A new perspective on anti-Toxoplasma gondii immunity

Lead Research Organisation: MRC National Inst for Medical Research

Abstract

Toxoplasma gondii is a pathogen related to the malaria parasite with a wide range of host specificity that includes humans. It can cause brain disease and is a serious health threat to individuals with a poorly functioning immune system, including newborns. No vaccines or drugs to fight the disease are available and once infected the parasite remains with the host its whole life. We don't exactly know how Toxoplasma maintains the intricate balance between survival inside a host and elimination.
We study how the host cell redesigns and controls the composition of its membrane system and the sites to which the parasite has access directly following infection. These same controls enable the immune system to "see" which cells are infected. Proteins called large GTPases are upregulated in infected cells and brought to the membrane of the enclosure in which the parasites multiply. We investigate how this recruitment contributes to the parasites’s survival or elimination.
Moreover, we have created mice that bear large numbers of immune cells, capable of specific recognition of Toxoplasma. We will ask how these specific immune cells control the infection with Toxoplasma in the chronic phase. This is especially important, as clinically the parasite lies permanently dormant due to the pressure these cells exert in normal individuals.
Understanding these parameters of parasite control will contribute to the design of effective countermeasures against Toxoplasma.

Technical Summary

The protozoan parasite Toxoplasma gondii infects a broad range of hosts, with a seroprevalence in man of about 30%. It is unclear how Toxoplasma maintains the intricate balance between survival and host defence. IFN?, the main cytokine responsible for its control, activates cells to restrict intracellular parasite replication or to kill intracellular Toxoplasma. Cell-mediated immunity, driven mostly by CD8 T cells, confers resistance to the chronic phase of the parasite. The outcome of an infection with Toxoplasma is determined not only by the host's immune status, but also by the genotype of the infecting strain. The major cause of Toxoplasma pathogenesis results from parasite burden, concurrent with an over-stimulation of the immune system in the form of high levels of T helper cell type 1 cytokines, increased apoptosis and organ damage.

Among the proteins highly upregulated as a consequence of interferon-stimulated transcription are the p65 GTPases (GBPs), members of the family of large GTPases that mediate resistance to intracellular pathogens. Contrary to the p47 class of GTPases, the GBPs are present in multiple copies in both the mouse and human genome. Even though the structure of human GBP1 has been solved and the ability of some family members to hydrolyze GTP to GDP and then to GMP is established, limited in vivo role for this class of enzymes have been assigned and few interaction partners are known. We will investigate the mode of action as well as the function of this class of GTPases on the vacuole of Toxoplasma in a parasite virulence-dependent fashion.

Through their production of IFN?, CD8 T cells are important in conferring resistance against Toxoplasma. After an infection with Toxoplasma, both CD4 and CD8 T cells infiltrate the brain. Thus far only transgenic parasites expressing model antigens in combination with transgenic T cells have been used as models for infection. Obvious shortcomings of this approach include difficulties in controlling expression levels and localization of the antigen. Moreover, the model transgenic T cell receptors might not recognise the model antigen with physiological affinity.

We and others have identified 3 H-2Ld and 1 H-2Kb restricted CD8 T cell epitope derived from Toxoplasma proteins. Using flow cytometry to sort antigen-specific CD8 T cells from Toxoplasma-infected BL/6 x Balb/c F1 mice, transnuclear (TN) Toxoplasma-specific CD8 T cell receptor (TCR) mice for all four of these Toxoplasma epitope-MHC complexes (Ld-Rop7, Ld-Gra4, Ld-Gra6 and Kb-tgd057) have been generated by somatic cell nuclear transfer. We have three lines of mice with unique TCR sequences that all recognize Ld-Rop7, thus enabling further studies on how the binding affinity of the TCR to the MHC controls parasite burden and dictates memory CD8 T cell properties during the chronic phase of infection.

Our long-term goal is to identify novel pathways and mechanisms of host resistance to Toxoplasma.

Publications

10 25 50
 
Description Boehringer Ingelheim PhD Fellowship
Amount £45,000 (GBP)
Organisation Boehringer Ingelheim 
Sector Private
Country Germany
Start 06/2012 
End 01/2014
 
Description Wellcome Trust Career Development Award
Amount £700,000 (GBP)
Organisation Wellcome Trust 
Sector Charity/Non Profit
Country United Kingdom
Start 07/2011 
End 06/2016
 
Title iPSC-derived macrophages 
Description We have successfully set up the production of induced pluripotent stem cell-derived macrophages. My postdoc Rabia Khan did this work receiving help and guidance from Lyn Healy at the Crick. They used protocols sourced from Sally Cowley in Oxford in an agreement of collaboration. The iPSC line is under MTA with the Sanger. 
Type Of Material Cell line 
Year Produced 2017 
Provided To Others? Yes  
Impact We will use these cells in the future to study the impact of human immune defence mechanisms on Salmonella and Toxoplasma. Additionally, we will use these cells to study general mechanisms of action of the Guanylate Binding Proteins. 
 
Description NIMR Centenary Day 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Other academic audiences (collaborators, peers etc.)
Results and Impact Talk enabled a broader recognition of our work and lead to discussions with peer scientists.

The talk lead to further discussions with scientists within the institute and from abroad.
Year(s) Of Engagement Activity 2014
 
Description NIMR Open Day 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Type Of Presentation Keynote/Invited Speaker
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact This was part of the NIMR open day meant to engage the public in North London. I gave a 15 minute presentation followed by questions from the audience.

The open day including my talk was advertised and later summarised on social media.
Year(s) Of Engagement Activity 2013
 
Description NIMR School visit 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Schools
Results and Impact Talk stimulated students to ask questions.
Year(s) Of Engagement Activity 2013,2014
 
Description St Paul School visit 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Schools
Results and Impact Talk sparked questions from the students.
Year(s) Of Engagement Activity 2013,2014
 
Description Teaching an MSc module for London School of Hygiene and Tropical Medicine 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Postgraduate students
Results and Impact I teach a 1.5h MSc level class on Immunity to Toxoplasma gondii.
Year(s) Of Engagement Activity 2017
 
Description Teaching an MSc module for London School of Hygiene and Tropical Medicine 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Postgraduate students
Results and Impact 1.5h lecture on Immunity to Toxoplasma gondii to MSc students at the London School of Hygiene and Tropical Medicine
Year(s) Of Engagement Activity 2014,2015,2016
 
Description UCL LMCB Immunology and Infection Master's Course 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Postgraduate students
Results and Impact I teach a 2h class based on literature reading discussing a topic within Immunity to Toxoplasma gondii.
Year(s) Of Engagement Activity 2017