Responsiveness to interleukin-15 therapy is shared between tissue-resident and circulating memory CD8+ T cell subsets.

Interleukin-15 (IL-15) is often considered a central regulator of memory CD8+ T cells, based primarily on studies of recirculating subsets. However, recent work identified IL-15-independent CD8+ T cell memory populations, including tissue-resident memory CD8+ T cells (TRM) in some nonlymphoid tissues (NLTs). Whether this reflects the existence of IL-15-insensitive memory CD8+ T cells is unclear. We report that IL-15 complexes (IL-15c) stimulate rapid proliferation and expansion of both tissue-resident and circulating memory CD8+ T cell subsets across lymphoid and nonlymphoid tissues with varying magnitude by tissue and memory subset, in some sites correlating with differing levels of the IL-2Rß. This was conserved for memory CD8+ T cells recognizing distinct antigens and elicited by different pathogens. Following IL-15c-induced expansion, divided cells contracted to baseline numbers and only slowly returned to basal proliferation, suggesting a mechanism to transiently amplify memory populations. Through parabiosis, we showed that IL-15c drive local proliferation of TRM, with a degree of recruitment of circulating cells to some NLTs. Hence, irrespective of homeostatic IL-15 dependence, IL-15 sensitivity is a defining feature of memory CD8+ T cell populations, with therapeutic potential for expansion of TRM and other memory subsets in an antigen-agnostic and temporally controlled fashion.

Extricating human tumour immune alterations from tissue inflammation.

Immunotherapies have achieved remarkable successes in the treatment of cancer, but major challenges remain1,2. An inherent weakness of current treatment approaches is that therapeutically targeted pathways are not restricted to tumours, but are also found in other tissue microenvironments, complicating treatment3,4. Despite great efforts to define inflammatory processes in the tumour microenvironment, the understanding of tumour-unique immune alterations is limited by a knowledge gap regarding the immune cell populations in inflamed human tissues. Here, in an effort to identify such tumour-enriched immune alterations, we used complementary single-cell analysis approaches to interrogate the immune infiltrate in human head and neck squamous cell carcinomas and site-matched non-malignant, inflamed tissues. Our analysis revealed a large overlap in the composition and phenotype of immune cells in tumour and inflamed tissues. Computational analysis identified tumour-enriched immune cell interactions, one of which yields a large population of regulatory T (Treg) cells that is highly enriched in the tumour and uniquely identified among all haematopoietically-derived cells in blood and tissue by co-expression of ICOS and IL-1 receptor type 1 (IL1R1). We provide evidence that these intratumoural IL1R1+ Treg cells had responded to antigen recently and demonstrate that they are clonally expanded with superior suppressive function compared with IL1R1- Treg cells. In addition to identifying extensive immunological congruence between inflamed tissues and tumours as well as tumour-specific changes with direct disease relevance, our work also provides a blueprint for extricating disease-specific changes from general inflammation-associated patterns.

Inflammatory signals are sufficient to elicit TOX expression in mouse and human CD8+ T cells.

T cell receptor (TCR) stimulation leads to the expression of the transcription factor thymocyte selection-associated high-mobility group box (TOX). Prolonged TCR signaling, such as encountered during chronic infections or in tumors, leads to sustained TOX expression, which is required for the induction of a state of exhaustion or dysfunction. Although CD8+ memory T (Tmem) cells in mice typically do not express TOX at steady state, some human Tmem cells express TOX but appear fully functional. This seeming discrepancy between mouse and human T cells has led to the speculation that TOX is differentially regulated between these species, which could complicate the interpretation of preclinical mouse model studies. We report here that, similar to TCR-mediated signals, inflammatory cytokines are also sufficient to increase TOX expression in human and mouse Tmem cells. Thus, TOX expression is controlled by the environment, which provides an explanation for the different TOX expression patterns encountered in T cells isolated from specific pathogen-free laboratory mice versus humans. Finally, we report that TOX is not necessary for cytokine-driven expression of programmed cell death 1. Overall, our data highlight that the mechanisms regulating TOX expression are conserved across species and indicate that TOX expression reflects a T cell's activation state and does not necessarily correlate with T cell dysfunction.

The Ugly Duckling Turned to Swan: A Change in Perception of Bystander-Activated Memory CD8 T Cells.

Memory T cells (Tmem) rapidly mount Ag-specific responses during pathogen reencounter. However, Tmem also respond to inflammatory cues in the absence of an activating TCR signal, a phenomenon termed bystander activation. Although bystander activation was first described over 20 years ago, the physiological relevance and the consequences of T cell bystander activation have only become more evident in recent years. In this review, we discuss the scenarios that trigger CD8 Tmem bystander activation including acute and chronic infections that are either systemic or localized, as well as evidence for bystander CD8 Tmem within tumors and following vaccination. We summarize the possible consequences of bystander activation for the T cell itself, the subsequent immune response, and the host. We highlight when T cell bystander activation appears to benefit or harm the host and briefly discuss our current knowledge gaps regarding regulatory signals that can control bystander activation.

Human Tissue-Resident Memory T Cells in the Maternal-Fetal Interface. Lost Soldiers or Special Forces?

The immune system plays a critical role during pregnancy, but the specific mechanisms and immune cell function needed to support pregnancy remain incompletely understood. Despite decades of research efforts, it is still unclear how the immune system maintains tolerance of fetal-derived tissues, which include most cells of the placenta and of course the fetus itself, without forfeiting the ability to protect against harmful infections. T cells recognize antigen in the context of major histocompatibility complex (MHC) encoded proteins, but classical MHC class I and II expression are diminished in fetal-derived cells. Can T cells present at the maternal-fetal interface (MFI) protect these cells from infection? Here we review what is known in regard to tissue-resident memory T (Trm) cells at the MFI. We mainly focus on how Trm cells can contribute to protection in the context of the unique features of the MFI, such as limited MHC expression as well as the temporary nature of the MFI, that are not found in other tissues.

Inflammatory Cytokines Induce Sustained CTLA-4 Cell Surface Expression on Human MAIT Cells.

Mucosal-associated invariant T (MAIT) cells acquire effector function in response to proinflammatory signals, which synergize with TCR-mediated signals. We asked if cell-intrinsic regulatory mechanisms exist to curtail MAIT cell effector function akin to the activation-induced expression of inhibitory receptors by conventional T cells. We examined human MAIT cells from blood and oral mucosal tissues by RNA sequencing and found differential expression of immunoregulatory genes, including CTLA-4, by MAIT cells isolated from tissue. Using an ex vivo experimental setup, we demonstrate that inflammatory cytokines were sufficient to induce CTLA-4 expression on the MAIT cell surface in the absence of TCR signals. Even brief exposure to the cytokines IL-12, IL-15, and IL-18 was sufficient for sustained CTLA-4 expression by MAIT cells. These data suggest that control of CTLA-4 expression is fundamentally different between MAIT cells and conventional T cells. We propose that this mechanism serves to limit MAIT cell-mediated tissue damage.

CXCR3 enables recruitment and site-specific bystander activation of memory CD8+ T cells.

Bystander activation of memory T cells occurs in the absence of cognate antigen during infections that elicit strong systemic inflammatory responses, which subsequently affect host immune responses. Here we report that memory T cell bystander activation is not limited to induction by systemic inflammation. We initially observe potential T cell bystander activation in a cohort of human vaccine recipients. Using a mouse model system, we then find that memory CD8+ T cells are specifically recruited to sites with activated antigen-presenting cells (APCs) in a CXCR3-dependent manner. In addition, CXCR3 is also necessary for T cell clustering around APCs and T cell bystander activation, which temporospatially overlaps with the subsequent antigen-specific T cell response. Our data thus suggest that bystander activation is part of the initial localized immune response, and is mediated by a site-specific recruitment process of memory T cells.

Controlled Human Malaria Infection Leads to Long-Lasting Changes in Innate and Innate-like Lymphocyte Populations.

Animal model studies highlight the role of innate-like lymphocyte populations in the early inflammatory response and subsequent parasite control following Plasmodium infection. IFN-γ production by these lymphocytes likely plays a key role in the early control of the parasite and disease severity. Analyzing human innate-like T cell and NK cell responses following infection with Plasmodium has been challenging because the early stages of infection are clinically silent. To overcome this limitation, we examined blood samples from a controlled human malaria infection (CHMI) study in a Tanzanian cohort, in which volunteers underwent CHMI with a low or high dose of Plasmodium falciparum sporozoites. The CHMI differentially affected NK, NKT (invariant NKT), and mucosal-associated invariant T cell populations in a dose-dependent manner, resulting in an altered composition of this innate-like lymphocyte compartment. Although these innate-like responses are typically thought of as short-lived, we found that changes persisted for months after the infection was cleared, leading to significantly increased frequencies of mucosal-associated invariant T cells 6 mo postinfection. We used single-cell RNA sequencing and TCR αß-chain usage analysis to define potential mechanisms for this expansion. These single-cell data suggest that this increase was mediated by homeostatic expansion-like mechanisms. Together, these data demonstrate that CHMI leads to previously unappreciated long-lasting alterations in the human innate-like lymphocyte compartment. We discuss the consequences of these changes for recurrent parasite infection and infection-associated pathologies and highlight the importance of considering host immunity and infection history for vaccine design.

Silymarin suppresses basal and stimulus-induced activation, exhaustion, differentiation, and inflammatory markers in primary human immune cells.

Silymarin (SM), and its flavonolignan components, alter cellular metabolism and inhibit inflammatory status in human liver and T cell lines. In this study, we hypothesized that SM suppresses both acute and chronic immune activation (CIA), including in the context of HIV infection. SM treatment suppressed the expression of T cell activation and exhaustion markers on CD4+ and CD8+ T cells from chronically-infected, HIV-positive subjects. SM also showed a trend towards modifying CD4+ T cell memory subsets from HIV+ subjects. In the HIV-negative setting, SM treatment showed trends towards suppressing pro-inflammatory cytokines from non-activated and pathogen-associated molecular pattern (PAMP)-activated primary human monocytes, and non-activated and cytokine- and T cell receptor (TCR)-activated mucosal-associated invariant T (MAIT) cells. The data suggest that SM elicits broad anti-inflammatory and immunoregulatory activity in primary human immune cells. By using novel compounds to alter cellular inflammatory status, it may be possible to regulate inflammation in both non-disease and disease states.

Crosstalk between sentinel and helper macrophages permits neutrophil migration into infected uroepithelium.

The phagocytes of the innate immune system, macrophages and neutrophils, contribute to antibacterial defense, but their functional specialization and cooperation is unclear. Here, we report that three distinct phagocyte subsets play highly coordinated roles in bacterial urinary tract infection. Ly6C(-) macrophages acted as tissue-resident sentinels that attracted circulating neutrophils and Ly6C(+) macrophages. Such Ly6C(+) macrophages played a previously undescribed helper role: once recruited to the site of infection, they produced the cytokine TNF, which caused Ly6C(-) macrophages to secrete CXCL2. This chemokine activated matrix metalloproteinase-9 in neutrophils, allowing their entry into the uroepithelium to combat the bacteria. In summary, the sentinel macrophages elicit the powerful antibacterial functions of neutrophils only after confirmation by the helper macrophages, reminiscent of the licensing role of helper T cells in antiviral adaptive immunity. These findings identify helper macrophages and TNF as critical regulators in innate immunity against bacterial infections in epithelia.

Robust suppression of env-SHIV viremia in Macaca nemestrina by 3-drug ART is independent of timing of initiation during chronic infection.

BACKGROUND: Nonhuman primates (NHPs) are an important model organism for studies of HIV pathogenesis and preclinical evaluation of anti-HIV therapies. The successful translation of NHP-derived data to clinically relevant anti-HIV studies will require better understanding of the viral strains and NHP species used and their responses to existing antiretroviral therapies (ART). METHODS: Five pigtailed macaques (Macaca nemestrina) were productively infected with the SIV/HIV chimeric virus SHIV-1157 ipd3N4 following intravenous challenge. After 8 or 27 weeks, ART (PMPA, FTC, raltegravir) was initiated. Viral load, T-cell counts, and production of SHIV-specific antibodies were monitored throughout the course of infection and ART. RESULTS: ART led to a rapid and sustained decrease in plasma viral load. Suppression of plasma viremia by ART was independent of the timing of initiation during chronic infection. CONCLUSIONS: We present a new NHP model of HIV infection on antiretroviral therapy, which should prove applicable to multiple clinically relevant anti-HIV approaches.

Positive selection of mC46-expressing CD4+ T cells and maintenance of virus specific immunity in a primate AIDS model.

Despite continued progress in the development of novel antiretroviral therapies, it has become increasingly evident that drug-based treatments will not lead to a functional or sterilizing cure for HIV(+) patients. In 2009, an HIV(+) patient was effectively cured of HIV following allogeneic transplantation of hematopoietic stem cells (HSCs) from a CCR5(-/-) donor. The utility of this approach, however, is severely limited because of the difficulty in finding matched donors. Hence, we studied the potential of HIV-resistant stem cells in the autologous setting in a nonhuman primate AIDS model and incorporated a fusion inhibitor (mC46) as the means for developing infection-resistant cells. Pigtail macaques underwent identical transplants and Simian-Human Immunodeficiency Virus (SHIV) challenge procedures with the only variation between control and mC46 macaques being the inclusion of a fusion-inhibitor expression cassette. Following SHIV challenge, mC46 macaques, but not control macaques, showed a positive selection of gene-modified CD4(+) T cells in peripheral blood, gastrointestinal tract, and lymph nodes, accounting for >90% of the total CD4(+) T-cell population. mC46 macaques also maintained high frequencies of SHIV-specific, gene-modified CD4(+) T cells, an increase in nonmodified CD4(+) T cells, enhanced cytotoxic T lymphocyte function, and antibody responses. These data suggest that HSC protection may be a potential alternative to conventional antiretroviral therapy in patients with HIV/AIDS.

Silibinin inhibits HIV-1 infection by reducing cellular activation and proliferation.

Purified silymarin-derived natural products from the milk thistle plant (Silybum marianum) block hepatitis C virus (HCV) infection and inhibit T cell proliferation in vitro. An intravenous formulation of silibinin (SIL), a major component of silymarin, displays anti-HCV effects in humans and also inhibits T-cell proliferation in vitro. We show that SIL inhibited replication of HIV-1 in TZM-bl cells, PBMCs, and CEM cells in vitro. SIL suppression of HIV-1 coincided with dose-dependent reductions in actively proliferating CD19+, CD4+, and CD8+ cells, resulting in fewer CD4+ T cells expressing the HIV-1 co-receptors CXCR4 and CCR5. SIL inhibition of T-cell growth was not due to cytotoxicity measured by cell cycle arrest, apoptosis, or necrosis. SIL also blocked induction of the activation markers CD38, HLA-DR, Ki67, and CCR5 on CD4+ T cells. The data suggest that SIL attenuated cellular functions involved in T-cell activation, proliferation, and HIV-1 infection. Silymarin-derived compounds provide cytoprotection by suppressing virus infection, immune activation, and inflammation, and as such may be relevant for both HIV mono-infected and HIV/HCV co-infected subjects.

Combining different design strategies for rational affinity maturation of the MICA-NKG2D interface.

We redesigned residues on the surface of MICA, a protein that binds the homodimeric immunoreceptor NKG2D, to increase binding affinity with a series of rational, incremental changes. A fixed-backbone RosettaDesign protocol scored a set of initial mutations, which we tested by surface plasmon resonance for thermodynamics and kinetics of NKG2D binding, both singly and in combination. We combined the best four mutations at the surface with three affinity-enhancing mutations below the binding interface found with a previous design strategy. After curating design scores with three cross-validated tests, we found a linear relationship between free energy of binding and design score, and to a lesser extent, enthalpy and design score. Multiple mutants bound with substantial subadditivity, but in at least one case full additivity was observed when combining distant mutations. Altogether, combining the best mutations from the two strategies into a septuple mutant enhanced affinity by 50-fold, to 50 nM, demonstrating a simple, effective protocol for affinity enhancement.