Peripherally induced brain tissue-resident memory CD8+ T cells mediate protection against CNS infection.

The central nervous system (CNS) is classically viewed as immune-privileged; however, recent advances highlight interactions between the peripheral immune system and CNS in controlling infections and tissue homeostasis. Tissue-resident memory (TRM) CD8+ T cells in the CNS are generated after brain infections, but it is unknown whether CNS infection is required to generate brain TRM cells. We show that peripheral infections generate antigen-specific CD8+ memory T cells in the brain that adopt a unique TRM signature. Upon depletion of circulating and perivascular memory T cells, this brain signature was enriched and the surveilling properties of brain TRM cells was revealed by intravital imaging. Notably, peripherally induced brain TRM cells showed evidence of rapid activation and enhanced cytokine production and mediated protection after brain infections. These data reveal that peripheral immunizations can generate brain TRM cells and will guide potential use of T cells as therapeutic strategies against CNS infections and neurological diseases.

A T Cell Receptor Locus Harbors a Malaria-Specific Immune Response Gene.

Immune response (Ir) genes, originally proposed by Baruj Benacerraf to explain differential antigen-specific responses in animal models, have become synonymous with the major histocompatibility complex (MHC). We discovered a non-MHC-linked Ir gene in a T cell receptor (TCR) locus that was required for CD8+ T cell responses to the Plasmodium berghei GAP5040-48 epitope in mice expressing the MHC class I allele H-2Db. GAP5040-48-specific CD8+ T cell responses emerged from a very large pool of naive Vß8.1+ precursors, which dictated susceptibility to cerebral malaria and conferred protection against recombinant Listeria monocytogenes infection. Structural analysis of a prototypical Vß8.1+ TCR-H-2Db-GAP5040-48 ternary complex revealed that germline-encoded complementarity-determining region 1ß residues present exclusively in the Vß8.1 segment mediated essential interactions with the GAP5040-48 peptide. Collectively, these findings demonstrated that Vß8.1 functioned as an Ir gene that was indispensable for immune reactivity against the malaria GAP5040-48 epitope.

Discrete class I molecules on brain endothelium differentially regulate neuropathology in experimental cerebral malaria.

Cerebral malaria is the deadliest complication that can arise from Plasmodium infection. CD8 T-cell engagement of brain vasculature is a putative mechanism of neuropathology in cerebral malaria. To define contributions of brain endothelial cell major histocompatibility complex (MHC) class I antigen-presentation to CD8 T cells in establishing cerebral malaria pathology, we developed novel H-2Kb LoxP and H-2Db LoxP mice crossed with Cdh5-Cre mice to achieve targeted deletion of discrete class I molecules, specifically from brain endothelium. This strategy allowed us to avoid off-target effects on iron homeostasis and class I-like molecules, which are known to perturb Plasmodium infection. This is the first endothelial-specific ablation of individual class-I molecules enabling us to interrogate these molecular interactions. In these studies, we interrogated human and mouse transcriptomics data to compare antigen presentation capacity during cerebral malaria. Using the Plasmodium berghei ANKA model of experimental cerebral malaria (ECM), we observed that H-2Kb and H-2Db class I molecules regulate distinct patterns of disease onset, CD8 T-cell infiltration, targeted cell death and regional blood-brain barrier disruption. Strikingly, ablation of either molecule from brain endothelial cells resulted in reduced CD8 T-cell activation, attenuated T-cell interaction with brain vasculature, lessened targeted cell death, preserved blood-brain barrier integrity and prevention of ECM and the death of the animal. We were able to show that these events were brain-specific through the use of parabiosis and created the novel technique of dual small animal MRI to simultaneously scan conjoined parabionts during infection. These data demonstrate that interactions of CD8 T cells with discrete MHC class I molecules on brain endothelium differentially regulate development of ECM neuropathology. Therefore, targeting MHC class I interactions therapeutically may hold potential for treatment of cases of severe malaria.

Out-of-sequence signal 3 as a mechanism for virus-induced immune suppression of CD8 T cell responses.

Virus infections are known to induce a transient state of immune suppression often associated with an inhibition of T cell proliferation in response to mitogen or cognate-antigen stimulation. Recently, virus-induced immune suppression has been linked to responses to type 1 interferon (IFN), a signal 3 cytokine that normally can augment the proliferation and differentiation of T cells exposed to antigen (signal 1) and co-stimulation (signal 2). However, pre-exposure of CD8 T cells to IFN-inducers such as viruses or poly(I∶C) prior to antigen signaling is inhibitory, indicating that the timing of IFN exposure is of essence. We show here that CD8 T cells pretreated with poly(I∶C) down-regulated the IFN receptor, up-regulated suppressor of cytokine signaling 1 (SOCS1), and were refractory to IFNß-induced signal transducers and activators of transcription (STAT) phosphorylation. When exposed to a viral infection, these CD8 T cells behaved more like 2-signal than 3-signal T cells, showing defects in short lived effector cell differentiation, reduced effector function, delayed cell division, and reduced levels of survival proteins. This suggests that IFN-pretreated CD8 T cells are unable to receive the positive effects that type 1 IFN provides as a signal 3 cytokine when delivered later in the signaling process. This desensitization mechanism may partially explain why vaccines function poorly in virus-infected individuals.

The N-terminus of vaccinia virus host range protein C7L is essential for function.

Vaccinia virus (VACV), a member of the Poxviridae family of large double-stranded DNA viruses, is being used as a smallpox vaccine as well as an expression vector for immunization against other infectious diseases and cancer. The host range of wild type VACV is very broad among mammalian cells. C7L is a host range gene identified in VACV and is well conserved in mammalian poxviruses except for parapoxviruses and molluscum contagiosum virus. The molecular mechanisms by which the C7L gene exerts host range function are not well understood. The C7L protein does not have any known conserved domains or show sequence similarity to cellular proteins or viral proteins other than the C7L homologs in mammalian poxviruses. We generated recombinant vaccinia viruses carrying deletion mutants of the C7L gene using NYVAC as a parental strain and found that the N-terminus is essential for host range function of C7L, which is consistent with a previous report that showed that homology among C7L homologs are greater near the N-terminus than the C-terminus.

Virus-induced transient immune suppression and the inhibition of T cell proliferation by type I interferon.

Vaccine-induced memory is necessary for protective immunity to pathogens, but many viruses induce a state of transient immune suppression that might contribute to the inability of a vaccine to elicit immunity. We evaluated here the fate of bystander T cells activated by third party cognate antigens during acute viral infections in vivo, using distinct models to track and specifically activate HY and P14 transgenic bystander CD8 T cells in vivo during acute arenavirus infections of mice. Viral infections acted as stimulatory adjuvants when bystander T cells were exposed to an inflammatory milieu and cognate antigens at the beginning of infections, but bystander CD8 T cell proliferation in response to cognate antigen was inhibited 3 to 9 days after virus infection. Reduced proliferation was not dependent on Fas-FasL- or tumor necrosis factor (TNF)-induced activation-induced cell death or on deficiencies of antigen presentation. Instead, reduced proliferation was associated with a delayed onset of division that was an intrinsic defect of T cells. Inhibition of proliferation could be simulated by exposure of T cells to the Toll-like receptor agonist and type I interferon (IFN) inducer poly(I · C). T cells lacking IFN-α/ß receptors resisted both the suppressive effects of preexposure to poly(I · C) and the stimulatory effects of type I IFN, indicating that the timing of exposure to IFN can have negative or positive effects on T cell proliferation. Inhibition of T cell receptor-stimulated bystander CD8 T cell proliferation during acute viral infections may reflect the reduced ability of vaccines to elicit protective immunity when administered during an acute illness.

Expeditious recruitment of circulating memory CD8 T cells to the liver facilitates control of malaria.

Circulating memory CD8 T cell trafficking and protective capacity during liver-stage malaria infection remains undefined. We find that effector memory CD8 T cells (Tem) infiltrate the liver within 6 hours after malarial or bacterial infections and mediate pathogen clearance. Tem recruitment coincides with rapid transcriptional upregulation of inflammatory genes in Plasmodium-infected livers. Recruitment requires CD8 T cell-intrinsic LFA-1 expression and the presence of liver phagocytes. Rapid Tem liver infiltration is distinct from recruitment to other non-lymphoid tissues in that it occurs both in the absence of liver tissue resident memory "sensing-and-alarm" function and ∼42 hours earlier than in lung infection by influenza virus. These data demonstrate relevance for Tem in protection against malaria and provide generalizable mechanistic insights germane to control of liver infections.

Prolonged microglial cell activation and lymphocyte infiltration following experimental herpes encephalitis.

Experimental murine herpes simplex virus (HSV)-1 brain infection stimulates microglial cell-driven proinflammatory chemokine production which precedes the presence of brain-infiltrating systemic immune cells. In the present study, we investigated the phenotypes and infiltration kinetics of leukocyte trafficking into HSV-infected murine brains. Using real-time bioluminescence imaging, the infiltration of luciferase-positive splenocytes, transferred via tail vein injection into the brains of HSV-infected animals, was followed over an 18-day time course. Flow cytometric analysis of brain-infiltrating leukocytes at 5, 8, 14, and 30 days postinfection (d.p.i.), was performed to assess their phenotype. A predominantly macrophage (CD45(high)CD11b(+)Ly6C(high)) and neutrophil (CD45(high)CD11b(+)Ly6G(+)) infiltration was seen early during infection, with elevated levels of TNF-alpha mRNA expression. By 14 d.p.i., the phenotypic profile shifted to a predominantly lymphocytic (CD45(high)CD3(+)) infiltrate. This lymphocyte infiltrate was detected until 30 d.p.i., when infectious virus could not be recovered, with CD8(+) and CD4(+) T cells present at a 3:1 ratio, respectively. This T lymphocyte infiltration paralleled increased IFN-gamma mRNA expression in the brain. Activation of resident microglia (CD45(int)CD11b(+)) was also detected until 30 d.p.i., as assessed by MHC class II expression. Activated microglial cells were further identified as the predominant source of IL-1beta. In addition, infected mice given primed immunocytes at 4 d.p.i. showed a significant increase in mortality. Taken together, these results demonstrate that intranasal infection results in early macrophage and neutrophil infiltration into the brain followed by prolonged microglial activation and T lymphocyte retention. Similar prolonged neuroimmune activation may contribute to the neuropathological sequelae observed in herpes encephalitis patients.

Type 1 interferon licenses naïve CD8 T cells to mediate anti-viral cytotoxicity.

Naïve T cells, unlike memory T cells, exhibit very limited effector function in response to cognate antigen, but exposure to type 1 interferon (IFN) prior to cognate antigen allows for rapid manifestation of effector functions. A full assessment of the functions of these IFN-sensitized otherwise naïve T cells has not been made, nor has their capacity to be effector cells in vivo. We describe here that IFN-sensitized naïve T cells in the absence of cognate antigen adopt a partial activated phenotype distinguished by the upregulation of the surface activation marker CD69, effector-associated transcription factors Eomes and IRF4, and cytotoxicity effector molecule granzyme B. IFN-sensitized naive T cells lysed target cells in vivo and responded to low concentrations and affinities of cognate ligands. We suggest that this rapid and sensitive effector function of IFN-conditioned naïve CD8 T cells may play a role in pathogen control and help ward off superinfections.