Research Report: Long-Term Effects of Class II Invariant Peptide (CLIP) Antagonism on CLIP+ B Cells in the Meninges, Neuropathology, and Neurobehavioral Deficits in 5xFAD Mice

Research Background

Traumatic brain injury (TBI) is a significant public health problem affecting millions of individuals worldwide each year. More importantly, TBI is a significant risk factor for Alzheimer’s disease (AD). There is a vast body of evidence suggesting that B cells and T cells within the adaptive immune system play crucial roles in the pathogenesis of both TBI and AD. Previous studies have found that B cells, and Major Histocompatibility Complex Class II (MHCII)-associated Class II Invariant Peptide (CLIP)-positive B cells in particular, expand after TBI. Furthermore, inhibiting the binding of CLIP with the MHCII antigen presentation groove acutely reduces the number of CLIP+ splenic B cells following TBI and has a neuroprotective effect. Based on this, the current study investigates the long-term effects of utilizing a CLIP antagonist (CAP) in a 5xFAD mouse model of Alzheimer’s disease, with or without TBI.

Study Source

This paper is authored by Jaclyn Iannucci et al., from various departments at Texas A&M University, including the Department of Neuroscience and Experimental Therapeutics and the Department of Medical Physiology. The paper was published in the “Journal of Neuroinflammation” in 2024.

Research Process

The study included male 5xFAD Alzheimer’s disease mouse models and wild-type (WT) mice, both 12 weeks of age. Each group of mice was randomly assigned to receive a single dose of CLIP antagonist (CAP, 1 mg/kg) or vehicle control treatment 30 minutes after sham surgery or lateral fluid percussion injury (FPI). Over the next six months, the mice were analyzed using various methods including flow cytometry, histopathological analysis, magnetic resonance diffusion tensor imaging (MRI DTI), cerebral angiography, and assessments of motor and neurobehavioral functions.

Experimental Procedures and Methods

1. Flow Cytometry: Immune cells collected including the meninges were analyzed for changes in the type and quantity of immune cells, especially CLIP+ B cells.
2. Histopathological Analysis: Immunohistochemistry was used for detailed analysis of neuropathological features such as beta-amyloid plaques and CD74+ cells in brain tissue.
3. Magnetic Resonance Diffusion Tensor Imaging (MRI DTI): Assessed alterations in brain white matter connections.
4. Cerebral Angiography: Vascular staining methods were used to assess morphological changes in brain vessels.
5. Neurobehavioral Function Assessment: Included Digigait testing for motor function, the pattern separation test (PST), and Barnes maze test for cognitive function, as well as assessments for social interaction and burrowing behavior.

Results

Pathological Evaluation

1. Immune Cell Analysis

   • 9-month-old 5xFAD mice exhibited a significantly higher proportion of CLIP+ B cells in the meninges compared to WT mice of the same age. CAP treatment effectively reduced the number of CLIP+ B cells and the level of CLIP expression on B cells in 5xFAD mice.
   • FPI did not further increase the number of CLIP+ B cells, but inhibited the capacity of CAP to reduce these cells.

2. Brain CD74+ Cells

   • A significant increase in CD74+ cells was observed in the hippocampus of 5xFAD mice. CAP treatment significantly reduced the number of these cells, especially in the dentate gyrus region.

3. Beta-Amyloid Plaque Deposition

   • The number of beta-amyloid plaques in the CA3 region of the hippocampus of 5xFAD mice increased due to FPI, while CAP treatment significantly reduced the number of these plaques.

4. Microglial Cells

   • The number of microglial cells in the hippocampus of 5xFAD mice increased significantly and was particularly reduced after FPI, while CAP treatment was able to restore the number of microglial cells.

5. Vascular Structure Changes

   • FPI led to reduced brain vessel connection density and length in 5xFAD mice, which was significantly improved by CAP treatment.

6. Brain Connectivity Analysis

   • MRI DTI showed altered cerebral cortical connectivity in 5xFAD mice. FPI further altered these connections, but CAP treatment restored them to some extent.

Behavioral Evaluation

1. Motor Function

   • The Digigait test performed on the first day after FPI showed impaired motor function in 5xFAD mice, as evidenced by an increased stand/swing ratio, but CAP treatment improved these acute motor deficits.

2. Social Interaction and Depression-Related Behavior

   • Social interaction tests showed that CAP treatment increased social activity in 5xFAD mice, while FPI weakened this effect.
   • Burrowing behavior tests showed a trend towards reduced burrowing activity in 5xFAD mice after FPI, which was not significantly changed by CAP treatment.

3. Cognitive Function

   • The pattern separation test found impaired performance in 5xFAD mice in object recognition, negatively correlated with the number of CLIP+ B cells in the meninges.
   • The Barnes maze test showed a significantly increased escape latency during the first four days of training for 5xFAD mice, which was not significantly altered by CAP treatment.

Conclusion

The study demonstrated that the long-term impact of CLIP antagonism on 5xFAD mice depended on the presence of FPI. In the absence of TBI, CLIP antagonism reduced CLIP+ B cells in the meninges and CD74+ cells in the hippocampus, enhanced neurogenesis, and improved neurobehavioral functions. Following TBI, even though the effectiveness of CLIP antagonism was attenuated in reducing the number of meningeal CLIP+ B cells, it still improved certain aspects of neuropathological changes caused by TBI. Overall, this study identifies meningeal CLIP+ B cells as a potential pathogenic mediator in the development of AD pathology and suggests that this unique immune cell population may become a new target for future AD therapy.