Carol Shoshkes Reiss

Innate immune responses to pathogens are evolutionarily ancient and are found in the most primitive organisms. These are highly conserved and are not pathogen-specific, but are in response to classes of molecular structures. Infections can be perceived both extracellularly and intracellularly by Pathogen Associated Molecular Patterns (PAMPs) and their host cell ligands, Pathogen Recognition Receptors (PRRs), among them, Toll-Like Receptors (TLRs). The innate immune response to infection includes the release of soluble preformed mediators, or synthesis of cytoplasmic enzymes, cytokines, chemokines, interferons (IFNs), lipid mediators, proteins of the complement cascade, neurotransmitters, nucleotides, and components of transcription factors (High Mobility Group B1, receptors for sex hormones/steroids). Directed cellular migration of parenchymal astrocytes and microglia, as well as recruitment across the blood brain barrier (BBB) of circulating neutrophils, natural killer, monocytes, macrophages, dendritic cells, and ultimately T lymphocytes to the site of infection are also hallmarks of innate responses to infections. These responding cells contribute their own secreted effector molecules and effector activities (such as phagocytosis). Distinct viruses are capable of infecting every cell type (endothelial cells, ependymal cells, perivascular macrophages and pericytes, astrocytes, microglia, oligodendrocytes, Schwann cells, and neurons) in the central nervous system (CNS). These CNS infections challenge the host with a different set of problems than do peripheral viral infections. Among the complications are (a) neurons that rarely express Class I or Class II Major Histocompatibility Complex (MHC) molecules and are thus not suitable targets for either CD4⁺ or CD8⁺ MHC-restricted T cells, (b) an enclosed volume that is constrained from swelling during inflammation, as well as poorly developed lymphatic drainage, and (c) the immunologic privilege of the CNS which leads to extremely limited immune surveillance for pathogens. Therefore, the role of innate immunity, both from CNS-resident cells and their products, and from circulating inflammatory cells and molecules which traverse the BBB are essential to “buy time,” inhibiting viral replication and dissemination, until the host can marshal an adaptive immune response. The immune responses are crucial for host survival from the infection. Consequently, successful pathogens, especially those that persist, have developed a wide variety of evasive approaches to limit the inhibition of replication. Many of these pathways are highlighted in individual chapters that precede this one. These evasive measures range from neutralizing host secreted molecules (cytokines and chemokines) with soluble receptors, encoding anti-inflammatory proteins in their genome, preventing signal transduction, blocking inhibition of protein synthesis, degradation of essential antiviral molecules, preventing apoptosis, and blockade of the nuclear pore complex. In this chapter, I will attempt to cover the breadth of the innate immune response to viral infection, but will also devote more space to generally under-considered aspects than to the well-known components. There are some caveats to consider, as most experiments have been performed in the murine model and not in man; further, conclusions from experiments using in vitro cultured cells (whether primary or established lines) may not reflect physiological conditions in an undisturbed CNS. Lastly, we now appreciate the complexities imposed on hosts by polymorphisms in genes of critical pathways, leading to increased susceptibility or resistance of that individual but not others.

Summary: Objective: Genetic loss of Tsc1/Tsc2 function in tuberous sclerosis complex (TSC) results in altered mammalian target of rapamycin (mTOR) signaling and abnormal brain development. Although earlier studies have focused on characterization of cortical tubers, in this study we sought to examine the unique cellular and molecular features of the perituberal cortex in order to better understand its contribution to epileptogenesis, cognitive dysfunction, and autism.Methods: Standard histologic and immunohistochemical labeling was used to assess structural abnormalities and cell‐specific pattern of mTORC1 activation in surgically resected cortical tubers and perituberal cortex. Western blotting was performed to quantify the expression of the mTORC1 and mTORC2 biomarkers phospho‐S6 (Ser235/236), phospho‐S6 (Ser240/244), and phospho‐Akt (Ser473), in addition to evaluating the differential expression levels of several neuronal and glial‐specific proteins in tubers and peritubers, as compared to non‐TSC epilepsy specimens.Results: Tubers demonstrated mild to severe disruption of cortical lamination, the presence of pS6‐positive dysplastic neurons and giant cells, an overall increase in mTORC1 and a decrease in mTORC2 activity, increased axonal connectivity and growth, and hypomyelination. Perituberal cortex presented similar histologic, immunohistochemical, and molecular features; however, they were overall milder. Axonal growth was specific for TSC and was negatively correlated with deficient myelination.Significance: Our results show an extension of cellular dysplasia and dysregulated mTOR signaling in the perituberal tissue, and demonstrate for the first time aberrant connectivity in human TSC brain. This study provides new insights into the pathophysiology of neurologic dysfunction associated with TSC and supports the intrinsic epileptogenicity of normal‐appearing perituberal cortex.A PowerPoint slide summarizing this article is available for download in the Supporting Information section here.