Julia A. Wagner

Single-molecule analysis of fluorescently labeled G-protein–coupled receptors reveals complexes with distinct dynamics and organization

G-protein–coupled receptors (GPCRs) constitute the largest family of receptors and major pharmacological targets. Whereas many GPCRs have been shown to form di-/oligomers, the size and stability of such complexes under physiological conditions are largely unknown. Here, we used direct receptor labeling with SNAP-tags and total internal reflection fluorescence microscopy to dynamically monitor single receptors on intact cells and thus compare the spatial arrangement, mobility, and supramolecular organization of three prototypical GPCRs: the β1-adrenergic receptor (β1AR), the β2-adrenergic receptor (β2AR), and the γ-aminobutyric acid (GABAB) receptor. These GPCRs showed very different degrees of di-/oligomerization, lowest for β1ARs (monomers/dimers) and highest for GABAB receptors (prevalently dimers/tetramers of heterodimers). The size of receptor complexes increased with receptor density as a result of transient receptor–receptor interactions. Whereas β1-/β2ARs were apparently freely diffusing on the cell surface, GABAB receptors were prevalently organized into ordered arrays, via interaction with the actin cytoskeleton. Agonist stimulation did not alter receptor di-/oligomerization, but increased the mobility of GABAB receptor complexes. These data provide a spatiotemporal characterization of β1-/β2ARs and GABAB receptors at single-molecule resolution. The results suggest that GPCRs are present on the cell surface in a dynamic equilibrium, with constant formation and dissociation of new receptor complexes that can be targeted, in a ligand-regulated manner, to different cell-surface microdomains.

Cytokine-induced memory-like natural killer cells exhibit enhanced responses against myeloid leukemia

Cytokine-induced differentiation of memory-like natural killer cells enhances antileukemia responses. Natural killers of leukemia Natural killer cells, part of the innate immune system, play a role in immune responses against exogenous pathogens as well as cancer. Recent studies have identified the existence of memory-like characteristics in some natural killer cells, and Romee et al. investigated these memory cells’ potential as a cancer therapy. The authors compared human natural killer memory cells to non-memory control cells, then demonstrated their effectiveness against myeloid leukemia models in vitro and in mice. They also performed a clinical trial in human patients with acute myeloid leukemia, where the natural killer memory-like cells again demonstrated antileukemia effects, some of which produced clinical remissions. Natural killer (NK) cells are an emerging cellular immunotherapy for patients with acute myeloid leukemia (AML); however, the best approach to maximize NK cell antileukemia potential is unclear. Cytokine-induced memory-like NK cells differentiate after a brief preactivation with interleukin-12 (IL-12), IL-15, and IL-18 and exhibit enhanced responses to cytokine or activating receptor restimulation for weeks to months after preactivation. We hypothesized that memory-like NK cells exhibit enhanced antileukemia functionality. We demonstrated that human memory-like NK cells have enhanced interferon-γ production and cytotoxicity against leukemia cell lines or primary human AML blasts in vitro. Using mass cytometry, we found that memory-like NK cell functional responses were triggered against primary AML blasts, regardless of killer cell immunoglobulin-like receptor (KIR) to KIR-ligand interactions. In addition, multidimensional analyses identified distinct phenotypes of control and memory-like NK cells from the same individuals. Human memory-like NK cells xenografted into mice substantially reduced AML burden in vivo and improved overall survival. In the context of a first-in-human phase 1 clinical trial, adoptively transferred memory-like NK cells proliferated and expanded in AML patients and demonstrated robust responses against leukemia targets. Clinical responses were observed in five of nine evaluable patients, including four complete remissions. Thus, harnessing cytokine-induced memory-like NK cell responses represents a promising translational immunotherapy approach for patients with AML.

Human Cytokine-Induced Memory-Like Natural Killer Cells

Natural killer (NK) cells are innate lymphoid cells that are important for host defense against infection and mediate antitumor responses. Recent reports from several laboratories have identified that NK cells can remember a prior activation event and consequently respond more robustly when restimulated, a property termed innate NK cell memory. NK cell memory has now been identified following hapten exposure, viral infection, and combined cytokine preactivation with IL-12, IL-15, and IL-18. Many questions in the field remain regarding the cellular and molecular mechanisms regulating memory NK cells and their responses, as well as their formation and function in mice and humans. Here we review our current understanding of cytokine-induced memory-like (CIML) NK cells that are generated by combined preactivation with IL-12, IL-15, and IL-18. These cells exhibit enhanced NK cell effector functions weeks after the initial cytokine preactivation. Further, we highlight the preclinical rationale and ongoing therapeutic application of CIML NK cells for adoptive immunotherapy in patients with hematologic malignancies.

CD56bright NK cells exhibit potent antitumor responses following IL-15 priming

NK cells, lymphocytes of the innate immune system, are important for defense against infectious pathogens and cancer. Classically, the CD56dim NK cell subset is thought to mediate antitumor responses, whereas the CD56bright subset is involved in immunomodulation. Here, we challenge this paradigm by demonstrating that brief priming with IL-15 markedly enhanced the antitumor response of CD56bright NK cells. Priming improved multiple CD56bright cell functions: degranulation, cytotoxicity, and cytokine production. Primed CD56bright cells from leukemia patients demonstrated enhanced responses to autologous blasts in vitro, and primed CD56bright cells controlled leukemia cells in vivo in a murine xenograft model. Primed CD56bright cells from multiple myeloma (MM) patients displayed superior responses to autologous myeloma targets, and furthermore, CD56bright NK cells from MM patients primed with the IL-15 receptor agonist ALT-803 in vivo displayed enhanced ex vivo functional responses to MM targets. Effector mechanisms contributing to IL-15–based priming included improved cytotoxic protein expression, target cell conjugation, and LFA-1–, CD2-, and NKG2D-dependent activation of NK cells. Finally, IL-15 robustly stimulated the PI3K/Akt/mTOR and MEK/ERK pathways in CD56bright compared with CD56dim NK cells, and blockade of these pathways attenuated antitumor responses. These findings identify CD56bright NK cells as potent antitumor effectors that warrant further investigation as a cancer immunotherapy.

PTEN regulates natural killer cell trafficking in vivo

Significance Natural killer (NK) cells are critical players in the response to viruses and transformed cells, but the molecular mechanisms controlling their functions are incompletely understood. A major pathway leading to NK cell activation is the phosphoinositide 3-kinase pathway. However, the impact of phosphatase and tensin homolog (PTEN), a key phosphatase opposing this pathway, on NK cells has not been reported. We generated a previously unreported NK cell-intrinsic PTEN-deletion mouse model to evaluate its role in NK cell biology. In contrast to other lymphocytes, we demonstrate that the primary effects of PTEN loss are marked perturbation in NK cell trafficking and distribution during both homeostasis and malignancy. These findings indicate that PTEN plays an essential role in NK cell localization in vivo. Phosphatase and tensin homolog (PTEN) is a critical negative regulator of the phosphoinositide-3 kinase pathway, members of which play integral roles in natural killer (NK) cell development and function. However, the functions of PTEN in NK cell biology remain unknown. Here, we used an NK cell-specific PTEN-deletion mouse model to define the ramifications of intrinsic NK cell PTEN loss in vivo. In these mice, there was a significant defect in NK cell numbers in the bone marrow and peripheral organs despite increased proliferation and intact peripheral NK cell maturation. Unexpectedly, we observed a significant expansion of peripheral blood NK cells and the premature egress of NK cells from the bone marrow. The altered trafficking of NK cells from peripheral organs into the blood was due to selective hyperresponsiveness to the blood localizing chemokine S1P. To address the importance of this trafficking defect to NK cell immune responses, we investigated the ability of PTEN-deficient NK cells to traffic to a site of tumor challenge. PTEN-deficient NK cells were defective at migrating to distal tumor sites but were more effective at clearing tumors actively introduced into the peripheral blood. Collectively, these data identify PTEN as an essential regulator of NK cell localization in vivo during both homeostasis and malignancy.

Cytokine-Induced Memory-Like Differentiation Enhances Unlicensed Natural Killer Cell Antileukemia and FcγRIIIa-Triggered Responses.

Cytokine-induced memory-like natural killer (NK) cells differentiate after short-term preactivation with IL-12, IL-15, and IL-18 and display enhanced effector function in response to cytokines or tumor targets for weeks after the initial preactivation. Conventional NK cell function depends on a licensing signal, classically delivered by an inhibitory receptor engaging its cognate MHC class I ligand. How licensing status integrates with cytokine-induced memory-like NK cell responses is unknown. We investigated this interaction using killer cell immunoglobulin-like receptor- and HLA-genotyped primary human NK cells. Memory-like differentiation resulted in enhanced IFN-γ production triggered by leukemia targets or FcγRIIIa ligation within licensed NK cells, which exhibited the highest functionality of the NK cell subsets interrogated. IFN-γ production by unlicensed memory-like NK cells was also enhanced to a level comparable with that of licensed control NK cells. Mechanistically, differences in responses to FcγRIIIa-based triggering were not explained by alterations in key signaling intermediates, indicating that the underlying biology of memory-like NK cells is distinct from that of adaptive NK cells in human cytomegalovirus-positive individuals. Additionally, memory-like NK cells responded robustly to cytokine receptor restimulation with no impact of licensing status. These results demonstrate that both licensed and unlicensed memory-like NK cell populations have enhanced functionality, which may be translated to improve leukemia immunotherapy.

Human Adaptive Natural Killer Cells: Beyond NKG2C

Paradigm-shifting studies have identified NKG2C(+) adaptive natural killer (NK) cells in individuals infected with cytomegalovirus. Recently in Cell Reports, Liu et al. demonstrate that NKG2C(-/-) HCMV(+) individuals also generate adaptive NK cells, and reveal CD2 as a major co-stimulatory receptor for these NK cells specialized to respond via FcγRIIIa/CD16.

Transcriptional and post-transcriptional regulation of NK cell development and function.

Natural killer (NK) cells are specialized innate lymphoid cells that survey against viral infections and malignancy. Numerous advances have improved our understanding of the molecular mechanisms that control NK cell development and function over the past decade. These include both studies on the regulatory effects of transcription factors and translational repression via microRNAs. In this review, we summarize our current knowledge of DNA-binding transcription factors that regulate gene expression and thereby orchestrate NK cell development and activation, with an emphasis on recent discoveries. Additionally, we highlight our understanding of how RNA-binding microRNAs fine tune the NK cell molecular program. We also underscore the large number of open questions in the field that are now being addressed using new technological approaches and genetically engineered model organisms. Ultimately, a deeper understanding of the basic molecular biology of NK cells will facilitate new strategies to manipulate NK cells for the treatment of human disease.

Single-Molecule Fluorescence Microscopy for the Analysis of Fast Receptor Dynamics

Assessing the dynamics of individual membrane proteins in living cells is a powerful approach to investigate their assembly, mobility, and function. Here, we describe how to image single G protein-coupled receptors (GPCRs), both in the active and inactive state. This is achieved by combining labeling of GPCRs with bright organic fluorophores and fluorescent imaging by total internal reflection fluorescence microscopy. Using this method, individual tracks of single molecules can be analyzed in parallel with high spatial precision and with frame rates up to 50/s.