Plasma membrane abundance dictates phagocytic capacity and functional crosstalk in myeloid cells

Professional phagocytes like neutrophils and macrophages tightly control what they eat, how much they eat, and when they move after eating. We show that plasma membrane abundance is a key arbiter of these cellular behaviors. Neutrophils and macrophages lacking the G-protein subunit Gb4 exhibit profound plasma membrane expansion due to enhanced production of sphingolipids. This increased membrane allocation dramatically enhances phagocytosis of bacteria, fungus, apoptotic corpses, and cancer cells. Gb4 deficient neutrophils are also defective in the normal inhibition of migration following cargo uptake. In Gb4 knockout mice, myeloid cells exhibit enhanced phagocytosis of inhaled fungal conidia in the lung but also increased trafficking of engulfed pathogens to other organs. These results reveal an unexpected, biophysical control mechanism lying at the heart of myeloid functional decision-making.

Apoptotic contraction drives target cell release by cytotoxic T cells

Cytotoxic T lymphocytes (CTLs) fight intracellular pathogens and cancer by identifying and destroying infected or transformed target cells1. To kill, CTLs form a specialized cytotoxic immune synapse (IS) with a target of interest and then release toxic perforin and granzymes into the interface to elicit programmed cell death2-5. The IS then dissolves, enabling CTLs to search for additional prey and professional phagocytes to clear the corpse6. While the mechanisms governing IS assembly have been studied extensively, far less is known about target cell release. Here, we applied time-lapse imaging to explore the basis for IS dissolution and found that it occurred concomitantly with the cytoskeletal contraction of apoptotic targets. Genetic and pharmacological perturbation of this contraction response indicated that it was both necessary and sufficient for CTL dissociation. We also found that mechanical amplification of apoptotic contractility promoted faster CTL detachment and serial killing. Collectively, these results establish a biophysical basis for IS dissolution and highlight the importance of mechanosensory feedback in the regulation of cell-cell interactions.

Topographical analysis of immune cell interactions reveals a biomechanical signature for immune cytolysis

Immune cells live intensely physical lifestyles characterized by structural plasticity, mechanosensitivity, and force exertion. Whether specific immune functions require stereotyped patterns of mechanical output, however, is largely unknown. To address this question, we used super-resolution traction force microscopy to compare cytotoxic T cell immune synapses with contacts formed by other T cell subsets and macrophages. T cell synapses were globally and locally protrusive, which was fundamentally different from the coupled pinching and pulling of macrophage phagocytosis. By spectrally decomposing the force exertion patterns of each cell type, we associated cytotoxicity with compressive strength, local protrusiveness, and the induction of complex, asymmetric interfacial topographies. These features were further validated as cytotoxic drivers by genetic disruption of cytoskeletal regulators, direct imaging of synaptic secretory events, and in silico analysis of interfacial distortion. We conclude that T cell-mediated killing and, by implication, other effector responses are supported by specialized patterns of efferent force.

Phollow the phosphoinositol: Actin dynamics at the B cell immune synapse

Actin remodeling promotes B cell activation by enabling B cell antigen receptor clustering in the immune synapse. In the current issue of JCB, Droubi et al. (2022. J. Cell Biol.https://doi.org/10.1083/jcb.202112018) find that this process is initiated by the lipid phosphatase INPP5B, which shapes synaptic actin architecture by locally depleting phosphatidylinositol 4,5 bisphosphate.

Mechanoregulation of Metastasis Beyond The Matrix

Epithelial transformation and carcinogenesis are characterized by profound alterations in cell mechanics that significantly impact multiple steps of the metastatic cascade. The ability of cancer cells to grow in the primary tumor, to locally invade through the confining extracellular matrix, to survive in circulation, and to extravasate into distant vital organs all depend on specific mechanical characteristics. Importantly, recent studies have shown that the mechanical properties of cancer cells also influence their interactions with immune and stromal cells. Here, we discuss the mechanical changes that cancer cells undergo during metastasis, how these changes impact immune and stromal responses, and the implications of these new insights for therapeutic intervention.

Abstract 3731: Epigenetic control of tumor cell killing by natural killer cells

Immunotherapies including the activation of endogenous immune cells have stimulated great interest for their anti-tumor functions. An initial focus has been on genetic lesions such as loss of MHC class I and class II molecules that facilitate tumor cell escape. More recently, epigenetic mechanisms such as the disruption of the SWI/SNF remodeling complex PBAF have been implicated in regulating tumor sensitivity to CD8 T cell-mediated killing. Here, I used an unbiased genetic screening approach to examine resistance and sensitivity of breast cancer cells to Natural Killer (NK) cells. Among expected hits including caspases, interferon receptors, and cell death executioners, the screen revealed a surprising role of alternate SWI/SNF complexes in controlling NK cell-mediated cytotoxicity that relate to interferon-γ and perforin/granzyme B, and may be pharmacologically reversible. Altogether, I will discuss a new epigenetic mechanism that affect NK cell-mediated cytotoxicity with a focus on SWI/SNF complexes.

Mechanically active integrins target lytic secretion at the immune synapse to facilitate cellular cytotoxicity

Cytotoxic lymphocytes fight pathogens and cancer by forming immune synapses with infected or transformed target cells and then secreting cytotoxic perforin and granzyme into the synaptic space, with potent and specific killing achieved by this focused delivery. The mechanisms that establish the precise location of secretory events, however, remain poorly understood. Here we use single cell biophysical measurements, micropatterning, and functional assays to demonstrate that localized mechanotransduction helps define the position of secretory events within the synapse. Ligand-bound integrins, predominantly the αLβ2 isoform LFA-1, function as spatial cues to attract lytic granules containing perforin and granzyme and induce their fusion with the plasma membrane for content release. LFA-1 is subjected to pulling forces within secretory domains, and disruption of these forces via depletion of the adaptor molecule talin abrogates cytotoxicity. We thus conclude that lymphocytes employ an integrin-dependent mechanical checkpoint to enhance their cytotoxic power and fidelity.

TCR signal strength defines distinct mechanisms of T cell dysfunction and cancer evasion

T cell receptor (TCR) signal strength is a key determinant of T cell responses. We developed a cancer mouse model in which tumor-specific CD8 T cells (TST cells) encounter tumor antigens with varying TCR signal strength. High-signal-strength interactions caused TST cells to up-regulate inhibitory receptors (IRs), lose effector function, and establish a dysfunction-associated molecular program. TST cells undergoing low-signal-strength interactions also up-regulated IRs, including PD1, but retained a cell-intrinsic functional state. Surprisingly, neither high- nor low-signal-strength interactions led to tumor control in vivo, revealing two distinct mechanisms by which PD1hi TST cells permit tumor escape; high signal strength drives dysfunction, while low signal strength results in functional inertness, where the signal strength is too low to mediate effective cancer cell killing by functional TST cells. CRISPR-Cas9-mediated fine-tuning of signal strength to an intermediate range improved anti-tumor activity in vivo. Our study defines the role of TCR signal strength in TST cell function, with important implications for T cell-based cancer immunotherapies.

Harder, better, faster, stronger: biochemistry and biophysics in the immunosurveillance concert

Antitumor immunosurveillance is triggered by immune cell recognition of characteristic biochemical signals on the surfaces of cancer cells. Recent data suggest that the mechanical properties of cancer cells influence the strength of these signals, with physically harder target cells (more rigid) eliciting better, faster, and stronger cytotoxic responses against metastasis. Using analogies to a certain electronic music duo, we argue that the biophysical properties of cancer cells and their environment can adjust the volume and tone of the antitumor immune response. We also consider the potential influence of biomechanics-based immunosurveillance in disease progression and posit that targeting the biophysical properties of cancer cells in concert with their biochemical features could increase the efficacy of immunotherapy.

HLA-independent T cell receptors for targeting tumors with low antigen density

Chimeric antigen receptors (CARs) are receptors for antigen that direct potent immune responses. Tumor escape associated with low target antigen expression is emerging as one potential limitation of their efficacy. Here we edit the TRAC locus in human peripheral blood T cells to engage cell-surface targets through their T cell receptor-CD3 complex reconfigured to utilize the same immunoglobulin heavy and light chains as a matched CAR. We demonstrate that these HLA-independent T cell receptors (HIT receptors) consistently afford high antigen sensitivity and mediate tumor recognition beyond what CD28-based CARs, the most sensitive design to date, can provide. We demonstrate that the functional persistence of HIT T cells can be augmented by constitutive coexpression of CD80 and 4-1BBL. Finally, we validate the increased antigen sensitivity afforded by HIT receptors in xenograft mouse models of B cell leukemia and acute myeloid leukemia, targeting CD19 and CD70, respectively. Overall, HIT receptors are well suited for targeting cell surface antigens of low abundance.

132 HLA-independent T cell receptors effectively target low abundance antigens

Background

Chimeric antigen receptors (CARs) engage antigen independently of HLA and enable sustained T cell proliferation when they are endowed with both activating and costimulatory functions. While remission rates have been noticeably elevated in numerous clinical trials targeting CD19, CD22 or BCMA, relapses are common. One of the several underlying relapse mechanisms is antigen escape, which refers to a relapsing tumor that is either negative for the targeted antigen or expresses the latter at a low level. Failure to eliminate antigen-low tumors raises questions about the sensitivity of CARs and the minimum antigen density that is required for effective tumor eradication. Unlike CARs, TCRs engage antigen in an HLA-dependent manner, and they do so with high sensitivity. We hypothesized that a TCR/CD3 complex containing the same heavy and light immunoglobulin chains as a CAR will display increased sensitivity to the target antigen.

Methods

We edited the TRAC locus in human primary T cells to establish a novel antigen receptor structure, termed HLA-independent TCR or HIT receptor, by incorporating into the TCR/CD3 complex the same heavy and light chains as those of a corresponding CAR. We assessed their antigen sensitivity against a panel of cell lines expressing different antigen levels, analyzing their cytotoxicity, cytokine secretion, signaling response and degranulation activity. HIT and CAR T cells were further evaluated for their anti-tumor response using established ALL and AML mouse models.

Results

CD19-TRAC-HIT and CD19-TRAC-CAR T cells lysed wild-type NALM6 (~27,000 CD19 molecules) and NALM6 variants with 100-fold less CD19. As CD19 levels decreased further, CAR T cells no longer killed their target, in contrast to HIT T cells. HIT T cells showed increased expression of IFN-gamma, IL-2 and TNF-alpha upon exposure to NALM6 cells expressing ~20 CD19 molecules per cell, compared to CAR T cells. This increased sensitivity of HIT receptors correlated to their greater signaling response, upon exposure to the low-antigen-density NALM6. Phospho-proteomic analyses further confirmed this increased response of HIT T cells to low antigen levels. Altogether, these results confirm that HIT receptors endow T cells with greater antigen sensitivity than canonical CARs. We further showed that HIT T cells have higher in vivo anti-tumor activity compared to CAR T cells in mice bearing low-antigen-density ALL or AML.

Conclusions

HIT receptors consistently afford high antigen sensitivity and mediate tumor recognition beyond what current CARs can provide. HIT receptors open new prospects for targeting cell surface antigens of low abundance.

Ethics Approval

Eight- to 12-week-old NOD/SCID/IL-2Rgamma-null (NSG) male mice (Jackson Laboratory) were used under a protocol approved by the MSKCC Institutional Animal Care and Use Committee.

Ectopic activation of the miR-200c-EpCAM axis enhances antitumor T cell responses in models of adoptive cell therapy

Adoptive T cell therapy (ACT) is a promising strategy for treating cancer, but it often fails because of cell intrinsic regulatory programs that limit the degree or duration of T cell function. In this study, we found that ectopic expression of microRNA-200c (miR-200c) markedly enhanced the antitumor activity of CD8+ cytotoxic T lymphocytes (CTLs) during ACT in multiple mouse models. CTLs transduced with miR-200c exhibited reduced apoptosis during engraftment and enhanced in vivo persistence, accompanied by up-regulation of the transcriptional regulator T cell factor 1 (TCF1) and the inflammatory cytokine tumor necrosis factor (TNF). miR-200c elicited these changes by suppressing the transcription factor Zeb1 and thereby inducing genes characteristic of epithelial cells. Overexpression of one of these genes, Epcam, was sufficient to augment therapeutic T cell responses against both solid and liquid tumors. These results identify the miR-200c–EpCAM axis as an avenue for improving ACT and demonstrate that select genetic perturbations can produce phenotypically distinct T cells with advantageous therapeutic properties.

Abstract 2825: Mechanosurveillance by immunity targets mechanical compliance of metastatic cells

Metastatic dissemination of cancer cells to vital secondary organs pose a lifelong lethal threat unless they can successfully be eliminated by the immune system. Interestingly, despite their ability to evade immunity at primary organs, the vast majority of disseminated cancer cells are killed at secondary organs, suggesting that the process of dissemination renders cancer cells sensitive to immunity. Understanding how dissemination renders cancer cells sensitive to immunity would therefore provide exciting new opportunities to target metastases, but the relationship between dissemination and the immune response is not clear. In our effort to study this relationship, we manipulated cancer cells' expression of myocardin related transcription factors A and B (MRTFA/B), which are central in regulation of actin cytoskeleton and are indispensable for metastatic invasion, dissemination and colonization. Surprisingly, despite MRTFA/B's established roles in promoting metastasis, we found that MRTFA/B expression in cancer cells downregulated metastatic colonization in fully immuno-competent mouse models. MRTFA/B mediated decrease in colonization was completely reversed upon immune cell depletion suggesting that the immune cells exploit MRTFA/B expression in cancer cells for cytotoxicity. Among various cancer cell parameters involved in immune cell activation, T- and NK cell activation most prominently correlated with MRTFA/B driven mechanical compliance of cancer cells. Mechanical compliance, also known as stiffness, of antigen presenting cells and synthetic substrates are well-known to activate T- and NK cells through biophysical and mechano-chemical signal transduction. As such, perturbing cancer cells' stiffness downregulated the immune response and cytotoxicity. Thus, we termed this process mechanosurveillance. Our work on mechanosurveillance will help us understand how disseminated cancer cells are targeted by immunity and how they evade it at secondary organs. Importantly the concept of mechanosurveillance will help guide the use of existing therapeutics that alter tissue stiffness and immune boosting therapies to combat metastases.

Citation Format: Maria Tello-Lafoz, Katja Srpan, Jing Hu, Yevgeniy Romin, Annalisa Calo, Katharine C. Hsu, Joan Massagué, Morgan Huse, Ekrem Emrah Er. Mechanosurveillance by immunity targets mechanical compliance of metastatic cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2825.

Interdomain spacing and spatial configuration drive the potency of IgG-[L]-scFv T cell bispecific antibodies

T cell-bispecific antibodies (BsAbs) couple cytotoxic T lymphocytes to tumor cells, inducing their destruction. Although there are more than 60 classes of BsAbs in development, the relative importance of parameters such as interdomain spacing or spatial configuration is largely unknown. Here, we dissected a symmetric dual bivalent BsAb platform (IgG-[L]-scFv: antitumor IgG with anti-CD3 scFv fused to the light chains) to explore the importance of valency and spatial configuration for BsAb-induced T cell cytotoxicity. Our results revealed that placing tumor and T cell binding domains on the same side of a BsAb (cis-configuration) elicited substantially stronger antitumor activity, in vitro and in vivo, compared to positioning them on opposite sides (trans-configuration). Moreover, using two cis-modules in the same BsAb further improved cytotoxicity (up to 2000-fold). In addition, separating antigen-binding components with a single Ig domain (C_L) markedly enhanced cytokine release and in vivo tumor responses compared to smaller (G₄S₁) or larger (C_H1-C_H2-C_H3) spacers. These findings provide guidelines for improving BsAb function and highlight the importance of spatial configuration and dual bivalency as development parameters.

Cytotoxic lymphocytes target characteristic biophysical vulnerabilities in cancer

Immune cells identify and destroy tumors by recognizing cellular traits indicative of oncogenic transformation. In this study, we found that myocardin-related transcription factors (MRTFs), which promote migration and metastatic invasion, also sensitize cancer cells to the immune system. Melanoma and breast cancer cells with high MRTF expression were selectively eliminated by cytotoxic lymphocytes in mouse models of metastasis. This immunosurveillance phenotype was further enhanced by treatment with immune checkpoint blockade (ICB) antibodies. We also observed that high MRTF signaling in human melanoma is associated with ICB efficacy in patients. Using biophysical and functional assays, we showed that MRTF overexpression rigidified the filamentous actin cytoskeleton and that this mechanical change rendered mouse and human cancer cells more vulnerable to cytotoxic T lymphocytes and natural killer cells. Collectively, these results suggest that immunosurveillance has a mechanical dimension, which we call mechanosurveillance, that is particularly relevant for the targeting of metastatic disease.

Modulating T Cell Activation Using Depth Sensing Topographic Cues

This report examines how sensing of substrate topography can be used to modulate T cell activation, a key coordinating step in the adaptive immune response. Inspired by the native T cell-antigen presenting cell interface, micrometer scale pits with varying depth are fabricated into planar substrates. Primary CD4⁺ T cells extend actin-rich protrusions into the micropits. T cell activation, reflected in secretion of cytokines interleukin-2 and interferon gamma, is sensitive to the micropit depth. Surprisingly, arrays of micropits with 4 μm depth enhance activation compared to flat substrates but deeper micropits are less effective at increasing cell response, revealing a biphasic dependence in activation as a function of feature dimensions. Inhibition of cell contractility abrogates the enhanced activation associated with the micropits. In conclusion, this report demonstrates that the 3D, microscale topography can be used to enhance T cell activation, an ability that most directly can be used to improve production of these cells for immunotherapy.

Interfacial actin protrusions mechanically enhance killing by cytotoxic T cells

Cytotoxic T lymphocytes (CTLs) kill by forming immunological synapses with target cells and secreting toxic proteases and the pore-forming protein perforin into the intercellular space. Immunological synapses are highly dynamic structures that boost perforin activity by applying mechanical force against the target cell. Here, we used high-resolution imaging and microfabrication to investigate how CTLs exert synaptic forces and coordinate their mechanical output with perforin secretion. Using micropatterned stimulatory substrates that enable synapse growth in three dimensions, we found that perforin release occurs at the base of actin-rich protrusions that extend from central and intermediate locations within the synapse. These protrusions, which depended on the cytoskeletal regulator WASP and the Arp2/3 actin nucleation complex, were required for synaptic force exertion and efficient killing. They also mediated physical deformation of the target cell surface during CTL-target cell interactions. Our results reveal the mechanical basis of cellular cytotoxicity and highlight the functional importance of dynamic, three-dimensional architecture in immune cell-cell interfaces.

Microparticle traction force microscopy reveals subcellular force exertion patterns in immune cell-target interactions

Force exertion is an integral part of cellular behavior. Traction force microscopy (TFM) has been instrumental for studying such forces, providing spatial force measurements at subcellular resolution. However, the applications of classical TFM are restricted by the typical planar geometry. Here, we develop a particle-based force sensing strategy for studying cellular interactions. We establish a straightforward batch approach for synthesizing uniform, deformable and tuneable hydrogel particles, which can also be easily derivatized. The 3D shape of such particles can be resolved with superresolution (<50 nm) accuracy using conventional confocal microscopy. We introduce a reference-free computational method allowing inference of traction forces with high sensitivity directly from the particle shape. We illustrate the potential of this approach by revealing subcellular force patterns throughout phagocytic engulfment and force dynamics in the cytotoxic T-cell immunological synapse. This strategy can readily be adapted for studying cellular forces in a wide range of applications.

Actin clearance promotes polarized dynein accumulation at the immunological synapse

Immunological synapse (IS) formation between a T cell and an antigen-presenting cell is accompanied by the reorientation of the T cell centrosome toward the interface. This polarization response is thought to enhance the specificity of T cell effector function by enabling the directional secretion of cytokines and cytotoxic factors toward the antigen-presenting cell. Centrosome reorientation is controlled by polarized signaling through diacylglycerol (DAG) and protein kinase C (PKC). This drives the recruitment of the motor protein dynein to the IS, where it pulls on microtubules to reorient the centrosome. Here, we used T cell receptor photoactivation and imaging methodology to investigate the mechanisms controlling dynein accumulation at the synapse. Our results revealed a remarkable spatiotemporal correlation between dynein recruitment to the synaptic membrane and the depletion of cortical filamentous actin (F-actin) from the same region, suggesting that the two events were causally related. Consistent with this hypothesis, we found that pharmacological disruption of F-actin dynamics in T cells impaired both dynein accumulation and centrosome reorientation. DAG and PKC signaling were necessary for synaptic F-actin clearance and dynein accumulation, while calcium signaling and microtubules were dispensable for both responses. Taken together, these data provide mechanistic insight into the polarization of cytoskeletal regulators and highlight the close coordination between microtubule and F-actin architecture at the IS.

Abstract 209: Metastatic colonization in the perivascular niche: A mechanotransduction perspective

Metastases are responsible for majority cancer related deaths. Disseminated tumor cells (DTCs) gain a diverse array of metastatic traits in order to adapt to and survive within different microenvironments, making them very difficult to therapeutically target. Remarkably, regardless of the secondary organ, DTCs survive and thrive in perivascular niches. Using xenograft and syngeneic models of metastasis in mouse, we found that upon extravasation, neuronal cell adhesion molecule L1 (L1CAM) expressed by DTCs engages the vascular basal lamina in order to amplify integrin β1 and integrin linked kinase (ILK) signaling in DTCs. ILK promotes actin polymerization for cell spreading and subsequent activation of mechanotransduction effector transcription factors YAP and MRTF. We found that activation of YAP and MRTF coincides with DTCs ability to dislodge resident pericytes from the perivascular niche and is an essential step in the metastatic colonization regardless of dormancy state. Therefore, these results provided us a unifying molecular mechanism that governs cancer progression from metastatic seeding to overt colonization. Ongoing studies will determine how mechanotransduction and pericyte-DTC interactions can be manipulated to prevent metastatic colonization by potentiating the immune response and exposing their biophysical vulnerabilities.

Citation Format: Ekrem Emrah Er, Maria Tello-Lafoz, Jing Hu, Morgan Huse, Joan Massagué. Metastatic colonization in the perivascular niche: A mechanotransduction perspective [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 209.

Cytomegalovirus Infection Drives Avidity Selection of Natural Killer Cells

The process of affinity maturation, whereby T and B cells bearing antigen receptors with optimal affinity to the relevant antigen undergo preferential expansion, is a key feature of adaptive immunity. Natural killer (NK) cells are innate lymphocytes capable of "adaptive" responses after cytomegalovirus (CMV) infection. However, whether NK cells are similarly selected on the basis of their avidity for cognate ligand is unknown. Here, we showed that NK cells with the highest avidity for the mouse CMV glycoprotein m157 were preferentially selected to expand and comprise the memory NK cell pool, whereas low-avidity NK cells possessed greater capacity for interferon-γ (IFN-γ) production. Moreover, we provide evidence for avidity selection occurring in human NK cells during human CMV infection. These results delineate how heterogeneity in NK cell avidity diversifies NK cell effector function during antiviral immunity, and how avidity selection might serve to produce the most potent memory NK cells.

Cytomegalovirus infection drives avidity selection of natural killer cells

Adaptive lymphocyte clones with the greatest receptor affinity tend to dominate primary and secondary immune responses. Strength of receptor signal and receptor abundance, along with competition for antigen and cytokines, drive preferential outgrowth of certain clones, shaping the effector and memory pool in selection processes known as affinity and avidity maturation. Natural killer (NK) cells are innate lymphocytes capable of “adaptive” responses following infectious challenges. However, whether NK cells undergo a similar selection process on the basis of their avidity for cognate ligand is not known. Here, we show that NK cells with a broad range of avidities for the mouse cytomegalovirus (MCMV) glycoprotein m157 are initially recruited after virus infection, but those with highest avidity are selected to undergo the greatest clonal expansion to comprise the memory NK cell population. Furthermore, pre-established levels of Ly49H receptor expression within the Ly49H+ NK cell pool dictated the functional contribution of a given NK cell during MCMV infection, with lower avidity NK cells possessing greater capacity for IFN-γ production, and higher avidity NK cells possessing greater capacity for cytotoxicity and adaptive responses. Moreover, we provide evidence for avidity selection also occurring in human NK cells during human CMV (HCMV) infection. These results provide a mechanistic understanding of how heterogeneity in NK cell avidity underlies the diversification of NK cell effector function during a primary antiviral immune response, and how the process of avidity selection may serve to produce the most potent memory NK cell pool.

Abstract B190: WASP-dependent actin protrusions mechanically potentiate killing by cytotoxic T-cells

Cytotoxic T Lymphocytes (CTLs) play a central role in immune responses against intracellular pathogens and cancer. Cytotoxic responses are induced by the formation of the immunologic synapse after the recognition of the peptide-major histocompatibility complex by the T-cell receptor. Synapse formation is associated dramatic reorganization of both microtubules and filamentous actin (F-actin) within the T-cell. This promotes the directional secretion of toxic perforin and granzymes into the intercellular space, enhancing both the potency and the specificity of targeT-cell killing. We have previously demonstrated that force exertion by CTLs at the immunologic synapse was strongly correlated with the cytotoxic potential. Indeed, force exertion enhances cytotoxicity by increasing membrane tension on the target cell, which in turn promotes the pore-forming activity of secreted perforin. This correlation between applied force and biochemical responses, which we referr to as mechanopotentiation, raised the prospect that CTLs might use three-dimensional structures at the immunologic synapse to coordinate force exertion and lytic granules secretion. In the present study, we investigated the mechanisms underlying mechanopotentiation in CTLs using a combination of three-dimensional micropatterned stimulatory substrates and high-resolution imaging. Our results revealed that CTLs couple lytic granule release with the formation of highly dynamic F-actin rich protrusions. These protrusions, which are generated by the Wiskott-Aldrich Syndrome protein (WASP) and the Arp2/3 actin nucleation complex, are required for coordinating force exertion with cytolytic secretion allowing efficient killing. Our results provide insight into how cells organize mechanical output and emphasize the importance of studying complex, communicative interfaces in three dimensions.

Citation Format: Fella Tamzalit, Mitchell S. Wang, Weiyang Jin, Vitaly Boyko, John M. Heddleston, Charles T. Black, Lance C. Kam, Morgan Huse. WASP-dependent actin protrusions mechanically potentiate killing by cytotoxic T-cells [abstract]. In: Proceedings of the Fourth CRI-CIMT-EATI-AACR International Cancer Immunotherapy Conference: Translating Science into Survival; Sept 30-Oct 3, 2018; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2019;7(2 Suppl):Abstract nr B190.

Spatial and Temporal Control of T Cell Activation Using a Photoactivatable Agonist

T lymphocytes engage in rapid, polarized signaling, occurring within minutes following TCR activation. This induces formation of the immunological synapse, a stereotyped cell-cell junction that regulates T cell activation and directionally targets effector responses. To study these processes effectively, an imaging approach that is tailored to capturing fast, polarized responses is necessary. This protocol describes such a system, which is based on a photoactivatable peptide-major histocompatibility complex (pMHC) that is non-stimulatory until it is exposed to ultraviolet light. Targeted decaging of this reagent during videomicroscopy experiments enables precise spatiotemporal control of TCR activation and high-resolution monitoring of subsequent cellular responses by total internal reflection (TIRF) imaging. This approach is also compatible with genetic and pharmacological perturbation strategies. This allows for the assembly of well-defined molecular pathways that link TCR signaling to the formation of the polarized cytoskeletal structures that underlie the immunological synapse.

Mechanical forces in the immune system

Leukocytes can completely reorganize their cytoskeletal architecture within minutes. This structural plasticity, which facilitates their migration and communicative function, also enables them to exert a substantial amount of mechanical force against the extracellular matrix and the surfaces of interacting cells. In recent years, it has become increasingly clear that these forces have crucial roles in immune cell activation and subsequent effector responses. Here, I review our current understanding of how mechanical force regulates cell-surface receptor activation, cell migration, intracellular signalling and intercellular communication, highlighting the biological ramifications of these effects in various immune cell types.

Sorting nexin 27 interactome in T-lymphocytes identifies zona occludens-2 dynamic redistribution at the immune synapse

T Lymphocyte recognition of antigens leads to the formation of a highly organized structure termed immune synapse (IS) by analogy with the neuronals synapse. Sorting nexin 27 (SNX27) controls the endosomal traffic of PSD95, Dlg1, ZO-1 (PDZ) domain-interacting proteins, and its alteration is associated with impaired synaptic function and neurological diseases. In T-lymphocytes, SNX27-positive vesicles polarize to the IS, the identity of SNX27 interactors in these conditions nonetheless remains unknown. Here we used proteomics to analyze the SNX27 interactome purified from IS-forming T cells, and confirmed the conserved nature of the SNX27/WASH/retromer association in hematopoietic cells. Furthermore, our comparative interactome analysis of SNX27 wild-type and a mutant-deficient for PDZ cargo recognition identified the epithelial cell-cell junction protein zona occludens-2 (ZO-2) as an IS component. Biochemistry and microscopy approaches in T cells confirmed SNX27/ZO-2 PDZ-dependent interaction, and demonstrated its role controlling the dynamic localization of ZO-2 at the IS. This study broadens our knowledge of SNX27 function in T lymphocytes, and suggests that pathways that delimit polarized structures in nervous and epithelial systems also participate in IS regulation.

A Tunable Diffusion-Consumption Mechanism of Cytokine Propagation Enables Plasticity in Cell-to-Cell Communication in the Immune System

Immune cells communicate by exchanging cytokines to achieve a context-appropriate response, but the distances over which such communication happens are not known. Here, we used theoretical considerations and experimental models of immune responses in vitro and in vivo to quantify the spatial extent of cytokine communications in dense tissues. We established that competition between cytokine diffusion and consumption generated spatial niches of high cytokine concentrations with sharp boundaries. The size of these self-assembled niches scaled with the density of cytokine-consuming cells, a parameter that gets tuned during immune responses. In vivo, we measured interactions on length scales of 80-120 μm, which resulted in a high degree of cell-to-cell variance in cytokine exposure. Such heterogeneous distributions of cytokines were a source of non-genetic cell-to-cell variability that is often overlooked in single-cell studies. Our findings thus provide a basis for understanding variability in the patterning of immune responses by diffusible factors.

Probing Synaptic Biomechanics Using Micropillar Arrays

Recent insights into the importance of mechanosensing and force transmission at the immune synapse have spurred increased interest in the mechanical properties of leukocyte cell-cell interactions. In this chapter, we describe an imaging-based strategy for measuring cellular forces that utilizes optically transparent arrays of flexible micropillars. This approach has several distinct advantages over standard traction force microscopy, and we anticipate that it will prove very useful for investigators who wish not only to quantify ligand-induced forces with high spatiotemporal resolution but also to place those forces within the context of a broader cell biological response.

Mechanical Communication at the Immunological Synapse

T and B lymphocytes communicate by forming immunological synapses with antigen-presenting target cells. These highly dynamic contacts are characterized by continuous cytoskeletal remodeling events, which not only structure the interface but also exert a considerable amount of mechanical force. In recent years, it has become increasingly clear that synaptic forces influence information transfer both into and out of the lymphocyte. Here, we review our current understanding of synapse mechanics, focusing on its role as an avenue for intercellular communication.

Abstract A070: The role of the centrosome in cytotoxic T cell function

Cytotoxic T lymphocytes (CTLs) are essential for productive immune responses and recent studies have demonstrated their strong clinical potential as immunotherapeutic agents against established tumors. Cytotoxic responses in CTLs are induced by the T cell receptor, which recognizes cognate peptide-major histocompatibility complex molecules on the surfaces of infected or transformed cells, which induces the formation the immunological synapse. Synapse formation is accompanied by a series of actin and microtubule remodeling events, the most striking of which is the dramatic reorientation of the centrosome to a position just beneath the center of the contact site. It has been proposed that lytic granules containing perforin and granzyme cluster around the centrosome in activated CTLs. This polarization toward the synapse promotes the selective fusion of these granules with the synaptic membrane, leading to directional release of their contents toward the target cell.

In order to study more precisely how the centrosome influences secretory responses and how these contributions affect CTL activity, we have developed an approach using OT-I mice bearing a conditional allele of both p53 (p53flox) as well as SAS4 (SAS4flox), a scaffolding protein that is absolutely required for the formation and maintenance of the centrosome. OT-I SAS4−/−p53−/− cells were generated by transducing SAS4flox/floxp53flox/flox cells with a retrovirus expressing Cre. Using gamma-tubulin and pericentrin staining, we were able to show that SAS4 deletion effectively eliminates interphase centrosomes in CTLs. OT-I SAS4−/− p53−/− cells exhibited a marked defect in target cell killing after 24h, implying a crucial role of the centrosome in cytotoxic responses. This killing defect was accompanied by a decrease in both granzyme B and perforin production. IFN-gamma production and secretion were not altered by the loss of the centrosome. Importantly, TCR induced signaling was normal in these cells.

In summary, we were able to show that centrosome deletion impairs target cell killing without altering their overall magnitude. Given the importance of centrosome polarization for lytic granules delivery, we can assume that the centrosome ablation selectively alters cytotoxic efficiency by disrupting the directional synaptic secretion.

Citation Format: Fella Tamzalit, Ariella Kepecs, Hisham Bazzi, Kathryn Anderson, Morgan Huse. The role of the centrosome in cytotoxic T cell function [abstract]. In: Proceedings of the Second CRI-CIMT-EATI-AACR International Cancer Immunotherapy Conference: Translating Science into Survival; 2016 Sept 25-28; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(11 Suppl):Abstract nr A070.

Successful engineering of a highly potent single-chain variable-fragment (scFv) bispecific antibody to target disialoganglioside (GD2) positive tumors

Engineering potent bispecific antibodies from single-chain variable fragments (scFv) remains difficult due to the inherent instability and insufficient binding of scFv's compared to their parental immunoglobulin format. Previously, we described a scFv-based bispecific antibody (scBA) against disialoganglioside (GD2) based on the anti-GD2 murine 5F11-scFv and the anti-CD3 huOKT3-scFv (5F11-scBA). In this study, we substituted the 5F11-scFv with the higher affinity (13-fold) hu3F8-scFv to form hu3F8-scBA. With this modification, hu3F8-scBA redirected T cells to kill GD2(+) cancer cell lines with up to 5,000-fold higher potency (femtomolar EC₅₀) compared with 5F11-scBA (picomolar EC₅₀) in cytotoxicity assays, even against target cells with low GD2 densities. Furthermore, hu3F8-scBA induced stronger T-cell activation than 5F11-scBA, as measured by Ca²⁺ flux and cytokine release. Additionally, in vivo, hu3F8-scBA suppressed tumor growth and prolonged mice survival much more effectively than 5F11-scBA, in both neuroblastoma and melanoma xenograft models. We conclude that the functional properties of scBA's can be increased substantially by relatively modest increases in antigen affinity.

Cancer Immunosurveillance by Tissue-resident Innate Lymphoid Cells and Innate-like T Cells

Malignancy can be suppressed by the immune system in a process termed immunosurveillance. However, to what extent immunosurveillance occurs in spontaneous cancers and the composition of participating cell types remain obscure. Here we show that cell transformation triggers a tissue-resident lymphocyte response in oncogene-induced murine cancer models. Non-circulating cytotoxic lymphocytes, derived from innate, TCRαβ and TCRγδ lineages, expand in early tumors. Characterized by high expression of NK1.1, CD49a and CD103, these cells share a gene expression signature distinct from those of conventional NK cells, T cells and invariant NKT cells. Generation of these lymphocytes is dependent on the cytokine IL-15, but not the transcription factor Nfil3 that is required for the differentiation of tumor-infiltrating NK cells, and IL-15, but not Nfil3, deficiency results in accelerated tumor growth. These findings reveal a tumor-elicited immunosurveillance mechanism that engages unconventional type 1-like innate lymphoid cells and type 1 innate-like T cells.

Cytotoxic T Cells Use Mechanical Force to Potentiate Target Cell Killing

The immunological synapse formed between a cytotoxic T lymphocyte (CTL) and an infected or transformed target cell is a physically active structure capable of exerting mechanical force. Here, we investigated whether synaptic forces promote the destruction of target cells. CTLs kill by secreting toxic proteases and the pore forming protein perforin into the synapse. Biophysical experiments revealed a striking correlation between the magnitude of force exertion across the synapse and the speed of perforin pore formation on the target cell, implying that force potentiates cytotoxicity by enhancing perforin activity. Consistent with this interpretation, we found that increasing target cell tension augmented pore formation by perforin and killing by CTLs. Our data also indicate that CTLs coordinate perforin release and force exertion in space and time. These results reveal an unappreciated physical dimension to lymphocyte function and demonstrate that cells use mechanical forces to control the activity of outgoing chemical signals.

Cancer Immunosurveillance by Tissue-Resident Innate Lymphoid Cells and Innate-like T Cells

Malignancy can be suppressed by the immune system in a process termed immunosurveillance. However, to what extent immunosurveillance occurs in spontaneous cancers and the composition of participating cell types remains obscure. Here, we show that cell transformation triggers a tissue-resident lymphocyte response in oncogene-induced murine cancer models. Non-circulating cytotoxic lymphocytes, derived from innate, T cell receptor (TCR)αβ, and TCRγδ lineages, expand in early tumors. Characterized by high expression of NK1.1, CD49a, and CD103, these cells share a gene-expression signature distinct from those of conventional NK cells, T cells, and invariant NKT cells. Generation of these lymphocytes is dependent on the cytokine IL-15, but not the transcription factor Nfil3 that is required for the differentiation of tumor-infiltrating NK cells, and IL-15 deficiency, but not Nfil3 deficiency, results in accelerated tumor growth. These findings reveal a tumor-elicited immunosurveillance mechanism that engages unconventional type-1-like innate lymphoid cells and type 1 innate-like T cells.

A Generalizable Platform for the Photoactivation of Cell Surface Receptors

Polarized signal transduction from cell surface receptors plays a central role in the development and homeostasis of multicellular organisms, and it also contributes to cellular dysfunction in many disease states. Understanding the molecular and cellular bases of polarized signaling requires experimental methods that provide precise spatiotemporal control of receptor activation. However, we currently lack strategies for inducing both sustained and spatially constrained signal transduction. In the present study, we combined synthetic and cell biological tools to develop a generalizable photoactivation approach for the stimulation of cell surface receptors. Our system, which is based upon the local decaging of a "universal" peptide ligand, is particularly well suited for the live imaging of single cells. We anticipate that it will greatly facilitate future mechanistic analyses of polarized signal transduction in a variety of cell types.

Retargeting T cells to GD2 pentasaccharide on human tumors using Bispecific humanized antibody

Anti-disialoganglioside GD2 IgG antibodies have shown clinical efficacy in solid tumors that lack human leukocyte antigens (e.g., neuroblastoma) by relying on Fc-dependent cytotoxicity. However, there are pain side effects secondary to complement activation. T-cell retargeting bispecific antibodies (BsAb) also have clinical potential, but it is thus far only effective against liquid tumors. In this study, a fully humanized hu3F8-BsAb was developed, in which the anti-CD3 huOKT3 single-chain Fv fragment (ScFv) was linked to the carboxyl end of the anti-GD2 hu3F8 IgG1 light chain, and was aglycosylated at N297 of Fc to prevent complement activation and cytokine storm. In vitro, hu3F8-BsAb activated T cells through classic immunologic synapses, inducing GD2-specific tumor cytotoxicity at femtomolar EC50 with >10⁵-fold selectivity over normal tissues, releasing Th1 cytokines (TNFα, IFNγ, and IL2) when GD2⁺ tumors were present. In separate murine neuroblastoma and melanoma xenograft models, intravenous hu3F8-BsAb activated T cells in situ and recruited intravenous T cells for tumor ablation, significantly prolonging survival from local recurrence or from metastatic disease. Hu3F8-BsAb, but not control BsAb, drove T cells and monocytes to infiltrate tumor stroma. These monocytes were necessary for sustained T-cell proliferation and/or survival and contributed significantly to the antitumor effect. The in vitro and in vivo antitumor properties of hu3F8-BsAb and its safety profile support its further clinical development as a cancer therapeutic, and provide the rationale for exploring aglycosylated IgG-scFv as a structural platform for retargeting human T cells.

Diacylglycerol kinase α establishes T cell polarity by shaping diacylglycerol accumulation at the immunological synapse

Polarization of the T cell microtubule-organizing center (MTOC) to the immunological synapse between the T cell and an antigen-presenting cell (APC) maintains the specificity of T cell effector responses by enabling directional secretion toward the APC. The reorientation of the MTOC is guided by a sharp gradient of the second messenger diacylglycerol (DAG), which is centered at the immunological synapse. We used a single-cell photoactivation approach to demonstrate that diacylglycerol kinase α (DGK-α), which catalyzes the conversion of DAG to phosphatidic acid, determined T cell polarity by limiting the diffusion of DAG. DGK-α-deficient T cells exhibited enlarged accumulations of DAG at the immunological synapse, as well as impaired reorientation of the MTOC. In contrast, T cells lacking the related isoform DGK-ζ did not display polarization defects. We also found that DGK-α localized preferentially to the periphery of the immunological synapse, suggesting that it constrained the area over which DAG accumulated. Phosphoinositide 3-kinase activity was required for the peripheral localization pattern of DGK-α, which suggests a link between DAG and phosphatidylinositol signaling during T cell activation. These results reveal a previously unappreciated function of DGK-α and provide insight into the mechanisms that determine lymphocyte polarity.

From lipid second messengers to molecular motors: microtubule-organizing center reorientation in T cells

In T lymphocytes, polarization of the microtubule-organizing center (MTOC) to the immunological synapse enables the directional secretion of cytokines, cytolytic factors, and other soluble molecules toward the antigen-presenting cell. This is likely to be crucial for maintaining the specificity of T-cell effector responses. Here, we review recent advances in our understanding of MTOC reorientation in T cells, focusing first on the importance of diacylglycerol and protein kinase C isozymes and then on the molecular motor proteins that function downstream to drive MTOC movement.

The variable hinge region of novel PKCs determines localization to distinct regions of the immunological synapse

The immunological synapse (IS) formed between a T cell and its cognate antigen-presenting cell (APC) enables the directional secretion of cytolytic and inflammatory molecules. Synaptic architecture is established in part by a two-step cascade of novel protein kinase C (nPKC) isozymes. PKCε and PKCη arrive at the IS first, and occupy the entire synaptic membrane. Then, PKCθ accumulates in a smaller zone at the center of the contact. We investigated the molecular basis for this differential recruitment behavior using chimeric nPKC constructs and total internal reflection fluorescence microscopy. Our studies revealed that the V3 linker just N-terminal to the kinase domain plays a crucial role in specifying nPKC localization. Substitution of this linker switched the scope and the kinetics of PKCθ accumulation to that of PKCε and PKCη, and vice versa. Although the V3 was necessary for synaptic compartmentalization, it was not sufficient, as the tandem C1 domains were also required to mediate membrane association. Together, these results suggest a model whereby the V3 linker controls nPKC sub-compartmentalization after initial C1 domain-mediated accumulation at the IS.

Inhibitory receptor signaling destabilizes immunological synapse formation in primary NK cells

Upon engagement of their cognate class I major histocompatibility complex ligands, receptors containing immunotyrosine-based inhibitory motifs (ITIMs) transduce signals that block cytolytic and inflammatory responses. In this manner, ITIM-coupled receptors play a crucial role in maintaining natural killer (NK) cell tolerance toward normal, healthy tissue. A number of studies, mostly using immortalized NK cell lines, have demonstrated that ITIM signaling functions by disrupting the cytolytic immunological synapse formed between an NK cell and its target. However, more recent imaging experiments using primary NK cells have suggested that inhibitory receptor engagement does not antagonize contact formation, casting doubt on the hypothesis that ITIM signals destabilize the synapse. To resolve this issue, we analyzed primary NK cell activation and contact formation on supported lipid bilayers containing controlled combinations of activating and inhibitory ligands. Under these conditions, we observed that ITIM signaling clearly inhibited adhesion, cell arrest, and calcium influx, three hallmarks of synapse formation. These results are consistent with previous reports showing that inhibitory receptors deliver a “reverse stop” signal, and confirm that ITIM signaling functions at least in part by destabilizing cytolytic synapse formation.

Annular PIP3 accumulation controls actin architecture and modulates cytotoxicity at the immunological synapse

In T cells, PI3K activation in the periphery of the immune synapse leads to PIP₃ accumulation that promotes actin polymerization in a pathway important for cytotoxic function.

The immunological synapse formed by a T lymphocyte on the surface of a target cell contains a peripheral ring of filamentous actin (F-actin) that promotes adhesion and facilitates the directional secretion of cytokines and cytolytic factors. We show that growth and maintenance of this F-actin ring is dictated by the annular accumulation of phosphatidylinositol trisphosphate (PIP₃) in the synaptic membrane. PIP₃ functions in this context by recruiting the exchange factor Dock2 to the periphery of the synapse, where it drives actin polymerization through the Rho-family GTPase Rac. We also show that synaptic PIP₃ is generated by class IA phosphoinositide 3-kinases that associate with T cell receptor microclusters and are activated by the GTPase Ras. Perturbations that inhibit or promote PIP₃-dependent F-actin remodeling dramatically affect T cell cytotoxicity, demonstrating the functional importance of this pathway. These results reveal how T cells use lipid-based signaling to control synaptic architecture and modulate effector responses.

Building tolerance by dismantling synapses: inhibitory receptor signaling in natural killer cells

Cell surface receptors bearing immunotyrosine-based inhibitory motifs (ITIMs) maintain natural killer (NK) cell tolerance to normal host tissues. These receptors are difficult to analyze mechanistically because they block activating responses in a rapid and comprehensive manner. The advent of high-resolution single cell imaging techniques has enabled investigators to explore the cell biological basis of the inhibitory response. Recent studies using these approaches indicate that ITIM-containing receptors function at least in part by structurally undermining the immunological synapse between the NK cell and its target. In this review, we discuss these new advances and how they might relate to what is known about the biochemistry of inhibitory signaling in NK cells and other cell types.

IL-2-dependent tuning of NK cell sensitivity for target cells is controlled by regulatory T cells

The emergence of the adaptive immune system took a toll in the form of pathologies mediated by self-reactive cells. Regulatory T cells (T reg cells) exert a critical brake on responses of T and B lymphocytes to self- and foreign antigens. Here, we asked whether T reg cells are required to restrain NK cells, the third lymphocyte lineage, whose features combine innate and adaptive immune cell properties. Although depletion of T reg cells led to systemic fatal autoimmunity, NK cell tolerance and reactivity to strong activating self- and non-self-ligands remained largely intact. In contrast, missing-self responses were increased in the absence of T reg cells as the result of heightened IL-2 availability. We found that IL-2 rapidly boosted the capacity of NK cells to productively engage target cells and enabled NK cell responses to weak stimulation. Our results suggest that IL-2-dependent adaptive-innate lymphocyte cross talk tunes NK cell reactivity and that T reg cells restrain NK cell cytotoxicity by limiting the availability of IL-2.

NK cell tolerance of self-specific activating receptor KIR2DS1 in individuals with cognate HLA-C2 ligand

NK cells are regulated by inhibiting and activating cell surface receptors. Most inhibitory receptors recognize MHC class I Ags and protect healthy cells from NK cell-mediated autoaggression. However, certain activating receptors, including the human activating killer cell Ig-like receptor (KIR) 2DS1, also recognize MHC class I. This fact raises the question of how NK cells expressing such activating receptors are tolerized to host tissues. We investigated whether the presence of HLA-C2, the cognate ligand for 2DS1, induces tolerance in 2DS1-expressing NK cells. Anti-HLA-C2 activity could be detected in vitro in some 2DS1 positive NK clones irrespective of the presence or absence of HLA-C2 ligand in the donor. The frequency of anti-HLA-C2 reactivity was high in donors homozygous for HLA-C1. Surprisingly, no significant difference was seen in the frequency of anti-HLA-C2 cytotoxicity in donors heterozygous for HLA-C2 and donors without HLA-C2 ligand. However, donors homozygous for HLA-C2, compared with all other donors, had significantly reduced frequency of anti-HLA-C2 reactive clones. The 2DS1 positive clones that express inhibitory KIR for self-HLA class I were commonly noncytotoxic, and anti-HLA-C2 cytotoxicity was nearly exclusively restricted to 2DS1 single positive clones lacking inhibitory KIR. 2DS1 single positive NK clones with anti-HLA-C2 reactivity were also present posttransplantation in HLA-C2 positive recipients of hematopoietic stem cell transplants from 2DS1 positive donors. These results demonstrate that many NK cells with anti-HLA-C2 reactivity are present in HLA-C1 homozygous and heterozygous donors with 2DS1. In contrast, 2DS1 positive clones from HLA-C2 homozygous donors are frequently tolerant to HLA-C2.