Nonconventional CD8+ T cell responses to Listeria infection in mice lacking MHC class Ia and H2-M3

A single-cell transcriptomic landscape of mouse testicular aging

INTRODUCTION

Advanced paternal age of reproduction is an increasing trend, especially in developed countries and areas. This trend results in elevated risks of adverse reproductive outcomes such as reduced fertility rates, increased pregnancy loss, and poor childhood health. Yet, a systematic profiling of aging-associated molecular and cellular alterations in testicular tissue is still missing.

OBJECTIVES

We aimed to dissect aging-associated molecular characteristics in testes of mice.

METHODS

Single-cell transcriptomic sequencing and analysis were conducted in testes of young (2 months old) and old mice (24 months old). Immunofluorescences and immunochemistry were used to characterize aging-associated phenotypes and verify single cell sequence results.

RESULTS

Here, we constructed the first single-cell transcriptomic atlases of testes of young and old mice. In-depth dissection of aging-dependent transcriptional alterations in specific cell types revealed multiple dysregulated biological processes such as increased 'senescence-associated secretory phenotype' and 'inflammation', which were major aging-associated characteristics. Further analysis of aging-related differentially expressed genes uncovered a disrupted balance of undifferentiated and differentiated spermatogonia stem cells in spermatogonia, indicative of a potential role of spermatogonia stem cells in aging-associated subfertility. Importantly, for the first time, our results identified an increased subtype of aging-specific macrophages, which may contribute to a hostile proinflammatory microenvironment during testicular aging.

CONCLUSION

Taken together, our findings depict the distinct single-cell transcriptional features of the aged mouse testes and provide enormous resources for a comprehensive understanding of the cell-type-specific molecular mechanisms underlying mouse testicular aging, which may shed light on developing novel potential diagnostic biomarkers and therapeutic targets for age-associated male subfertility.

Innate and Adaptive Immune Responses during Listeria monocytogenes Infection

It could be argued that we understand the immune response to infection with Listeria monocytogenes better than the immunity elicited by any other bacteria. L. monocytogenes are Gram-positive bacteria that are genetically tractable and easy to cultivate in vitro, and the mouse model of intravenous (i.v.) inoculation is highly reproducible. For these reasons, immunologists frequently use the mouse model of systemic listeriosis to dissect the mechanisms used by mammalian hosts to recognize and respond to infection. This article provides an overview of what we have learned over the past few decades and is divided into three sections: "Innate Immunity" describes how the host initially detects the presence of L. monocytogenes and characterizes the soluble and cellular responses that occur during the first few days postinfection; "Adaptive Immunity" discusses the exquisitely specific T cell response that mediates complete clearance of infection and immunological memory; "Use of Attenuated Listeria as a Vaccine Vector" highlights the ways that investigators have exploited our extensive knowledge of anti-Listeria immunity to develop cancer therapeutics.

Nonclassical MHC Ib-restricted CD8+ T Cells Recognize Mycobacterium tuberculosis-Derived Protein Antigens and Contribute to Protection Against Infection

MHC Ib-restricted CD8+ T cells have been implicated in host defense against Mycobacterium tuberculosis (Mtb) infection. However, the relative contribution of various MHC Ib-restricted T cell populations to anti-mycobacterial immunity remains elusive. In this study, we used mice that lack MHC Ia (Kb-/-Db-/-), MHC Ia/H2-M3 (Kb-/-Db-/-M3-/-), or β2m (β2m-/-) to study the role of M3-restricted and other MHC Ib-restricted T cells in immunity against Mtb. Unlike their dominant role in Listeria infection, we found that M3-restricted CD8+ T cells only represented a small proportion of the CD8+ T cells responding to Mtb infection. Non-M3, MHC Ib-restricted CD8+ T cells expanded preferentially in the lungs of Mtb-infected Kb-/-Db-/-M3-/- mice, exhibited polyfunctional capacities and conferred protection against Mtb. These MHC Ib-restricted CD8+ T cells recognized several Mtb-derived protein antigens at a higher frequency than MHC Ia-restricted CD8+ T cells. The presentation of Mtb antigens to MHC Ib-restricted CD8+ T cells was mostly β2m-dependent but TAP-independent. Interestingly, a large proportion of Mtb-specific MHC Ib-restricted CD8+ T cells in Kb-/-Db-/-M3-/- mice were Qa-2-restricted while no considerable numbers of MR1 or CD1-restricted Mtb-specific CD8+ T cells were detected. Our findings indicate that nonclassical CD8+ T cells other than the known M3, CD1, and MR1-restricted CD8+ T cells contribute to host immune responses against Mtb infection. Targeting these MHC Ib-restricted CD8+ T cells would facilitate the design of better Mtb vaccines with broader coverage across MHC haplotypes due to the limited polymorphism of MHC class Ib molecules.

The burgeoning family of unconventional T cells

While most studies of T lymphocytes have focused on T cells reactive to complexes of peptide and major histocompatibility complex (MHC) proteins, many other types of T cells do not fit this paradigm. These include CD1-restricted T cells, MR1-restricted mucosal associated invariant T cells (MAIT cells), MHC class Ib-reactive T cells, and γδ T cells. Collectively, these T cells are considered 'unconventional', in part because they can recognize lipids, small-molecule metabolites and specially modified peptides. Unlike MHC-reactive T cells, these apparently disparate T cell types generally show simplified patterns of T cell antigen receptor (TCR) expression, rapid effector responses and 'public' antigen specificities. Here we review evidence showing that unconventional T cells are an abundant component of the human immune system and discuss the immunotherapeutic potential of these cells and their antigenic targets.

CXCR3 expression defines a novel subset of innate CD8+ T cells that enhance immunity against bacterial infection and cancer upon stimulation with IL-15

Innate CD8(+) T cells are a heterogeneous population with developmental pathways distinct from conventional CD8(+) T cells. However, their biology, classification, and functions remain incompletely understood. We recently demonstrated the existence of a novel population of chemokine (C-X-C motif) receptor 3 (CXCR3)-positive innate CD8(+) T cells. Here, we investigated the functional properties of this subset and identified effector molecules and pathways which mediate their function. Adoptive transfer of IL-15 activated CXCR3(+) innate CD8(+) T cells conferred increased protection against Listeria monocytogenes infection in susceptible IFN-γ(-/-) mice compared with similarly activated CXCR3(-) subset. This was associated with enhanced proliferation and IFN-γ production in CXCR3(+) cells. Further, CXCR3(+) innate cells showed enhanced cytotoxicity against a tumor cell line in vitro. In depth analysis of the CXCR3(+) subset showed increased gene expression of Ccl5, Klrc1, CtsW, GP49a, IL-2Rβ, Atp5e, and Ly6c but reduced IFN-γR2 and Art2b. Ingenuity pathway analysis revealed an up-regulation of genes associated with T-cell activation, proliferation, cytotoxicity, and translational initiation in CXCR3(+) populations. Our results demonstrate that CXCR3 expression in innate CD8(+) T cells defines a subset with enhanced cytotoxic potential and protective antibacterial immune functions. Immunotherapeutic approaches against infectious disease and cancer could utilize CXCR3(+) innate CD8(+) T-cell populations as novel clinical intervention strategies.

Expression of the mouse MHC class Ib H2-T11 gene product, a paralog of H2-T23 (Qa-1) with shared peptide-binding specificity

The mouse MHC class Ib gene H2-T11 is 95% identical at the DNA level to H2-T23, which encodes Qa-1, one of the most studied MHC class Ib molecules. H2-T11 mRNA was observed to be expressed widely in tissues of C57BL/6 mice, with the highest levels in thymus. To circumvent the availability of a specific mAb, cells were transduced with cDNA encoding T11 with a substituted α3 domain. Hybrid T11D3 protein was expressed at high levels similar to control T23D3 molecules on the surface of both TAP(+) and TAP(-) cells. Soluble T11D3 was generated by folding in vitro with Qa-1 determinant modifier, the dominant peptide presented by Qa-1. The circular dichroism spectrum of this protein was similar to that of other MHC class I molecules, and it was observed to bind labeled Qa-1 determinant modifier peptide with rapid kinetics. By contrast to the Qa-1 control, T11 tetramers did not react with cells expressing CD94/NKG2A, supporting the conclusion that T11 cannot replace Qa-1 as a ligand for NK cell inhibitory receptors. T11 also failed to substitute for Qa-1 in the presentation of insulin to a Qa-1-restricted T cell hybridoma. Despite divergent function, T11 was observed to share peptide-loading specificity with Qa-1. Direct analysis by tandem mass spectrometry of peptides eluted from T11D3 and T23D3 isolated from Hela cells demonstrated a diversity of peptides with a clear motif that was shared between the two molecules. Thus, T11 is a paralog of T23 encoding an MHC class Ib molecule that shares peptide-binding specificity with Qa-1 but differs in function.

Mitochondria are required for antigen-specific T cell activation through reactive oxygen species signaling

It is widely appreciated that T cells increase glycolytic flux during activation, but the role of mitochondrial flux is unclear. Here, we have shown that mitochondrial metabolism in the absence of glucose metabolism is sufficient to support interleukin-2 (IL-2) induction. Furthermore, we used mice with reduced mitochondrial reactive oxygen species (mROS) production in T cells (T-Uqcrfs(-/-) mice) to show that mitochondria are required for T cell activation to produce mROS for activation of nuclear factor of activated T cells (NFAT) and subsequent IL-2 induction. These mice could not induce antigen-specific expansion of T cells in vivo, but Uqcrfs1(-/-) T cells retained the ability to proliferate in vivo under lymphopenic conditions. This suggests that Uqcrfs1(-/-) T cells were not lacking bioenergetically but rather lacked specific ROS-dependent signaling events needed for antigen-specific expansion. Thus, mitochondrial metabolism is a critical component of T cell activation through the production of complex III ROS.

Cutting edge: innate memory CD8+ T cells are distinct from homeostatic expanded CD8+ T cells and rapidly respond to primary antigenic stimuli

Innate memory phenotype (IMP) CD8(+) T cells are nonconventional αβ T cells exhibiting features of innate immune cells and are significantly increased in the absence of ITK. Their developmental path and function are not clear. In this study, we show hematopoietic MHC class I (MHCI)-dependent generation of Ag-specific IMP CD8(+) T cells using bone marrow chimeras. Wild-type bone marrow gives rise to IMP CD8(+) T cells in MHCI(-/-) recipients, resembling those in Itk(-/-) mice, but distinct from those derived via homeostatic proliferation, and independent of recipient thymus. In contrast, MHCI(-/-) bone marrow does not lead to IMP CD8(+) T cells in wild-type recipients. OTI IMP CD8(+) T cells generated via this method exhibited enhanced early response to Ag without prior primary stimulation. Our findings suggest a method to generate Ag-specific "naive" CD8(+) IMP T cells, as well as demonstrate that they are not homeostatic proliferation cells and can respond promptly in an Ag-specific fashion.

Distinct populations of innate CD8+ T cells revealed in a CXCR3 reporter mouse

CXCR3, expressed mainly on activated T and NK cells, is implicated in a host of immunological conditions and can contribute either to disease resolution or pathology. We report the generation and characterization of a novel CXCR3 internal ribosome entry site bicistronic enhanced GFP reporter (CIBER) mouse in which enhanced GFP expression correlates with surface levels of CXCR3. Using CIBER mice, we identified two distinct populations of innate CD8(+) T cells based on constitutive expression of CXCR3. We demonstrate that CXCR3(+) innate CD8(+) T cells preferentially express higher levels of Ly6C and CD122, but lower levels of CCR9 compared with CXCR3(-) innate CD8(+) T cells. Furthermore, we show that CXCR3(+) innate CD8(+) T cells express higher transcript levels of antiapoptotic but lower levels of proapoptotic factors, respond more robustly to IL-2 and IL-15, and produce significantly more IFN-γ and granzyme B. Interestingly, CXCR3(+) innate CD8(+) T cells do not respond to IL-12 or IL-18 alone, but produce significant amounts of IFN-γ on stimulation with a combination of these cytokines. Taken together, these findings demonstrate that CXCR3(+) and CXCR3(-) innate CD8(+) T cells are phenotypically and functionally distinct. These newly generated CIBER mice provide a novel tool for studying the role of CXCR3 and CXCR3-expressing cells in vivo.

SAP is required for the development of innate phenotype in H2-M3--restricted Cd8(+) T cells

H2-M3--restricted T cells have a preactivated surface phenotype, rapidly expand, and produce cytokines upon stimulation, and, as such, are classified as innate T cells. Unlike most innate T cells, M3-restricted T cells also express CD8αβ coreceptors and a diverse TCR repertoire: hallmarks of conventional MHC Ia-restricted CD8(+) T cells. Although invariant NKT cells are also innate T cells, they are selected exclusively on hematopoietic cells (HC), whereas M3-restricted T cells can be selected on either hematopoietic or thymic epithelial cells. Moreover, their phenotypes differ depending on what cells mediate their selection. Although there is a clear correlation between selection on HC and development of innate phenotype, the underlying mechanism remains unclear. Signaling lymphocyte activation molecule-associated protein (SAP) is required for the development of invariant NKT cells and mediates signals from signaling lymphocyte activation molecule receptors that are exclusively expressed on HC. Based on their dual selection pathway, M3-restricted T cells present a unique model for studying the development of innate T cell phenotype. Using both polyclonal and transgenic mouse models, we demonstrate that although M3-restricted T cells are capable of developing in the absence of SAP, SAP is required for HC-mediated selection, development of preactivated phenotype, and heightened effector functions of M3-restricted T cells. These findings are significant because they directly demonstrate the need for SAP in HC-mediated acquisition of innate T cell phenotype and suggest that, due to their SAP-dependent HC-mediated selection, M3-restricted T cells develop a preactivated phenotype and an intrinsic ability to proliferate faster upon stimulation, allowing for an important role in the early response to infection.

Early events regulating immunity and pathogenesis during Listeria monocytogenes infection

Listeria monocytogenes (Lm) is both a life-threatening pathogen of humans and a model organism that is widely used to dissect the mechanisms of innate and adaptive immune resistance to infection. Specific aspects of the immune response to systemic Lm infection can be protective, neutral, or in some cases deleterious. In this review, we seek to provide an overview of the early events during Lm infection that dictate or regulate host innate and adaptive immune responses. We highlight several recent developments that add to our understanding of the complex interplay between inflammatory responses, host susceptibility to infection, and the development of protective immunity.

Polyclonal mucosa-associated invariant T cells have unique innate functions in bacterial infection

Mucosa-associated invariant T (MAIT) cells are a unique population of αβ T cells in mammals that reside preferentially in mucosal tissues and express an invariant Vα paired with limited Vβ T-cell receptor (TCR) chains. Furthermore, MAIT cell development is dependent upon the expression of the evolutionarily conserved major histocompatibility complex (MHC) class Ib molecule MR1. Using in vitro assays, recent studies have shown that mouse and human MAIT cells are activated by antigen-presenting cells (APCs) infected with diverse microbes, including numerous bacterial strains and yeasts, but not viral pathogens. However, whether MAIT cells play an important, and perhaps unique, role in controlling microbial infection has remained unclear. To probe MAIT cell function, we show here that purified polyclonal MAIT cells potently inhibit intracellular bacterial growth of Mycobacterium bovis BCG in macrophages (MΦ) in coculture assays, and this inhibitory activity was dependent upon MAIT cell selection by MR1, secretion of gamma interferon (IFN-γ), and an innate interleukin 12 (IL-12) signal from infected MΦ. Surprisingly, however, the cognate recognition of MR1 by MAIT cells on the infected MΦ was found to play only a minor role in MAIT cell effector function. We also report that MAIT cell-deficient mice had higher bacterial loads at early times after infection compared to wild-type (WT) mice, demonstrating that MAIT cells play a unique role among innate lymphocytes in protective immunity against bacterial infection.

Nonclassical MHC class Ib-restricted cytotoxic T cells monitor antigen processing in the endoplasmic reticulum

The ER aminopeptidase associated with antigen processing, ERAAP, is essential for trimming peptides presented by MHC I molecules. ERAAP inhibition by cytomegalovirus causes immune evasion, and ERAAP polymorphisms are associated with autoimmune disorders. How normal ERAAP function is monitored is unknown. We found that ERAAP inhibition rapidly induced presentation of the FL9 peptide by the Qa-1^b MHC Ib molecule. Antigen-experienced T cells specific for the Qa-1^b-FL9 complex were frequent in naïve mice. Wild-type mice immunized with ERAAP-deficient cells mounted a potent CD8⁺ T cell response specific for the Qa-1^b-FL9- complex. MHC Ib-restricted cytolytic effectors specifically eliminated ERAAP-deficient cells in vitro and in vivo. Thus, non-classical peptide-Qa-1^b complexes direct cytotoxic T cells to targets with defective antigen processing in the ER.

An MHC class Ib-restricted CD8+ T cell response to lymphocytic choriomeningitis virus

Conventional MHC class Ia-restricted CD8(+) T cells play a dominant role in the host response to virus infections, but recent studies indicate that T cells with specificity for nonclassical MHC class Ib molecules may also participate in host defense. To investigate the potential role of class Ib molecules in anti-viral immune responses, K(b-/-)D(b-/-)CIITA(-/-) mice lacking expression of MHC class Ia and class II molecules were infected with lymphocytic choriomeningitis virus (LCMV). These animals have a large class Ib-selected CD8(+) T cell population and they were observed to mediate partial (but incomplete) virus clearance during acute LCMV infection as compared with K(b-/-)D(b-/-)β(2)-microglobulin(-/-) mice that lack expression of both MHC class Ia and class Ib molecules. Infection was associated with expansion of splenic CD8(+) T cells and induction of granzyme B and IFN-γ effector molecules in CD8(+) T cells. Partial virus clearance was dependent on CD8(+) cells. In vitro T cell restimulation assays demonstrated induction of a population of β(2)-microglobulin-dependent, MHC class Ib-restricted CD8(+) T cells with specificity for viral Ags and yet to be defined nonclassical MHC molecules. MHC class Ib-restricted CD8(+) T cell responses were also observed after infection of K(b-/-)D(b-/-)mice despite the low number of CD8(+) T cells in these animals. Long-term infection studies demonstrated chronic infection and gradual depletion of CD8(+) T cells in K(b-/-)D(b-/-)CIITA(-/-) mice, demonstrating that class Ia molecules are required for viral clearance. These findings demonstrate that class Ib-restricted CD8(+) T cells have the potential to participate in the host immune response to LCMV.

Positive selecting cell type determines the phenotype of MHC class Ib-restricted CD8+ T cells

Several studies have demonstrated an apparent link between positive selection on hematopoietic cells (HCs) and an "innate" T-cell phenotype. Whereas conventional CD8(+) T cells are primarily selected on thymic epithelial cells (TECs) and certain innate T cells are exclusively selected on HCs, MHC class Ib-restricted CD8(+) T cells appear to be selected on both TECs and HCs. However, whether TEC- and HC-selected T cells represent distinct lineages or whether the same T-cell precursors have the capacity to be selected on either cell type is unknown. Using an M3-restricted T-cell receptor transgenic mouse model, we demonstrate that not only are MHC class Ib-restricted CD8(+) T cells capable of being selected on either cell type but that selecting cell type directly affects the phenotype of the resulting CD8(+) T cells. M3-restricted CD8(+) T cells selected on HCs acquire a more activated phenotype and possess more potent effector functions than those selected on TECs. Additionally, these two developmental pathways are active in the generation of the natural pool of M3-restricted CD8(+) T cells. Our results suggest that these two distinct populations may allow MHC class Ib-restricted CD8(+) T cells to occupy different immunological niches playing unique roles in immune responses to infection.