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The meningeal space is an important structure in the brain borders, which provides immunosurveillance for the central nervous system, but the impact of infections on the meningeal immune landscape is far from being fully understood. The extracellular protozoan parasiteTrypanosoma brucei, which causes Human African Trypanosomiasis (HAT) or sleeping sickness, accumulate in the meningeal spaces, ultimately inducing severe meningitis and resulting in death if left untreated. Thus, sleeping sickness represents an attractive model to study immunological dynamics in the meninges during infection. Here, combining single cell transcriptomics and mass cytometry by time of flight (CyTOF), coupled within vivointerventions, we found that chronicT. bruceiinfection triggers the development of ectopic lymphoid aggregates (ELAs) in the murine meninges during chronic infection. These infection-induced ectopic structures are defined by the presence of ER-TR7+fibroblastic reticular cells (FRCs) and follicular dendritic cells (FDCs) that initiate a signalling cascade driving local T cell activation towards a T follicular helper (TFH)-like phenotype, as well as B cell class switching. Furthermore, the GC-like B cells found in the infected meninges produce high-affinity autoantibodies able to recognise mouse brain antigens. We found that systemic lymphotoxin β (LTβ) signalling blockade led to a significant depletion of meningeal FDC-like cells and autoreactive B cells, indicating that LTβ signalling is critical to induce and maintain local responses in the meninges. In humans, we identified the presence of autoreactive IgG antibodies able to recognise human brain lysates in the cerebrospinal fluid of second stage HAT patients compared to first stage HAT patients, consistent with our findings in experimental infections. Taken together, our data provide evidence that the meningeal immune response results in the acquisition of lymphoid tissue-like properties during chronicT. bruceiinfection, broadening our understanding of meningeal immunity in the context of chronic infections. These findings have wider implications for understanding the mechanisms underlying the formation ELAs during chronic inflammation resulting in autoimmunity in mice and humans, as observed in other autoimmune neurodegenerative disorders such as neuropsychiatric lupus and multiple sclerosis.

Human African trypanosomiasis, or sleeping sickness, is caused by the protozoan parasiteTrypanosoma bruceiand induces profound reactivity of glial cells and neuroinflammation when the parasites colonise the central nervous system. However, the transcriptional and functional responses of the brain to chronicT. bruceiinfection remain poorly understood. By integrating single cell and spatial transcriptomics of the mouse brain, we identified that glial responses triggered by infection are readily detected in the proximity to the circumventricular organs, including the lateral and 3rdventricle. This coincides with the spatial localisation of both slender and stumpy forms ofT. brucei. Furthermore,in silicopredictions and functional validations led us to identify a previously unknown crosstalk between homeostaticCx3cr1+microglia andCd138+plasma cells mediated by IL-10 and B cell activating factor (BAFF) signalling. This study provides important insights and resources to improve understanding of the molecular and cellular responses in the brain during infection with African trypanosomes.

Microglia play key roles in brain homeostasis as well as responses to neurodegeneration and neuroinflammatory processes caused by physical disease and psychosocial stress. The pig is a physiologically-relevant model species for studying human neurological disorders, many of which are associated with microglial dysfunction. Furthermore, pigs are an important agricultural species, and there is a need to understand how microglial function affects their welfare. As a basis for improved understanding to enhance biomedical and agricultural research, we sought to characterise pig microglial identity at genome-wide scale and conduct inter-species comparisons.We isolated pig hippocampal tissue and microglia from frontal cortex, hippocampus and cerebellum, as well as alveolar macrophages from the lungs and conducted RNA-sequencing (RNAseq). By comparing the transcriptomic profiles between microglia, macrophages, and hippocampal tissue, we derived a set of 365 highly-enriched genes defining the porcine core microglial signature. We found brain regional heterogeneity based on 215 genes showing significant (adjusted pOur data provide a valuable resource defining the pig microglial transcriptome signature that highlights pigs as a useful large animal species bridging between rodents and humans in which to study the role of microglia during homeostasis and disease.Main Points- Defined a pig microglial transcriptome signature comprising 365 genes.- Demonstrated regional variance in the pig microglial transcriptome across the brain.- Revealed greater similarity between pig and human microglia than mouse.

Prion diseases are transmissible, neurodegenerative disorders associated with misfolding of the prion protein. Previous studies show that reduction of microglia accelerates CNS prion disease and increases the accumulation of prions in the brain, suggesting that microglia provide neuroprotection by phagocytosing and destroying prions. In Csf1rΔFIRE mice, the deletion of an enhancer within Csf1r specifically blocks microglia development, however, their brains develop normally and show none of the deficits reported in other microglia-deficient models. Csf1rΔFIRE mice were used as a refined model in which to study the impact of microglia-deficiency on CNS prion disease. Although Csf1rΔFIRE mice succumbed to CNS prion disease much earlier than wild-type mice, the accumulation of prions in their brains was reduced. Instead, astrocytes displayed earlier, non-polarized reactive activation with enhanced synaptic pruning and unfolded protein responses. Our data suggest that rather than simply phagocytosing and destroying prions, the microglia instead provide host-protection during CNS prion disease and restrict the harmful activities of reactive astrocytes.Main pointsCNS prion disease is accelerated in mice completely lacking microglia. The rate of prion accumulation in the brain was unaltered in absence of microglia. Microglia provide host-protection during CNS prion disease independent of prion clearance.

Ageing results in a decline in immune function. We showed previously that healthy older humans (>65 years old) have reduced antigen-specific cutaneous immunity to varicella zoster virus (VZV) antigen challenge. This was associated with p38 MAP kinase driven inflammation that was induced by mild tissue injury caused by the injection of the antigen itself. Here we show that non-specific injury induced by injection of air or saline into the skin of older adults recruits CCR2+CD14+monocytes by CCL2 produced by senescent fibroblasts. These monocytes reduced TRMproliferation via secretion of prostaglandin E2 (PGE2). Pre-treatment with a p38-MAPK inhibitor (Losmapimod) in older adultsin vivosignificantly decreased CCL2 expression, recruitment of monocyte into the skin, COX2 expression and PGE2production. This enhanced the VZV response in the skin. Therefore, local inflammation arising from interaction between senescent cells and monocytes leads to immune decline in the skin during ageing, a process that can be reversed.SummaryInflammation resulting from tissue injury blocks antigen-specific cutaneous immunity during ageing. Monocytes recruited to the skin inhibit TRMfunction through COX2-derived prostaglandin E2production. Blocking inflammation and resulting prostaglandin E2production with a p38-MAP kinase inhibitor significantly enhances cutaneous antigen-specific responses.

Summary Aging has a profound effect on the immune system, termed immunosenescence, resulting in increased incidence and severity of infections and decreased efficacy of vaccinations. We previously showed that immunosurveillance in the intestine, achieved primarily through antigen sampling M cells in the follicle associated epithelium (FAE) of Peyer's patches, was compromised during aging due to a decline in M cell functional maturation. The intestinal microbiota also changes significantly with age, but whether this affects M cell maturation was not known. We show that housing of aged mice on used bedding from young mice, or treatment with bacterial flagellin, were each sufficient to enhance the functional maturation of M cells in Peyer's patches. An understanding of the mechanisms underlying the influence of the intestinal microbiota on M cells has the potential to lead to new methods to enhance the efficacy of oral vaccination in aged individuals., Graphical Abstract, Highlights • Exposure of aged mice to a young microbiota restores M cell density and function • Systemic flagellin treatment enhances M cell density and function in aged mice • Young microbiota exposure and flagellin treatment improve intestinal crypt function • Microbial stimulation can restore the aging-related decline in M cells, Microbiology; Microbiome