The Human Cutaneous Chemokine System

Irrespective of the immune status, the vast majority of all lymphocytes reside in peripheral tissues whereas those present in blood only amount to a small fraction of the total. It has been estimated that T cells in healthy human skin outnumber those present in blood by at least a factor of two. How lymphocytes within these two compartments relate to each other is not well understood. However, mounting evidence suggest that the study of T cell subsets present in peripheral blood does not reflect the function of their counterparts at peripheral sites. This is especially true under steady-state conditions whereby long-lived memory T cells in healthy tissues, notably those in epithelial tissues at body surfaces, are thought to fulfill a critical immune surveillance function by contributing to the first line of defense against a series of local threats, including microbes, tumors, and toxins, and by participating in wound healing. The relative scarcity of information regarding peripheral T cells and the factors regulating their localization is primarily due to inherent difficulties in obtaining healthy tissue for the extraction and study of immune cells on a routine basis. This is most certainly true for humans. Here, we review our current understanding of T cell homing to human skin and compare it when possible with gut-selective homing. We also discuss candidate chemokines that may account for the tissue selectivity in this process and present a model whereby CCR8, and its ligand CCL1, selectively regulate the homeostatic migration of memory lymphocytes to skin tissue

Immune mobilising T cell receptors redirect polyclonal CD8+ T cells in chronic HIV infection to form immunological synapses

T cell exhaustion develops in human immunodeficiency virus (HIV) infection due to chronic viral antigenic stimulation. This adaptive response primarily affects virus-specific CD8+ T cells, which may remain dysfunctional despite viral load-reducing antiretroviral therapy; however, abnormalities may also be evident in non-HIV-specific populations. Both could limit the efficacy of cell therapies against viral reservoirs. Here, we show that bulk (polyclonal) CD8+ T cells from people living with HIV (PLWH) express proposed markers of dysfunctional HIV-specific T cells at high levels yet form lytic immunological synapses (IS) and eliminate primary resting infected (HIV Gaglo) CD4+ T cells, when redirected by potent bispecific T cell-retargeting molecules, Immune mobilising monoclonal T cell receptors (TCR) Against Virus (ImmTAV). While PLWH CD8+ T cells are functionally impaired when compared to CD8+ T cells from HIV-naïve donors, ImmTAV redirection enables them to eliminate Gaglo CD4+ T cells that are insensitive to autologous HIV-specific cytolytic T cells. ImmTAV molecules may therefore be able to target HIV reservoirs, which represent a major barrier to a cure

CCR8 Expression Defines Tissue-Resident Memory T Cells in Human Skin

Human skin harbors two major T cell compartments of equal size that are distinguished by expression of the chemokine receptor CCR8. In vitro studies have demonstrated that CCR8 expression is regulated by TCR engagement and the skin tissue microenvironment. To extend these observations, we examined the relationship between CCR8+ and CCR8− skin T cells in vivo. Phenotypic, functional, and transcriptomic analyses revealed that CCR8+ skin T cells bear all the hallmarks of resident memory T cells, including homeostatic proliferation in response to IL-7 and IL-15, surface expression of tissue localization (CD103) and retention (CD69) markers, low levels of inhibitory receptors (programmed cell death protein 1, Tim-3, LAG-3), and a lack of senescence markers (CD57, killer cell lectin-like receptor subfamily G member 1). In contrast, CCR8− skin T cells are heterogeneous and comprise variable numbers of exhausted (programmed cell death protein 1+), senescent (CD57+, killer cell lectin-like receptor subfamily G member 1+), and effector (T-bethi, Eomeshi) T cells. Importantly, conventional and high-throughput sequencing of expressed TCR β-chain (TRB) gene rearrangements showed that these CCR8-defined populations are clonotypically distinct, suggesting unique ontogenies in response to separate antigenic challenges and/or stimulatory conditions. Moreover, CCR8+ and CCR8− skin T cells were phenotypically stable in vitro and displayed similar levels of telomere erosion, further supporting the likelihood of a nonlinear differentiation pathway. On the basis of these results, we propose that long-lived memory T cells in human skin can be defined by the expression of CCR8

Skin metabolites define a new paradigm in the localization of skin tropic memory T cells

The localization of memory T cells to human skin is essential for long-term immune surveillance and the maintenance of barrier integrity. The expression of CCR8 during naive T cell activation is controlled by skin-specific factors derived from epidermal keratinocytes and not by resident dendritic cells. In this study, we show that the CCR8-inducing factors are heat stable and protease resistant and include the vitamin D3 metabolite 1α,25-dihydroxyvitamin D3 and PGE2. The effect of either metabolite alone on CCR8 expression was weak, whereas their combination resulted in robust CCR8 expression. Elevation of intracellular cAMP was essential because PGE2 could be substituted with the adenylyl cyclase agonist forskolin, and CCR8 expression was sensitive to protein kinase A inhibition. For effective induction, exposure of naive T cells to these epidermal factors needed to occur either prior to or during T cell activation even though CCR8 was only detected 4–5 d later in proliferating T cells. The importance of tissue environments in maintaining cellular immune surveillance networks within distinct healthy tissues provides a paradigm shift in adaptive immunity. Epidermal-derived vitamin D3 metabolites and PGs provide an essential cue for the localization of CCR8+ immune surveillance T cells within healthy human skin

Epithelial chemokine CXCL14 synergizes with CXCL12 via allosteric modulation of CXCR4

The chemokine receptor, CXC chemokine receptor 4 (CXCR4), is selective for CXC chemokine ligand 12 (CXCL12), is broadly expressed in blood and tissue cells, and is essential during embryogenesis and hematopoiesis. CXCL14 is a homeostatic chemokine with unknown receptor selectivity and preferential expression in peripheral tissues. Here, we demonstrate that CXCL14 synergized with CXCL12 in the induction of chemokine responses in primary human lymphoid cells and cell lines that express CXCR4. Combining subactive concentrations of CXCL12 with 100–300 nM CXCL14 resulted in chemotaxis responses that exceeded maximal responses that were obtained with CXCL12 alone. CXCL14 did not activate CXCR4-expressing cells (i.e., failed to trigger chemotaxis and Ca2+ mobilization, as well as signaling via ERK1/2 and the small GTPase Rac1); however, CXCL14 bound to CXCR4 with high affinity, induced redistribution of cell-surface CXCR4, and enhanced HIV-1 infection by >3-fold. We postulate that CXCL14 is a positive allosteric modulator of CXCR4 that enhances the potency of CXCR4 ligands. Our findings provide new insights that will inform the development of novel therapeutics that target CXCR4 in a range of diseases, including cancer, autoimmunity, and HIV.—Collins, P. J., McCully, M. L., Mart´ınez-Muñoz, L., Santiago, C.,Wheeldon, J., Caucheteux, S., Thelen, S., Cecchinato, V., Laufer, J.M., Purvanov, V.,Monneau, Y. R., Lortat-Jacob, H., Legler, D. F., Uguccioni, M., Thelen, M., Piguet, V., Mellado, M., Moser, B. Epithelial chemokine CXCL14 synergizes with CXCL12 via allosteric modulation of CXCR4. FASEB J. 31, 000–000 (2017). www.fasebj.or

The Future of RIP2/RICK/CARDIAK as a Biomarker of the Inflammatory Response to Infection

Biological markers of disease have become increasingly important for the clinician to diagnose, predict and monitor progression, and assess the therapeutic effect of interventions on underlying pathogenic mechanisms. Robust and specific biomarkers would be very useful in inflammation, where they may facilitate early identification of tissue injury, predict disease progression and help to modify disease outcomes. However, at present, there are no robust biomarkers to predict the course of inflammation. Here, we discuss emerging data indicating that RIP2, a putative serine/threonine protein kinase, may serve as a biomarker for the resolution of peritoneal dialysis-associated peritonitis and, more generally, of the acute inflammatory response to infection

RIP2 Is Required for NOD Signaling But Not for Th1 Cell Differentiation and Cellular Allograft Rejection

Two previous reports that receptor-interacting protein (RIP)-2 knockout (RIP2-/-) mice had defective nuclear factor-kappa B (NF-kappaB) signaling and T helper (Th)1 immune responses had led us to believe that this putative serine-threonine kinase might be a possible target for transplant immunosuppression. Thus, we tested whether RIP2-/- mice were able to reject vascularized allografts. Surprisingly, we found that T cells from RIP2-/- mice proliferated and produced interferon (IFN)-gamma after allostimulation in vitro. Moreover, naïve RIP2-/- CD4+ T cells differentiated normally into Th1 or Th2 cells under appropriate cytokine microenvironments. Consistent with these findings, no difference in allograft survival was observed between wild-type and RIP2-/- recipient mice, and rejection had similar pathology and cytokine profiles in both types of recipients. RIP2 deficiency was associated with defective NOD signaling, but this did not affect T-cell receptor (TCR)-dependent activation of the canonical NF-kappaB signaling or expression of NF-kappaB genes in rejecting allografts. Our data demonstrate that RIP2-deficient mice have intact canonical NF-kappaB signaling and can mount Th1-mediated alloresponses and reject vascularized allografts as efficiently as wild-type mice, thus arguing against RIP2 as a primary target for immunosuppression

Epidermis instructs skin homing receptor expression in human T cells

The localization of memory T cells to human skin is essential for long-term immune surveillance and the maintenance of barrier integrity. Although the mechanisms controlling memory T-cell migration to peripheral tissues are poorly understood, the current paradigm includes the localized secretion of “imprinting” signals from tissue-resident dendritic cells in the draining lymph nodes. Here we show that CCR8 expression by newly activated naive T cells is regulated by skin-specific factor(s) derived primarily from epidermal keratinocytes, thereby providing a mechanism for the preferential expression of CCR8 by skin-resident memory T cells. Importantly, no such effects were observed after coculture with primary cells from skin-unrelated epithelia, including mesothelium and small intestine. The keratinocyte-derived CCR8-inducing factor(s) were soluble, and independent of vitamins A and D. Furthermore, the induction of CCR8 under these conditions correlated with an increase in cutaneous lymphocyte-associated antigen expression. Our findings challenge current tissue homing paradigms, especially those involving CCR10, and emphasize the importance of steady-state epidermis rather than tissue-resident dendritic cells in controlling the localization of memory T cells within human skin

Skin Metabolites Define a New Paradigm in the Localization of Skin Tropic Memory T Cells

The localization of memory T cells to human skin is essential for long-term immune surveillance and the maintenance of barrier integrity. The expression of CCR8 during naive T cell activation is controlled by skin-specific factors derived from epidermal keratinocytes and not by resident dendritic cells. In this study, we show that the CCR8-inducing factors are heat stable and protease resistant and include the vitamin D3 metabolite 1α,25-dihydroxyvitamin D3 and PGE2. The effect of either metabolite alone on CCR8 expression was weak, whereas their combination resulted in robust CCR8 expression. Elevation of intracellular cAMP was essential because PGE2 could be substituted with the adenylyl cyclase agonist forskolin, and CCR8 expression was sensitive to protein kinase A inhibition. For effective induction, exposure of naive T cells to these epidermal factors needed to occur either prior to or during T cell activation even though CCR8 was only detected 4–5 d later in proliferating T cells. The importance of tissue environments in maintaining cellular immune surveillance networks within distinct healthy tissues provides a paradigm shift in adaptive immunity. Epidermal-derived vitamin D3 metabolites and PGs provide an essential cue for the localization of CCR8+ immune surveillance T cells within healthy human skin