Abstract

BackgroundBiomarker discovery remains a major challenge for predictive medicine, in particular, in the context of chronic diseases. This is true for the widespread protozoan Toxoplasma gondii which establishes long-lasting parasitism in metazoans, humans included. This microbe successively unfolds distinct genetic programs that direct the transition from high to low replicative potential inside host cells. As a slow-replicating cell, the T. gondii bradyzoite developmental stage persists enclosed in a cyst compartment within tissues including the nervous system, being held by a sustained immune equilibrium which accounts for the prolonged clinically silent phase of parasitism. Serological surveys indicate that nearly one third of the human population has been exposed to T. gondii and possibly host bradyzoites. Because any disruption of the immune balance drives the reverse transition from bradyzoite to fast replicating tachyzoite and uncontrolled growth of the latter, these people are at risk for life-threatening disease. While serological tests for discriminating recent from past infection are available, there is yet no immunogenic biomarker used in the serological test to allow ascertaining the presence of persistent bradyzoites.ResultsCapitalizing on genetically engineered parasites induced to produce mature bradyzoites in vitro, we have identified the BCLA/MAG2 protein being restricted to the bradyzoite and the cyst envelope. Using laboratory mice as relevant T. gondii host models, we demonstrated that BCLA/MAG2 drives the generation of antibodies that recognize bradyzoite and the enveloping cyst structure. We have designed an ELISA assay based on a bacterially produced BCLA recombinant polypeptide, which was validated using a large collection of sera from mice of different genetic backgrounds and infected with bcla+ or bcla-null cystogenic and non-cystogenic T. gondii strains. To refine the design of the ELISA assay, we applied high-resolution BCLA epitope mapping and identified a specific combination of peptides and accordingly set up a selective and sensitive ELISA assay which allowed the detection of anti-BCLA/MAG2 antibodies in the sera of human patients with various forms of toxoplasmosis.ConclusionsWe brought proof of principle that anti-BCLA/MAG2 antibodies serve as specific and sensitive serological markers in the perspective of a combinatorial strategy for detection of persistent T. gondii parasitism.

Highlights

  • Biomarker discovery remains a major challenge for predictive medicine, in particular, in the context of chronic diseases

  • We identified a specific cyst-resident protein, hereafter referred as brain cyst-loaded antigen (BCLA) (Brain Cyst Load-associated Antigen), independently reported as the matrix antigen 2 (MAG2) [17, 18], the expression of which is regulated at the transcriptional level by MORC

  • Depletion of MORC phenocopies the inhibition of HDAC3 in inducing the expression of bradyzoite- and cystassociated immunodominant antigens We have already reported that the T. gondii in vitro conversion of the intracellular dividing tachyzoites into bradyzoites could be achieved through the pharmacological inactivation of HDAC3 using the HDAC inhibitor FR235222 [20,21,22]

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Summary

Introduction

Biomarker discovery remains a major challenge for predictive medicine, in particular, in the context of chronic diseases. While the acute expansion of the tachyzoite population is usually restrained by the host immune system [2], the latter directs the differentiation of few tachyzoites into slow-replicating and host-to-host transmissible bradyzoites These bradyzoites reside within intracellular compartments called cysts within tissues including the brain, the retina, and the skeletal muscles, being held quasi silent by a IFNγ-orchestrated immune response [3]; they account for the subsequent prolonged and so-called clinically silent phase of parasitism. Disruption of this immune balance drives the reverse transition from bradyzoite to tachyzoite and uncontrolled growth of the latter, thereby causing life-debilitating and threatening damages in the aforementioned tissues, and exemplified with the encephalitis of AID patients [4]. A significant risk of subsequent systemic or/and chronicrelated illnesses exists for the ever-increasing number of patients undergoing transient or sustained immunosuppressive therapies [7]

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