Microglia and astrocyte responses to neuropathogenic protozoan parasites

Parasite infections: how inflammation alters brain function

Parasitic infections can profoundly impact brain function through inflammation within the central nervous system (CNS). Once viewed as an immune-privileged site, the CNS is now recognized as vulnerable to immune disruptions from both local and systemic infections. Recent studies reveal that certain parasites, such as Toxoplasma gondii and Plasmodium falciparum, can invade the CNS or influence it indirectly by triggering neuroinflammation. These processes may disrupt brain homeostasis, influence neurotransmission, and lead to significant behavioral or cognitive changes. This review discusses the pathways by which parasites disrupt CNS function and highlights systemic inflammation as a critical link between peripheral infections and neuroinflammatory conditions, advancing understanding of parasite-associated neurological complications.

Pathological mechanisms of glial cell activation and neurodegenerative and neuropsychiatric disorders caused by Toxoplasma gondii infection

Toxoplasma gondii is an intracellular opportunistic parasite that exists in a latent form within the human central nervous system (CNS), even in immune-competent hosts. During acute infection, T. gondii traverses the blood-brain barrier (BBB). In the subsequent chronic infection phase, the infiltration of immune cells into the brain, driven by T. gondii infection and the formation of parasitic cysts, leads to persistent activation and proliferation of astrocytes and microglia. This process results in neuronal damages that are fatal in some cases. Through inducing systemic immune responses, T. gondii infection can dramatically alter the behavior of rodents and increase the risk of various neuropsychiatric disorders in humans. In this review, we explore some recent research progress on the major events involved in BBB disruption, glial cell activation and neuronal damage following T. gondii infection in hosts. It further discusses potential pathological mechanisms and the feasible treatment approaches for the neurodegenerative and neuropsychiatric disorders caused by T. gondii infection to extend our understanding for pathogenesis and preventive control of toxoplasmosis in humans.

Astrocytes Can Be Key Players Against Cerebral Leishmaniasis: In Vitro Co-Culture Model for the Assessment of Infection

Leishmaniasis is a neglected tropical disease, caused by protozoan parasites of Leishmania (L.), and is transmitted by bite of phlebotomine sandflies. There are several studies on central nervous system infection to indicate that Leishmania can cross the blood-brain barrier, resulting in neurological manifestations, known as "cerebral leishmaniasis." This study highlighted the notions: (i) polarisation of bone marrow-derived macrophages (BMDM) incubated following stimulation with lipopolysaccharide (LPS) or soluble Leishmania antigen (SLA), (ii) quantification of parasites within co-culture of Leishmania-infected macrophages, and astrocytes, and (iii) effect of interferon-gamma (IFN-γ) on the infection rate of co-culture populations. Accordingly, 83% of overall macrophage population was identified on day 7 for CD11b and F4/80 macrophage markers. Flow cytometry analysis revealed significant increases in CD11b and F4/80 surface markers in LPS and SLA-stimulated BMDMs at 24 h, compared to untreated cells. TNF-α levels increased significantly in both LPS and SLA-treated BMDMs after 48 h. Additionally, SLA treatment induced a more elongated, spindle-like shape in the cells, indicative of M2 macrophage polarisation over the M1 phenotype. When non-infected astrocytes with/without stimulation with IFN-γ before co-culture, gp63 FITC-labelled parasite populations (%) in co-culture decreased to 25% at 72 h, thus indicating a lower infection rate in a time-dependent manner. IFN-γ and IL-6 levels significantly increased to 71.66 ± 3.51 and 184 ± 14.42 pg/mL, resulting in the inflammatory response in the co-culture system at 48 h (p ≤ 0.0001), when compared to the control (30 ± 2.52 pg/mL for IFN-γ and 8.66 ± 2.37 pg/mL for IL-6) at 0 h of the incubation. It is the first study to emphasize the communication between Leishmania-infected macrophages and astrocytes regarding Leishmania parasite load. The results suggest that astrocytes can lead to the reduction in Leishmania parasites, thereby controlling the incidence of cerebral leishmaniasis.

Type-1 diabetes mellitus down-regulated local cerebral glial fibrillary acidic protein expression in experimental toxoplasmosis

Cerebral toxoplasmosis is an opportunistic infection, occurring mostly in immunosuppressed patients due to the reactivation of latent Toxoplasma cysts. The cerebral comorbidity in diabetic patients tends to intensify the burden of pathogenic infection within the brain. The aim of this work was to study the effect of cerebral toxoplasmosis in experimentally infected hyperglycemic mice, on histopathology and glial fibrillary acidic protein (GFAP) expression, compared to normoglycemic mice at different time intervals. Vasculopathy was exclusively observed in diabetic groups, with features of increased severity during Toxoplasma infection. Gliosis was observed in diabetic groups, while hyperactive astroglial activity was detected in normoglycemic groups, especially at 6 weeks of infection. GFAP expression showed significant up-regulation in normoglycemic mice at 6 weeks of infection (40.03 ± 1.41) afterwards, it decreased to 22.22 ± 3.14 at 12 weeks which was statistically insignificant to the normal level, possibly indicating the successful Toxoplasma stage transformation (to bradyzoite), thereby limiting the infection within the brain. In hyperglycemic infected groups, GFAP was significantly down-regulated, in both acute and chronic phases of infection, most likely indicating failure of stage transformation and infection limitation. This may expose those vulnerable groups to the risk of dissemination, resulting in life-threatening diffuse encephalitis. The current study emphasized the importance of rapid diagnosis of Toxoplasma infection in diabetic subjects, and highlighted the value of using GFAP as a neurological indicator of disease progression in those comorbid cases.

Multimodal Assessment of Bottlenose Dolphin Auditory Nuclei Using 7-Tesla MRI, Immunohistochemistry and Stereology

The importance of assessing neurochemical processes in the cetacean brain as a tool for monitoring their cognitive health and to indirectly model human neurodegenerative conditions is increasingly evident, although available data are largely semiquantitative. High-resolution MRI for post-mortem brains and stereology allow for quantitative assessments of the cetacean brain. In this study, we scanned two brains of bottlenose dolphins in a 7-Tesla (7T) MR scanner and assessed the connectivity of the inferior colliculi and ventral cochlear nuclei using diffusion tensor imaging (DTI). Serial thick sections were investigated stereologically in one of the dolphins to generate rigorous quantitative estimates of identifiable cell types according to their morphology and expression of molecular markers, yielding reliable cell counts with most coefficients of error <10%. Fibronectin immunoreactivity in the dolphin resembled the pattern in a human chronic traumatic encephalopathy brain, suggesting that neurochemical compensation for insults such as hypoxia may constitute a noxious response in humans, while being physiological in dolphins. These data contribute to a growing body of knowledge on the morphological and neurochemical properties of the dolphin brain and highlight a stereological and neuroimaging workflow that may enable quantitative and translational assessment of pathological processes in the dolphin brain in the future.

The Defensive Interactions of Prominent Infectious Protozoan Parasites: The Host's Complement System

The complement system exerts crucial functions both in innate immune responses and adaptive humoral immunity. This pivotal system plays a major role dealing with pathogen invasions including protozoan parasites. Different pathogens including parasites have developed sophisticated strategies to defend themselves against complement killing. Some of these strategies include the employment, mimicking or inhibition of host's complement regulatory proteins, leading to complement evasion. Therefore, parasites are proven to use the manipulation of the complement system to assist them during infection and persistence. Herein, we attempt to study the interaction´s mechanisms of some prominent infectious protozoan parasites including Plasmodium, Toxoplasma, Trypanosoma, and Leishmania dealing with the complement system. Moreover, several crucial proteins that are expressed, recruited or hijacked by parasites and are involved in the modulation of the host´s complement system are selected and their role for efficient complement killing or lysis evasion is discussed. In addition, parasite's complement regulatory proteins appear as plausible therapeutic and vaccine targets in protozoan parasitic infections. Accordingly, we also suggest some perspectives and insights useful in guiding future investigations.

Behavioral and Neuropathological Changes After Toxoplasma gondii Ocular Conjunctival Infection in BALB/c Mice

Ocular infection with Toxoplasma gondii causes toxoplasmosis in mice. However, following ocular infection with tachyzoites, the cause of the accompanying progressive changes in hippocampal-dependent tasks, and their relationship with the morphology and number of microglia, is less well understood. Here, in 6-month-old, female BALB/c mice, 5 μl of a suspension containing 48.5 × 10⁶ tachyzoites/ml was introduced into the conjunctival sac; control received an equal volume of saline. Before and after instillation, all mice were subject to an olfactory discrimination (OD) test, using predator (cat) feces, and to an open-field (OF) task. After the behavioral tests, the animals were culled at either 22 or 44 days post-instillation (dpi), and the brains and retinas were dissected and processed for immunohistochemistry. The total number of Iba-1-immunolabeled microglia in the molecular layer of the dentate gyrus was estimated, and three-dimensional reconstructions of the cells were evaluated. Immobility was increased in the infected group at 12, 22, and 43 dpi, but the greatest immobility was observed at 22 dpi and was associated with reduced line crossing in the OF and distance traveled. In the OD test, infected animals spent more time in the compartment with feline fecal material at 14 and at 43 dpi. No OD changes were observed in the control group. The number of microglia was increased at 22 dpi but returned to control levels by 44 dpi. These changes were associated with the differentiation of T. gondii tachyzoites into bradyzoite-enclosed cysts within the brain and retina. Thus, infection of mice with T. gondii alters exploratory behavior, gives rise to a loss in predator’s odor avoidance from 2 weeks after infection, increased microglia number, and altered their morphology in the molecular layer of the dentate gyrus.