Toxoplasma-host interactions

Toxoplasma infects an extraordinary range of hosts, however, the host genetic background significantly impacts the outcome of toxoplasmosis. For example, mice usually succumb to acute infection, whereas most hosts (including humans) only develop asymptomatic chronic toxoplasmosis. Given the host differences in their response to Toxoplasma, the Wang lab is motivated to set a long-term research goal of understanding the immune response that determines host resistance to Toxoplasma and demonstrating the molecular basis of parasite effector-mediated host immune modulation. There remain critical gaps in our knowledge on (i) what are the host mechanism that determine Toxoplasma resistance; (ii) how Toxoplasma secreted effectors contribute to infection in various hosts. The answers to these questions will help us to further understand the determinants of infection outcome of toxoplasmosis.

Host immunity - Elucidate host innate immune machinery determining Toxoplasma resistance.

The status quo is that the field has a decent understanding of anti-Toxoplasma responses and parasite pathogenesis in mice, but a limited understanding of how other less susceptible hosts (e.g., humans) respond to Toxoplasma infection. Therefore, the Wang lab aims to exploit the host restriction mechanism in a Toxoplasma-resistant laboratory animal model. As a host resistant to acute Toxoplasma infection, rat provides the opportunity to better understand anti-Toxoplasma mechanisms in humans (perhaps other animals) because (i) rats’ natural resistance to Toxoplasma is more relevant to human toxoplasmosis (Dubey and Frenkel, 1998); (ii) a clinical human-lethal Toxoplasma strain causes similar pathology and lethality in rats (Loeuillet et al., 2019); and (iii) a single locus linked to Toxoplasma susceptibility between rat strains is associated with human susceptibility to congenital toxoplasmosis (Witola et al., 2011; Witola et al., 2014). Unlike in mice, Toxoplasma infection in rats is rapidly controlled at the site of infection during the acute infection stage (Fig 1). Macrophages are the cell type preferentially infected by Toxoplasma in vivo (Jensen et al., 2011), and rat primary macrophages exhibit a higher anti-Toxoplasma activity (Chinchilla et al., 1982; Li et al., 2012). Thus, we will focus on rat macrophage-intrinsic immune machinery and elucidate their roles in inhibiting Toxoplasma proliferation. Understanding the molecular basis of how these innate immune mechanisms endow rats with resistance to acute toxoplasmosis will help uncover the human restriction factors and have implications for human biology.

Parasite virulence - Understand Toxoplasma effector-mediated immune modulation in macrophages.

Macrophages are essential for the first-line defense against Toxoplasma infection (Park and Hunter, 2020). To avoid clearance, Toxoplasma utilizes its secretory effectors to modulate host macrophages and change the cellular microenvironment toward a pro-Toxoplasma state (Hakimi et al., 2017). Given macrophages from different hosts exhibit distinct anti-Toxoplasma responses (Park and Hunter, 2020), we will study the function of Toxoplasma secretory effectors that mediate host-specific immune modulation and maintain parasite proliferation in macrophages from a particular host species. As a proof-of-concept to identify host species-specific parasite effectors, we performed genome-wide CRISPR screens (Fig 2) and identified Toxoplasma secreted effectors that specifically determine parasite fitness in murine BMDMs (Wang et al., 2020) or rat BMDMs (unpublished data). To expand our view, the Wang lab will initiate another CRISPR screen to discover fitness-conferring Toxoplasma secretory effectors in human macrophages. By comparing the results of these screens, we will be able to identify Toxoplasma effector-coding genes belonging to different categories, such as murine-specific genes, rat-specific genes, human-specific genes, and genes determining parasite fitness in macrophages from all tested hosts (referred to as common genes) (Fig 2). Functional analysis of effector-coding genes from different categories will not only aid in revealing the strategies Toxoplasma uses to infect various host species but also complement the discoveries from the "Host Immunity" project by offering a perspective on host-pathogen interactions.

Overall, these projects will advance the field by (i) providing a clear view of how host innate immune machinery restricts Toxoplasma infection and contributes to Toxoplasma resistance and (ii) identifying novel Toxoplasma effectors important for modulating immune responses and establishing infection in different hosts, which will likely contribute to the future rational design of novel anti-Toxoplasma therapeutic strategies.