Immune response to Toxoplasma gondii at the choroid plexus and the the immunomodulatory impact of the neuropeptide PACAP

Abstract

Toxoplasma gondii (T. gondii) is a highly successful parasite able to cross all biological barriers of the body. The parasites invade through the intestinal epithelium the body, where they cause acute infection, thereafter they cross the borders of the central nervous system (CNS), entering the brain and inducing neuroinflammation. Previous studies have described the critical involvement of the CNS borders during T. gondii invasion and the development of neuroinflammation. While many studies have focused on the blood-brain barrier (BBB), the contribution of the choroid plexus (CP), the main structure forming the blood-cerebrospinal fluid (CSF)-barrier (BCSFB) has remained largely unknown. In this present study, confocal imaging of immuno-stained brain sections, tissue whole mount preparations, and determination of T. gondii gDNA levels were applied to reveal that CP is infected by T. gondii. In addition, flow cytometric and gene expression analysis showed that CP elicits an immune response prior to the BBB. Moreover, imaging and gene expression of tight junctions (TJ) in CP and primary cultured CP epithelial cells indicate a loss of function and integrity of the BCSFB, which was confirmed by in vivo FITC-dextran permeability assay [Publication 1]. Generally, albeit the elicited immune response during infection is essential for parasite control, continuous activation and recruitment of immune cells to infected organs entail tissue damage and cause detrimental alterations to their function. To mitigate these alterations, this study additionally set out to investigate the effects of the neuropeptide PACAP upon acute [Publication 2] and chronic T. gondii infection [Publication 3], as PACAP has shown immunomodulatory and neuroprotective properties in other disease models. Here, the infection of mice with GFP-reporter parasites indicate that PACAP displayed anti-parasitic activity, and further in vitro assays showed that this is an immune cell-mediated effect rather than a direct elimination of the parasites. Overall, flow cytometric and gene expression analysis showed that PACAP treatment reduced immune cell infiltration, activation, and cytokine production in the periphery and in the CNS. Additionally, a cross-talk between PACAP and neurotrophin signaling pathways was suggested to affect the immune as well as the nervous systems, and to promote neuroprotection during T. gondii infection. Altogether, those findings reveal a close interaction between T. gondii infection at the CP and the impairment of the BCSFB function, indicating that infection-related neuroinflammation is initiated in the CP. Moreover, this study highlights the beneficial effects of PACAP application upon acute and chronic toxoplasmosis, providing new insights into the potential use of neuropeptides to counteract infection-induced inflammation.