Jeroen Saeij

We are interested in host-parasite interactions and the genetics of susceptibility to infectious disease. We study host-parasite interactions between the obligate intracellular eukaryotic parasite Toxoplasma gondii and its hosts. Toxoplasma virulence differs, often quite dramatically, depending on the infecting strain and the host. The focus of the Saeij laboratory over the last years has been to identify genes of Toxoplasma that modulate the host cell and/or determine virulence, host genes and pathways that determine resistance/susceptibility, and to characterize their specific interactions. To achieve this we use a combination of genetics, genomics, biochemistry, microscopy, immunology and computational tools.

Toxoplasma biology, life cycle and disease
Toxoplasma gondii is an obligate intracellular parasite capable of infecting virtually any warm-blooded animal. In humans, Toxoplasma infections are widespread (~between 20-85% of humans are chronically infected, depending on the region) and can lead to severe disease (toxoplasmosis) in individuals with an immature (fetus) or suppressed immune system (AIDS patients). The life cycle of Toxoplasma is complex and includes both sexual and asexual stages. While the sexual cycle is limited to the gut of felines, the asexual cycle can occur in a wide range of hosts and has two major forms: the rapidly growing tachyzoite and the slowly growing encysted bradyzoite. Bradyzoite cysts can persist within the infected tissue for the life of the host, hidden from the immune system and anti-parasitic drugs. In addition to studying Toxoplasma biology in order to develop anti-Toxoplasma agents, researchers are studying Toxoplasma as an important model of the pathogenesis of other disease-causing Apicomplexan parasites such as Plasmodium (malaria), and Cryptosporidium parvum, another opportunistic pathogen associated with AIDS.

Toxoplasma population biology
The majority of Toxoplasma isolates from Europe and North America belong to three distinct clonal lines, referred to as types I, II and III.  The three types have been shown to differ widely in a number of phenotypes in mice such as virulence, persistence, migratory capacity, attraction of different cell-types and induction of cytokine expression.  Recent data suggest that such differences may also exist in human infection. In South America many other Toxoplasma strains exist, some of which can cause severe disease even in healthy individuals. One of our long-term goals is to understand how distinct Toxoplasma strains differ in their ability to cause disease in humans. Determining how particular Toxoplasma genotypes differ in their capacity to induce pathology in a particular animal species could enable prediction of the outcome of infection based on the genotype of the infecting organism. For example, not all seropositive AIDS patients develop toxoplasmic encephalitis; the ones that do might be infected with a particular subset of parasite strains. Similarly, seroconversion during pregnancy does not always lead to infection of the fetus; this might be a result of variability in the ability of different strains to cross the placental barrier. We have sequenced whole genomes of strains with different phenotypes and compared the differences and commonalities. This approach allowed us to correlate genotype with phenotype and have led to the identification of Toxoplasma loci and genes that affect fitness, clonality, virulence and modulation of host signaling pathways.
Identification of Toxoplasma virulence genes
Toxoplasma has a haploid genome with 14 chromosomes that total 65 Mbp in size, representing ~7900 genes ( Classical genetic crosses can be performed in Toxoplasma and several experimental crosses have been used to generate genetic linkage maps. To identify the Toxoplasma loci involved in virulence, we mapped virulence in F1 progeny derived from crosses between type II and type III strains. Five virulence loci were thus identified, and for three of these, genetic complementation showed that protein kinases (ROP18 and ROP16, respectively) and pseudokinases (ROP5)  are the key molecules (Saeij et al. 2006, Saeij et al. 2007, Reese et al. 2011). These are hypervariable rhoptryproteins that are secreted into the host cell upon invasion.

We have also used linkage mapping to identify Toxoplasma loci involved in modulation of host gene expression. Our initial analysis of the data identified that most of the strain-specific differences in the modulation of host cell transcription are mediated by two genes ROP16 and GRA15. Upon invasion by the parasite, ROP16 is injected into the host cell, where it ultimately affects the activation of signal transducer and activator of transcription (STAT) signaling pathways (Saeij et al. 2007). GRA15, encodes a dense granule protein, which affects activation of NF-kB, a transcription factor involved in the activation of the inflammatory response (Rosowski et al. 2011). Different Toxoplasma strains (e.g. type I, II and III) have different alleles of ROP16 and GRA15. Our results showed that the precise allelic combination of ROP16 and GRA15 determines how distinct Toxoplasma strains modulate the host immune response (Jensen et al. 2011). For example, type II strains can kill susceptible mice because they induce a hyper-inflammatory response that damages host tissue (Jensen et al. 2011). We are now combining genetic and biochemical approaches to identify host factors that interact with GRA15. Furthermore, we are continuing to investigate the host genes and pathways that mediate ROP16’s strong anti-inflammatory effects.

Overall, these results suggest that analogous to bacterial pathogens and their secretion system, it seems that Toxoplasma can secrete effectors into host cells to subvert host-cell signaling pathways. ROP16, ROP18 and ROP5 are members of a large protein family suggesting that Toxoplasma has a wide arsenal of effectors to modulate diverse host cell signaling pathways. The detailed characterization of these effectors represents a major focus of the laboratory.

Identification of host genes and pathways that influence resistance or susceptibility to Toxoplasma
In addition to parasite strain differences, host genetic differences also determine the outcome of toxoplasmosis. We found that inflammatory pathways are differentially regulated in macrophages from susceptible and resistant mice. Using a genetic approach we have identified the mouse genomic regions that determine these differences and we are now in the position to identify the exact causative genes.

Current projects in the lab:
1) Pathogenesis: what properties make certain strains more virulent than others and what proteins are involved?
2) How does Toxoplasma co-opt host-cell gene expression and how does this differ between strains?
3) How does Toxoplasma modulate the NF-κB signaling pathway?
4) How can we use Toxoplasma whole genome data to understand its population biology?
5) What are the host genes and pathways that influence resistance or susceptibility to Toxoplasma?

($Denotes shared first author. *Denotes graduate students from my lab. #Denotes Co-corresponding authors)

From MIT


44. Gorfu G$, Cirelli KM$*, Melo MB, Mayer-Barber K, Crown D, Koller BH, Masters S, Sher A, Leppla SH, Moayeri M#, Saeij JP#, Grigg ME#. A dual role for inflammasome sensors NLRP1 and NLRP3 in murine resistance to Toxoplasma gondii. mBio. 2014. Accepted.


43. Cirelli KM*, Gorfu G, Hassan MA, Printz M, Crown D, Leppla SH, Grigg ME#, Saeij JP#, Moayeri M#. Inflammasome sensor NLRP1 controls rat macrophage susceptibility to Toxoplasma gondii. PLoS Pathogens. 2013. Accepted.

42. Hassan MA, Butty V, Jensen KD, Saeij JP. The genetic basis for individual differences in mRNA splicing and Apobec1 editing activity in murine macrophages. Genome Research. Nov 18. [Epub ahead of print]; 2013.

41. Rosowski EE*, Nguyen QP, Camejo A, Spooner E, Saeij JP. Toxoplasma gondii inhibits IFN-γ- and IFN-β-induced host cell STAT1 transcriptional activity by increasing the association of STAT1 with DNA. Infection and Immunity. Nov 25. [Epub ahead of print]; 2013.

40. Melo MB, Nguyen QP, Cordeiro C, Hassan MA, Yang N*, McKell R*, Rosowski EE*, Julien L, Butty V, Darde M-L, Ajzenberg D, Fitzgerald K. Young LH, Saeij JP. Transcriptional analysis of murine macrophages infected with different Toxoplasma strains identifies novel regulation of host signaling pathways. PLoS Pathogens. 9(12):e1003779; 2013. PDF

39. Lim D, Gold DA, Julien L, Rosowski EE*, Niedelman W*, Yaffe MB#, Saeij JP#. Structure of the Toxoplasma gondii ROP18 kinase domain reveals a second ligand binding pocket required for acute virulence. The Journal of Biological Chemistry. 288(48):34968-80; 2013. PDF

38. Niedelman W*, Sprokholt JK, Clough B, Frickel EM, Saeij JP. Cell death of interferon-gamma stimulated human fibroblasts upon Toxoplasma gondii infection induces early parasite egress and limits parasite replication. Infection and Immunity. 81(12):4341-9; 2013. PDF

37. Camejo A, Gold DA, Lu D*, McFetridge K, Julien L, Yang N*, Jensen KD, Saeij JP. Identification of three novel Toxoplasma gondii rhoptry proteins. International Journal for Parasitology. S0020-7519(13)00222-1; 2013. [Epub ahead of print]. PDF

36. Yang N*, Farrell A, Niedelman W*, Melo MB, Lu D*, Julien L, Marth GT, Gubbels MJ, Saeij JP. Genetic basis for phenotypic differences between different Toxoplasma gondii type I strains. BMC Genomics. 14:467; 2013. PDF

35. Jensen KD, Hu K, Whitmarsh RJ, Hassan MA, Julien L, Lu D*, Chen L, Hunter CA, Saeij JP. Toxoplasma gondii rhoptry 16 kinase promotes host resistance to oral infection and intestinal inflammation only in the context of the dense granule protein GRA15. Infection and Immunity. 81(6):2156-67; 2013. PDF
‘Article of significant interest’ selected by the editors, see Research Spotlight: Infection and Immunity, 81(6): 1859; 2013.


34. Rosowski EE*, Saeij JP. Toxoplasma gondii clonal strains all inhibit STAT1 transcriptional activity but polymorphic effectors differentially modulate IFNγ induced gene expression and STAT1 phosphorylation. PLoS One. 7(12):e51448; 2012. PDF

33. Hassan MA, Melo MB, Haas B, Jensen KD, Saeij JP. De novo reconstruction of the Toxoplasma gondii transcriptome improves on the current genome annotation and reveals alternatively spliced transcripts and putative long non-coding RNAs. BMC Genomics. 13(1):696; 2012. PDF

32. Minot S$, Melo MB$, Li F, Lu D*, Niedelman W*, Levine SS, Saeij JP. Admixture and recombination among Toxoplasma gondii lineages explains global genome diversity. PNAS. 14;109(33):13458-63; 2012. PDF

31. Niedelman W*, Gold DA, Rosowski EE*, Sprokholt J, Lim D, Farid A, Melo MB, Spooner E, Yaffe MB, Saeij JP. The rhoptry proteins ROP18 and ROP5 mediate Toxoplasma gondii evasion of the murine, but not the human, interferon-gamma response. PLoS Pathogens. 8(6):e1002784; 2012. PDF

30. Szatanek T, Anderson-White BR, Faugno-Fusci DM, White M, Saeij JP, Gubbels MJ. Cactin is essential for G1 progression in Toxoplasma gondii. Molecular Microbiology. 84(3):566-77; 2012.

29. Lim DC, Cooke BM, Doerig C, Saeij JP. Toxoplasma and Plasmodium protein kinases: roles in invasion and host cell remodelling. International Journal for Parasitology. 42(1):21-32; 2012. [Review article] PDF


28. Virreira-Winter S, Niedelman W*, Jensen KD, Rosowski EE*, Julien L, Spooner E, Caradonna K, Burleigh BA, Saeij JP, Ploegh HL, Frickel E. Determinants of GBP recruitment to Toxoplasma gondii vacuoles and the parasitic factors that control it. PLoS One. 6(9):e24434; 2011. PDF

27. Reese ML, Zeiner GM, Saeij JP, Boothroyd JC, Boyle JP. Polymorphic family of injected pseudokinases is paramount in Toxoplasma virulence. PNAS 108(23):9625-30; 2011. PDF
Selected by Faculty of 1000.

26. Jensen KD, Wang Y, Tait ED, Shastri AJ, Hu K, Cornel L, Boedec E, Ong Y, Chien Y, Hunter CA, Boothroyd JC, Saeij JP. Toxoplasma polymorphic effectors determine macrophage polarization and intestinal inflammation. Cell Host & Microbe 9(6):472-83; 2011. PDF
Highlighted in: Macrophages as a battleground for Toxoplasma pathogenesis. Cell Host & Microbe 9(6):445-7; 2011.

25. Melo MB$, Jensen KD$, Saeij JP. Toxoplasma gondii effectors are master regulators of the inflammatory response. Trends in Parasitology. 27(11):487-95; 2011. [Review article] PDF

24. Rosowski EE$*, Lu D$*, Julien L, Rodda L, Gaiser R, Jensen KD, Saeij JP. Strain-specific activation of the NF-κB pathway by GRA15, a novel Toxoplasma dense granule protein. Journal of Experimental Medicine 208(1):195-212; 2011. PDF
Selected by Faculty of 1000. Highlighted in: It takes II to induce NF-κB. Nature Reviews Microbiology 9, 147, 2011.


23. Blader IJ, Saeij JP. Communication between Toxoplasma gondii and its host: impact on parasite growth, development, immune evasion, and virulence. APMIS (acta pathologica, microbiologica, et immunologica Scandinavica). 117(5-6):458-76; 2009. [Review article] PDF

From Postdoctoral work


22. Boyle JP, Saeij JP, Harada SY, Ajioka JW, Boothroyd JC. Expression quantitative trait locus mapping of Toxoplasma genes reveals multiple mechanisms for strain-specific differences in gene expression. Eukaryotic Cell. 7(8):1403-14; 2008.

21. Saeij JP, Arrizabalaga G, Boothroyd JC. A cluster of four surface antigen genes specifically expressed in bradyzoites, SAG2CDXY, plays an important role in Toxoplasma gondii persistence. Infection and Immunity. 76(6): 2402-2410; 2008.


20. Boyle JP, Saeij JP, Boothroyd JC. Inconsistent dissemination patterns of Toxoplasma gondii following oral infection. Experimental Parasitology. 116(3):302-305; 2007.

19. Saeij JP$, Coller S$, Boyle JP, Jerome ME, White MW, Boothroyd JC. Toxoplasma co-opts host gene expression by injection of a polymorphic kinase homologue. Nature. 445(7125): 324-327; 2007.
Selected by Faculty of 1000. Highlighted in: Toxo researchers spot the difference. Nature Reviews Microbiology 5, 86, 2007.

18. Saeij JP$, Boyle JP$, Coller SC, Taylor S, Sibley LD, Brooke-Powell ET, Ajioka JW, Boothroyd JC. Polymorphic secreted kinases are key virulence factors in toxoplasmosis. Science. 314(5806):1780-1783; 2006.
Selected by Faculty of 1000. Highlighted in: Toxo researchers spot the difference. Nature Reviews Microbiology 5, 86, 2007; Nature Immunology 8, 129, 2007.


17. Boyle, JP, Rajasekar B, Saeij JP, Ajioka JW, Berriman M, Paulsen I, Roos DS, Sibley D, White M, Boothroyd JC. Just one cross appears capable of dramatically altering the population biology of a eukaryotic pathogen like Toxoplasma gondii. PNAS. 103(27): 10514-10519; 2006.
Selected by Faculty of 1000.

16. Boyle JP, Saeij JP, Cleary MD, Boothroyd JC. Analysis of gene expression during development: lessons from the Apicomplexa. Microbes and Infection. 8(6): 1623-1630; 2006. [Review article]


15. Saeij JP, Boyle JP, Boothroyd JC. Differences among the three major strains of Toxoplasma gondii and their specific interactions with the infected host. Trends in Parasitology. 21(10): 476-81; 2005.

14. Saeij JP$, Boyle JP$, Grigg ME, Arrizabalaga G, Boothroyd JC. Bioluminescence imaging of Toxoplasma gondii infection in living mice reveals dramatic differences between strains. Infection and Immunity. 73(2): 695-702; 2005.
Selected by Faculty of 1000.

From graduate work

13. Huttenhuis HBT, Taverne-Thiele AJ, Grou CPO, Bergsma J, Saeij JP, Nakayasu C, Rombout JHWM. Ontogeny of the common carp (Cyprinus carpio L.) innate immune system. Developmental & Comparative Immunology. 30(6): 557-574; 2006.


12. Joerink M, Saeij JP, Stafford JL, Belosevic M, Wiegertjes GF. Animal models for the study of innate immunity: protozoan infections in fish. Symposia of the Society for Experimental Biology. 55:67-89; 2004.


11. Saeij JP, Groeneveld A, Van Rooijen N, Haenen OLM, Wiegertjes GF. Minor effect of depletion of resident macrophages from peritoneal cavity on resistance of common carp Cyprinus carpio to blood flagellates. Diseases of aquatic organisms. 57: 67-75; 2003.

10. Engelsma MY, Stet RJM, Saeij JP, Verburg-van Kemenade BML. Differential expression and haplotypic variation of two interleukin-1 beta genes in the common carp (Cyprinus carpio L.). Cytokine. 22: 21-32; 2003.

9. Saeij JP, Stet RJM, de Vries B, van Muiswinkel WB, Wiegertjes GF. Molecular and functional characterization of carp TNF: a link between TNF polymorphism and trypanotolerance? Developmental & Comparative Immunology. 27: 29-41; 2003.

8. Saeij JP, van Muiswinkel WB, van de Meent M, Amaral C, Wiegertjes GF. Different capacities of carp leukocytes to encounter nitric oxide-mediated stress: a role for the intracellular reduced glutathione pool. Developmental & Comparative Immunology. 27: 555-568; 2003.

7. Saeij JP, Verburg-van Kemenade BML, van Muiswinkel WB, Wiegertjes GF. Daily handling reduces resistance of carp to Trypanoplasma borreli: in vitro modulatory effects of cortisol on leukocyte function and apoptosis. Developmental & Comparative Immunology. 27: 233-245; 2003.

6. Saeij JP, de Vries BJ, Wiegertjes GF. The immune response of carp to Trypanoplasma borreli: kinetics of immune gene expression and polyclonal lymphocyte activation. Developmental & Comparative Immunology. 27: 859-874; 2003.


5. Saeij JP, Stet RJM, Wiegertjes GF. Immune modulation by fish kinetoplastid parasites: a role for nitric oxide. Parasitology. 124: 77-86; 2002.


4. Saeij JP, Stet RJM, Groeneveld A, Verburg-van Kemenade BML, van Muiswinkel WB, Wiegertjes GF. Molecular and functional characterization of a fish inducible-type nitric oxide synthase. Immunogenetics. 51: 339-346; 2000.


3. Saeij JP, Wiegertjes GF, Stet RJM. Identification and characterization of a fish natural resistance-associated macrophage protein (NRAMP) cDNA. Immunogenetics. 50: 60-66; 1999.


2. Stet RJM, Kruiswijk CP, Saeij JP, Wiegertjes GF. Major histocompatibility genes in cyprinid fishes: theory and practice. Immunological Reviews. 166: 301-316; 1998. [Review article]

1. Verburg-van Kemenade BML, Saeij JP, Flik G, Willems PHGM. Ca2+ signals during early lymphocyte activation in carp Cyprinus carpio L. The Journal of Experimental Biology. 201: 591-598; 1998.