The Effects of Exoskeleton Assistance at the Ankle on Sensory Integration During Standing Balance

Exoskeleton devices can reduce metabolic cost, increase walking speed, and augment load-carrying capacity. However, little is known about the effects of powered assistance on the sensory information required to achieve these tasks. To learn how to use an assistive device, humans must integrate novel sensory information into their internal model. This process may be disrupted by challenges to the sensory systems used for posture. We investigated the exoskeleton-induced changes to balance performance and sensory integration during quiet standing. We asked 11 unimpaired adults to perform a virtual reality-based test of sensory integration in balance (VRSIB) on two days while wearing the exoskeleton either unpowered, using proportional myoelectric control, or with regular shoes. We measured postural biomechanics, muscle activity, equilibrium scores, postural control strategy, and sensory ratios. Results showed improvement in balance performance when wearing the exoskeleton on firm ground. The opposite occurred when standing on an unstable platform with eyes closed or when the visual information was non-veridical. The balance performance was equivalent when the exoskeleton was powered versus unpowered in all conditions except when both the support surface and the visual information were altered. We argue that in stable ground conditions, the passive stiffness of the device dominates the postural task. In contrast, when the ground becomes unstable the passive stiffness negatively affects balance performance. Furthermore, when the visual input to the user is non-veridical, exoskeleton assistance can magnify erroneous muscle inputs and negatively impact the user's postural control.

Ankle Exoskeleton Assistance can Affect Step Regulation during Self-Paced Walking

Exoskeleton assistance can reduce metabolic cost and increase preferred walking speed in unimpaired and impaired groups, but individual outcomes are highly variable. Assistance may influence step regulation, leading to individual modulation of gait variability, energetic cost, and balance control. In this study, we aimed to understand the effects of a powered ankle exoskeleton on step regulation and its relationship to self-selected walking speed, cost of transport, and gait variability. We asked 12 unimpaired young adults to walk at their comfortable walking speed on a self-paced treadmill in their regular shoes, with the exoskeleton tracking zero torque, and in two trials using proportional myoelectric control. We measured preferred walking speed, cost of transport (COT), mean and standard deviation of gait parameters, (step length, step time, and step width) and computed long-term correlations via detrended fluctuation analysis (DFA). In all exoskeleton trials, subjects walked significantly slower than in their shoes. However, the COT was equivalent between shoes and both proportional myoelectric control trials. Subjects also increased medio-lateral balance control by increasing their mean step width and reducing both short-term variability and long-term auto-correlation for this parameter. In the second powered trial subjects returned to the levels of control over step width exhibited during regular shoe walking. During the unpowered condition subjects showed a significant association between step width regulation, walking speed, and COT. However, these parameters were not significantly associated when the assistance was turned on. Together, these results demonstrate that the response to assistance is closely related to the stepping strategy, especially in the initial stages of learning.

Non-radiometric Cell-free Assay to Measure the Effect of Molecular Chaperones on AMP-activated Kinase Activity

AMP-activated kinase (AMPK) is a trimeric protein holoenzyme with kinase activity. AMPK plays an important role in cellular metabolism and is thought to function as a fuel sensor within the cell, exerting kinase activity to activate energy-conserving pathways and simultaneously inhibit energy-consuming pathways. Traditional in vitro methods to measure AMPK activity to test potential agonists or antagonists utilize radiolabeled ATP with a peptide substrate. Although radiolabeling provides a high level of sensitivity, this approach is not ideal for medium to high-throughput screening, dose-response curves, or kinetic analyses. Our protocol utilizes Invitrogen's Z'-LYTE™ Kinase Assay Kit (Ser/Thr 23 Peptide) to measure changes in the enzymatic activity of AMPKɑ2β1γ1 in the presence of a molecular chaperone. The Z'-LYTE™ platform is based on Fluorescence Resonance Energy Transfer (FRET). The AMPK peptide substrate (S/T 23 peptide: MRPRKRQGSVRRRV) is a self-contained FRET system, using coumarin as the donor and fluorescein as the acceptor. When the peptide is phosphorylated, it is sensitive to cleavage by a site-specific protease. The cleavage of the phospho-peptide eliminates the FRET pair, and the ratiometric analysis of FRET is used as an indirect measure of AMPK kinase activity. This method does not require the use of radiolabeling or antibodies and is used in a multi-well format, with high reproducibility and throughput capabilities.

Type I and Type II Interferon Coordinately Regulate Suppressive Dendritic Cell Fate and Function during Viral Persistence

Persistent viral infections are simultaneously associated with chronic inflammation and highly potent immunosuppressive programs mediated by IL-10 and PDL1 that attenuate antiviral T cell responses. Inhibiting these suppressive signals enhances T cell function to control persistent infection; yet, the underlying signals and mechanisms that program immunosuppressive cell fates and functions are not well understood. Herein, we use lymphocytic choriomeningitis virus infection (LCMV) to demonstrate that the induction and functional programming of immunosuppressive dendritic cells (DCs) during viral persistence are separable mechanisms programmed by factors primarily considered pro-inflammatory. IFNγ first induces the de novo development of naive monocytes into DCs with immunosuppressive potential. Type I interferon (IFN-I) then directly targets these newly generated DCs to program their potent T cell immunosuppressive functions while simultaneously inhibiting conventional DCs with T cell stimulating capacity. These mechanisms of monocyte conversion are constant throughout persistent infection, establishing a system to continuously interpret and shape the immunologic environment. MyD88 signaling was required for the differentiation of suppressive DCs, whereas inhibition of stimulatory DCs was dependent on MAVS signaling, demonstrating a bifurcation in the pathogen recognition pathways that promote distinct elements of IFN-I mediated immunosuppression. Further, a similar suppressive DC origin and differentiation was also observed in Mycobacterium tuberculosis infection, HIV infection and cancer. Ultimately, targeting the underlying mechanisms that induce immunosuppression could simultaneously prevent multiple suppressive signals to further restore T cell function and control persistent infections.

Persistent virus infections induce host derived immunosuppressive factors that attenuate the immune response and prevent control of infection. Although the mechanisms of T cell exhaustion are being defined, we know surprisingly little about the underlying mechanisms that induce the immunosuppressive state and the origin and functional programming of the cells that deliver these signals to the T cells. We recently demonstrated that type I interferon (IFN-I) signaling was responsible for many of the immune dysfunctions associated with persistent virus infection and in particular the induced expression of the suppressive factors IL-10 and PDL1 by dendritic cells (DCs). Yet, mechanistically how IFN-I signaling specifically generates and programs cells to become immunosuppressive is still unknown. Herein, we define the underlying mechanisms of IFN-I mediated immunosuppression and establish that the induction of factors and the generation of the DCs that express them are separable events integrally reliant on additional inflammatory factors. Further, we demonstrate a similar derivation of the suppressive DCs that emerge in other diseases associated with prolonged inflammation and immunosuppression, specifically in HIV infection, Mycobacterium tuberculosis, and cancer, indicating a conserved origin of immunosuppression and suggesting that targeting the pathways that underlie expression of immunosuppressive cells and factors could be beneficial to treat multiple chronic diseases.

Controlled infection with a therapeutic virus defines the activation kinetics of human natural killer cells in vivo

Human natural killer (NK) cells play an important role in anti-viral immunity. However, studying their activation kinetics during infection is highly problematic. A clinical trial of a therapeutic virus provided an opportunity to study human NK cell activation in vivo in a controlled manner. Ten colorectal cancer patients with liver metastases received between one and five doses of oncolytic reovirus prior to surgical resection of their tumour. NK cell surface expression of the interferon-inducible molecules CD69 and tetherin peaked 24-48 h post-infection, coincident with a peak of interferon-induced gene expression. The interferon response and NK cell activation were transient, declining by 96 h post-infection. Furthermore, neither NK cell activation nor the interferon response were sustained in patients undergoing multiple rounds of virus treatment. These results show that reovirus modulates human NK cell activity in vivo and suggest that this may contribute to any therapeutic effect of this oncolytic virus. Detection of a single, transient peak of activation, despite multiple treatment rounds, has implications for the design of reovirus-based therapy. Furthermore, our results suggest the existence of a post-infection refractory period when the interferon response and NK cell activation are blunted. This refractory period has been observed previously in animal models and may underlie the enhanced susceptibility to secondary infections that is seen following viral infection.

Decoding the complexity of type I interferon to treat persistent viral infections

Type I interferons (IFN-I) are a broad family of cytokines that are central to the innate immune response. These proteins have long been appreciated for the critical roles they play in restraining viral infections and shaping antiviral immune responses. However, in recent years there has been increased awareness of the immunosuppressive actions of these proteins as well. Although there are many current therapeutic applications to manipulate IFN-I pathways, we have limited understanding of the mechanisms by which these therapies are actually functioning. In this review, we highlight the diversity and temporal impact of IFN-I signaling, discuss the current therapeutic uses of IFN-I, and explore the strategy of blocking IFN-I to alleviate immune dysfunction in persistent virus infections.

Blockade of chronic type I interferon signaling to control persistent LCMV infection

Type I interferons (IFN-I) are critical for antiviral immunity; however, chronic IFN-I signaling is associated with hyperimmune activation and disease progression in persistent infections. We demonstrated in mice that blockade of IFN-I signaling diminished chronic immune activation and immune suppression, restored lymphoid tissue architecture, and increased immune parameters associated with control of virus replication, ultimately facilitating clearance of the persistent infection. The accelerated control of persistent infection induced by blocking IFN-I signaling required CD4 T cells and was associated with enhanced IFN-γ production. Thus, we demonstrated that interfering with chronic IFN-I signaling during persistent infection redirects the immune environment to enable control of infection.

Inflammation makes T cells sensitive

Inflammatory cytokines shape CD8(+) T cell responses. In this issue of Immunity, Richer et al. (2013) and Raue et al. (2013) demonstrate that inflammatory cytokines dynamically fine-tune antigen sensitivity of CD8(+) T cells to potently detect and better eliminate infected cells.

Emergence of distinct multiarmed immunoregulatory antigen-presenting cells during persistent viral infection

During persistent viral infection, adaptive immune responses are suppressed by immunoregulatory factors, contributing to viral persistence. Although this suppression is mediated by inhibitory factors, the mechanisms by which virus-specific T cells encounter and integrate immunoregulatory signals during persistent infection are unclear. We show that a distinct population of IL-10-expressing immunoregulatory antigen-presenting cells (APCs) is amplified during chronic versus acute lymphocytic choriomeningitis virus (LCMV) infection and suppresses T cell responses. Although acute LCMV infection induces the expansion of immunoregulatory APCs, they subsequently decline. However, during persistent LCMV infection, immunoregulatory APCs are amplified and parallel the viral replication kinetics. Further characterization demonstrates that immunoregulatory APCs are molecularly and metabolically distinct, and exhibit increased expression of T cell-interacting molecules and negative regulatory factors that suppress T cell responses. Thus, immunoregulatory APCs are amplified during viral persistence and deliver inhibitory signals that suppress antiviral T cell immunity and likely contribute to persistent infection.

Translating insights from persistent LCMV infection into anti-HIV immunity

Human immunodeficiency virus (HIV) is a major global health concern with more than 30 million individuals currently infected worldwide. To date, attempts to stimulate protective immunity to viral components of HIV have been unsuccessful in preventing or clearing infection. Lymphocytic choriomeningitis virus (LCMV) is an established murine model of persistent viral infection that has been instrumental in illuminating several critical aspects of antiviral immunity. Although virologically the course of LCMV infection differs significantly from HIV, the immune responses and regulatory mechanisms elicited by these two viruses are markedly similar. In this review we discuss important recent findings in the LCMV model, highlighting the role of host-derived proteins in shaping immune responses to persistent infections, and explore the therapeutic potential of manipulating these pathways to enhance HIV vaccination strategies.

Viral persistence redirects CD4 T cell differentiation toward T follicular helper cells

Persistent virus infection drives follicular T helper cell differentiation.

CD4 T cell responses are crucial to prevent and control viral infection; however, virus-specific CD4 T cell activity is considered to be rapidly lost during many persistent viral infections. This is largely caused by the fact that during viral persistence CD4 T cells do not produce the classical Th1 cytokines associated with control of acute viral infections. Considering that CD4 T cell help is critical for both CD8 T cell and B cell functions, it is unclear how CD4 T cells can lose responsiveness but continue to sustain long-term control of persistent viral replication. We now demonstrate that CD4 T cell function is not extinguished as a result of viral persistence. Instead, viral persistence and prolonged T cell receptor stimulation progressively redirects CD4 T cell development away from the Th1 response induced during an acute infection toward T follicular helper cells. Importantly, this sustained CD4 T cell functionality is critical to maintain immunity and ultimately aid in the control of persistent viral infection.

Type I interferon supports primary CD8+ T-cell responses to peptide-pulsed dendritic cells in the absence of CD4+ T-cell help

CD8(+) T-cell responses to non-pathogen, cell-associated antigens such as minor alloantigens or peptide-pulsed dendritic cells (DC) are usually strongly dependent on help from CD4(+) T cells. However, some studies have described help-independent primary CD8(+) T-cell responses to cell-associated antigens, using immunization strategies likely to trigger natural killer (NK) cell activation and inflammatory cytokine production. We asked whether NK cell activation by MHC I-deficient cells, or administration of inflammatory cytokines, could support CD4(+) T-cell help-independent primary responses to peptide-pulsed DC. Injection of MHC I-deficient cells cross-primed CD8(+) T-cell responses to the protein antigen ovalbumin (OVA) and the male antigen HY, but did not stimulate CD8(+) T-cell responses in CD4-depleted mice; hence NK cell stimulation by MHC I-deficient cells did not replace CD4(+) T-cell help in our experiments. Dendritic cells cultured with tumour necrosis factor-α (TNF-α) or type I interferon-α (IFN-α) also failed to prime CD8(+) T-cell responses in the absence of help. Injection of TNF-α increased lymph node cellularity, but did not generate help-independent CD8(+) T-cell responses. In contrast, CD4-depleted mice injected with IFN-α made substantial primary CD8(+) T-cell responses to peptide-pulsed DC. Mice deficient for the type I IFN receptor (IFNR1) made CD8(+) T-cell responses to IFNR1-deficient, peptide-pulsed DC; hence IFN-α does not appear to be a downstream mediator of CD4(+) T-cell help. We suggest that primary CD8(+) T-cell responses will become help-independent whenever endogenous IFN-α secretion is stimulated by tissue damage, infection, or autoimmune disease.

The evolution of robotic general surgery

Surgical robotics in general surgery has a relatively short but very interesting evolution. Just as minimally invasive and laparoscopic techniques have radically changed general surgery and fractionated it into subspecialization, robotic technology is likely to repeat the process of fractionation even further. Though it appears that robotics is growing more quickly in other specialties, the changes digital platforms are causing in the general surgical arena are likely to permanently alter general surgery. This review examines the evolution of robotics in minimally invasive general surgery looking forward to a time where robotics platforms will be fundamental to elective general surgery. Learning curves and adoption techniques are explored. Foregut, hepatobiliary, endocrine, colorectal, and bariatric surgery will be examined as growth areas for robotics, as well as revealing the current uses of this technology.

Unravelling the mechanisms of help for CD8+ T cell responses

CD8+ T cells are critically important for immune defense against many viral and bacterial pathogens, and are also key components of cancer immunotherapy. Help from CD4+ T cells is usually essential for optimal CD8+ T cell responses, driving the primary response, the survival of memory cells, and the generation of protective and therapeutic immunity. Understanding the mechanisms of help is thus essential for vaccine design, and for restoring protective immunity in immunosuppressed individuals. Our laboratory has developed an immunization protocol using peptide-pulsed dendritic cells to stimulate help-dependent primary, memory, and secondary CD8+ T cell responses. We have used gene-targeted and T cell receptor transgenic mice to identify two distinct pathways that generate help-dependent and help-independent CD8+ T cell responses, respectively, and are now starting to define the molecular mechanisms underlying these two pathways.

Cutting edge: CD4+ T cell-derived IL-2 is essential for help-dependent primary CD8+ T cell responses

CD4(+) T cell help is essential for primary CD8(+) T cell responses to noninflammatory Ags. IL-2 is one of the principal cytokines made by naive CD4(+) T cells, and we show in this study that it is an essential component of help. Adoptively transferred naive CD4(+) TCR-transgenic OT-II cells supported endogenous primary CD8(+) T cell responses, but IL-2-deficient OT-II cells were unable to provide help, although they responded to Ag in vivo and up-regulated CD40 ligand in vitro. Wild -type OT-II cells helped endogenous CD8(+) T cell responses in IL-2-deficient mice, but not in IL-2Ralpha-deficient mice. Thus, CD4(+) T cell-derived IL-2 is essential for CD8(+) T cell responses to noninflammatory, cell-associated Ags. We suggest that it is also a critical component of help for CD8(+) T cell responses to pathogens, because protective memory also requires CD8(+) T cell stimulation by IL-2 during priming.