A mechanism of interleukin-12 unresponsiveness associated with thermal injury

Scald Injury-Induced T Cell Dysfunction Can Be Mitigated by Gr1+ Cell Depletion and Blockage of CD47/CD172a Signaling

Infection is a common and severe complication of burn injury: Sepsis accounts for 47% of postburn mortality. Burn-induced T cell suppression likely contributes to the increased infection susceptibility in burn patients. However, little is known about the kinetics of T cell dysfunction after burn and its underlying mechanisms. In this study, we show in a murine scald injury model that T cell activation of both CD4⁺ and CD8⁺ T cells as well as T cell cytokine production is suppressed acutely and persistently for at least 11 days after burn injury. Purified T cells from scald-injured mice exhibit normal T cell functions, indicating an extrinsically mediated defect. We further show that T cell dysfunction after burn appears to be cell-to-cell contact dependent and can be ameliorated by depletion of myeloid-derived suppressor cells. These cells expand after burn injury, particularly a subset expressing the checkpoint inhibitor CD172a, and infiltrate germinal centers. Expression of CD172a appears to be driven by ingestion of immature reticulocytes. Immature reticulocytes are drastically increased in the spleen of scald mice and may contribute to immunosuppression through more direct mechanisms as well. Overall, our study newly identifies two cell populations, myeloid-derived suppressor cells and immature reticulocytes, as well as the CD47/CD172a-signaling pathways as mediators of T cell suppressors after burn and thus opens up new research opportunities in the search for new therapies to combat increased infection susceptibility and the associated morbidity and mortality in burn victims.

Intestine immune homeostasis after alcohol and burn injury

Traumatic injury remains one of the most prevalent reasons for patients to be hospitalized. Burn injury accounts for 40,000 hospitalizations in the United States annually, resulting in a large burden on both the health and economic system and costing millions of dollars every year. The complications associated with postburn care can quickly cause life-threatening conditions including sepsis and multiple organ dysfunction and failure. In addition, alcohol intoxication at the time of burn injury has been shown to exacerbate these problems. One of the biggest reasons for the onset of these complications is the global suppression of the host immune system and increased susceptibility to infection. It has been hypothesized that infections after burn and other traumatic injury may stem from pathogenic bacteria from within the host's gastrointestinal tract. The intestine is the major reservoir of bacteria within the host, and many studies have demonstrated perturbations of the intestinal barrier after burn injury. This article reviews the findings of these studies as they pertain to changes in the intestinal immune system after alcohol and burn injury.

M2b monocytes predominated in peripheral blood of severely burned patients

Severely burned patients were shown to be carriers of M2 monocytes, and all of the monocytes isolated from peripheral blood of severely burned patients (19 of 19 patients) were demonstrated as M2b monocytes (IL-12(-)IL-10(+)CCL1(+) monocytes). Low levels of M2a (IL-12(-)IL-10(+)CCL17(+) monocytes) and M2c monocytes (IL-12(-)IL-10(+)CXCL13(+) monocytes) were demonstrated in peripheral blood of severely burned patients (M2a, 2 of 19 patients; M2c, 5 of 19 patients). M2b, M2a, and M2c monocytes were not detected in peripheral blood of healthy donors. However, M2b monocytes appeared when healthy donor monocytes were cultured in media supplemented with burn patient serum (15%). CCL2 was detected in sera of all burn patients, and M2b monocytes were not generated from healthy donor monocytes cultured with media containing 15% burn patient sera that were previously treated with anti-CCL2 mAb. In addition, M2b monocytes were generated from healthy donor monocytes in cultures supplemented with rCCL2. These results indicate that M2b monocytes are predominant in peripheral blood of severely burned patients who are carriers of CCL2 that functions to stimulate monocyte conversion from resident monocytes to M2b monocytes.

Linking the "two-hit" response following injury to enhanced TLR4 reactivity

Severe injury can initiate an exaggerated systemic inflammatory response and multiple organ failure (MOF) if a subsequent immune stimulus, "second hit", occurs. Using a mouse thermal injury model, we tested whether changes in innate immune cell reactivity following injury can contribute to the development of heightened inflammation and MOF. Using high-purity Escherichia coli lipopolysaccharide (LPS) to selectively stimulate Toll-like receptor 4 (TLR4), we demonstrate augmented interleukin (IL)-1beta, tumor necrosis factor alpha (TNF-alpha), and IL-6 production by 1 day but particularly, at 7 days after injury. The in vivo significance of enhanced TLR4 responsiveness was explored by challenging sham or burn mice with LPS at 1 or 7 days after injury and determining mortality along with in vivo cytokine and chemokine levels. Mortality was high (75%) in LPS-challenged burn but not sham mice at 7 days, although not at 1 day, after injury. Death was associated with leukocyte sequestration in the lungs and livers along with increased proinflammatory cytokine and chemokine levels in these organs. Blocking TNF-alpha activity prevented this mortality, suggesting that excessive TNF-alpha production contributes to this lethal response. These findings demonstrate the potential lethality of excessive TLR4 reactivity after injury and provide an explanation for the exaggerated inflammatory response to a second hit, which can occur following severe injury.

Macrophage migration inhibitory factor mediates late cardiac dysfunction after burn injury

We have recently demonstrated that macrophage migration inhibitory factor (MIF) is a myocardial depressant protein and that MIF mediates late, prolonged cardiac dysfunction after endotoxin challenge in mice. Because many factors, including endotoxin, have been implicated in the pathogenesis of cardiac dysfunction after burn injury, we tested the hypothesis that MIF might also be the mediator of prolonged cardiac dysfunction in this model. At 4 h after 40% total body surface area burn in anesthetized mice, serum MIF levels increased significantly compared with baseline (2.2-fold). This increase was accompanied by a significant decrease in cardiac tissue MIF levels (2.1-fold decrease compared with controls). This pattern was consistent with MIF release from preformed cytoplasmic stores in the heart and other organs. To determine whether MIF mediates cardiac dysfunction after burn injury, mice were pretreated with anti-MIF neutralizing monoclonal antibodies or isotype control antibodies. Beginning 4 h after burn injury (and continuing through 48 h), burned mice demonstrated a significantly depressed left ventricular shortening fraction of 38.6 +/- 1.8%, compared with the normal controls (56.0 +/- 2.6%). Mice treated with anti-MIF displayed an initial depression of cardiac function similar to nontreated animals but then showed rapid restoration of cardiac function with complete recovery by 24 h, which persisted for the duration of the protocol. This study is the first to demonstrate that MIF mediates late, prolonged cardiac dysfunction after burn injury and suggests that MIF blockade should be considered a therapeutic target for the treatment of burn injury.

Mechanisms of postburn intestinal barrier dysfunction in the rat: Roles of epithelial cell renewal, E-cadherin, and neutrophil extravasation*

OBJECTIVE

Our group has previously shown that the intestinal epithelium exhibits increased postburn barrier permeability and bacterial translocation associated with deranged neutrophil activity. The purpose of this investigation is to explore possible underlying intestinal structural mechanisms, leading to those functional changes with emphasis on (1) neutrophil influx and extravasation in the intestinal lamina propria 1-3 days after burn and (2) enterocyte proliferation, migration, apoptosis, and E-cadherin junctional epithelium levels 3 days after burn.

DESIGN

Freshly isolated ileum specimens were quick frozen, then cut by a cryostat into 30-micron-thick sections. Sections from day 1 postburn rats were immunostained with (1) anti-granulocyte or anti-elastase antibodies to assess neutrophil influx or (2) combined anti-granulocyte and anti-von Willebrand factor double immunolabeling to compare levels of neutrophil extravasation. Sections from day 3 postburn rats were immunostained with (1) bromodeoxyuridine immunohistochemistry 1, 3, 6, or 18 hrs after bromodeoxyuridine injection to assess enterocyte proliferation and migration, (2) cytokeratin-18 M30-immunohistochemistry to compare levels of enterocyte apoptosis, and (3) E-cadherin immunohistochemistry to compare junctional E-cadherin integrity. Ileal myeloperoxidase activity and bacterial translocation of Enterococcus faecalis were assessed biochemically and by E. faecalis-specific bacterial cultures, respectively, in day 3 postburn rats.

SETTING

: Research laboratories in a medical center and an academic institution.

SUBJECTS

Male Sprague-Dawley rats given sham treatment or treatment as a burn model with full-thickness skin scald over 30% total body surface area.

CONCLUSIONS

We report (1) increased levels of neutrophil influx and extravasation in villi lamina propriae, including elastase-positive cells (postburn day 1), (2) heightened levels of intestinal myeloperoxidase activity (postburn day 3), (3) decreased levels of epithelial cell proliferation, migration, and E-cadherin (postburn day 3), and (4) increased enterocyte apoptosis and E. faecalis bacterial translocation (postburn day 3). Based on these structural and functional abnormalities, we propose a mechanism for burn injury-related intestinal barrier dysfunction that includes increased trans- and para-cellular leakage caused by impaired enterocyte renewal and decreased junctional E-cadherin levels subsequent to increased neutrophil influx and extravasation within the villus lamina propria microenvironment.

Mechanisms of immune resolution

Initially after injury, the innate/proinflammatory and some aspects of the acquired immune response are up-regulated to maintain a defense against foreign pathogens, clear tissue debris present at the wound site, and orchestrate aspects of tissue remodeling, cell proliferation and angiogenic process, associated with the wound response. However, for proper wound healing to progress, this initial inflammatory response has to be regulated or shut down so as to allow for the reestablishment of matrix, recellularization, and tissue remodeling. Inability to properly resolve the extent of innate/acquired response at a site of injury can lead to poor wound healing, immune suppression, and recurrent infectious episodes. This review attempts to summarize information on regulatory mechanisms that are thought to be involved in controlling/resolving innate or acquired immune responses so as to provide a framework for use in thinking about the impact these processes and their manipulation may have on wound healing and its potential management.

Soluble IL-4 receptor improves the skin-graft-associated cytomegalovirus infection in thermally injured mice

The susceptibility of thermally injured mice (TI-mice) to murine cytomegalovirus (MCMV) infection is markedly influenced by burn-associated type 2 T cell responses, which are common with severe thermal injuries. In the present study, the effect of soluble IL-4 receptor (sIL-4R) on the skin-graft-associated MCMV infection was investigated. The marked growth of MCMV was demonstrated in the salivary glands of TI-mice grafted with MCMV seropositive [MCMV sero(+)] skin. However, the growth of MCMV was not demonstrated in the salivary glands of TI-mice grafted with MCMV sero(+) skin and treated with 50ng per mouse of sIL-4R. Compared with grafted normal mice, production of type 1 cytokines was markedly decreased when splenic T cells from TI-mice grafted with MCMV sero(+) skin were stimulated with anti-CD3 monoclonal antibody (mAb). The impaired type 1 cytokine production was recovered in cultures of splenic T cells from grafted TI-mice previously treated with sIL-4R. After grafting with MCMV sero(+) skin, the growth of MCMV was markedly inhibited in the salivary glands of severe combined immunodeficient (SCID) mice inoculated with T cells from TI-mice treated with sIL-4R. These results suggest that sIL-4R regulates the skin-graft-associated MCMV infection in TI-mice.

Macrophages and post-burn immune dysfunction

The activation of a pro-inflammatory cascade after burn injury appears to be important in the development of subsequent immune dysfunction, susceptibility to sepsis and multiple organ failure. Macrophages are major producers of pro-inflammatory mediators and their productive capacity for these mediators is markedly enhanced following thermal injury. Thus, macrophage hyperactivity (as defined by increased productive capacity for pro-inflammatory mediators) appears to be of critical importance in the development of post-burn immune dysfunction. This review will focus on the current state of knowledge with regards to the role of macrophages in the development of post-burn immune dysfunction. Particular areas of discussion include: nitric oxide synthase (NOS) and cyclooxygenase (COX) enzyme systems, macrophages and the T-helper (Th)-1/Th-2 cytokine responses, alterations in macrophages signal transduction and a potential role for gamma/delta T-cells in the development of macrophage hyperactivity following thermal injury. A more comprehensive understanding of the relationship between macrophage activity and post-burn immune dysfunction will hopefully provide the basis for improved therapeutic regimes in the treatment of burn patients.

Therapeutic effects of IL-12 combined with benzoylmesaconine, a non-toxic aconitine-hydrolysate, against herpes simplex virus type 1 infection in mice following thermal injury

IL-12 is an inducer of type 1 T cell responses, which are essential in host defense against herpes simplex virus type 1 (HSV-1) infection. However, type 1 T cell responses are not elicited by IL-12 in thermally injured mice (TI-mice) that routinely have a predominance of burn-associated type 2 T cell responses. In our previous studies, benzoylmesaconine (BEN, an aconitine derivative extracted from heated-Aconiti tuber) induced the generation of CD4+ T cells antagonistic to type 2 T cells (BEN-CD4+ T cells). In the present study, the effects of a combination therapy using IL-12 and BEN to treat severe HSV-1 infection in TI-mice were investigated. When TI-mice were treated with either IL-12 (500 U per mouse) or BEN (1 microg/kg) alone, they did not resist HSV-1 infection. However, 60-80% of TI-mice exposed to HSV-1 survived after they received IL-12 and BEN or BEN-CD4+ T cells in combination. After stimulation with anti-CD3 mAb in vitro, IFN- was not produced in cultures of splenic T cells from TI-mice exposed to HSV-1 and treated with either IL-12, BEN or BEN-CD4+ T cell alone. However, IFN- production was induced by the mAb stimulation in the cultures of T cells from infected mice treated with IL-12 and BEN or BEN-CD4+ T cells in combination. These results suggest that the combination therapy of IL-12 (an inducer of type 1 T cell responses) and BEN (an inhibitor of type 2 T cell responses) may protect TI-mice from severe HSV-1 infection.

Effect of a combination therapy between IL-12 and soluble IL-4 receptor (sIL-4R) on Candida albicans and herpes simplex virus type I infections in thermally injured mice

The effectiveness of a combination using IL-12 and soluble IL-4 receptor (sIL-4R) to treat severe infections of herpes simplex virus type 1 (HSV-1) and Candida albicans in thermally injured mice was investigated. Although sIL-4R decreased burn-associated type 2 T-cell responses, the effect of sIL-4R was minimal on the morbidity and mortality of thermally injured mice exposed to 250 times LD50 of HSV-1 or 10 times LD50 of C. albicans. Compared with 100% mortality in control mice, mortality for HSV-1 and C. albicans was 40 and 20%, respectively, in thermally injured mice that received IL-12 and sIL-4R in combination. After stimulation with anti-CD3 monoclonal antibody, splenic T cells from thermally injured mice exposed to large amounts of HSV-1 or C. albicans did not produce gamma interferon (IFN-gamma) into their culture fluids. However, IFN-gamma was produced by splenic T cells from thermally injured and infected mice treated with IL-12 and sIL-4R in combination. These results suggest that therapeutic treatment with a combination of IL-12 and sIL-4R may be effective by inducing type 1 T-cell responses in thermally injured mice exposed to large amounts of HSV-1 or C. albicans.

Differential expression and tissue compartmentalization of the inflammatory response following thermal injury

Studies have shown that both animal tissue-fixed immune cells and human peripheral blood mononuclear cell (PBMC) functions are altered after burn injury. Additional studies suggest that the burn injury-induced alterations in these divergent cell populations from different species are similar. It remains unknown, however, whether the observed changes in animal tissue-fixed immune cell function following thermal injury also occurs to a similar extent in the PBMC population. The aim of our study was to compare PBMC and tissue-fixed immune cell functions from the same animal using a murine burn model. At 7 days post-burn, mice were more susceptible to sepsis and delayed type hypersensitivity responses were suppressed. Splenocytes isolated from injured mice displayed suppressed proliferation and increased IL-10 production. In contrast, PBMC from injured mice displayed suppressed proliferation, IL-2 and IFN-gamma production. Splenic macrophage nitric oxide, PGE(2), TNF-alpha, IL-6 and IL-10 production was enhanced post-burn and IL-12 production was suppressed. PBMC from such animals displayed enhanced PGE(2) production and suppressed IL-6 and IL-12 production. These results indicate that while an immunosuppressive Th(2) phenotype (increased IL-10 and/or suppressed IL-2, IFN-gamma) was induced in both the splenic and PBMC compartments post-injury, differential expression and dimorphism in the response also exists. Thus, the assessment of only PBMC function in burn patients may not accurately reflect the patient's actual immune status at the tissue level.

Cyclooxygenase 2-mediated suppression of macrophage interleukin-12 production after thermal injury

Macrophage (Mphi) prostaglandin (PG)E(2) production has been implicated in immunosuppression and increased susceptibility to sepsis after thermal injury. Deficient interleukin (IL)-12 production has also been implicated in these postburn complications. The present study examined the relationship between Mphi cyclooxygenase (COX)-2 activity and IL-12 production after thermal injury. C57BL/6 female mice were subjected to a 25% total body surface area full-thickness burn. Mphi were isolated 7 days later, or the mice were subjected to sepsis by cecal ligation and puncture (CLP). IL-12 production by Mphi from injured mice was suppressed by >50%, whereas COX-2 expression and PGE(2) production were increased twofold. The COX-2 inhibitor NS-398 suppressed PGE(2) production and normalized IL-12 production in the injury group, whereas it had no effect on IL-10 production. Injured mice subjected to CLP had lower IL-12 plasma levels compared with sham-treated mice subjected to CLP. NS-398 treatment prevented the suppression in plasma IL-12 levels in the injury group. Thus elevated Mphi COX-2 activity, independent of IL-10, suppresses Mphi IL-12 production after thermal injury and may play an important role in the observed immunosuppression under such conditions.

Glycyrrhizin restores the impaired IL-12 production in thermally injured mice

Mice 6 days after thermal injury (TI-mice) did not respond to lipopolysaccharide (LPS) stimulation for production of serum interleukin 12 (IL-12; 2 h after LPS stimulation, <20 pg/ml in TI-mice; 1091+/-162 pg/ml in normal mice). However, 2 h after LPS stimulation, 1456+/-118 pg/ml of IL-12 were demonstrated in sera of TI-mice previously treated with a 10 mg/kg i.p. dose of glycyrrhizin (GR). IL-12 was not induced by LPS in sera of normal mice inoculated with burn-associated type 2 T cells (IL-4/IL-10-producing CD8+CD11b+TCRgamma/delta+T cells isolated from spleens of TI-mice). However, IL-12 production was induced by LPS in sera of these mice previously treated with GR or a mixture of monoclonal antibodies (mAbs) for type 2 cytokines. Also, IL-12 production was induced by LPS in TI-mice inoculated with CD4+T cells from spleens of GR-treated normal mice (GR-CD4+T cells, 5x10(6)cells/mouse). Since GR-CD4+T cells have been shown to be antagonistic cells against production of type 2 cytokines by burn-associated type 2 T cells, these results indicate that IL-12 unresponsiveness shown in TI-mice is recovered by GR through the regulation of burn-associated type 2 T cell responses.