i>T. gondii</i> tachyzoites, or medium without tachyzoites. Both developing (1 month post-isolation) and matured (6 months post-isolation) organoids were susceptible to infection. Spread of live parasites from the margin to the entire organoid over 1 week was indicated by immunolabelling for <i>T. gondii</i> surface antigen 1. This progression was accompanied by changes in the levels of selected tachyzoite transcripts-<i>SAG1</i>, <i>GRA6</i>, and <i>ROP16</i>-and human cytokine transcripts-<i>CCL2</i>, <i>CXCL8</i>, <i>CXCL10</i>, and <i>IL6</i>-in infected versus control conditions. Our human retinal organoid model of ocular toxoplasmosis offers the opportunity for many future lines of study, including tachyzoite interactions with retinal cell populations and leukocyte subsets, parasite stage progression, and disease processes of different <i>T. gondii</i> strains, as well as drug testing.