Protective and Pathological Immunity during Central Nervous System Infections

Abstract

The concept of immune privilege of the central nervous system (CNS) has dominated the study of inflammatory processes in the brain. However, clinically relevant models have highlighted that innate pathways limit pathogen invasion of the CNS and adaptive immunity mediates control of many neural infections. As protective responses can result in bystander damage, there are regulatory mechanisms that balance protective and pathological inflammation, but these mechanisms might also allow microbial persistence. The focus of this review is to consider the host-pathogen interactions that influence neurotropic infections and to highlight advances in our understanding of innate and adaptive mechanisms of resistance as key determinants of the outcome of CNS infection. Advances in these areas have broadened our comprehension of how the immune system functions in the brain and can readily overcome immune privilege.

Keywords: Brain; T cell; astrocyte; blood brain barrier; central nervous system; encephalitis; infection; meningitis; monocyte; neuron.

Figure 1

Pathogen Mechanisms for Crossing the Blood Brain Barrier (A) The NVU is composed of astrocyte endfeet, pericytes, and endtothelial cells and the local production of type I IFNs has a major role in controlling opening of the BBB (Daniels et al., 2017, Lazear et al., 2015). Micro-organisms utilize three main mechanisms to directly transit the BBB. (B) Trans-cellular route by which viruses or infected cells directly cross through EC (Dahm et al., 2016). (C) Para-cellular route whereby the breakdown of the tight junctions allows access of infected cells or extracellular pathogens to access the peri-vascular unit (Santiago-Tirado et al., 2017, Shi et al., 2010, Vu et al., 2013). (D) Lytic mechanisms in which the ability to infect EC and lyse these cells allows pathogens to access the brain (Konradt et al., 2016).

Figure 2

Viral Modulation of BBB Function Viruses might impact on TJ integrity via direct and indirect mechanisms: (1) virus detection by brain endothelial TAM receptors in conjunction with type I IFN signaling enhances TJ integrity (Miner et al., 2015); (2) nonstructural protein (NS)1 secretion by flaviviruses might decrease TJ integrity, increasing barrier permeability; (3) PRR signaling during viral infection of astrocytes leads to type I IFN expression which has been shown to improve BBB function via intercellular crosstalk with brain endothelium (Daniels et al., 2014); (Pfefferkorn et al., 2015); and (4) secretion of transactivator protein (TAT) during CNS infection with HIV-1 induces pericyte expression of PDGFβ, which disrupts BBB junctional proteins and promotes pericyte migration away from the vasculature (Niu et al., 2014), (Winkler et al., 2010).

Figure 3

Effector Mechanisms Used by T Cells to Control CNS Infection (A) The ability of pathogen-specific CD4⁺ and CD8⁺ T cells to exit the circulation, cross the BBB, and access the parenchyma precedes the search for relevant infected CNS-specific targets. (B) The local production of IFN-γ by CD4⁺ and CD8⁺ T cells has a prominent role in the activation of STAT1 and the anti-microbial activities of astrocytes and microglia. While CD8⁺ T cells can interact with infected astrocytes and microglia to mediate elimination of infected cells or control of pathogen replication it is unclear whether their interactions with neurons is direct through MHC-TCR interactions or indirect through the activation of microglia. (C) The development of Th17 responses characterized by the production of IL-17 and/or GM-CSF is correlated with the development of immunopathology and the recruitment of inflammatory monocytes.