Thanks for the memories: Low-avidity T cells shine against escape variants.

T cell responses against foreign antigens are clonally diverse, but the significance of this diversity is unclear. In this issue of Immunity, Straub et al.1 show that recruitment of low-avidity T cells during primary infection can provide protection against subsequent encounter with escape variants.

The Impact of MHC Class I Dose on Development and Maintenance of the Polyclonal Naive CD8+ T Cell Repertoire.

Naive CD8+ T cell survival in the periphery is critically dependent on tonic TCR signaling through peptide + MHC class I (MHCI) recognition; however, little is known about how natural variation in MHCI levels impacts the naive CD8+ T cell repertoire. Using mice that are hemizygous or homozygous for a single MHCI allele, we showed that despite a reduction in peripheral CD8+ T cell numbers of ∼50% in MHCI hemizygous mice, MHCI levels had no notable impact on the rate of thymic generation or emigration of CD8 single-positive T cells. Moreover, the peripheral T cell repertoire in hemizygous mice showed selective retention of T cell clonotypes with a greater competitive advantage as evidenced by increased expression of CD5 and IL-7Rα. The qualitative superiority of CD8+ T cells retained in hemizygous mice was also seen during influenza A virus infection, in which epitope-specific CD8+ T cells from hemizygous mice had a higher avidity for pMHCI and increased cytokine polyfunctionality, despite a reduced response magnitude. Collectively, this study suggests that natural variation in MHCI expression levels has a notable and biologically relevant impact on the maintenance, but not generation, of the naive CD8+ T cell repertoire.

Overlapping Peptides Elicit Distinct CD8+ T Cell Responses following Influenza A Virus Infection.

The presentation of pathogen-derived peptides on MHC class I molecules is essential for the initiation of adaptive CD8+ T cell immunity, which in turn is critical for effective control of many significant human infections. The identification of immunogenic pathogen-derived epitopes and a detailed understanding of how they are recognized by TCRs is essential for the design of effective T cell-based vaccines. In this study, we have characterized the T cell recognition and immune responses in mice to two naturally presented influenza A virus-derived peptides previously identified from virally infected cells via mass spectrometry. These neuraminidase-derived peptides, NA181-190 (SGPDNGAVAV) and NA181-191 (SGPDNGAVAVL), are completely overlapping with the exception of a 1 aa extension at the C terminus of the longer peptide. This minor peptidic difference results in the induction of two completely independent and non-cross-reactive T cell populations that show distinct functional characteristics after influenza A virus infection of B6 mice. We show that the unique TCR reactivity to the overlapping peptides is present in the naive repertoire prior to immune expansion in B6 mice. Moreover, we provide a structural explanation underlying the distinct CD8+ T cell reactivities, which reinforces the concept that peptide length is a key determinant of Ag specificity in CD8+ T cell responses.

Amelioration of experimental autoimmune encephalomyelitis by clozapine is not associated with defective CD4 T cell responses.

Atypical antipsychotic agents, such as clozapine, are used for treating psychosis and depression and have recently been found to modulate neuroinflammation. We have shown previously that treatment of mice with the atypical antipsychotic agents, clozapine or risperidone, attenuates disease severity in experimental autoimmune encephalomyelitis (EAE); however, the mechanism by which they are protective is unknown. In this study, we investigated the effects of clozapine on CD4+ T cell responses and found that clozapine did not significantly affect the expansion of myelin-specific T cells, their differentiation into pathogenic subsets, or their encephalitogenic capacity to induce EAE. Interestingly, although clozapine enhanced differentiation of regulatory T (Treg) cells, in vivo neutralization of Tregs indicated that Tregs were not responsible for the protective effects of clozapine during the induction and effector phase of EAE. Taken together, our studies indicate that clozapine does not mediate its protective effects by directly altering CD4 T cells.

Covalent TCR-peptide-MHC interactions induce T cell activation and redirect T cell fate in the thymus.

Interactions between a T cell receptor (TCR) and a peptide-major histocompatibility complex (pMHC) ligand are typically mediated by noncovalent bonds. By studying T cells expressing natural or engineered TCRs, here we describe covalent TCR-pMHC interactions that involve a cysteine-cysteine disulfide bond between the TCR and the peptide. By introducing cysteines into a known TCR-pMHC combination, we demonstrate that disulfide bond formation does not require structural rearrangement of the TCR or the peptide. We further show these disulfide bonds still form even when the initial affinity of the TCR-pMHC interaction is low. Accordingly, TCR-peptide disulfide bonds facilitate T cell activation by pMHC ligands with a wide spectrum of affinities for the TCR. Physiologically, this mechanism induces strong Zap70-dependent TCR signaling, which triggers T cell deletion or agonist selection in the thymus cortex. Covalent TCR-pMHC interactions may thus underlie a physiological T cell activation mechanism that has applications in basic immunology and potentially in immunotherapy.

Checkpoint inhibitor immunotherapy diminishes oocyte number and quality in mice.

Loss of fertility is a major concern for female reproductive-age cancer survivors, since a common side-effect of conventional cytotoxic cancer therapies is permanent damage to the ovary. While immunotherapies are increasingly becoming a standard of care for many cancers-including in the curative setting-their impacts on ovarian function and fertility are unknown. We evaluated the effect of immune checkpoint inhibitors blocking programmed cell death protein ligand 1 and cytotoxic T lymphocyte-associated antigen 4 on the ovary using tumor-bearing and tumor-free mouse models. We find that immune checkpoint inhibition increases immune cell infiltration and tumor necrosis factor-α expression within the ovary, diminishes the ovarian follicular reserve and impairs the ability of oocytes to mature and ovulate. These data demonstrate that immune checkpoint inhibitors have the potential to impair both immediate and future fertility, and studies in women should be prioritized. Additionally, fertility preservation should be strongly considered for women receiving these immunotherapies, and preventative strategies should be investigated in future studies.

Evaluation of inflammation and follicle depletion during ovarian ageing in mice.

Reproductive ageing in females is defined by a progressive decline in follicle number and oocyte quality. This is a natural process that leads to the loss of fertility and ovarian function, cycle irregularity and eventually menopause or reproductive senescence. The factors that underlie the natural depletion of follicles throughout reproductive life are poorly characterised. It has been proposed that inflammatory processes and fibrosis might contribute to ovarian ageing. To further investigate this possibility, we evaluated key markers of inflammation and immune cell populations in the ovaries of 2, 6, 12 and 18-month-old C57BL/6 female mice. We report that the decrease in follicle numbers over the reproductive lifespan was associated with an increase in the intra-ovarian percentage of CD4 + T cells, B cells and macrophages. Serum concentration and intra-ovarian mRNA levels of several pro-inflammatory cytokines, including IL-1α/ß, TNF-α, IL-6, and inflammasome genes ASC and NLRP3, were significantly increased with age. Fibrosis levels, as determined by picrosirius red staining for collagen I and III, were unchanged up to 18 months of age. Collectively, these data suggest that inflammation could be one of the mechanisms responsible for the age-related regulation of follicle number, but the role of fibrosis is unclear. Further studies are now required to determine if there is a causative relationship between inflammation and follicle depletion as females age.

Canonical T cell receptor docking on peptide-MHC is essential for T cell signaling.

T cell receptor (TCR) recognition of peptide-major histocompatibility complexes (pMHCs) is characterized by a highly conserved docking polarity. Whether this polarity is driven by recognition or signaling constraints remains unclear. Using "reversed-docking" TCRß-variable (TRBV) 17+ TCRs from the naïve mouse CD8+ T cell repertoire that recognizes the H-2Db-NP366 epitope, we demonstrate that their inability to support T cell activation and in vivo recruitment is a direct consequence of reversed docking polarity and not TCR-pMHCI binding or clustering characteristics. Canonical TCR-pMHCI docking optimally localizes CD8/Lck to the CD3 complex, which is prevented by reversed TCR-pMHCI polarity. The requirement for canonical docking was circumvented by dissociating Lck from CD8. Thus, the consensus TCR-pMHC docking topology is mandated by T cell signaling constraints.

The shared susceptibility epitope of HLA-DR4 binds citrullinated self-antigens and the TCR.

Individuals expressing HLA-DR4 bearing the shared susceptibility epitope (SE) have an increased risk of developing rheumatoid arthritis (RA). Posttranslational modification of self-proteins via citrullination leads to the formation of neoantigens that can be presented by HLA-DR4 SE allomorphs. However, in T cell-mediated autoimmunity, the interplay between the HLA molecule, posttranslationally modified epitope(s), and the responding T cell repertoire remains unclear. In HLA-DR4 transgenic mice, we show that immunization with a Fibß-74cit69-81 peptide led to a population of HLA-DR4Fibß-74cit69-81 tetramer+ T cells that exhibited biased T cell receptor (TCR) ß chain usage, which was attributable to selective clonal expansion from the preimmune repertoire. Crystal structures of pre- and postimmune TCRs showed that the SE of HLA-DR4 represented a main TCR contact zone. Immunization with a double citrullinated epitope (Fibß-72,74cit69-81) altered the responding HLA-DR4 tetramer+ T cell repertoire, which was due to the P2-citrulline residue interacting with the TCR itself. We show that the SE of HLA-DR4 has dual functionality, namely, presentation and a direct TCR recognition determinant. Analogous biased TCR ß chain usage toward the Fibß-74cit69-81 peptide was observed in healthy HLA-DR4+ individuals and patients with HLA-DR4+ RA, thereby suggesting a link to human RA.

Enhanced disease reduction using clozapine, an atypical antipsychotic agent, and glatiramer acetate combination therapy in experimental autoimmune encephalomyelitis.

BACKGROUND: Atypical antipsychotic agents (AAP) alleviate the symptoms of severe mental health disorders, such as schizophrenia, by antagonizing dopamine and serotonin receptors. Recently, AAP have also been shown to exhibit immunomodulatory properties in the central nervous system (CNS). OBJECTIVE: Building on research which demonstrated the ability of the AAP risperidone and clozapine to modify the disease course of experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis (MS), we aimed to more fully investigate the potential of clozapine as a possible treatment for MS. RESULTS: We report that orally administered clozapine significantly reduced the disease severity of EAE in a dose-dependent manner and was effective when administered prophylactically and therapeutically. In comparison to risperidone, quetiapine, and olanzapine, clozapine was the best at reducing disease severity. While clozapine-treated mice had only modest changes to peripheral leukocytes and cytokine responses, these animals had significantly fewer CNS-infiltrating CD4 T cells and myeloid cells. Furthermore, the CNS myeloid cells displayed a less activated phenotype in mice treated with clozapine. Finally, we found that co-administration of clozapine with glatiramer acetate enhanced disease protection compared to either treatment alone. CONCLUSION: These studies indicate that clozapine is an effective immunomodulatory agent with the potential to treat immune-mediated diseases such as MS.

Treatment with the antipsychotic agent, risperidone, reduces disease severity in experimental autoimmune encephalomyelitis.

Recent studies have demonstrated that atypical antipsychotic agents, which are known to antagonize dopamine D2 and serotonin 5-HT2a receptors, have immunomodulatory properties. Given the potential of these drugs to modulate the immune system both peripherally and within the central nervous system, we investigated the ability of the atypical anti-psychotic agent, risperidone, to modify disease in the animal model of multiple sclerosis (MS)4, experimental autoimune encephalomyelitis (EAE). We found that chronic oral administration of risperidone dose-dependently reduced the severity of disease and decreased both the size and number of spinal cord lesions. Furthermore, risperidone treatment substantially reduced antigen-specific interleukin (IL)-17a, IL-2, and IL-4 but not interferon (IFN)-γ production by splenocytes at peak disease and using an in vitro model, we show that treatment of macrophages with risperidone alters their ability to bias naïve T cells. Another atypical antipsychotic agent, clozapine, showed a similar ability to modify macrophages in vitro and to reduce disease in the EAE model but this effect was not due to antagonism of the type 1 or type 2 dopamine receptors alone. Finally, we found that while risperidone treatment had little effect on the in vivo activation of splenic macrophages during EAE, it significantly reduced the activation of microglia and macrophages in the central nervous system. Together these studies indicate that atypical antipsychotic agents like risperidone are effective immunomodulatory agents with the potential to treat immune-mediated diseases such as MS.