The characterization of CD8+ T-cell responses in COVID-19.

This review gives an overview of the protective role of CD8+ T cells in SARS-CoV-2 infection. The cross-reactive responses intermediated by CD8+ T cells in unexposed cohorts are described. Additionally, the relevance of resident CD8+ T cells in the upper and lower airway during infection and CD8+ T-cell responses following vaccination are discussed, including recent worrisome breakthrough infections and variants of concerns (VOCs). Lastly, we explain the correlation between CD8+ T cells and COVID-19 severity. This review aids in a deeper comprehension of the association between CD8+ T cells and SARS-CoV-2 and broadens a vision for future exploration.

Intrinsic factors and CD1d1 but not CD1d2 expression levels control invariant natural killer T cell subset differentiation.

Invariant natural killer T (NKT) cell subsets are defined based on their cytokine-production profiles and transcription factors. Their distribution is different in C57BL/6 (B6) and BALB/c mice, with a bias for NKT1 and NKT2/NKT17 subsets, respectively. Here, we show that the non-classical class I-like major histocompatibility complex CD1 molecules CD1d2, expressed in BALB/c and not in B6 mice, could not account for this difference. We find however that NKT cell subset distribution is intrinsic to bone marrow derived NKT cells, regardless of syngeneic CD1d-ligand recognition, and that multiple intrinsic factors are likely involved. Finally, we find that CD1d expression levels in combination with T cell antigen receptor signal strength could also influence NKT cell distribution and function. Overall, this study indicates that CD1d-mediated TCR signals and other intrinsic signals integrate to influence strain-specific NKT cell differentiation programs and subset distributions.

The role of regulatory T cells and follicular T helper cells in HBV infection.

Hepatitis B has become one of the major global health threats, especially in developing countries and regions. Hepatitis B virus infection greatly increases the risk for liver diseases such as cirrhosis and cancer. However, treatment for hepatitis B is limited when considering the huge base of infected people. The immune response against hepatitis B is mediated mainly by CD8+ T cells, which are key to fighting invading viruses, while regulatory T cells prevent overreaction of the immune response process. Additionally, follicular T helper cells play a key role in B-cell activation, proliferation, differentiation, and formation of germinal centers. The pathogenic process of hepatitis B virus is generally the result of a disorder or dysfunction of the immune system. Therefore, we present in this review the critical functions and related biological processes of regulatory T cells and follicular T helper cells during HBV infection.

The role of Raptor in lymphocytes differentiation and function.

Raptor, a key component of mTORC1, is required for recruiting substrates to mTORC1 and contributing to its subcellular localization. Raptor has a highly conserved N-terminus domain and seven WD40 repeats, which interact with mTOR and other mTORC1-related proteins. mTORC1 participates in various cellular events and mediates differentiation and metabolism. Directly or indirectly, many factors mediate the differentiation and function of lymphocytes that is essential for immunity. In this review, we summarize the role of Raptor in lymphocytes differentiation and function, whereby Raptor mediates the secretion of cytokines to induce early lymphocyte metabolism, development, proliferation and migration. Additionally, Raptor regulates the function of lymphocytes by regulating their steady-state maintenance and activation.

The role of dendritic cells in COVID-19 infection.

The persistent pandemic of coronavirus disease in 2019 (COVID-19) caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) currently poses a major infectious threat to public health around the world. COVID-19 is an infectious disease characterized by strong induction of inflammatory cytokines, progressive lung inflammation, and potential multiple organs dysfunction. SARS-CoV-2 infection is closely related to the innate immune system and adaptive immune system. Dendritic cells (DCs), as a "bridge" connecting innate immunity and adaptive immunity, play many important roles in viral diseases. In this review, we will pay special attention to the possible mechanism of dendritic cells in human viral transmission and clinical progression of diseases, as well as the reduction and dysfunction of DCs in severe SARS-CoV-2 infection, so as to understand the mechanism and immunological characteristics of SARS-CoV-2 infection.

B cell-T cell interplay in immune regulation: A focus on follicular regulatory T and regulatory B cell functions.

B cells are the core components of humoral immunity. A mature B cell can serve in multiple capacities, including antibody production, antigen presentation, and regulatory functions. Forkhead box P3 (FoxP3)-expressing regulatory T cells (Tregs) are key players in sustaining immune tolerance and keeping inflammation in check. Mounting evidence suggests complex communications between B cells and Tregs. In this review, we summarize the yin-yang regulatory relationships between B cells and Tregs mainly from the perspectives of T follicular regulatory (Tfr) cells and regulatory B cells (Bregs). We discuss the regulatory effects of Tfr cells on B cell proliferation and the germinal center response. Additionally, we review the indispensable role of B cells in ensuring homeostatic Treg survival and describe the function of Bregs in promoting Treg responses. Finally, we introduce a new subset of Tregs, termed Treg-of-B cells, which are induced by B cells, lake the expression of FoxP3 but still own immunomodulatory effects. In this article, we also enumerate a sequence of research from clinical patients and experimental models to clarify the role of Tfr cells in germinal centers and the role of convention B cells and Bregs to Tregs in the context of different diseases. This review offers an updated overview of immunoregulatory networks and unveils potential targets for therapeutic interventions against cancer, autoimmune diseases and allograft rejection.

The role of B cells in COVID-19 infection and vaccination.

B cells secrete antibodies and mediate the humoral immune response, making them extremely important in protective immunity against SARS-CoV-2, which caused the coronavirus disease 2019 (COVID-19) pandemic. In this review, we summarize the positive function and pathological response of B cells in SARS-CoV-2 infection and re-infection. Then, we structure the immunity responses that B cells mediated in peripheral tissues. Furthermore, we discuss the role of B cells during vaccination including the effectiveness of antibodies and memory B cells, viral evolution mechanisms, and future vaccine development. This review might help medical workers and researchers to have a better understanding of the interaction between B cells and SARS-CoV-2 and broaden their vision for future investigations.

Identification of Rare Thymic NKT Cell Precursors by Multiparameter Flow Cytometry.

Mouse invariant natural killer T (NKT) cells are a subset of T lymphocytes which have been shown to play a significant role in innate and adaptive immune responses. Features of innate responses are attributed to these cells because they can be stimulated simultaneously with the same ligand to produce quickly and in large amount cytokines without prior immunization. Because these characteristics could be exploited for clinical applications, NKT cells have attracted considerable interest. Many studies have investigated the molecular mechanisms through which they are selected and differentiate. These studies are based on developmental models that serve as a scaffold to understand the specific roles played by various factors and to identify checkpoints during cellular development. Analysis of NKT cell precursors at the HSAhigh stage, stage 0, can reveal potential selection defects, whereas analysis of NKT cells at the HSAlow stage can shed light on defects in the maturation/differentiation of the different NKT cell subsets (NKT1, 2, and 17). Unlike HSAlow NKT cell subsets, HSAhigh NKT cell precursors are not accurately identified by flow cytometry because of their extreme rarity. Here, we describe an NKT cell enrichment strategy to identify unambiguously NKT cell precursors at the HSAhigh stage that can be used to assess their distribution and characteristics by multicolor flow cytometry.

PLZF Acetylation Levels Regulate NKT Cell Differentiation.

The transcription factor promyelocytic leukemia zinc finger (PLZF) is encoded by the BTB domain-containing 16 (Zbtb16) gene. Its repressor function regulates specific transcriptional programs. During the development of invariant NKT cells, PLZF is expressed and directs their effector program, but the detailed mechanisms underlying PLZF regulation of multistage NKT cell developmental program are not well understood. This study investigated the role of acetylation-induced PLZF activation on NKT cell development by analyzing mice expressing a mutant form of PLZF mimicking constitutive acetylation (PLZFON) mice. NKT populations in PLZFON mice were reduced in proportion and numbers of cells, and the cells present were blocked at the transition from developmental stage 1 to stage 2. NKT cell subset differentiation was also altered, with T-bet+ NKT1 and RORγt+ NKT17 subsets dramatically reduced and the emergence of a T-bet-RORγt- NKT cell subset with features of cells in early developmental stages rather than mature NKT2 cells. Preliminary analysis of DNA methylation patterns suggested that activated PLZF acts on the DNA methylation signature to regulate NKT cells' entry into the early stages of development while repressing maturation. In wild-type NKT cells, deacetylation of PLZF is possible, allowing subsequent NKT cell differentiation. Interestingly, development of other innate lymphoid and myeloid cells that are dependent on PLZF for their generation is not altered in PLZFON mice, highlighting lineage-specific regulation. Overall, we propose that specific epigenetic control of PLZF through acetylation levels is required to regulate normal NKT cell differentiation.

Cortical Thymocytes Along With Their Selecting Ligands Are Required for the Further Thymic Maturation of NKT Cells in Mice.

Following positive selection, NKT cell precursors enter an "NK-like" program and progress from an NK- to an NK+ maturational stage to give rise to NKT1 cells. Maturation takes place in the thymus or after emigration of NK- NKT cells to the periphery. In this study, we followed the fate of injected NKT cells at the NK- stage of their development in the thymus of a series of mice with differential CD1d expression. Our results indicate that CD1d-expressing cortical thymocytes, and not epithelial cells, macrophages, or dendritic cells, are necessary and sufficient to promote the maturation of thymic NKT1 cells. Migration out of the thymus of NK- NKT cells occurred in the absence of CD1d expression, however, CD1d expression is required for maturation in peripheral organs. We also found that the natural ligand Isoglobotriosylceramide (iGb3), and the cysteine protease Cathepsin L, both localizing with CD1d in the endosomal compartment and crucial for NKT cell positive selection, are also required for NK- to NK+ NKT cell transition. Overall, our study indicates that the maturational transition of NKT cells require continuous TCR/CD1d interactions and suggest that these interactions occur in the thymic cortex where DP cortical thymocytes are located. We thus concluded that key components necessary for positive selection of NKT cells are also required for subsequent maturation.

A focus on natural killer T-cell subset characterization and developmental stages.

Almost 20 years ago, CD1d tetramers were developed to track invariant natural killer T (NKT) cells based on their specificity, and to define developmental steps during which differentiation markers and functional features are progressively acquired from early NKT cell precursor to fully mature NKT cell subsets. Based on these findings, a linear developmental model was proposed and subsequently used by all studies investigating the specific role of factors that control NKT cell development. More recently, based on intracellular staining patterns of lineage-specific transcription factors such as T-bet, GATA-3, promyelocytic leukemia zinc finger and RORγt, a lineage differentiation model was proposed for NKT cell development. Currently, studies on NKT cells development present lineage differentiation model data in addition to the linear maturation model. In the perspective presented here, we discuss current knowledge relating to NKT cell developmental models and particularly focus on the approaches and strategies, some of which appear nebulous, used to define NKT cell developmental stages and subsets.

Characterization of the developmental landscape of murine RORγt+ iNKT cells.

Invariant natural killer T (iNKT) cells expressing the retinoic acid receptor-related orphan receptor γt (RORγt) and producing IL-17 represent a minor subset of CD1d-restricted iNKT cells (iNKT17) in C57BL/6J (B6) mice. We aimed in this study to define the reasons for their low distribution and the sequence of events accompanying their normal thymic development. We found that RORγt+ iNKT cells have higher proliferation potential and a greater propensity to apoptosis than RORγt- iNKT cells. These cells do not likely reside in the thymus indicating that thymus emigration, and higher apoptosis potential, could contribute to RORγt+ iNKT cell reduced thymic distribution. Ontogeny studies suggest that mature HSAlow RORγt+ iNKT cells might develop through developmental stages defined by a differential expression of CCR6 and CD138 during which RORγt expression and IL-17 production capabilities are progressively acquired. Finally, we found that RORγt+ iNKT cells perceive a strong TCR signal that could contribute to their entry into a specific 'Th17 like' developmental program influencing their survival and migration. Overall, our study proposes a hypothetical thymic developmental sequence for iNKT17 cells, which could be of great use to study molecular mechanisms regulating this developmental program.

TCR density in early iNKT cell precursors regulates agonist selection and subset differentiation in mice.

It is established that iNKT cells are a cell type that require strong TCR signal for their proper development and represent a model for thymic agonist selection. The nature of the signal perceived by iNKT cells promoting their specification is not well understood. To address this question, we analyzed iNKT cell development in relevant TCR Vα14-Jα18 alpha chain transgenic mice (Vα14Tg). In CD4-Vα14Tg mice, where the transgene is driven by CD4 promoter, we identified a block in iNKT cell development at early developmental stages due to a reduced expression of key transcription factors accompanied with a reduced TCR expression levels. This indicates that TCR signal strength control iNKT cell differentiation. Importantly, we found in WT mice that early precursors of iNKT cells express higher TCR levels compared to positively selected precursors of mainstream T cells showing that TCR levels could contribute to the strength of iNKT cell TCR signaling. Overall, our study highlights TCR signal strength associated with a higher TCR density as an important regulator of iNKT cell lineage specification.

Critical Contribution of NK Group 2 Member D Expressed on Invariant Natural Killer T Cells in Concanavalin A-Induced Liver Hepatitis in Mice.

Natural killer group 2D (NKG2D) is a well-characterized activating receptor expressed on many immune cells, including invariant natural killer T (iNKT) cells. These cells were shown to be responsible of liver injury in the model of concanavalin A (Con A)-induced hepatitis, considered to be an experimental model of human autoimmune hepatitis. In this study, we investigated whether NKG2D plays a role in the hepatitis induced by iNKT cell-mediated immune response to Con A. By using killer cell lectin-like receptor subfamily K, member 1 deficient (Klrk1-/-) mice, we found that the absence of NKG2D reduced the hepatic injury upon Con A administration. This was not due to an intrinsic functional defect of NKG2D-deficient iNKT cells as mice missing NKG2D have normal distribution and function of iNKT cells. Furthermore, increased resistance to Con A-induced hepatitis was confirmed using neutralizing anti-NKG2D antibodies. The reduced pathogenic effect of Con A in the absence of NKG2D correlates with a reduction in pathogenic cytokine production and FAS-Ligand (FAS-L) expression by iNKT cells. We also found that Con A administration led to an increase in the retinoic acid early inducible (RAE-1) surface expression on wild-type hepatocytes. Finally, we found that Con A has no direct action on FAS-L expression or cytokine production by iNKT cells and thus propose that NKG2D-L expression on stressed hepatocytes promote cytotoxic activity of iNKT cells via its interaction with NKG2D contributing to hepatic injury. In conclusion, our results highlight NKG2D as an essential receptor required for the activation of iNKT cells in Con A-induced hepatitis and indicate that it represents a potential drug target for prevention of autoimmune hepatitis.

Potential role of IL-17-producing iNKT cells in type 1 diabetes.

We explored in this study the status and potential role of IL-17-producing iNKT cells (iNKT17) in type 1 diabetes (T1D) by analyzing these cells in patients with T1D, and in NOD mice, a mouse model for T1D. Our analysis in mice showed an increase of iNKT17 cells in NOD vs control C57BL/6 mice, partly due to a better survival of these cells in the periphery. We also found a higher frequency of these cells in autoimmune-targeted organs with the occurrence of diabetes, suggesting their implication in the disease development. In humans, though absent in fresh PMBCs, iNKT17 cells are detected in vitro with a higher frequency in T1D patients compared to control subjects in the presence of the proinflammatory cytokine IL-1ß, known to contribute to diabetes occurrence. These IL-1ß-stimulated iNKT cells from T1D patients keep their potential to produce IFN-γ, a cytokine that drives islet ß-cell destruction, but not IL-4, with a reverse picture observed in healthy volunteers. On the whole, our results argue in favour of a potential role of IL-17-producing iNKT cells in T1D and suggest that inflammation in T1D patients could induce a Th1/Th17 cytokine secretion profile in iNKT cells promoting disease development.

Development and function of murine RORγt+ iNKT cells are under TGF-ß signaling control.

Invariant natural killer T (iNKT) cells have the ability to rapidly secret cytokines in response to diverse stimuli, and therefore influence numerous immune reactions. Although IFN-γ and IL-4 are thought to dominate iNKT cytokine production, a distinct subset of iNKT cells, expressing RORγt and producing IL-17, has now been identified in both mice and humans. Although a role in pathogen and allergic responses has been assigned to the RORγt(+) iNKT subset, factors controlling their development and function remain illusive. Here, we demonstrate that RORγt(+) iNKT-cell differentiation obeys transforming growth factor-ß (TGF-ß) signaling control, different from that described for conventional iNKT cells. We reveal that TGF-ß signaling, and particularly its SMAD4-dependent pathway, is required for both the survival of RORγt(+) iNKT cells during their development and IL-17 production at the periphery. Moreover, constitutive TGF-ß signaling in RORγt(+) iNKT cells drives higher peripheral numbers and increased tissue distribution. Finally, we found that SMAD4-dependent TGF-ß signaling is mandatory for the peripheral expansion of the RORγt(+) iNKT cells responding to inflammatory signals. Thus, this work demonstrates that both the development and responsiveness of the newly described IL-17-producing iNKT cell subset is under the control of a dedicated TGF-ß signaling pathway.

Contribution of IL-17-producing gamma delta T cells to the efficacy of anticancer chemotherapy.

By triggering immunogenic cell death, some anticancer compounds, including anthracyclines and oxaliplatin, elicit tumor-specific, interferon-γ-producing CD8(+) αß T lymphocytes (Tc1 CTLs) that are pivotal for an optimal therapeutic outcome. Here, we demonstrate that chemotherapy induces a rapid and prominent invasion of interleukin (IL)-17-producing γδ (Vγ4(+) and Vγ6(+)) T lymphocytes (γδ T17 cells) that precedes the accumulation of Tc1 CTLs within the tumor bed. In T cell receptor δ(-/-) or Vγ4/6(-/-) mice, the therapeutic efficacy of chemotherapy was compromised, no IL-17 was produced by tumor-infiltrating T cells, and Tc1 CTLs failed to invade the tumor after treatment. Although γδ T17 cells could produce both IL-17A and IL-22, the absence of a functional IL-17A-IL-17R pathway significantly reduced tumor-specific T cell responses elicited by tumor cell death, and the efficacy of chemotherapy in four independent transplantable tumor models. Adoptive transfer of γδ T cells restored the efficacy of chemotherapy in IL-17A(-/-) hosts. The anticancer effect of infused γδ T cells was lost when they lacked either IL-1R1 or IL-17A. Conventional helper CD4(+) αß T cells failed to produce IL-17 after chemotherapy. We conclude that γδ T17 cells play a decisive role in chemotherapy-induced anticancer immune responses.

Cutting edge: crucial role of IL-1 and IL-23 in the innate IL-17 response of peripheral lymph node NK1.1- invariant NKT cells to bacteria.

We have shown previously that peripheral lymph node-resident retinoic acid receptor-related orphan receptor γt(+) NK1.1(-) invariant NKT (iNKT) cells produce IL-17A independently of IL-6. In this study, we show that the concomitant presence of IL-1 and IL-23 is crucial to induce a rapid and sustained IL-17A/F and IL-22 response by these cells that requires TCR-CD1d interaction and partly relies on IL-23-mediated upregulation of IL-23R and IL-1R1 expression. We further show that IL-1 and IL-23 produced by pathogen-associated molecular pattern-stimulated dendritic cells induce this response from NK1.1(-) iNKT cells in vitro, involving mainly TLR2/4-signaling pathways. Finally, we found that IL-17A production by these cells occurs very early and transiently in vivo in response to heat-killed bacteria. Overall, our study indicates that peripheral lymph node NK1.1(-) iNKT cells could be a source of innate Th17-related cytokines during bacterial infections and supports the hypothesis that they are able to provide an efficient first line of defense against bacterial invasion.