Expression of rearranged TCRgamma genes in natural killer cells suggests a minor thymus-dependent pathway of lineage commitment

Extrinsic and intrinsic drivers of natural killer cell clonality

Clonal expansion of antigen-specific lymphocytes is the fundamental mechanism enabling potent adaptive immune responses and the generation of immune memory. Accompanied by pronounced epigenetic remodeling, the massive proliferation of individual cells generates a critical mass of effectors for the control of acute infections, as well as a pool of memory cells protecting against future pathogen encounters. Classically associated with the adaptive immune system, recent work has demonstrated that innate immune memory to human cytomegalovirus (CMV) infection is stably maintained as large clonal expansions of natural killer (NK) cells, raising questions on the mechanisms for clonal selection and expansion in the absence of re-arranged antigen receptors. Here, we discuss clonal NK cell memory in the context of the mechanisms underlying clonal competition of adaptive lymphocytes and propose alternative selection mechanisms that might decide on the clonal success of their innate counterparts. We propose that the integration of external cues with cell-intrinsic sources of heterogeneity, such as variegated receptor expression, transcriptional states, and somatic variants, compose a bottleneck for clonal selection, contributing to the large size of memory NK cell clones.

Thymic NK-Cells and Their Potential in Cancer Immunotherapy

Natural killer (NK)-cells are innate immune cells with potent anti-tumor capacity, capable of recognizing target cells without prior exposure. For this reason, NK-cells are recognized as a useful source of cell therapy. Although most NK-cells are derived from the bone marrow (BM), a separate developmental pathway in the thymus also exists, producing so-called thymic NK-cells. Unlike conventional NK-cells, thymic NK (tNK)-cells have a combined capacity for cytokine production and a natural ability to kill tumor cells in the presence of NK-cell receptor stimulatory ligands. Furthermore, tNK-cells are reported to express CD3 subunits intracellularly, without the presence of a rearranged T-cell receptor (TCR). This unique feature may enable harnessing of these cells with a TCR to combine NK- and T-cell effector properties in one cell type. The development, phenotype, and function of tNK-cells, and potential as a cell therapy is, however, poorly explored. In this review, we provide an overview of current literature on both murine and human tNK-cells in comparison to conventional BM-derived NK-cells, and discuss the potential applications of this cellular subset in the context of cancer immunotherapy.

Parallel origins and functions of T cells and ILCs

Innate lymphoid cells (ILCs) are tissue resident cells that are triggered through a relatively broad spectrum of alarmins, inflammatory cues, neuropeptides, and hormones. Functionally, ILCs are akin to subsets of helper T cells and are characterized by a similar effector cytokine profile. They also share a dependency on many of the same essential transcription factors identified for the maintenance and survival of T cells. The key distinguishing factor between the ILC family and T cells is the lack of antigen-specific T cell receptor (TCR) on ILCs and, thus, they can be considered the "ultimate invariant T cells". ILCs, like T cells, orchestrate downstream effector inflammatory responses by adjusting the cytokine microenvironment in a fashion that promotes protection, health, and homeostasis at mucosal barrier sites. But also, like T cells, ILCs have recently been implicated in several pathological inflammatory disease states. This review focuses on the selective role of ILCs in the development of allergic airway inflammation (AAI) and fibrosis in the gut where a complex ILC interplay has been shown to either attenuate or worsen disease. Finally, we discuss new data on TCR gene rearrangements in subsets of ILCs that challenge the current dogma linking their origin to committed bone marrow progenitors and instead propose a thymic origin for at least some ILCs. In addition, we highlight how naturally occurring TCR rearrangements and the expression of major histocompatibility (MHC) molecules in ILCs provide a useful natural barcode for these cells and may prove instrumental in studying their origins and plasticity.

Speed and navigation control of thymocyte development by the fetal T-cell gene regulatory network

T-cell differentiation is a tightly regulated developmental program governed by interactions between transcription factors (TFs) and chromatin landscapes and affected by signals received from the thymic stroma. This process is marked by a series of checkpoints: T-lineage commitment, T-cell receptor (TCR)β selection, and positive and negative selection. Dynamically changing combinations of TFs drive differentiation along the T-lineage trajectory, through mechanisms that have been most extensively dissected in adult mouse T-lineage cells. However, fetal T-cell development differs from adult in ways that suggest that these TF mechanisms are not fully deterministic. The first wave of fetal T-cell differentiation occurs during a unique developmental window during thymic morphogenesis, shows more rapid kinetics of differentiation with fewer rounds of cell division, and gives rise to unique populations of innate lymphoid cells (ILCs) and invariant γδT cells that are not generated in the adult thymus. As the characteristic kinetics and progeny biases are cell-intrinsic properties of thymic progenitors, the differences could be based on distinct TF network circuitry within the progenitors themselves. Here, we review recent single-cell transcriptome data that illuminate the TF networks involved in T-cell differentiation in the fetal and adult mouse thymus.

The case of T-ALL presenting with NK phenotype after COVID-19 vaccination

NK-lymphoblastic leukemia/lymphoma (NK-LL) is an extremely rare hematopoietic tumor consisting of natural killer (NK) precursor cells, and their lineage overlaps with T-cells, making it challenging to diagnose. COVID-19 vaccination is recommended for people with a risk of aggravation such as cancer-bearing patients, including hematopoietic tumors. We present a 55-year-old man who had cervical lymph node swelling post vaccination for COVID-19. Hematological malignancy was suspected due to the presence of atypical lymphoid cells with an elevated IL-2R in laboratory data. Tumor cells were positive for CD7, CD56, cyCD3, and terminal deoxynucleotidyl transferase (TdT) evidenced through flow cytometry of the bone marrow and the lymph node. The histopathological findings showed monotonous tumor cell proliferation, the cells being positive for CD3 and TdT in the bone marrow and they were positive for CD3, TdT, and CD56 in lymph node. Even though these findings suggested NK-LL, clonal T-cell receptor (TCR) β gene rearrangement by Southern blot hybridization was observed in the bone marrow. TCRβ rearrangement led to the final diagnosis of T-cell lymphoblastic leukemia (T-ALL). The causal relationship between COVID-19 vaccination and carcinogenesis is not clear, and more cases need to be studied in order to elucidate the relationship between the two factors.

Surfing on the waves of the human γδ T cell ontogenic sea

While γδ T cells are present virtually in all vertebrates, there is a remarkable lack of conservation of the TRG and TRD loci underlying the generation of the γδ T cell receptor (TCR), which is associated with the generation of species-specific γδ T cells. A prominent example is the human phosphoantigen-reactive Vγ9Vδ2 T cell subset that is absent in mice. Murine γδ thymocyte cells were among the first immune cells identified to follow a wave-based layered development during embryonic and early life, and since this initial observation, in-depth insight has been obtained in their thymic ontogeny. By contrast, less is known about the development of human γδ T cells, especially regarding the generation of γδ thymocyte waves. Here, after providing an overview of thymic γδ wave generation in several vertebrate classes, we review the evidence for γδ waves in the human fetal thymus, where single-cell technologies have allowed the breakdown of human γδ thymocytes into functional waves with important TCR associations. Finally, we discuss the possible mechanisms contributing to the generation of waves of γδ thymocytes and their possible significance in the periphery.

Waves of layered immunity over innate lymphoid cells

Innate lymphoid cells (ILCs) harbor tissue-resident properties in border zones, such as the mucosal membranes and the skin. ILCs exert a wide range of biological functions, including inflammatory response, maintenance of tissue homeostasis, and metabolism. Since its discovery, tremendous effort has been made to clarify the nature of ILCs, and scientific progress revealed that progenitor cells of ILC can produce ILC subsets that are functionally reminiscent of T-cell subsets such as Th1, Th2, and Th17. Thus, now it comes to the notion that ILC progenitors are considered an innate version of naïve T cells. Another important discovery was that ILC progenitors in the different tissues undergo different modes of differentiation pathways. Furthermore, during the embryonic phase, progenitor cells in different developmental chronologies give rise to the unique spectra of immune cells and cause a wave to replenish the immune cells in tissues. This observation leads to the concept of layered immunity, which explains the ontology of some cell populations, such as B-1a cells, γδ T cells, and tissue-resident macrophages. Thus, recent reports in ILC biology posed a possibility that the concept of layered immunity might disentangle the complexity of ILC heterogeneity. In this review, we compare ILC ontogeny in the bone marrow with those of embryonic tissues, such as the fetal liver and embryonic thymus, to disentangle ILC heterogeneity in light of layered immunity.

The divergence between T cell and innate lymphoid cell fates controlled by E and Id proteins

T cells develop in the thymus from lymphoid primed multipotent progenitors or common lymphoid progenitors into αβ and γδ subsets. The basic helix-loop-helix transcription factors, E proteins, play pivotal roles at multiple stages from T cell commitment to maturation. Inhibitors of E proteins, Id2 and Id3, also regulate T cell development while promoting ILC differentiation. Recent findings suggest that the thymus can also produce innate lymphoid cells (ILCs). In this review, we present current findings that suggest the balance between E and Id proteins is likely to be critical for controlling the bifurcation of T cell and ILC fates at early stages of T cell development.

Correlation between circulating innate lymphoid cell precursors and thymic function

The thymus has a high capacity to support the differentiation of ILCs, especially when E protein transcription factors are ablated. Whether it contributes to the homeostasis of ILC pools in tissues is not clear. Single-cell RNA sequencing analysis shows a substantial amount of ILC precursors in wild type but not athymic nude blood. The precursors express CD3 intracellularly (ic) but not on the surface. The abundance of Lin⁻CD127⁺CD62L⁺icCD3ε⁺ precursors varies with age, peaking at 2–3 months. These cells can differentiate into various ILC subsets on OP9-DL1 stroma in vitro. In the lung, small intestine, and epidermis, icCD3ε⁺ cells differentiate into diverse ILC subsets in different tissue environments in steady state. Helminth infection promotes their differentiation toward functional ILC2s. Thus, the thymus appears to play a role in replenishing ILC pools in different peripheral tissues. Because thymic activity is age-dependent, this finding may help explain age-related differences in immune responses.

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Single-cell RNA sequencing detects thymus-dependent (td) ILC precursors in the blood

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Intracellular (ic) but not surface CD3ε marks td-ILCs in the blood and tissues

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Blood td-ILCs differentiate into distinct ILC subsets in vitro

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Helminth infection promotes the maturation of icCD3ε⁺ ILC2s

Abortive γδTCR rearrangements suggest ILC2s are derived from T-cell precursors

Innate lymphoid cells (ILCs) are a recently identified subset of leukocytes that play a central role in pathogen surveillance and resistance, modulation of immune response, and tissue repair. They are remarkably similar to CD4+ T-helper subsets in terms of function and transcription factors required for their development but are distinguished by their lack of antigen-specific receptors. Despite their similarities, the absence of a surface T-cell receptor (TCR) and presence of ILCs and precursors in adult bone marrow has led to speculation that ILCs and T cells develop separately from lineages that branch at the point of precursors within the bone marrow. Considering the common lineage markers and effector cytokine profiles shared between ILCs and T cells, it is surprising that the status of the TCR loci in ILCs was not fully explored at the time of their discovery. Here, we demonstrate that a high proportion of peripheral tissue ILC2s have TCRγ chain gene rearrangements and TCRδ locus deletions. Detailed analyses of these loci show abundant frameshifts and premature stop codons that would encode nonfunctional TCR proteins. Collectively, these data argue that ILC2 can develop from T cells that fail to appropriately rearrange TCR genes, potentially within the thymus.

ILC-You in the Thymus: A Fresh Look at Innate Lymphoid Cell Development

The discovery of innate lymphoid cells (ILCs) has revolutionized our understanding of innate immunity and immune cell interactions at epithelial barrier sites. Their presence and maintenance are critical for modulating immune homeostasis, responding to injury or infection, and repairing damaged tissues. To date, ILCs have been defined by a set of transcription factors, surface antigens and cytokines, and their functions resemble those of three major classes of helper T cell subsets, Th1, Th2 and Th17. Despite this, the lack of antigen-specific surface receptors and the notion that ILCs can develop in the absence of the thymic niche have clearly set them apart from the T-cell lineage and promulgated a dogma that ILCs develop directly from progenitors in the bone marrow. Interestingly however, emerging studies have challenged the BM-centric view of adult ILC development and suggest that ILCs could arise neonatally from developing T cell progenitors. In this review, we discuss ILC development in parallel to T-cell development and summarize key findings that support a T-cell-centric view of ILC ontogeny.

Innate lymphoid cell development

Innate lymphoid cells (ILCs) mainly reside at barrier surfaces and regulate tissue homeostasis and immunity. ILCs are divided into 3 groups, group 1 ILCs, group 2 ILCs, and group 3 ILC3, on the basis of their similar effector programs to T cells. The development of ILCs from lymphoid progenitors in adult mouse bone marrow has been studied in detail, and multiple ILC progenitors have been characterized. ILCs are mostly tissue-resident cells that develop in the perinatal period. More recently, ILC progenitors have also been identified in peripheral tissues. In this review, we discuss the stepwise transcription factor-directed differentiation of mouse ILC progenitors into mature ILCs, the critical time windows in ILC development, and the contribution of bone marrow versus tissue ILC progenitors to the pool of mature ILCs in tissues.

A newly established canine NK-type cell line and its cytotoxic properties

We established a canine natural killer (NK)-type cell line called CNK-89 derived from a dog with NK cell neoplasia. Immunophenotyping analysis showed positive staining for CD5, CD8, CD45, CD56, CD79a and NKp46, while negative for CD3, CD4, CD14, CD20, CD21, CD34, Thy1, IgG, IgM and MHCII. Polymerase chain reaction analysis revealed the presence of CD56, NKG2D, NKp30, NKp44, NKp46 and perforin, but the absence of CD16, Ly49 and granzyme B mRNA. Treating CNK-89 cells with IL-2 did not change the expression of activating receptors, TNFα and IFNγ secretion and cytotoxic activity, however, treatment with IL-12 alone or in combinations with IL-15, IL-18 and IL-21 caused an increase in granzyme B and CD16 mRNA, IFNγ secretion and cytotoxic properties of the CNK-89 cell line.

γδTCR-independent origin of neonatal γδ T cells prewired for IL-17 production

A classical view of T cell lineages consists of two major clades of T cells expressing either the αβ or γδ T cell receptor (TCR). However, genome-wide assessments indicate molecular clusters segregating T cell subsets that are preprogrammed for effector function (innate) from those that mediate conventional adaptive response, regardless of the TCR types. Within this paradigm, γδ T cells remain the prototypic innate-like lymphocytes, many subsets of which are programmed during intrathymic development for committed peripheral tissue localization and effector responses. Emerging evidence for innate γδ T cell lineage choice dictated by developmental gene programs rather than the sensory TCR is discussed in this review.

Suppression of ILC2 differentiation from committed T cell precursors by E protein transcription factors

Qian et al. shows that ILC2s can be generated from not only thymic multipotent progenitors but also committed T cell precursors. These processes are greatly suppressed by E protein transcription factors. Thymic ILC2s show functional differences from those made elsewhere.

Current models propose that group 2 innate lymphoid cells (ILC2s) are generated in the bone marrow. Here, we demonstrate that subsets of these cells can differentiate from multipotent progenitors and committed T cell precursors in the thymus, both in vivo and in vitro. These thymic ILC2s exit the thymus, circulate in the blood, and home to peripheral tissues. Ablation of E protein transcription factors greatly promotes the ILC fate while impairing B and T cell development. Consistently, a transcriptional network centered on the ZBTB16 transcription factor and IL-4 signaling pathway is highly up-regulated due to E protein deficiency. Our results show that ILC2 can still arise from what are normally considered to be committed T cell precursors, and that this alternative cell fate is restrained by high levels of E protein activity in these cells. Thymus-derived lung ILC2s of E protein–deficient mice show different transcriptomes, proliferative properties, and cytokine responses from wild-type counterparts, suggesting potentially distinct functions.

Interleukin-17-Producing γδ T Cells Originate from SOX13+ Progenitors that Are Independent of γδTCR Signaling

Lineage-committed αβ and γδ T cells are thought to originate from common intrathymic multipotent progenitors following instructive T cell receptor (TCR) signals. A subset of lymph node and mucosal Vγ2⁺ γδ T cells is programmed intrathymically to produce IL-17 (Tγδ17 cells), however the role of the γδTCR in development of these cells remains controversial. Here we generated reporter mice for the Tγδ17 lineage-defining transcription factor SOX13 and identified fetal-origin, intrathymic Sox13⁺ progenitors. In organ culture developmental assays, Tγδ17 cells derived primarily from Sox13⁺ progenitors, and not from other known lymphoid progenitors. Single cell transcriptome assays of the progenitors found in TCR-deficient mice demonstrated that Tγδ17 lineage programming was independent of γδTCR. Instead, generation of the lineage committed progenitors and Tγδ17 cells was controlled by TCF1 and SOX13. Thus, T lymphocyte lineage fate can be prewired cell-intrinsically and is not necessarily specified by clonal antigen receptor signals.

Deciphering the transcriptional switches of innate lymphoid cell programming: the right factors at the right time

Innate lymphoid cells (ILCs) are increasingly recognised as an innate immune counterpart of adaptive T_H cells. In addition to their similar effector cytokine production, there is a strong parallel between the transcription factors that control the differentiation of T_H1, T_H2 and T_H17 cells and ILC Groups 1, 2 and 3, respectively. Here, we review the transcriptional circuit that specifies the development of a common ILC progenitor and its subsequent programming into distinct ILC groups. Notch, GATA-3, Nfil3 and Id2 are identified as early factors that suppress B and T cell potentials and are turned on in favour of ILC commitment. Natural killer cells, which are the cytotoxic ILCs, develop along a pathway distinct from the rest of the helper-like ILCs that are derived from a common progenitor to all helper-like innate lymphoid cells (CHILPs). PLZF⁻ CHILPs give rise to lymphoid tissue inducer cells while PLZF⁺ CHILPs have multi-lineage potential and could give rise to ILCs 1, 2 and 3. Such lineage specificity is dictated by the controlled expression of T-bet, RORα, RORγt and AHR. In addition to the type of transcription factors, the developmental stages at which these factors are expressed are crucial in specifying the fate of the ILCs.

CD1d-unrestricted NKT cells are endowed with a hybrid function far superior than that of iNKT cells

Invariant natural killer T (iNKT) cells to date represent the best example of cells known to have a hybrid function, representing both innate and adaptive immunity. Shared phenotypic similarities with NK cells together with a rapid response to a cytokine stimulus and a productive TCR engagement are the features that underline the hybrid nature of iNKT cells. Using these criteria, we provide molecular and functional evidence demonstrating that CD1d-independent (CD1d(ind)) NKT cells, a population of CD1d-unrestricted NKT cells, are endowed with a hybrid function far superior to that of iNKT cells: (i) an extensive shared program with NK cells, (ii) a closer Euclidian distance with NK cells, and (iii) the ability to respond to innate stimuli (Poly:IC) with cytotoxic potential in the same manner as NK cells identify a hybrid feature in CD1d(ind)NKT cells that truly fulfills the dual function of an NK and a T cell. Our finding that CD1d(ind)NKT cells are programmed to act like NK cells in response to innate signals while being capable of adaptive responses is unprecedented, and thus might reemphasize CD1d-unrestricted NKT cells as a subset of lymphocytes that could affect biological processes of antimicrobial and tumor immunity in a unique way.

Hepatosplenic and Hepatocytotropic T-Cell Lymphoma

The clinical, clinicopathologic, and pathological findings of 9 dogs with T-cell lymphoma that involved the liver in the absence of peripheral lymphadenopathy were assessed. Seven dogs had hepatosplenic T-cell lymphoma (HS-TCL). Dogs with HS-TCL presented with hepato- and/or splenomegaly, regenerative anemia, thrombocytopenia, and hypoproteinemia. The clinical course was rapidly progressive with all dogs but 1 dead within 24 days of initial presentation. Neoplastic lymphocytes were centered on hepatic and splenic sinusoids and had a CD3+ (5/7), TCRαβ– (5/5), TCRγδ+ (3/5), CD11d+ (6/7), granzyme B+ (5/7) immunophenotype. Bone marrow and lungs were consistently but variably involved. These findings closely resemble the human disease and support the classification of HS-TCL as a distinct World Health Organization entity in dogs. The remaining 2 dogs markedly differed in the pattern of hepatic involvement by neoplastic lymphocytes, which were not confined to hepatic sinusoids but invaded hepatic cords. In addition, neoplastic cells had a CD11d– immunophenotype, and clinicopathologic data indicated marked cholestasis and mild to absent anemia. Based on the distinct tropism of neoplastic lymphocytes for hepatocytes, the name hepatocytotropic T-cell lymphoma (HC-TCL) is proposed. Given the histomorphologic, clinicopathologic, and immunophenotypic differences, HC-TCL likely represents a separate biological entity rather than a histomorphologic variant of HS-TCL.

Extranodal NK/T-cell lymphoma, nasal type, includes cases of natural killer cell and αβ, γδ, and αβ/γδ T-cell origin: a comprehensive clinicopathologic and phenotypic study

Extranodal NK/T-cell lymphoma (ENKTL), nasal type, may be of NK or T-cell origin; however, the proportion of T-ENKTLs and whether they are of αβ or γδ type remains uncertain. To elucidate the cell of origin and detailed phenotype of ENKTL and assess any clinicopathologic associations, 67 cases of ENKTL from Thailand were investigated, together with 5 γδ enteropathy-associated T-cell lymphomas (EATLs) for comparison. In all, 70% of the ENKTL were T-cell receptor (TCR) β,γ and, in cases tested, δ negative (presumptive NK origin); 5% were TCR γδ, 3% were TCR αβ, 1% were TCR αβ/γδ, and 21% were indeterminate. Out of 17 presumptive NK-ENKTLs tested, 3 had clonal TCR rearrangements. All cases were EBV and TIA-1; >85% were positive for CD3, CD2, granzyme B, pSTAT3, and Lsk/MATK; and all were CD16. Presumptive NK-ENKTLs had significantly more frequent CD56 (83% vs. 33%) and CXCL13 (59% vs. 0%) but less frequent PD-1 (0% vs. 40%) compared with T-ENKTLs. Of the NK-ENKTLs, 38% were Oct-2 compared with 0% of T-ENKTLs, and 54% were IRF4/MUM1 compared with 20% of T-ENKTLs. Only αβ T-ENKTLs were CD5. Intestinal ENKTLs were EBV and had significantly more frequent CD30, pSTAT3, and IRF4/MUM1 expression but less frequent CD16 compared with γδ EATL. Significant adverse prognostic indicators included a primary non-upper aerodigestive tract site, high stage, bone marrow involvement, International Prognostic Index ≥2, lack of radiotherapy, Ki67 >40%, and CD25 expression. The upper aerodigestive tract ENKTLs of T-cell origin compared with those of presumptive NK origin showed a trend for better survival. Thus, at least 11% of evaluable ENKTLs are of T-cell origin. Although T-ENKTLs have phenotypic and some possible clinical differences, they share many similarities with ENKTLs that lack TCR expression and are distinct from intestinal γδ EATL.

Evidence for the divergence of innate and adaptive T-cell precursors before commitment to the αβ and γδ lineages

In addition to adaptive T cells, the thymus supports the development of unconventional T cells such as natural killer T (NKT) and CD8αα intraepithelial lymphocytes (IELs), which have innate functional properties, particular antigenic specificities, and tissue localization. Both conventional and innate T cells are believed to develop from common precursors undergoing instructive, TCR-mediated lineage fate decisions, but innate T cells are proposed to undergo positive instead of negative selection in response to agonistic TCR signals. In the present study, we show that, in contrast to conventional αβT cells, innate αβT cells are not selected against functional TCRγ rearrangements and express TCRγ mRNA. Likewise, in contrast to the majority of γδT cells, thymic innate γδT cells are not efficiently selected against functional TCRβ chains. In precursors of conventional T cells, autonomous TCR signals emanating from the pre-TCR or γδTCR in the absence of ligand mediate selection against the TCR of the opposite isotype and αβ/γδ lineage commitment. Our data suggest that developing innate T cells ignore such signals and rely solely on agonistic TCR interactions. Consistently, most innate T cells reacted strongly against autologous thymocytes. These results suggest that innate and adaptive T-cell lineages do not develop from the same pool of precursors and potentially diverge before αβ/γδ lineage commitment.

Unique progenitors in mouse lymph node develop into CD127+ NK cells: thymus-dependent and thymus-independent pathways

A subset of natural killer (NK) cells in normal mouse lymph node (LN) expresses CD127 (IL-7 receptor-α chain) and is thought to derive from the thymus. However, CD127(+) NK cells are found in the LN of athymic mice. Therefore, the origin of CD127(+) NK cells in the LN is unclear. Here, we have identified unique NK-cell progenitors (NKPs) in the LN that express the pan-NK cell marker CD49b and CD127 but lack CD122 and lineage markers. The LN NKPs develop in vitro into CD127(+) NK cells that display natural cytotoxicity and cytokine production capacity. They also become CD127(+) NK cells in lymphopenic mice that received a transplant. LN NKPs can be divided into stem cell antigen-1 (Sca-1)(hi) and Sca-1(lo) subsets. The latter comprise ∼ 60% of LN NKPs in normal mouse and < 10% of athymic mouse LN NKPs. Whereas both Sca-1(hi) and Sca-1(lo) NKPs develop into CD127(+) NK cells in vitro, only those derived from Sca-1(lo) LN NKPs have rearranged TCRγ genes. Thus, CD127(+) NK cells in the LN seem to be generated, at least in part, from both thymus-dependent Sca-1(lo) and thymus-independent Sca-1(hi) LN NKPs.

Cutting edge: Thymic NK cells develop independently from T cell precursors

Although NK cells in the mouse are thought to develop in the bone marrow, a small population of NK cells in the thymus has been shown to derive from a GATA3-dependent pathway. Characteristically, thymic NK cells express CD127 and few Ly49 molecules and lack CD11b. Because these NK cells develop in the thymus, the question of their relationship to the T cell lineage has been raised. Using several different mouse models, we find that unlike T cells, thymic NK cells are not the progeny of Rorc-expressing progenitors and do not express Rag2 or rearrange the TCRγ locus. We further demonstrate that thymic NK cells develop independently of the Notch signaling pathway, supporting the idea that thymic NK cells represent bona fide NK cells that can develop independently of all T cell precursors.

Endogenous TCR recombination in TCR Tg single RAG-deficient mice uncovered by robust in vivo T cell activation and selection

Recombination activating gene (RAG)-deficient TCR (T Cell Receptor) Tg (transgenic) mice are routinely used as sources of monoclonal T cells. We found that after the transfer of T cells from a RAG-2-deficient 5CC7 TCR Tg mice into allogeneic hosts we recovered a population of T cells expressing diverse alphabeta-TCRs. In fact, in the thymus and spleen of the 5CC7 RAG-2-deficient donor mice, we detected rare T cells expressing non-Tg TCR chains. Similar observations were obtained using T cells from two other TCR transgenic strains, namely RAG-2-deficient aHY and RAG-1-deficient OT-1 mice. The sequences of the endogenous TCR transcripts suggested that gene recombination could occur, albeit quite inefficiently, in the RAG-deficient mice we used. In agreement, we evidenced rare TCR Valpha and Vbeta-chain transcripts in non-Tg RAG-2-deficient mice. Since in these non-Tg RAG-deficient mice no mature T cells could ever be found, our findings suggested a role for the TCR Tg in rescuing rare recombined endogenous chains. Robust T-cell activation by the allogeneic environment favored the selection and expansion of the rare cells expressing endogenous TCRs. Potential mechanisms involved in the recombination of the endogenous TCR chains in the different strains of RAG-deficient mice used, and in particular the possibility of RAG-1 hypomorphism due to an incomplete knocking out procedure, are discussed. Our findings have important experimental implications for studies using TCR-Tg RAG-deficient cells as monoclonal T cell populations.

Peripheral Thy1+ lymphocytes rearranging TCR-gammadelta genes in LAT-deficient mice

Linker for activation of T cells (LAT) is an adaptor molecule indispensable for development of alphabeta and gammadelta T lymphocytes. Surprisingly, using a new model of LAT-deficient mice we found that despite arrested thymic development, a discrete population of cells with active Lat promoter, expressing Thy1 molecules, accumulated in peripheral lymphoid organs of homozygous (Lat(Inv/Inv)) mutant mice. By measuring frequencies of TCR gene rearrangements in conjunction with a panel of cell surface Ag, we dissected two subsets of these Thy1(+) cells. Thy1(dull) cells expressed markers of NK lymphocytes and contained low frequency of TCR-gamma gene rearrangements without detectable TCR-delta rearrangements. Thy1(high) cells resembled immature CD44(+)CD25(+) thymocytes and contained high frequency of non-productive TCR-gamma and TCR-delta rearrangements, indicating that cells displaying molecular signatures of commitment toward gammadelta T-cell lineage can develop and populate lymphoid tissues of LAT-deficient mice. Phenotypically similar Thy1(high) cells were also found in lymph nodes of lymphocyte-deficient (Rag2(-/-)) mice but not in T lymphocyte proficient, heterozygous Lat(+/Inv) mice suggesting that Thy1(high) cells of LAT-deficient mice identified in this study accumulate in peripheral lymphoid organs as a result of congenital lymphopenia.

B-cell co-receptor CD72 is expressed on NK cells and inhibits IFN-gamma production but not cytotoxicity

NK cells have two main functions, namely cell-mediated cytotoxicity and production of cytokines. Multiple inhibitory receptors that regulate NK-cell cytotoxicity have been characterized whereas little is known about receptors regulating cytokine production. Here we report that CD72, which is considered to be an important co-receptor regulating B-cell activation, is also expressed on mouse NK cells. NK cells expressing high levels of CD72, upon stimulation with IL-12 and IL-18 or target cells, produce significantly less IFN-gamma than those expressing low levels of CD72, whereas both subsets are equally cytotoxic. Ectopic expression of CD72 in the murine NK-cell line KY2 inhibits cytokine-induced IFN-gamma production, and the inhibitory effect is diminished by mutations in the inhibitory motifs in the intracellular domain or replacement of the extracellular domain of CD72. Thus, CD72 is an inhibitory receptor on NK cells regulating cytokine production.

Untangling the T branch of the hematopoiesis tree

T cells develop in the thymus. Previous work suggested an early separation of lymphoid from myeloerythroid lineages during hematopoiesis and hypothesized the thymus was settled exclusively by lymphoid-restricted hematopoietic progenitors. Recent data have instead established the existence of lymphoid-myeloid progenitors, which possess lymphoid and myeloid lineage potentials but lack erythroid potential. Myeloid and lymphoid potentials are present at the clonal level in early thymic progenitors, confirming that progenitors settling the thymus include lymphoid-myeloid progenitors. These results revise our view of the T lineage branch of hematopoiesis and focus attention on the generation, circulation, and homing of lymphoid-myeloid progenitors to the thymus.

Characterization of the role of TCR gammadelta in NK cell accumulation during viral liver inflammation

Polyinosinic-polyctidylic acid (Poly I:C) is a viral RNA mimic that can induce immune responses similar to that seen during viral infection. Although poly I:C administration into mice is associated increased NK cell infiltrates in the liver, the mechanisms underlying increased hepatic NK cell accumulation in response to poly I:C administration are incompletely defined. In the current study, we have identified a novel and important role for gammadelta T cells in driving the accumulation and activation of NK cells in the liver during poly I:C-mediated viral liver infection. Specifically, NK cell accumulation but not activation in gammadelta T cell deficient mice following poly I:C administration was significantly attenuated in comparison to that seen in poly I:C-treated wildtype mice. The ability of gammadelta T cells to promote NK cell accumulation and activation in the liver may be virus-specific since NK cell accumulation in the liver was not altered by TCR gammadelta deficiency following adenovirus administration.

The ontogeny and fate of NK cells marked by permanent DNA rearrangements

A subset of NK cells bears incomplete V(D)J rearrangements, but neither the consequence to cell activities nor the precise developmental stages in which recombination occurs is known. These are important issues, as recombination errors cause cancers of the B and T lineages. Using transgenic recombination reporter mice to examine NK cell dynamics in vivo, we show that recombination(+) NK cells have distinct developmental patterns in the BM, including reduced homeostatic proliferation and diminished Stat5 phosphorylation. In the periphery, both recombination(+) and recombination(-) NK cells mediate robust functional responses including IFN-gamma production, cytolysis, and tumor homing, suggesting that NK cells with distinct developmental histories can be found together in the periphery. We also show that V(D)J rearrangement marks both human cytolytic (CD56(dim)) and immunoregulatory (CD56(bright)) populations, demonstrating the distribution of permanent DNA rearrangements across major NK cell subsets in man. Finally, direct quantification of rag transcripts throughout NK cell differentiation in both mouse and man establishes the specific developmental stages that are susceptible to V(D)J rearrangement. Together, these data demonstrate that multipotent progenitors rather than lineage-specified NK progenitors are targets of V(D)J recombination and that NK cells bearing the relics of earlier V(D)J rearrangements have different developmental dynamics but robust biological capabilities in vivo.

Unique subset of natural killer cells develops from progenitors in lymph node

Natural killer (NK) cells have been thought to develop from committed progenitors in the bone marrow. However, a novel pathway of thymus-dependent NK-cell development that produces a unique subset of NK cells expressing CD127 has recently been reported. We now have identified 2 populations of NK progenitors, one in the thymus and the other in the lymph node (LN). Immature double-negative 2 (CD4(-)CD8(-)CD44(+)CD25(+)) thymocytes have potential to produce NK cells with rearranged T-cell receptor gamma genes (Tcrgamma(+)) in vitro. Tcrgamma(+) NK cells are rare in spleen but relatively abundant in the thymus and LN. Approximately 20% of LN NK cells are Tcrgamma(+), and they are found at similar levels in both CD127(+) and CD127(-) subsets. Moreover, a subpopulation of LN cells resembling immature thymocytes differentiates into Tcrgamma(+) NK cells in vitro and also repopulates the NK compartment in lymphopenic mice. Athymic mice lack the LN NK progenitors expressing CD127 as well as Tcrgamma(+) NK cells. These results suggest that Tcrgamma(+) NK cells may be generated from unique progenitors in the thymus as well as in the LN.

Novel insights into the relationships between dendritic cell subsets in human and mouse revealed by genome-wide expression profiling

Genome-wide expression profiling of mouse and human leukocytes reveal conserved transcriptional programs of plasmacytoid or conventional dendritic cell subsets.

Background

Dendritic cells (DCs) are a complex group of cells that play a critical role in vertebrate immunity. Lymph-node resident DCs (LN-DCs) are subdivided into conventional DC (cDC) subsets (CD11b and CD8α in mouse; BDCA1 and BDCA3 in human) and plasmacytoid DCs (pDCs). It is currently unclear if these various DC populations belong to a unique hematopoietic lineage and if the subsets identified in the mouse and human systems are evolutionary homologs. To gain novel insights into these questions, we sought conserved genetic signatures for LN-DCs and in vitro derived granulocyte-macrophage colony stimulating factor (GM-CSF) DCs through the analysis of a compendium of genome-wide expression profiles of mouse or human leukocytes.

Results

We show through clustering analysis that all LN-DC subsets form a distinct branch within the leukocyte family tree, and reveal a transcriptomal signature evolutionarily conserved in all LN-DC subsets. Moreover, we identify a large gene expression program shared between mouse and human pDCs, and smaller conserved profiles shared between mouse and human LN-cDC subsets. Importantly, most of these genes have not been previously associated with DC function and many have unknown functions. Finally, we use compendium analysis to re-evaluate the classification of interferon-producing killer DCs, lin^-CD16⁺HLA-DR⁺ cells and in vitro derived GM-CSF DCs, and show that these cells are more closely linked to natural killer and myeloid cells, respectively.

Conclusion

Our study provides a unique database resource for future investigation of the evolutionarily conserved molecular pathways governing the ontogeny and functions of leukocyte subsets, especially DCs.

Developmental pathways that generate natural-killer-cell diversity in mice and humans

Natural killer (NK) cells are large granular lymphocytes capable of producing inflammatory cytokines and spontaneously killing malignant, infected or 'stressed' cells. These NK-cell functions are controlled by cell-surface receptors that titrate stimulatory and inhibitory signals. However, we remain puzzled about where and when NK cells develop and differentiate, and this has fuelled the debate over the diversification of the peripheral NK-cell pool: are NK cells functionally homogeneous or are there subsets with specialized effector functions? In this Review, we consider the developmental relationships and biological significance of the diverse NK-cell subsets in mice and humans, and discuss how new humanized mouse models may help to characterize them further.

Back to the future--defining NK cells and T cells

In this issue of the European Journal of Immunology (EJI), Stewart et al. report about a population of gammadelta-T cells expressing an extensive repertoire of NK cell receptors, and the presence of non-rearranged germline TCRdelta transcripts in conventional NK cells. These findings and other recent studies highlight the similarities of NK cells and T cells and the problems in discriminating between these two lineages.

Germ-line and rearranged Tcrd transcription distinguish bona fide NK cells and NK-like gammadelta T cells

NK cells and gammadelta T cells are distinct subsets of lymphocytes that contextually share multiple phenotypic and functional characteristics. However, the acquisition and the extent of these similarities remain poorly understood. Here, using T cell receptor delta locus-histone 2B-enhanced GFP (Tcrd-H2BEGFP) reporter mice, we show that germ-line transcription of Tcrd occurs in all maturing NK cells. We also describe a population of mouse NK-like cells that are indistinguishable from "bona fide" NK cells using standard protocols. Requirements for V(D)J recombination and a functional thymus, along with very low-level expression of surface TCRgammadelta but high intracellular CD3, define these cells as gammadelta T cells. "NK-like gammadelta T cells" are CD127+, have a memory-activated phenotype, express multiple NK cell receptors and readily produce interferon-gamma in response to IL-12/IL-18 stimulation. The close phenotypic resemblance between NK cells and NK-like gammadelta T cells is a source of experimental ambiguity in studies bridging NK and T cell biology, such as those on thymic NK cell development. Instead, it ascribes chronic TCRgammadelta engagement as a means of acquiring NK-like function.

10th Meeting of the Society for Natural Immunity, Cambridge, UK, 11-14 April 2007

Natural killer (NK) cells are important members of the innate immune system being involved in the detection and clearance of transformed or virus-infected cells. The 10th meeting of the Society for Natural Immunity was recently held in Cambridge, UK, where leaders from around the globe gathered to discuss the latest developments and understandings in the field of NK cell biology. Among the topics covered in this meeting were NK cell development, the origin of thymic NK cells, contribution of NK cells to viral infections and subsequent tissue damage, crosstalk between NK cells and other immune cells and the role of NK cells at the foetal-maternal interface.

Zoned out: functional mapping of stromal signaling microenvironments in the thymus

All hematopoietic cells, including T lymphocytes, originate from stem cells that reside in the bone marrow. Most hematopoietic lineages also mature in the bone marrow, but in this respect, T lymphocytes differ. Under normal circumstances, most T lymphocytes are produced in the thymus from marrow-derived progenitors that circulate in the blood. Cells that home to the thymus from the marrow possess the potential to generate multiple T and non-T lineages. However, there is little evidence to suggest that, once inside the thymus, they give rise to anything other than T cells. Thus, signals unique to the thymic microenvironment compel multipotent progenitors to commit to the T lineage, at the expense of other potential lineages. Summarizing what is known about the signals the thymus delivers to uncommitted progenitors, or to immature T-committed progenitors, to produce functional T cells is the focus of this review.

Characterization of developmental pathway of natural killer cells from embryonic stem cells in vitro

In vitro differentiation of embryonic stem (ES) cells is often used to study hematopoiesis. However, the differentiation pathway of lymphocytes, in particular natural killer (NK) cells, from ES cells is still unclear. Here, we used a multi-step in vitro ES cell differentiation system to study lymphocyte development from ES cells, and to characterize NK developmental intermediates. We generated embryoid bodies (EBs) from ES cells, isolated CD34(+) EB cells and cultured them on OP9 stroma with a cocktail of cytokines to generate cells we termed ES-derived hematopoietic progenitors (ES-HPs). EB cell subsets, as well as ES-HPs derived from EBs, were tested for NK, T, B and myeloid lineage potentials using lineage specific cultures. ES-HPs derived from CD34(+) EBs differentiated into NK cells when cultured on OP9 stroma with IL-2 and IL-15, and into T cells on Delta-like 1-transduced OP9 (OP9-DL1) with IL-7 and Flt3-L. Among CD34(+) EB cells, NK and T cell potentials were detected in a CD45(-) subset, whereas CD45(+) EB cells had myeloid but not lymphoid potentials. Limiting dilution analysis of ES-HPs generated from CD34(+)CD45(-) EB cells showed that CD45(+)Mac-1(-)Ter119(-) ES-HPs are highly enriched for NK progenitors, but they also have T, B and myeloid potentials. We concluded that CD45(-)CD34(+) EB cells have lymphoid potential, and they differentiate into more mature CD45(+)Lin(-) hematopoietic progenitors that have lymphoid and myeloid potential. NK progenitors among ES-HPs are CD122(-) and they rapidly acquire CD122 as they differentiate along the NK lineage.

Transient Notch signaling induces NK cell potential in Pax5-deficient pro-B cells

Unlike early B/T cell development, NK cell lineage commitment is not well understood, with a major limitation being the lack of a robust culture system to assay NK cell progenitors. Here we have exploited the multi-lineage potential of Pax5(-/-) pro-B cells to establish an effective system to direct differentiation of progenitors into the NK cell lineage. Cultivation of Pax5(-/-) pro-B cells on OP9 cells expressing the Notch ligand Delta-Like1 (OP9-DL1) in the presence of IL-7 efficiently induced T and NK cell potential. For NK cells, Notch was only transiently required, as prolonged signaling decreased NK and increased T cell development. Pure NK cell populations could be obtained by the culture of these Notch signal-experienced cells onto OP9 stroma and IL-15. A similar transient exposure to Notch was also compatible with the differentiation of NK cells from hematopoietic progenitors, while sustained Notch signaling impaired NK cell generation. Pax5(-/-) pro-B cell-derived NK cells were cytotoxic, secreted cytokines and expressed all the expected NK cell-specific surface markers examined except the Ly49 family, a phenotype similar to fetal NK cells. These data indicate that Notch signaling induces T/NK cell differentiation in Pax5(-/-) pro-B cells that is strikingly similar to early thymopoiesis.

Bone marrow versus thymic pathways of natural killer cell development

Understanding natural killer (NK) cell developmental pathways is crucial for harnessing the potential therapeutic benefits of this specialized lymphocyte subset. The bone marrow (BM) plays a major role in NK cell development, providing the appropriate environmental cues for NK cell commitment and subsequent NK cell differentiation. Nevertheless, the molecular signals provided in this context remain enigmatic. It is widely assumed that BM seeds the periphery with NK cells. However, the precise origins of NK cells found in lymphoid organs and tissues are not defined. Recently, we found that thymic NK cells bear molecular markers and functional attributes that distinguish them from most peripheral NK cells. We find that NK cells are actively exported from the thymus to the periphery, suggesting that thymus-derived NK cells may have unique roles both intrathymically and in secondary lymphoid organs. Here we compare the properties of thymic NK cells with properties of other NK cell subsets that have been identified in the mouse. We propose that heterogeneity in NK cell function can be achieved through distinct thymic and bone marrow pathways of NK cell development.

The potential involvement of Notch signaling in NK cell development

NK cells constitute an essential element of the innate immune system; however, the cellular and molecular mechanisms that guide their early development are still poorly understood. Here, we demonstrate that in addition to its known crucial role in T cell development, Notch signaling can also be involved in NK cell development. Thus, upon co-culture on OP9 stroma expressing the Notch ligand Delta-like 1 (OP9-DL1), Pax5-deficient pro-B cells, which have multi-lineage potential, efficiently differentiate into T and NK cells. Upon DL-1 signaling, Pax5-deficient pro-B cells down-regulate both surface CD93 expression and transcripts for B cell-specific genes and concomitantly up-regulate T lineage gene transcripts. Subsequent transfer of DL-1-signaled Pax5-deficient pro-B cells onto OP9 stroma in the presence of IL-2 leads to their efficient differentiation into NK1.1(+), functional NK cells. Moreover, bone marrow early progenitor with lymphoid and myeloid differentiation potential (EPLM), which we have previously described as the normal in vivo-equivalent of Pax5-deficient pro-B cells, also gain the ability to differentiate into effector NK cells following transient DL1 Notch-mediated signaling. The potential involvement of Notch signaling in the generation of the NK cell repertoire in vivo is discussed.