Abstract
Toxoplasma gondii (TOXO) infection typically results in chronic latency due to its ability to form cysts in the brain and other organs. Latent toxoplasmosis could promote innate immune responses and impact brain function. A large body of evidence has linked TOXO infection to severe mental illness (SMI). We hypothesized that TOXO immunoglobulin G (IgG) seropositivity, reflecting previous infection and current latency, is associated with increased circulating neuron-specific enolase (NSE), a marker of brain damage, and interleukin-18 (IL-18), an innate immune marker, mainly in SMI. We included 735 patients with SMI (schizophrenia or bipolar spectrum) (mean age 32 years, 47% women), and 518 healthy controls (HC) (mean age 33 years, 43% women). TOXO IgG, expressed as seropositivity/seronegativity, NSE and IL-18 were measured with immunoassays. We searched for main and interaction effects of TOXO, patient/control status and sex on NSE and IL-18. In the whole sample as well as among patients and HC separately, IL-18 and NSE concentrations were positively correlated (p < 0.001). TOXO seropositive participants had significantly higher NSE (3713 vs. 2200 pg/ml, p < 0.001) and IL-18 levels (1068 vs. 674 pg/ml, p < 0.001) than seronegative participants, and evaluation within patients and HC separately showed similar results. Post-hoc analysis on cytomegalovirus and herpes simplex virus 1 IgG status showed no associations with NSE or IL-18 which may suggest TOXO specificity. These results may indicate ongoing inflammasome activation and neuronal injury in people with TOXO infections unrelated to diagnosis.
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Introduction
Toxoplasma gondii (TOXO) is an intracellular parasite with a high seroprevalence affecting 30–65% of the human population worldwide1. Human hosts contract the infection predominantly via ingestion of oocysts from environments contaminated with cat feces or ingestion of tissue cysts in undercooked meat, while another route of infection is the transplacental1,2. The TOXO primary infection of immunocompetent individuals is typically either asymptomatic or oligosymptomatic with lymphadenopathy and flu-like symptoms, whereas immunocompromised and congenitally infected hosts may develop severe disease including central nervous system (CNS) pathology2,3. Even when infecting immunocompetent hosts, TOXO typically establishes an inapparent and life-long latent infection due to its ability to form latent tissue cysts mostly in the brain, eyes, myocardium and skeletal muscles1. This latent infection can be complicated with symptomatic reactivations mainly in immunodeficient hosts3.
Schizophrenia (SZ) and bipolar disorder (BP) comprise severe mental illnesses (SMI) with multipart etiology where both genetic predisposition and environmental exposures are implicated4,5. SMI are linked to increased all-cause mortality6 and increased risk for suicide and self-perceived incapacity7. A large body of evidence has linked SZ and BP to TOXO infections8,9,10. However, the mechanisms underlying these associations are largely unknown.
The host innate immune system is the first line of defense against infections. It responds to pathogens, such as TOXO, by recognizing pathogen-associated molecular patterns and inducing among others activation of the Nod-like receptor (NLR) family pyrin domain containing 3 (NLRP3) inflammasomes, which results in the release of the highly pro-inflammatory cytokines interleukin-18 (IL-18) and IL-1β13. While our results on IL-18 are suggestive of a TOXO-induced systemic inflammasome activation in both patients and HC, the latent infection could have an additive impact on the already increased IL-18 levels of patients with SMI13 which may result in more detrimental consequences in these patients relative to HC.
Even though the TOXO-NSE association has also not been previously explored in human hosts, the increased NSE in TOXO+ participants is in accordance with an experimental study where TOXO-infected mice showed higher brain NSE expression relative to non-infected mice, with the highest increase early after the infection18. NSE is not only implicated in neuronal damage, but also in neuronal differentiation, maturation and migration14,15,16. We have recently reported, studying a sample of patients with SMI and HC overlap** with the present study, lower circulating NSE levels in both adults and adolescents with SMI relative to HC, suggestive of a NSE-related neurodevelopmental disturbance29. In the present study, patients had also lower NSE than HC, while among all participants as well as within each diagnostic group (patients and HC), TOXO+ participants had higher NSE concentrations than TOXO− participants. Our results may suggest that two different processes affect NSE levels in TOXO+ patients: a neurodevelopmental disturbance related to the disorder leads to lower NSE in this group while TOXO-related neuronal injury increases NSE.
In our post-hoc analyses, we studied two common herpesviruses (CMV and HSV1) which, as with TOXO, are neurotropic pathogens that typically establish life-long latency in human hosts37,38,39. CMV40,41,42 and HSV143,44,45 are the best studied herpesviruses in relation to cognitive and MRI measures in SMI. In the present study, we did not find any associations between CMV or HSV1 seropositivity and NSE or IL-18, which suggests some specificity of the TOXO infection. The non-elevated NSE and IL-18 levels in CMV and HSV1 seropositive participants relative to seronegative participants may be due to an absence of a substantial ongoing impact of the viruses on the CNS. The previously reported brain structure and cognitive aberrations might thereby be related to the primary viral infection or viral reactivations.
The present study has certain limitations. First, the study is cross-sectional and causality cannot be determined. Although rather unlikely, we cannot exclude a reverse causation, meaning that individuals with higher inflammasome activation might contract TOXO-infections more often than those with lower inflammasome activation. Further, TOXO as well as CMV and HSV1 IgG seropositivity indicates exposure to the pathogens, but the time of the primary infection or the subsequent reactivations cannot be specified. In addition, even though the studied sample was well-characterized, we cannot exclude that the TOXO-IL-18 and the TOXO-NSE associations may have been confounded by unidentified factors that we have not been able to control for. Further, even though NSE is thought to be the main biomarker indicating neuronal damage17 we cannot exclude alternative interpretations of the elevated NSE levels in TOXO+ participants. NSE is a glycolytic enzyme15 and further, it is present not only in neurons, but also in neuroendocrine cells14, with increased circulating levels found in patients with neuroendocrine tumours 15. The elevated NSE levels in TOXO+ participants may thereby suggest altered neuronal glycolysis or neuroendocrine system dysregulation. Finally, TOXO infection was associated with inflammasome activation, but the proposed connection to heightened vulnerability to psychiatric and medical conditions are necessarily speculative, as the observed IL-18 increase may lack clinical significance.
To conclude, TOXO seropositivity (which mirrors past infection and current latency) was linked to increased circulating IL-18 and NSE levels, irrespective of diagnostic status. IL-18 and NSE were correlated irrespective of diagnostic and TOXO status. The findings suggest novel relationships between TOXO and IL-18 and NSE, and given that these relationships are not different between patients with SMI and HC, the findings may reflect basic immunological mechanisms and neuronal damage not related to SMI. The elevated TOXO-related NSE can reflect progressive neuronal damage while the increased inflammasome activation might render TOXO-exposed individuals susceptible to both psychiatric and systemic diseases. Further scientific investigation is warranted to establish the connection between TOXO infections and such chronic illnesses, as well as to investigate whether exposed individuals might benefit from antiprotozoal medication.
Data availability
Data supporting the findings of the present study have repository at NORMENT/Oslo University Hospital. Restrictions apply to the availability of data and are thereby not publicly available. Data can be made available under reasonable request to the corresponding author and with permission of NORMENT/Oslo University Hospital, in accordance with the ethics agreements/research participants consent.
References
Sullivan, W. J. Jr. & Jeffers, V. Mechanisms of Toxoplasma gondii persistence and latency. FEMS Microbiol. Rev. 36, 717–733. https://doi.org/10.1111/j.1574-6976.2011.00305.x (2012).
McAuley, J. B. Congenital toxoplasmosis. J. Pediatr. Infect. Dis. Soc. 3(Suppl 1), S30-35. https://doi.org/10.1093/jpids/piu077 (2014).
Tenter, A. M., Heckeroth, A. R. & Weiss, L. M. Toxoplasma gondii: From animals to humans. Int. J. Parasitol. 30, 1217–1258. https://doi.org/10.1016/s0020-7519(00)00124-7 (2000).
Vieta, E. et al. Bipolar disorders. Nat. Rev. Dis. Primers 4, 18008. https://doi.org/10.1038/nrdp.2018.8 (2018).
Kahn, R. S. et al. Schizophrenia. Nat. Rev. Dis. Primers 1, 15067. https://doi.org/10.1038/nrdp.2015.67 (2015).
Hayes, J. F., Marston, L., Walters, K., King, M. B. & Osborn, D. P. J. Mortality gap for people with bipolar disorder and schizophrenia: UK-based cohort study 2000–2014. Br. J. Psychiatry 211, 175–181. https://doi.org/10.1192/bjp.bp.117.202606 (2017).
Costanza, A. et al. Demoralization in suicide: A systematic review. J. Psychosom. Res. 157, 110788. https://doi.org/10.1016/j.jpsychores.2022.110788 (2022).
Sutterland, A. L. et al. Beyond the association. Toxoplasma gondii in schizophrenia, bipolar disorder, and addiction: Systematic review and meta-analysis. Acta Psychiatr. Scand. 132, 161–179. https://doi.org/10.1111/acps.12423 (2015).
Sutterland, A. L. et al. Toxoplasma gondii infection and clinical characteristics of patients with schizophrenia: A systematic review and meta-analysis. Schizophrenia Bull. Open https://doi.org/10.1093/schizbullopen/sgaa042 (2020).
Del Grande, C., Galli, L., Schiavi, E., Dell’Osso, L. & Bruschi, F. Is Toxoplasma gondii a trigger of bipolar disorder?. Pathogens https://doi.org/10.3390/pathogens6010003 (2017).
Latz, E., **ao, T. S. & Stutz, A. Activation and regulation of the inflammasomes. Nat. Rev. Immunol. 13, 397–411. https://doi.org/10.1038/nri3452 (2013).
Gorfu, G. et al. Dual role for inflammasome sensors NLRP1 and NLRP3 in murine resistance to Toxoplasma gondii. mBio https://doi.org/10.1128/mBio.01117-13 (2014).
Szabo, A. et al. Increased circulating IL-18 levels in severe mental disorders indicate systemic inflammasome activation. Brain Behav. Immunol. 99, 299–306. https://doi.org/10.1016/j.bbi.2021.10.017 (2022).
Marangos, P. J. & Schmechel, D. E. Neuron specific enolase, a clinically useful marker for neurons and neuroendocrine cells. Annu. Rev. Neurosci. 10, 269–295. https://doi.org/10.1146/annurev.ne.10.030187.001413 (1987).
Isgro, M. A., Bottoni, P. & Scatena, R. Neuron-specific enolase as a biomarker: Biochemical and clinical aspects. Adv. Exp. Med. Biol. 867, 125–143. https://doi.org/10.1007/978-94-017-7215-0_9 (2015).
Haque, A., Polcyn, R., Matzelle, D. & Banik, N. L. New insights into the role of neuron-specific enolase in neuro-inflammation, neurodegeneration, and neuroprotection. Brain Sci. https://doi.org/10.3390/brainsci8020033 (2018).
Cheng, F., Yuan, Q., Yang, J., Wang, W. & Liu, H. The prognostic value of serum neuron-specific enolase in traumatic brain injury: Systematic review and meta-analysis. PLoS One 9, e106680. https://doi.org/10.1371/journal.pone.0106680 (2014).
Dincel, G. C. & Atmaca, H. T. Role of oxidative stress in the pathophysiology of Toxoplasma gondii infection. Int. J. Immunopathol. Pharmacol. 29, 226–240. https://doi.org/10.1177/0394632016638668 (2016).
Sasai, M. & Yamamoto, M. Innate, adaptive, and cell-autonomous immunity against Toxoplasma gondii infection. Exp. Mol. Med. 51, 1–10. https://doi.org/10.1038/s12276-019-0353-9 (2019).
Wohlfert, E. A., Blader, I. J. & Wilson, E. H. Brains and brawn: Toxoplasma infections of the central nervous system and skeletal muscle. Trends Parasitol. 33, 519–531. https://doi.org/10.1016/j.pt.2017.04.001 (2017).
First, M. B., Spitzer, R. L., Gibbon, M. & Williams, J. B. Structured clinical interview for DSM-IV axis I disorders (SCIDI), clinician version, administration booklet. (American Psychiatric Association Publishing, 2012).
Spitzer, R. L. et al. Utility of a new procedure for diagnosing mental disorders in primary care. The PRIME-MD 1000 study. JAMA 272, 1749–1756 (1994).
Galobardes, B., Shaw, M., Lawlor, D. A., Lynch, J. W. & Davey Smith, G. Indicators of socioeconomic position (part 1). J. Epidemiol. Community Health 60, 7–12. https://doi.org/10.1136/jech.2004.023531 (2006).
Andreasen, N. C., Pressler, M., Nopoulos, P., Miller, D. & Ho, B. C. Antipsychotic dose equivalents and dose-years: A standardized method for comparing exposure to different drugs. Biol. Psychiatry 67, 255–262. https://doi.org/10.1016/j.biopsych.2009.08.040 (2010).
Dickerson, F. B. et al. Association of serum antibodies to herpes simplex virus 1 with cognitive deficits in individuals with schizophrenia. Arch. Gen. Psychiatry 60, 466–472. https://doi.org/10.1001/archpsyc.60.5.466 (2003).
Dickerson, F. et al. Association between cytomegalovirus antibody levels and cognitive functioning in non-elderly adults. PLoS One 9, e95510. https://doi.org/10.1371/journal.pone.0095510 (2014).
Yolken, R. H. et al. Antibodies to Toxoplasma gondii in individuals with first-episode schizophrenia. Clin. Infect. Dis. 32, 842–844. https://doi.org/10.1086/319221 (2001).
Wang, H., Yolken, R. H., Hoekstra, P. J., Burger, H. & Klein, H. C. Antibodies to infectious agents and the positive symptom dimension of subclinical psychosis: The TRAILS study. Schizophr. Res. 129, 47–51. https://doi.org/10.1016/j.schres.2011.03.013 (2011).
Andreou, D. et al. Lower circulating neuron-specific enolase concentrations in adults and adolescents with severe mental illness. Psychol. Med. 20, 1–10 (2021).
Beyerlein, A. Quantile regression-opportunities and challenges from a user’s perspective. Am. J. Epidemiol. 180, 330–331. https://doi.org/10.1093/aje/kwu178 (2014).
Cheon, S. Y., Kim, J., Kim, S. Y., Kim, E. J. & Koo, B. N. Inflammasome and cognitive symptoms in human diseases: Biological evidence from experimental research. Int. J. Mol. Sci. https://doi.org/10.3390/ijms21031103 (2020).
Iwata, M., Ota, K. T. & Duman, R. S. The inflammasome: Pathways linking psychological stress, depression, and systemic illnesses. Brain Behav. Immunol. 31, 105–114. https://doi.org/10.1016/j.bbi.2012.12.008 (2013).
Kaufmann, F. N. et al. NLRP3 inflammasome-driven pathways in depression: Clinical and preclinical findings. Brain Behav. Immunol. 64, 367–383. https://doi.org/10.1016/j.bbi.2017.03.002 (2017).
Berardelli, I. et al. The involvement of hypothalamus–pituitary–adrenal (HPA) axis in suicide risk. Brain Sci. https://doi.org/10.3390/brainsci10090653 (2020).
Papuc, E. & Rejdak, K. Increased cerebrospinal fluid S100B and NSE reflect neuronal and glial damage in Parkinson’s disease. Front. Aging Neurosci. 12, 156. https://doi.org/10.3389/fnagi.2020.00156 (2020).
Cai, G., Kastelein, R. & Hunter, C. A. Interleukin-18 (IL-18) enhances innate IL-12-mediated resistance to Toxoplasma gondii. Infect. Immunol. 68, 6932–6938. https://doi.org/10.1128/IAI.68.12.6932-6938.2000 (2000).
Dupont, L. & Reeves, M. B. Cytomegalovirus latency and reactivation: Recent insights into an age old problem. Rev. Med. Virol. 26, 75–89. https://doi.org/10.1002/rmv.1862 (2016).
Wills, M. R., Poole, E., Lau, B., Krishna, B. & Sinclair, J. H. The immunology of human cytomegalovirus latency: Could latent infection be cleared by novel immunotherapeutic strategies?. Cell Mol. Immunol. 12, 128–138. https://doi.org/10.1038/cmi.2014.75 (2015).
Duarte, L. F. et al. Herpes simplex virus Type 1 infection of the central nervous system: Insights into proposed interrelationships with neurodegenerative disorders. Front. Cell. Neurosci. 13, 46. https://doi.org/10.3389/fncel.2019.00046 (2019).
Andreou, D. et al. Cytomegalovirus infection associated with smaller dentate gyrus in men with severe mental illness. Brain Behav. Immunol. https://doi.org/10.1016/j.bbi.2021.05.009 (2021).
Andreou, D. et al. Cytomegalovirus infection and IQ in patients with severe mental illness and healthy individuals. Psychiatry Res. 300, 113929. https://doi.org/10.1016/j.psychres.2021.113929 (2021).
Andreou, D. et al. Cytomegalovirus infection associated with smaller total cortical surface area in Schizophrenia spectrum disorders. Schizophr. Bull. https://doi.org/10.1093/schbul/sbac036 (2022).
Tucker, J. D. & Bertke, A. S. Assessment of cognitive impairment in HSV-1 positive schizophrenia and bipolar patients: Systematic review and meta-analysis. Schizophr. Res. 209, 40–47. https://doi.org/10.1016/j.schres.2019.01.001 (2019).
Prasad, K. M. et al. Progressive gray matter loss and changes in cognitive functioning associated with exposure to herpes simplex virus 1 in schizophrenia: A longitudinal study. Am. J. Psychiatry 168, 822–830. https://doi.org/10.1176/appi.ajp.2011.10101423 (2011).
Andreou, D. et al. Herpes simplex virus 1 infection on grey matter and general intelligence in severe mental illness. Transl. Psychiatry 12, 276. https://doi.org/10.1038/s41398-022-02044-3 (2022).
Acknowledgements
This work was supported by the Research Council of Norway (223273) and the South-Eastern Norway Regional Health Authority (2019-108).
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D.A. drafted the manuscript, performed the statistical analysis and interpreted the data. D.A., N.E.S., O.A.A., T.U. and .I.A conceptualized and designed the work. I.A. initiated and supervised the study. R.H.Y. was responsible for the serology assessments. All co-authors had substantial contributions to the interpretation of data, critically revised the manuscript for important intellectual content and approved the final version to be published.
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OAA is a consultant to HealthLytix, and received speaker’s honorarium from Lundbeck and Sunovion. SD and IA received speaker’s honoraria from Lundbeck. All other authors reported no potential conflicts of interest.
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Andreou, D., Steen, N.E., Mørch-Johnsen, L. et al. Toxoplasma gondii infection associated with inflammasome activation and neuronal injury. Sci Rep 14, 5327 (2024). https://doi.org/10.1038/s41598-024-55887-9
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DOI: https://doi.org/10.1038/s41598-024-55887-9
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