Rhoptry and Dense Granule Secreted Effectors Regulate CD8+ T Cell Recognition of Toxoplasma gondii Infected Host Cells.

Leah M Rommereim,Barbara A Fox,David J Bzik,Kiah L Butler,Viviana Cantillana,Gregory A Taylor

doi:10.3389/fimmu.2019.02104

Abstract

Toxoplasma gondii secretes rhoptry (ROP) and dense granule (GRA) effector proteins to evade host immune clearance mediated by interferon gamma (IFN-γ), immunity-related GTPase (IRG) effectors, and CD8+ T cells. Here, we investigated the role of parasite-secreted effectors in regulating host access to parasitophorous vacuole (PV) localized parasite antigens and their presentation to CD8+ T cells by the major histocompatibility class I (MHC-I) pathway. Antigen presentation of PV localized parasite antigens by MHC-I was significantly increased in macrophages and/or dendritic cells infected with mutant parasites that lacked expression of secreted GRA (GRA2, GRA3, GRA4, GRA5, GRA7, GRA12) or ROP (ROP5, ROP18) effectors. The ability of various secreted GRA or ROP effectors to suppress antigen presentation by MHC-I was dependent on cell type, expression of IFN-γ, or host IRG effectors. The suppression of antigen presentation by ROP5, ROP18, and GRA7 correlated with a role for these molecules in preventing PV disruption by IFN-γ-activated host IRG effectors. However, GRA2 mediated suppression of antigen presentation was not correlated with PV disruption. In addition, the GRA2 antigen presentation phenotypes were strictly co-dependent on the expression of the GRA6 protein. These results show that MHC-I antigen presentation of PV localized parasite antigens was controlled by mechanisms that were dependent or independent of IRG effector mediated PV disruption. Our findings suggest that the GRA6 protein underpins an important mechanism that enhances CD8+ T cell recognition of parasite-infected cells with damaged or ruptured PV membranes. However, in intact PVs, parasite secreted effector proteins that associate with the PV membrane or the intravacuolar network membranes play important roles to actively suppress antigen presentation by MHC-I to reduce CD8+ T cell recognition and clearance of Toxoplasma gondii infected host cells.

Highlights

Toxoplasma gondii [hereafter, Toxoplasma] frequently infects warm-blooded vertebrates including humans [1], yet infection by this parasite typically causes little disease burden due to the development of strong protective CD8+ T cell immunity
To investigate the role of IFN-γ priming, host cell Irgm1/Irgm3 molecules, and cell type in regulating the presentation of parasitophorous vacuole (PV) associated parasite antigens by major histocompatibility complex I (MHC-I), wild type (WT) and Irgm1/m3 deficient (Irgm1/m3−/−) bone marrow derived macrophages (BMM s) or bone marrow derived dendritic cells (BMDCs) were primed with IFN-γ, or left unprimed and host cells were infected with RH or RH-OVA
Antigen presentation by RH-OVA infected WT Bone marrow derived macrophages (BMM) s primed with IFN-γ was markedly increased in comparison to unprimed WT BMM s, unprimed Irgm1/m3−/− BMM s, as well as primed Irgm1/m3−/− BMM s infected with RH-OVA parasites (Figure 1, top panel)

Summary

Introduction

Toxoplasma gondii [hereafter, Toxoplasma] frequently infects warm-blooded vertebrates including humans [1], yet infection by this parasite typically causes little disease burden due to the development of strong protective CD8+ T cell immunity. The initiation of CD8+ T cell responses during infection is determined by the ability of professional antigen presenting cells to acquire and present antigens in the context of major histocompatibility complex I (MHC-I). Toxoplasma infected cells have been observed in vitro to present antigen to CD8+ T cells [3,4,5], and perforin mediated cytolysis of parasite infected cells suggests these cells present antigen in vivo to prime effector CD8+ T cells [6, 7]. Professional antigen presenting cells that phagocytosed Toxoplasma failed to initiate significant CD8+ or CD4+ T cell responses in vitro or during in vivo infection. Active invasion and formation of the parasitophorous vacuole (PV) in infected macrophages and dendritic cells is critical for the priming of significant CD4+ and CD8+ T cell responses [5]

Methods

Results

Conclusion