Interactions between CD8+ T cells and endothelial cells are important in both protective and pathologic immune responses. Endothelial cells regulate the recruitment of CD8 + T cells into tissues, and the activation of CD8 + T cells by antigen presentation and costimulatory signals. PD-L1 and PD-L2 are recently described B7-family molecules which bind to PD-1 on activated lymphocytes and down-regulate T cell activation. We found that PD-L1 is expressed on interferon-+ stimulated cultured human and mouse endothelial cells, while PD-L2 was found on stimulated human but not mouse endothelial cells. Expression was further upregulated by TNF- § . Antibody blockade of endothelial cell PD-L1 and PD-L2 enhanced endothelial cell costimulation of PHA-activated human CD8 + T cells. Antibody blockade of mouse endothelial cell PD-L1 enhanced both IFN-+ secretion and cytolytic activity of CD8 + T cells in response to endothelial cell antigen presentation. These results show that IFN-+ activated endothelial cells can inhibit T cell activation via expression of the immunoinhibitory PD-L1 and PD-L2 molecules. Endothelial expression of PD-ligands would allow activation and extravasation of T cells without excessive vessel damage. Our findings highlight a potentially important pathway by which endothelial cells down-regulate CD8 + T cellmediated immune responses.

Proteases function at every level in host defense, from regulating vascular hemostasis and inflammation to mobilizing the "rapid responder" leukocytes of the immune system by regulating the activities of various chemoattractants. Recent studies implicate proteolysis in the activation of a ubiquitous plasma chemoattractant, chemerin, a ligand for the G-protein-coupled receptor CMKLR1 present on plasmacytoid dendritic cells and macrophages. To define the pathophysiologic triggers of chemerin activity, we evaluated the ability of serum-and inflammation-associated proteases to cleave chemerin and stimulate CMKLR1-mediated chemotaxis. We showed that serine proteases factor XIIa and plasmin of the coagulation and fibrinolytic cascades, elastase and cathepsin G released from activated neutrophil granules and mast cell tryptase are all potent activators of chemerin. Activation results from cleavage of the labile carboxyl terminus of the chemoattractant at any of several different sites. Activation of chemerin by the serine protease cascades that trigger rapid defenses in the body may direct CMKLR1-positive plasmacytoid dendritic cell and tissue macrophage recruitment to sterile sites of tissue damage, as well as trafficking to sites of infectious and allergic inflammation.A network of serine proteases regulates the primary response to injury and infection in the host. Serine proteases of the coagulation and fibrinolytic cascades mediate the homeostatic response to blood vessel injury. Kallikrein and factor XIIa process kininogens to generate bradykinin, a potent vasodilator that triggers increased vascular permeability during inflammation. Serine proteases termed convertases release multiple pathogen-neutralizing components of activated complement. Serine protease cascades also regulate the recruitment of phagocytic and antigen-presenting cells to sites of inflammation and tissue damage. The complement cascade, for example, releases active components C5a and C3a, potent attractants for many leukocytes, including neutrophils and monocytes (1, 2). Thus serine proteases are critical participants in rapid defense mechanisms in the body.We and others have recently identified chemerin as a potent chemoattractant for cells expressing the G-protein-linked receptor chemokine-like receptor 1 (CMKLR1), 5 also known as ChemR23 or DEZ (3-5). CMKLR1 is expressed in vitro by monocyte-derived macrophages and dendritic cells (3,5,6) and in vivo by circulating plasmacytoid dendritic cells (pDCs) (5) and tissue macrophages.6 pDCs are major producers of ␣-interferons and can differentiate into antigen-presenting cells capable of triggering T effector or suppressor responses (7). Tissue macrophages have a major role as phagocytes but, similar to pDCs, are also implicated in bridging innate and adaptive immune responses and in regulating immunity in sterile versus infectious tissue injury (8). Importantly, chemerin is widely expressed and circulates in human plasma in an inactive state (5). Active forms of chemerin have been isolated from hu...

This study explores the influence of innate immunity on CD8؉ T-cell responses against heart tissue. Adoptive transfer of ovalbumin-specific CD8؉ effector T cells into CMy-mOva mice, which express ovalbumin in cardiac myocytes, results in a lethal acute myocarditis. The inflammatory infiltrate in the heart includes neutrophils as well as T cells. We used anti-Ly6G antibody to transiently deplete neutrophils at the time of onset of disease. By day 7 after receiving 5 ؋ 10 5 CD8؉ effector T cells, 100% of control Ig-treated CMymOva mice had died, while 85% of anti-Ly6G-treated mice survived indefinitely. CD8؉ T-cell infiltration and tissue damage were present in both groups, but the disease was limited in the anti-Ly6G-treated mice, with a rapid disappearance of the adoptively transferred CD8 ؉ T cells within 11 days. Recovery occurred even though blood neutrophil counts began to rise 48 hours after the last anti-Ly6G treatment. Recovery was associated with a chronic CD4 ؉ cell infiltrate, and a rapid decline in expression of IFN-␥ and IP-10 mRNA in the myocardium. Neutrophil depletion did not effect survival of CMy-mOva mice that received 3 ؋ 10 6 CD8 ؉ T cells. These data show that granulocytic inflammation sustains CD8 ؉ T-cell-mediated heart disease, which has important implications for the pathogenesis and treatment of acute myocarditis and allograft rejection.

Antigen-specific immune responses are impaired after allogeneic hematopoietic cell transplantation (HCT). The events contributing to this impairment include host hematolymphoid ablation and donor cell regeneration, which is altered by pharmacologic immune suppression to prevent graft-versus-host disease (GVHD). A generally accepted concept is that graft T cell depletion performed to avoid GVHD yields poorer immune recovery because mature donor T cells are thought to be the major mediators of protective immunity early post-HCT. Our findings contradict the idea that removal of mature donor cells worsens immune recovery post-HCT. By transplantation of purified hematopoietic stem cells (HSC) compared with bone marrow (BM) across donor and recipient pairs of increasing genetic disparity, we show that grafts composed of the purified progenitor population give uniformly superior lymphoid reconstitution, both qualitatively and quantitatively. Subclinical GVHD by T cells in donor BM likely caused this lymphodepleting GVHD. We further determined in the major histocompatibility complex (MHC)-mismatched pairs, that T cell restricted proliferative responses were dictated by donor rather than host elements. We interpret these latter findings to show the importance of peripheral antigen presentation in the selection and maintenance of the T cell repertoire.immune reconstitution ͉ mice ͉ T cell selection

Systemic metabolic reprogramming induced by infection exerts profound, pathogen-specific effects on infection outcome. Here, we detail the host immune and metabolic response during sickness and recovery in a mouse model of malaria. We describe extensive alterations in metabolism during acute infection, and identify increases in host-derived metabolites that signal through the aryl hydrocarbon receptor (AHR), a transcription factor with immunomodulatory functions. We find that Ahr-/- mice are more susceptible to malaria and develop high plasma heme and acute kidney injury. This phenotype is dependent on AHR in Tek-expressing radioresistant cells. Our findings identify a role for AHR in limiting tissue damage during malaria. Furthermore, this work demonstrates the critical role of host metabolism in surviving infection.

When we get sick, we want to be resilient and recover our original health. To measure resilience, we need to quantify a host's position along its disease trajectory. Here we present Looper, a computational method to analyze longitudinally gathered datasets and identify gene pairs that form looping trajectories when plotted in the space described by these phases. These loops enable us to track where patients lie on a typical trajectory back to health. We analyzed two publicly available, longitudinal human microarray datasets that describe self-resolving immune responses. Looper identified looping gene pairs expressed by human donor monocytes stimulated by immune elicitors, and in YF17D-vaccinated individuals. Using loops derived from training data, we found that we could predict the time of perturbation in withheld test samples with accuracies of 94% in the human monocyte data, and 65–83% within the same cohort and in two independent cohorts of YF17D vaccinated individuals. We suggest that Looper will be useful in building maps of resilient immune processes across organisms.

Antigen specific immune responses are known to be impaired following allogeneic hematopoietic cell transplantation (HCT). Some clinical studies suggest that graft T cell depletion for the prevention of graft-versus-host disease (GVHD) leads to poorer immune recovery, while other correlate GVHD with delayed immune reconstitution. Our studies sought to examine the degree to which the co-transplantation of GVHD-inducing mature cells mediated protective immunity post-HCT. We compared the transplant of FACS purified hematopoietic stem cells (HSC: cKit+Thy1.1loSca+Lin-) with bone marrow (BM) between congenic (BA to B6Ly5.2), minor-antigen mismatched (BA to BALB.B), haploidentical (BAxSWR F1 to BALB/cxSWR F1) and MHC-mismatched (BA to BALB/c) donor and host pairs. We show that grafts composed solely of purified HSC give uniformly superior lymphoid reconstitution across all mismatched pairs, both qualitatively and quantitatively. Although absolute blood lymphocytes counts were increased in recipients of BM compared to HSC, lymphoid reconstitution as measured by lymph node size, counts and architecture was significantly improved in the HSC groups regardless of minor or major mismatches between donor and host. Proliferative responses to the allele specific peptides of the antigen hen egg lysozyme were also significantly increased in the HSC as compared BM recipients (p=0.028), with fully MHC mismatched BM recipient cells showing almost no proliferative response. The use of MHC allele specific antigens also revealed that T cell responses post-HCT are dominated by donor-restricted elements. These data suggest that subclinical GVHD mediated by mature cells in the donor BM result in impaired immune reconstitution. While there may be an increase in absolute lymphocyte numbers, this increase does not correlate with an increase in immune cell function. These findings provide important insight into the benefits of purified HSC for preventing post-HCT infectious complication in addition to its known GVHD prophylaxis benefits. Our data highlights the benefit of transplanting a pure HSC population, as the relatively small number of T cells that are found in mouse bone marrow or that remain in the graft after T cell depletion in humans can still result in immune impairment due to subclinical GVHD.

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