Mouse CCL8 is a CC chemokine of the monocyte chemoattractant protein (MCP) family whose biological activity and receptor usage have remained elusive. Here we show that CCL8 is highly expressed in the skin, where it serves as an agonist for the chemokine receptor CCR8 but not for CCR2. This distinguishes CCL8 from all other MCP chemokines. CCL8 responsiveness defined a population of highly differentiated, CCR8-expressing inflammatory T helper type 2 (TH2) cells enriched for interleukin (IL)-5. Ccr8- and Ccl8-deficient mice had markedly less eosinophilic inflammation than wild-type or Ccr4-deficient mice in a model of chronic atopic dermatitis. Adoptive transfer studies established CCR8 as a key regulator of TH2 cell recruitment into allergen-inflamed skin. In humans, CCR8 expression also defined an IL-5–enriched TH2 cell subset. The CCL8-CCR8 chemokine axis is therefore a crucial regulator of TH2 cell homing that drives IL-5–mediated chronic allergic inflammation.

Staphylococcal superantigens are pyrogenic exotoxins that cause massive T cell activation leading to toxic shock syndrome and death. Despite the strong adaptive immune response induced by these toxins, infections by superantigen-producing staphylococci are very common clinical events. We hypothesized that this may be partly a result of staphylococcal strains having developed strategies that downregulate the T cell response to these toxins. Here we show that the human interleukin-2 response to staphylococcal superantigens is inhibited by the simultaneous presence of bacteria. Such a downregulatory effect is the result of peptidoglycan-embedded molecules binding to Toll-like receptor 2 and inducing interleukin-10 production and apoptosis of antigen-presenting cells. We corroborated these findings in vivo by showing substantial prevention of mortality after simultaneous administration of staphylococcal enterotoxin B with either heat-killed staphylococci or Staphylococcus aureus peptidoglycan in mouse models of superantigen-induced toxic shock syndrome.

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 skinresident 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. (Blood. 2012;120(23):4591-4598) IntroductionThe majority of lymphocytes reside in peripheral tissues, where they fulfill essential immune surveillance functions. 1 Peripheral tissue immune surveillance T cells (T PS ) are antigen-experienced sentinels that provide first-line defense against recurrent infections and are thought to control aberrant autoimmunity. 2 Early work in animal models provided clear evidence for the presence of tissue tropism in memory T cells that correlated with the site of infection or vaccine deposition. 3,4 In contrast to effector T cells, which are short-lived and can traffic to multiple unrelated sites, 5 such memory T cells persist in peripheral tissues long after resolution of the primary immune response, 1 exist in disequilibrium with circulating blood T cells, 6,7 and provide protection against reinfection in both mice and humans. [6][7][8][9][10] The mechanisms underlying the segregation of memory T cells to distinct, nonoverlapping sites are not well understood but appear to depend on the coordinated expression of specific adhesion molecules and chemokine receptors. Understanding this process is essential for the development of rational vaccination strategies and requires the processing of tissue samples from healthy peripheral organs.Recent studies have implicated vitamins A and D in the control of T-cell homing to the small intestine and skin tissue. 11,12 In brief, vitamin A was shown to play a crucial role in the induction of the "gut-homing" receptors CCR9 and ␣4␤7 in murine T cells, 13 and the production of active vitamin A metabolites was shown to be a feature of local CD103 ϩ dendritic cells (DCs), [14][15][16] findings that were subsequently replicated with human T cells. 14,17 The situation for ski...

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.