Systemic Listeria monocytogenes infection in aged mice induces long-term neuroinflammation: the role of miR-155

Microglial activation and neuroinflammation in acute and chronic cognitive deficits in sepsis

Sepsis is characterised by dysregulated immune responses to infection, leading to multi-organ dysfunction and high rates of mortality. With increasing survival rates in recent years long-term neurological and psychiatric consequences have become more apparent in survivors. Many patients develop sepsis associated encephalopathy (SAE) which encompasses the profound but usually transient neuropsychiatric syndrome delirium but also new brain injury that emerges in the months and years post-sepsis. It is now clear that systemic inflammatory signals reach the brain during sepsis and that very significant neuroinflammation ensues. The major brain resident immune cell population, the microglia, has been implicated in acute and chronic cognitive dysfunction in animal models of sepsis based on a growing number of studies using bacterial endotoxin and in polymicrobial sepsis models such as cecal ligation and puncture. The current review explores the effects of sepsis on the brain, focussing on how systemic insults translate to microglial activation and neuroinflammation and how this disrupts neuronal function and integrity. We examine what has been demonstrated specifically with respect to microglial activation, revealing robust evidence for a role for neuroinflammation in sepsis-induced brain sequelae but less clear information on the extent of the specific microglial contribution to this, arising from findings using global knockout mice, non-selective drugs and treatments that equally target peripheral and central compartments. There is, nonetheless, clear evidence that microglia do become activated and do contribute to brain consequences of sepsis thus arguing for improved understanding of these neuroinflammatory processes toward the prevention and treatment of sepsis-induced brain dysfunction.

Repetitive antigen stimulation in the periphery dictates the composition and recall responses of brain-resident memory CD8+ T cells

The human brain harbors virus-specific, tissue-resident memory (TRM) CD8+ T cells. However, the impact of repeated peripheral viral infection on the generation, phenotype, localization, and recall responses of brain TRM remains elusive. Here, utilizing two murine models of peripheral viral infection, we demonstrate that circulating memory CD8+ T cells with previous antigen exposure exhibit a markedly reduced capacity to form brain TRM compared to naive CD8+ T cells. Repetitively stimulated brain TRM also demonstrate differential inhibitory receptor expression, preserved functionality, and divergent localization patterns compared to primary memory counterparts. Despite these differences, repetitively stimulated brain TRM provide similar protection against intracranial infection as primary populations with superior recall-based recruitment of peripheral lymphocytes. As CD8+ T cells may distinctly seed the brain with each repeated infection of the same host, these findings point to heterogeneity in the brain TRM pool that is dictated by prior peripheral antigen stimulation history.

Meningeal regulation of infections: A double-edged sword

In the past 10 years, important discoveries have been made in the field of neuroimmunology, especially regarding brain borders. Indeed, meninges are protective envelopes surrounding the CNS and are currently in the spotlight, with multiple studies showing their involvement in brain infection and cognitive disorders. In this review, we describe the meningeal layers and their protective role in the CNS against bacterial, viral, fungal, and parasitic infections, by immune and nonimmune cells. Moreover, we discuss the neurological and cognitive consequences resulting from meningeal infections in neonates (e.g. infection with group B Streptococcus, cytomegalovirus, …) or adults (e.g. infection with Trypanosoma brucei, Streptococcus pneumoniae, …). We hope that this review will bring to light an integrated view of meningeal immune regulations during CNS infections and their neurological consequences.

Progressive cognitive impairment after recovery from neuroinvasive and non-neuroinvasive Listeria monocytogenes infection

Background

Neuro-cognitive impairment is a deleterious complication of bacterial infections that is difficult to treat or prevent. Listeria monocytogenes (Lm) is a neuroinvasive bacterial pathogen and commonly used model organism for studying immune responses to infection. Antibiotic-treated mice that survive systemic Lm infection have increased numbers of CD8⁺ and CD4⁺ T-lymphocytes in the brain that include tissue resident memory (T_RM) T cells, but post-infectious cognitive decline has not been demonstrated. We hypothesized that Lm infection would trigger cognitive decline in accord with increased numbers of recruited leukocytes.

Methods

Male C57BL/6J mice (age 8 wks) were injected with neuroinvasive Lm 10403s, non-neuroinvasive Δhly mutants, or sterile saline. All mice received antibiotics 2-16d post-injection (p.i.) and underwent cognitive testing 1 month (mo) or 4 mo p.i. using the Noldus PhenoTyper with Cognition Wall, a food reward-based discrimination procedure using automated home cage based observation and monitoring. After cognitive testing, brain leukocytes were quantified by flow cytometry.

Results

Changes suggesting cognitive decline were observed 1 mo p.i. in both groups of infected mice compared with uninfected controls, but were more widespread and significantly worse 4 mo p.i. and most notably after Lm 10403s. Impairments were observed in learning, extinction of prior learning and distance moved. Infection with Lm 10403s, but not Δhly Lm, significantly increased numbers of CD8⁺ and CD4⁺ T-lymphocytes, including populations expressing CD69 and T_RM cells, 1 mo p.i. Numbers of CD8⁺, CD69⁺CD8⁺ T-lymphocytes and CD8⁺ T_RM remained elevated at 4 mo p.i. but numbers of CD4⁺ cells returned to homeostatic levels. Higher numbers of brain CD8⁺ T-lymphocytes showed the strongest correlations with reduced cognitive performance.

Conclusions

Systemic infection by neuroinvasive as well as non-neuroinvasive Lm triggers a progressive decline in cognitive impairment. Notably, the deficits are more profound after neuroinvasive infection that triggers long-term retention of CD8⁺ T-lymphocytes in the brain, than after non-neuroinvasive infection, which does not lead to retained cells in the brain. These results support the conclusion that systemic infections, particularly those that lead to brain leukocytosis trigger a progressive decline in cognitive function and implicate CD8⁺ T-lymphocytes, including CD8⁺T_RM in the etiology of this impairment.

Keeping T cell memories in mind

The mammalian central nervous system (CNS) contains a vibrant community of resident adaptive immune cells at homeostasis. Among these are memory CD8+ and CD4+ T cells, which reside in the CNS in the settings of health, aging, and neurological disease. These T cells commonly exhibit a tissue-resident memory (TRM) phenotype, suggesting that they are antigen-experienced and remain separate from the circulation. Despite these characterizations, T cell surveillance of the CNS has only recently been studied through the lens of TRM immunology. In this Review, we outline emerging concepts of CNS TRM generation, localization, maintenance, function, and specificity. In this way, we hope to highlight roles of CNS TRM in health and disease to inform future studies of adaptive neuroimmunity.