Pamela A. Carpentier

Cutting Edge: CD4+CD25+ Regulatory T Cells Suppress Antigen-Specific Autoreactive Immune Responses and Central Nervous System Inflammation During Active Experimental Autoimmune Encephalomyelitis

Autoreactive CD4+ T cells exist in normal individuals and retain the capacity to initiate autoimmune disease. The current study investigates the role of CD4+CD25+ T-regulatory (TR) cells during autoimmune disease using the CD4+ T cell-dependent myelin oligodendrocyte glycoprotein (MOG)-specific experimental autoimmune encephalomyelitis model of multiple sclerosis. In vitro, TR cells effectively inhibited both the proliferation of and cytokine production by MOG35–55-specific Th1 cells. In vivo, adoptive transfer of TR cells conferred significant protection from clinical experimental autoimmune encephalomyelitis which was associated with normal activation of autoreactive Th1 cells, but an increased frequency of MOG35–55-specific Th2 cells and decreased CNS infiltration. Lastly, transferred TR cells displayed an enhanced ability to traffic to the peripheral lymph nodes and expressed increased levels of the adhesion molecules ICAM-1 and P-selectin that may promote functional interactions with target T cells. Collectively, these findings suggest that TR cells contribute notably to the endogenous mechanisms that regulate actively induced autoimmune disease.

Differential activation of astrocytes by innate and adaptive immune stimuli

The immunologic privilege of the central nervous system (CNS) makes it crucial that CNS resident cells be capable of responding rapidly to infection. Astrocytes have been reported to express Toll‐like receptors (TLRs), hallmark pattern recognition receptors of the innate immune system, and respond to their ligation with cytokine production. Astrocytes have also been reported to respond to cytokines of the adaptive immune system with the induction of antigen presentation functions. Here we have compared the ability of TLR stimuli and the adaptive immune cytokines interferon‐γ (IFN‐γ) and tumor necrosis factor‐α (TNF‐α) to induce a variety of immunologic functions of astrocytes. We show that innate signals LPS– and poly I:C lead to stronger upregulation of TLRs and production of the cytokines IL‐6 and TNF‐α as well as innate immune effector molecules IFN‐α4, IFN‐β, and iNOS compared with cytokine‐stimulated astrocytes. Both innate stimulation and adaptive stimulation induce similar expression of the chemokines CCL2, CCL3, and CCL5, as well as similar enhancement of adhesion molecule ICAM‐1 and VCAM‐1 expression by astrocytes. Stimulation with adaptive immune cytokines, however, was unique in its ability to induce upregulation of MHC II and the functional ability of astrocytes to activate CD4+ T cells. These results indicate potentially important and changing roles for astrocytes during the progression of CNS infection.

Immune Influence on Adult Neural Stem Cell Regulation and Function

Neural stem cells (NSCs) lie at the heart of central nervous system development and repair, and deficiency or dysregulation of NSCs or their progeny can have significant consequences at any stage of life. Immune signaling is emerging as one of the influential variables that define resident NSC behavior. Perturbations in local immune signaling accompany virtually every injury or disease state, and signaling cascades that mediate immune activation, resolution, or chronic persistence influence resident stem and progenitor cells. Some aspects of immune signaling are beneficial, promoting intrinsic plasticity and cell replacement, while others appear to inhibit the very type of regenerative response that might restore or replace neural networks lost in injury or disease. Here we review known and speculative roles that immune signaling plays in the postnatal and adult brain, focusing on how environments encountered in disease or injury may influence the activity and fate of endogenous or transplanted NSCs.

Distinct roles of protein kinase R and toll-like receptor 3 in the activation of astrocytes by viral stimuli

Impaired immune surveillance and constitutive immunosuppressive properties make the central nervous system (CNS) a particular challenge to immune defense, and require that CNS‐resident cells be capable of rapidly recognizing and responding to infection. We have previously shown that astrocytes respond to treatment with a TLR3 ligand, poly I:C, with the upregulation of innate immune functions. In the current study, we examine the activation of innate immune functions of astrocytes by Theilers murine encephalomyelitis virus (TMEV), a picornavirus, which establishes a persistent infection in the CNS of susceptible strains of mice and leads to the development of an autoimmune demyelinating disease that resembles human multiple sclerosis. Astrocytes infected with TMEV are activated to produce type I interferons, the cytokine IL‐6, and chemokines CCL2 and CXCL10. We further examined the mechanisms that are responsible for the activation of astrocytes in response to direct viral infection and treatment with poly I:C. We found that the cytoplasmic dsRNA‐activated kinase PKR is important for innate immune responses to TMEV infection, but has no role in their induction by poly I:C delivered extracellularly. In contrast, we found that TLR3 has only a minor role in responses to TMEV infection, but is important for responses to poly I:C. These results highlight the differences between responses induced by direct, nonlytic virus infection and extracellular poly I:C. The activation of astrocytes through these different pathways has implications for the initiation and progression of viral encephalitis and demyelinating diseases such as multiple sclerosis.

Placental TNF-α signaling in illness-induced complications of pregnancy.

Maternal infections are implicated in a variety of complications during pregnancy, including pregnancy loss, prematurity, and increased risk of neurodevelopmental disorders in the child. Here, we show in mice that even mild innate immune activation by low-dose lipopolysaccharide in early pregnancy causes hemorrhages in the placenta and increases the risk of pregnancy loss. Surviving fetuses exhibit hypoxia in the brain and impaired fetal neurogenesis. Maternal Toll-like receptor 4 signaling is a critical mediator of this process, and its activation is accompanied by elevated proinflammatory cytokines in the placenta. We evaluated the role of tumor necrosis factor-α (TNF-α) signaling and show that TNF receptor 1 (TNFR1) is necessary for the illness-induced placental pathology, accompanying fetal hypoxia, and neuroproliferative defects in the fetal brain. We also show that placental TNFR1 in the absence of maternal TNFR1 is sufficient for placental pathology to develop and that a clinically relevant TNF-α antagonist prevents placental pathology and fetal loss. Our observations suggest that the placenta is highly sensitive to proinflammatory signaling in early pregnancy and that TNF-α is an effective target for preventing illness-related placental defects and related risks to the fetus and fetal brain development.

Pro-Inflammatory Functions of Astrocytes Correlate with Viral Clearance and Strain-Dependent Protection from TMEV-Induced Demyelinating Disease

Intracerebral infection of susceptible strains of mice, e.g. SJL/J, with Theilers murine encephalomyelitis virus (TMEV) leads to a persistent CNS infection accompanied by development of a chronic-progressive inflammatory CNS autoimmune demyelinating disease which is clinically and pathologically similar to human multiple sclerosis. In contrast, resistant strains of mice, e.g. C57BL/6 (B6), effectively clear TMEV from the CNS and do not develop demyelinating disease. Although CD8(+) T cells are crucial for viral clearance in B6 mice, SJL mice also mount potent CD8(+) T cell responses against virus, thus the reason for the viral persistence in the CNS in these mice is unclear. Here, we examined innate anti-viral responses of CNS-resident astrocytes as a potential determinant of viral persistence and disease susceptibility. We demonstrate that B6 astrocytes produce significantly higher levels of cytokines, chemokines and adhesion molecules in response to TMEV infection, or stimulation with IFN-gamma and TNF-alpha or poly I:C than SJL mice. In addition, TMEV more effectively induces MHC I molecules on B6 astrocytes than SJL, corresponding with an increased ability to activate TMEV-specific CD8(+) T cells directly ex vivo. These results suggest that enhanced anti-viral responses of B6 astrocytes contribute to the ability of these mice to clear TMEV from the CNS and therefore to their resistance to the development of autoimmune demyelinating disease.

Stereotypical Alterations in Cortical Patterning Are Associated with Maternal Illness-Induced Placental Dysfunction

We have previously shown in mice that cytokine-mediated damage to the placenta can temporarily limit the flow of nutrients and oxygen to the fetus. The placental vulnerability is pronounced before embryonic day 11, when even mild immune challenge results in fetal loss. As gestation progresses, the placenta becomes increasingly resilient to maternal inflammation, but there is a narrow window in gestation when the placenta is still vulnerable to immune challenge yet resistant enough to allow for fetal survival. This gestational window correlates with early cortical neurogenesis in the fetal brain. Here, we show that maternal illness during this period selectively alters the abundance and laminar positioning of neuronal subtypes influenced by the Tbr1, Satb2, and Ctip2/Fezf2 patterning axis. The disturbances also lead to a laminar imbalance in the proportions of projection neurons and interneurons in the adult and are sufficient to cause changes in social behavior and cognition. These data illustrate how the timing of an illness-related placental vulnerability causes developmental alterations in neuroanatomical systems and behaviors that are relevant to autism spectrum disorders.

Pattern recognition receptors in CNS disease

Pattern recognition receptors (PRRs) are germline encoded receptors utilized by cells of the innate immune system for pathogen recognition. PRRs are classically activated by pathogen-associated molecular patterns (PAMPs) present in whole classes of pathogens, but not in mammalian cells, termed the “infectious nonself model” (Medzhitov and Janeway, 2002). It has also been recent appreciated that there are self-derived products released upon tissue injury or necrotic cell death that can activate PRRs (Morgan et al., 2005). PRR activation leads to opsonization, activation of complement and coagulation cascades, phagocytosis, activation of proinflammatory signaling pathways, and the induction of apoptosis (Janeway and Medzhitov, 2002; Matzinger, 2002). Additionally, activation of the innate immune system is crucial for the induction of adaptive immune responses and the eventual clearance of pathogens (Janeway and Medzhitov, 2002).