Dendritic cells and macrophages in lung allografts: A role in chronic rejection?

Immune landscape of the kidney allograft in response to rejection

Preventing kidney graft dysfunction and rejection is a critical step in addressing the nationwide organ shortage and improving patient outcomes. While kidney transplants (KT) are performed more frequently, the overall number of patients on the waitlist consistently exceeds organ availability. Despite improved short-term outcomes in KT, comparable progress in long-term allograft survival has not been achieved. Major cause of graft loss at 5 years post-KT is chronic allograft dysfunction (CAD) characterized by interstitial fibrosis and tubular atrophy (IFTA). Accordingly, proactive prevention of CAD requires a comprehensive understanding of the immune mechanisms associated with either further dysfunction or impaired repair. Allograft rejection is primed by innate immune cells and carried out by adaptive immune cells. The rejection process is primarily facilitated by antibody-mediated rejection (ABMR) and T cell-mediated rejection (TCMR). It is essential to better elucidate the actions of individual immune cell subclasses (e.g. B memory, Tregs, Macrophage type 1 and 2) throughout the rejection process, rather than limiting our understanding to broad classes of immune cells. Embracing multi-omic approaches may be the solution in acknowledging these intricacies and decoding these enigmatic pathways. A transition alongside advancing technology will better allow organ biology to find its place in this era of precision and personalized medicine.

Effector immune cells in Chronic Lung Allograft Dysfunction: a Systematic Review

Chronic lung allograft dysfunction (CLAD) remains the major barrier to long‐term survival after lung transplantation and improved insight into its underlying immunological mechanisms is critical to better understand the disease and to identify treatment targets. We systematically searched the electronic databases of PubMed and EMBASE for original research publications, published between January 2000 and April 2021, to comprehensively assess current evidence on effector immune cells in lung tissue and bronchoalveolar lavage fluid from lung transplant recipients with CLAD. Literature search revealed 1351 articles, 76 of which met the criteria for inclusion in our analysis. Our results illustrate significant complexity in both innate and adaptive immune cell responses in CLAD, along with presence of numerous immune cell products, including cytokines, chemokines and proteases associated with tissue remodelling. A clear link between neutrophils and eosinophils and CLAD incidence has been seen, in which eosinophils more specifically predisposed to restrictive allograft syndrome. The presence of cytotoxic and T‐helper cells in CLAD pathogenesis is well‐documented, although it is challenging to draw conclusions about their role in tissue processes from predominantly bronchoalveolar lavage data. In restrictive allograft syndrome, a more prominent humoral immune involvement with increased B cells, immunoglobulins and complement deposition is seen. Our evaluation of published studies over the last 20 years summarizes the complex multifactorial immunopathology of CLAD onset and progression. It highlights the phenotype of several key effector immune cells involved in CLAD pathogenesis, as well as the paucity of single cell resolution spatial studies in lung tissue from patients with CLAD.

Cathepsin B promotes collagen biosynthesis, which drives bronchiolitis obliterans syndrome

Bronchiolitis obliterans syndrome (BOS) is a major complication after lung transplantation (LTx). BOS is characterised by massive peribronchial fibrosis, leading to air trapping-induced pulmonary dysfunction. Cathepsin B, a lysosomal cysteine protease, has been shown to enforce fibrotic pathways in several diseases. However, the relevance of cathepsin B in BOS progression has not yet been addressed. The aim of the study was to elucidate the function of cathepsin B in BOS pathogenesis.We determined cathepsin B levels in bronchoalveolar lavage fluid (BALF) and lung tissue from healthy donors (HD) and BOS LTx patients. Cathepsin B activity was assessed via a fluorescence resonance energy transfer-based assay and protein expression was determined using Western blotting, ELISA and immunostaining. To investigate the impact of cathepsin B in the pathophysiology of BOS, we used an in vivo orthotopic left LTx mouse model. Mechanistic studies were performed in vitro using macrophage and fibroblast cell lines.We found a significant increase of cathepsin B activity in BALF and lung tissue from BOS patients, as well as in our murine model of lymphocytic bronchiolitis. Moreover, cathepsin B activity was associated with increased biosynthesis of collagen and had a negative effect on lung function. We observed that cathepsin B was mainly expressed in macrophages that infiltrated areas characterised by a massive accumulation of collagen deposition. Mechanistically, macrophage-derived cathepsin B contributed to transforming growth factor-β1-dependent activation of fibroblasts, and its inhibition reversed the phenotype.Infiltrating macrophages release active cathepsin B, thereby promoting fibroblast activation and subsequent collagen deposition, which drive BOS. Cathepsin B represents a promising therapeutic target to prevent the progression of BOS.

Genetically-engineered pigs as sources for clinical red blood cell transfusion: What pathobiological barriers need to be overcome?

An alternative to human red blood cells (RBCs) for clinical transfusion would be advantageous, particularly in situations of massive acute blood loss (where availability and compatibility are limited) or chronic hematologic diseases requiring frequent transfusions (resulting in alloimmunization). Ideally, any alternative must be neither immunogenic nor pathogenic, but readily available, inexpensive, and physiologically effective. Pig RBCs (pRBCs) provide a promising alternative due to their several similarities with human RBCs, and our increasing ability to genetically-modify pigs to reduce cellular immunogenicity. We briefly summarize the history of xenotransfusion, the progress that has been made in recent years, and the remaining barriers. These barriers include prevention of (i) human natural antibody binding to pRBCs, (ii) their phagocytosis by macrophages, and (iii) the T cell adaptive immune response (in the absence of exogenous immunosuppressive therapy). Although techniques of genetic engineering have advanced in recent years, novel methods to introduce human transgenes into pRBCs (which do not have nuclei) will need to be developed before clinical trials can be initiated.

A novel role of MMP-13 for murine DC function: its inhibition dampens T-cell activation

Dendritic cells (DCs) have been shown to express matrix metalloproteinase 13 (MMP-13), but little is known about its specific function in DCs and its role in inflammatory conditions. In the present study, we describe a novel role of MMP-13 in regulating the immunostimulatory function of murine DCs through moderating MHC-I surface presentation, endocytosis and cytokine/chemokine secretion. MMP-13 expression was confirmed in bone marrow-derived DCs at both the mRNA and the protein level and, furthermore, at the activity level. Remarkably, LPS treatment strongly enhanced MMP-13 mRNA expression as well as MMP-13 activity, indicating an important role of MMP-13 in inflammatory processes. Functionally, MMP-13 inhibition did not influence the DC migratory capacity, while endocytosis of ovalbumin was significantly decreased. Inhibition of MMP-13 lowered the capability of murine DCs to activate CD8⁺ T cells, apparently through reducing MHC-I surface presentation. Decreased surface expression of CD11c on DCs, as well as changes in the DC cytokine/chemokine profile after MMP-13 inhibition, emphasizes the influence of MMP-13 on DC function. Moreover, T-cell-targeting cytokines such as IL-12, IL-23 and IL-6 were significantly reduced. Collectively, our data reveal a novel involvement of MMP-13 in regulating DC immunobiology through moderating MHC-I surface presentation, endocytosis and cytokine/chemokine secretion. Furthermore, the reduced MHC-I surface presentation by DCs resulted in a poor CD8⁺ T-cell response in vitro This novel finding indicates that MMP-13 might be a promising target for therapeutic intervention in inflammatory diseases.

Toward managing chronic rejection after lung transplant: the fate and effects of inhaled cyclosporine in a complex environment

The fate and effects of inhaled cyclosporine A (CsA) are considered after deposition on the lung surface. Special emphasis is given to a post-lung transplant environment and to the potential effects of the drug on the various cell types it is expected to encounter. The known stability, metabolism, pharmacokinetics and pharmacodynamics of the drug have been reviewed and discussed in the context of the lung microenvironment. Arguments support the contention that the immuno-inhibitory and anti-inflammatory effects of CsA are not restricted to T-cells. It is likely that pharmacologically effective concentrations of CsA can be sustained in the lungs but due to the complexity of uptake and action, the elucidation of effective posology must ultimately rely on clinical evidence.

Bronchial epithelial cells cultured from clinically stable lung allograft patients promote the development of macrophages from monocytes rather than dendritic cells

BACKGROUND

It is understood that chronic allograft failure occurs as a result of alloimmune and non-alloimmune injury. Dendritic cells (DC) are thought to be crucial in regulating (allo)immune airway damage and interactions with epithelial cells are likely. Studies in human lung transplantation are limited, however, and the available literature on DC is inconsistent. This study focused on the ex vivo influence of primary bronchial epithelial cells derived from lung allografts on DC differentiation.

METHODS

Epithelial cell conditioned media (ECCM) were added to monocytes differentiating into DC under the influence of interleukin-4 and granulocyte macrophage-colony stimulating factor. The resultant cells were compared with DC cultured without ECCM and with monocyte-derived macrophages. Expression of typical DC (eg, CD1a) and macrophage (eg, CD14) markers was assessed by flow cytometry. Phenotypical assessments were complemented by functional studies of mannose receptor-mediated phagocytosis (FITC-dextran uptake) and antigen-presenting capability (mixed lymphocyte reactions).

RESULTS

Cells exposed to ECCM expressed significantly lower levels of CD1a than unexposed DC. CD14 expression and phagocytic function were increased. ECCM cultured cells also expressed lower levels of T cell co-stimulatory molecules, secreted an anti-inflammatory cytokine profile and had significantly reduced antigen-presenting capability.

CONCLUSION

Using phenotypic and functional approaches, this study has shown that ECCM from lung allografts drives the production of macrophage-like cells from monocytes rather than DC. The data suggest that epithelial cells may restrain airway DC and potential alloimmunity. It is unclear whether the observed effect is specifically seen in lung transplant recipients or is a general property of bronchial epithelial cells. This may reflect a homeostatic inter-relationship between airway epithelial and DC populations relevant both to lung allografts and the lung more generally.

Inducible costimulator blockade prolongs airway luminal patency in a mouse model of obliterative bronchiolitis

BACKGROUND

In human lung transplantation, chronic rejection is accompanied by obliterative bronchiolitis (OB), a fibrosing inflammatory condition that leads to occlusion of the bronchial lumen and graft failure. The pathogenesis of this disorder is poorly understood, but likely involves antigen presentation by dendritic cells (DC). We studied the presence and activation status of DCs in transplanted tracheas in a mouse model of OB and studied the effect on graft luminal patency of blocking the costimulatory B7RP-1/inducible costimulator (ICOS) pathway.

METHODS

Tracheas from Balb/C or from C57Bl/6 mice were transplanted heterotopically under the dorsal skin of C57Bl/6 mice. Histologic, fluorescence-activated cell sorter, and quantitative-polymerase chain reaction analyses were performed after 1, 2, or 4 weeks. In some groups, treatment with blocking rat anti-mICOS antibodies or irrelevant rat immunoglobulin G was administered during the entire observation period.

RESULTS

After heterotopic transplantation, both CD103+CD11b- and CD103- CD11b+ MHC II+ DCs accumulated in the airway epithelium as early as 1 week after allogeneic (mismatched) but not syngeneic (matched) transplantation. Four weeks after Tx, infiltration with CD11c+ MHCII+ DCs and CD8+ lymphocytes, luminal fibrosis and epithelial damage were more pronounced in the allogeneic than in the syngeneic setting. There was a 10-fold up-regulation of ICOS mRNA and of chemokines involved in T-cell influx in the mismatched setting compared with the matched setting. Strikingly, anti-ICOS treatment without other immunosuppression prevented luminal fibrosis in mismatched transplants.

CONCLUSIONS

Our results suggest that early infiltration by DC occurs in posttransplant OB. Blocking critical costimulatory molecules expressed on DCs, as in the B7RP1-ICOS pathway, prevents epithelial damage and luminal fibrosis.

Density of dendritic cells in the human tracheal mucosa is age dependent and site specific

BACKGROUND

The mucosal immune system undergoes extensive changes in early childhood in response to environmental stimuli. Dendritic cells (DC) play a major role in the development of the immune system. However, few data exist on the influence of continuous environmental stimulation on the distribution and phenotype of human airway DC.

METHODS

Human tissue samples are mostly paraffin embedded which limits the use of several antibodies, and respiratory tissue for cryopreservation is difficult to obtain. Human frozen post mortem tracheal tissue was therefore used for this study. Only samples with epithelial adherence to the basement membrane were included (n = 34). Immunohistochemical staining and sequential overlay immunofluorescence were performed with DC-SIGN and a panel of leucocyte markers co-expressed by DC.

RESULTS

DC detected in the human tracheal mucosa using DC-SIGN correlated with the expression of HLA-DR, co-stimulatory and adhesion molecules. Higher cell densities were found at the ventral tracheal site of patients older than 1 year than in infants in the first year of life.

CONCLUSION

The increasing population of mucosal DC with age could reflect immunological maturation.

Different roles for human lung dendritic cell subsets in pulmonary immune defense mechanisms

Dendritic cells (DC) have a central role in the initiation of adequate immune responses. They recognize pathogens by means of Toll-like receptors (TLR) and link innate to adaptive immune responses by releasing proinflammatory cytokines and inducing T cell proliferation. We conducted this study to evaluate the expression and function of TLR on human lung DC subsets and to study their T cell stimulatory capacity. TLR gene expression by human pulmonary DC was evaluated by RT-PCR, while protein expression was analyzed by flow cytometry. We investigated cytokine release by DC in response to different TLR ligands. T cell stimulatory capacity was evaluated by mixed leukocyte reactions of purified lung DC with allogeneic T cells. Myeloid dendritic cells type 1 (mDC1) and myeloid dendritic cells type 2 (mDC2) express mRNA transcripts for TLR1, TLR2, TLR3, TLR4, TLR6, and TLR8. Flow cytometric analysis demonstrated high TLR2 protein expression for mDC1 and moderate TLR4 expression for mDC2. mDC1 and mDC2 release proinflammatory cytokines (TNF-alpha, IL-1beta, IL-6, and IL-8) in response to TLR2 and TLR4 ligands. TLR3 ligands induce cytokine release in mDC1, but not in mDC2. Plasmacytoid DC (pDC) express TLR7 and TLR9 and release proinflammatory cytokines in response to imiquimod and IFN-alpha in response to CpG oligonucleotides. mDC1 are strong inducers of T cell proliferation, while pDC hardly induce any T cell proliferation. mDC2 have an intermediate T cell-stimulatory capacity. Our results show divergent roles for the different human lung DC subsets, both in innate and adaptive immune responses.

Modulation of monocyte-derived dendritic cell differentiation is associated with ischemic acute renal failure

BACKGROUND

Dendritic cells (DCs) play a central role in both stimulating and suppressing immune responses and are impacted by surgical injury, exercise, and other physiological stressors. This study aims to determine whether renal ischemia/reperfusion (I/R) injury alters the differentiation, maturation, and activation of DCs from peripheral blood monocytes (PBMo).

MATERIALS AND METHODS

Sprague-Dawley (SD) rats were subjected to I/R injury or sham-operated. Creatinine clearance (CCr) was monitored daily during the 14 days of reperfusion that followed the ischemic insult. At 2 and 14 days of reperfusion, the following properties of PBMo derived-DCs were assessed: the amount of generated DCs, surface markers [CD11c, CD80, CD86, and MHC-II (IA)], and functional status including magnitude of mixed lymphocyte reaction (MLR), production of IL-12 p70 by DCs, and production of IFN-gamma and IL-4 by DC-stimulated T cells.

RESULTS

CCr was greatly reduced in the injured rats 0 to 4 days after ischemia. Two days after I/R injury to kidney, the numbers of DCs differentiated from PBMo, IL-12 production by DCs, expression of MHC-II (IA), and IFN-gamma production by DC-stimulated T cells were significantly increased in the I/R injured group (compared to the sham-operated group). After 14 days of reperfusion, there was no between-group differences in the numbers of DCs derived from PBMo, MLR, expression of CD80, CD86, and MHC-II (IA), and production of IL-12, IFN-gamma, and IL-4.

CONCLUSIONS

The increases seen at 2 days of reperfusion may reflect a preparatory step in the renal I/R injury pathway. The relationship between up-regulation of DC differentiation and ischemic acute renal failure (ARF) remains to be elucidated.

Inhibition of Human Dendritic Cell Maturation and Function by the Novel Immunosuppressant FK778

BACKGROUND

FK778, a derivative of the active leflunomide-metabolite, A77 1726, has been shown to be a powerful immunosuppressant in several transplantation models, particularly efficient in the prevention of chronic allograft rejection. However, the cellular and molecular mechanisms underlying these effects of FK778 have not been investigated yet in detail. Because dendritic cells (DCs) are the most potent antigen-presenting cells (APCs) and are essential for the initiation of immune responses including acute and chronic allograft rejection, we investigated whether FK778 affects this particular cell type.

METHODS

Allogeneic T cell stimulation by FK778-treated human monocyte-derived DCs was determined by mixed leukocyte cultures. Surface molecule expression was analyzed by flow-cytometric analysis and cytokine production by ELISA from culture supernatants. Activation of NF-kappaB in DCs was assessed by electrophoretic mobility shift assays.

RESULTS

Treatment of DCs with FK778 inhibited their potency to stimulate allogeneic T cells. In line, LPS- and CD40L-induced upregulation of DC surface activation markers and production of IL-12 was significantly inhibited, irrespective of whether cells were treated during or after the monocyte to DC differentiation period. The effects of FK778 on DCs were not reversible by exogenous uridine indicating that FK778 acts independently of its action as an inhibitor of pyrimidine synthesis. On the signaling level, activation of NF-kappaB, the essential transcription factor involved in DC maturation and function, was markedly inhibited by FK778.

CONCLUSIONS

Inhibition of activation and function of DCs as the central APCs may significantly contribute to the immunosuppressive profile of FK778 when applied after allogeneic organ transplantation.

Rat cytomegalovirus and Listeria monocytogenes infection enhance chronic rejection after allogenic rat lung transplantation

The role of infection in the pathomechanism of obliterative bronchiolitis (OB) after human lung transplantation is controversial. In a rat lung transplantation model, we analyzed the effect of viral [rat cytomegalovirus (RCMV)] and bacterial infection [Listeria monocytogenes (LM)] on the development of chronic allograft rejection. Fisher rats underwent single left lung transplantation with allografts from Lewis rats. Postoperatively, animals were infected with either RCMV or LM, or served as noninfected controls. Animals were killed on day 120 and both lungs were evaluated histopathologically for chronic airway and chronic vascular rejection. Infection with RCMV produced a significant increase in the incidence of chronic airway rejection (66.7% vs. 20%), compared with noninfected long-term surviving animals. In rats with bacterial infection (LM) a similar increase of chronic airway changes as in viral infection (50% vs. 20%) was observed. Chronic rejection of allografts infected with either RCMV or LM was associated with significantly enhanced expression of intercellular adhesion molecule-1 (ICAM-1) on the endothelium. More infiltrating leukocytes (CD18, CD11a, CD44) and ED1-positive macrophages were found in allografts of infected animals. In this experimental model of chronic airway rejection in long-term surviving rats, not only viral but also bacterial infection resulted in enhanced development of chronic airway and vascular rejection. These results support our hypothesis that infectious complications have a substantial influence on the development of OB in human lung allografts.

Upregulation of CD40, CD80, CD83 or CD86 on alveolar macrophages after lung transplantation

BACKGROUND

Alveolar macrophages (AMs) are known to be poor antigen-presenting cells, and lack the accessory molecules such as CD40, CD80 or CD86 to activate T cells. The question raised is about the potential changes in phenotypes after lung transplantation, particularly during acute rejection episodes.

METHODS

The present study analyzed the phenotype of AMs longitudinally in 45 lung transplant patients, between August 1997 and April 2002, with a follow-up period of 27.2 +/- 2.5 (mean +/- SEM) months. There were 7.7 +/- 0.6 bronchoalveolar lavage (BAL) assessments performed per patient (i.e., 345 BALs), simultaneously with transbronchial biopsies. Transplantation was soon followed by a progressive upregulation of CD40 on 49.7 +/- 8% of AMs during the first month, and this marker remained elevated at 60 +/- 8% after 5 years.

RESULTS

Both CD86 and CD80, as well as CD83, a marker of dendritic cells, were enhanced for most AMs during Grade A2 and A3 rejection episodes. A correlation was found between expression of CD83 and CD86, but not between CD1a and CD86. Immunohistology confirmed that CD40-positive cells in the alveoli corresponded to AMs and to some dendritic cells in the basal layers of the airways. In vitro studies showed that harvested AMs with these enhanced accessory molecules remained poor stimulators of allogeneic cells, a phenomenon that may be related to the ongoing immunosuppressive treatments.

CONCLUSIONS

AM phenotypes showed marked changes during early or late acute rejection episodes, acquiring CD80, CD83 and CD86, while CD40 expression was further enhanced. This finding may provide clues on how to monitor the tolerance of transplanted lungs and may also provide new insights into the pathophysiology of lung transplantation.

Pulmonary dendritic cells

Dendritic cells (DCs) are leukocytes that are emerging as chief orchestrators of immune responses. The crucial task of DCs is the continuous surveillance of antigen-exposed sites throughout the body, and their unique responsibility is to decide whether to present sampled antigen in an immunogenic or tolerogenic way. Any misstep can either lead to a flawed immune defense or to allergy, even autoimmunity. It comes as no surprise that the lungs become increasingly the subject of DC-related investigations, as they represent a vast interface between the body and the outer world. This constitutes an enormous challenge for the immune system: "firing up" immune responses inappropriately could have devastating results for the fragile gas exchange structures. Evidence accumulates that DCs play a pivotal role in maintaining the delicate balance between tolerance and active immune response in our respiratory system. The exponentially growing body of DC-related publications is a big challenge. This article aims to provide researchers and clinicians with an up-to-date view on DC biology and its relevance to pulmonary medicine. A developing trend in the field of DCs is the shift from fundamental immunologic research toward exciting clinical insights and applications. For the pulmonary clinician, this heralds the dawn of promising therapies in various domains such as infections, allergy, and cancer.

Accurate and simple discrimination of mouse pulmonary dendritic cell and macrophage populations by flow cytometry: methodology and new insights

BACKGROUND

The need to accurately discriminate dendritic cells (DCs) and macrophages (Mphs) in mouse lungs is critical given important biological differences. However, a validated flow cytometry-based method is still lacking, resulting in much confusion between both cell types.

METHODS

Single-cell suspensions freshly obtained from collagenase-digested lung tissue were stained with a CD11c-specific monoclonal antibody, detected using a PE-Cy5 or APC-conjugated secondary reagent. Cellular immunophenotype was simultaneously explored using a panel of PE-conjugated markers. The FL1 or FITC-detection channel was reserved for the assessment of autofluorescence.

RESULTS

CD11c-bright cells were heterogeneous and displayed a bimodal distribution with regard to autofluorescence (AF). CD11c+/low-AF cells were lineage-negative and showed features compatible with myeloid DCs. This was confirmed by morphology, potent T-cell stimulatory function in a mixed-leukocyte reaction, surface expression of MHCII and costimulatory molecules, and further immunophenotypical criteria, including the expression of Mac-1 and absence of CD8alpha. In contrast, CD11c+/high-AF cells displayed the features of pulmonary Mphs, including typical Mph morphology, very weak induction of T-cell proliferation, low to absent expression of MHCII and costimulatory molecules, and very low levels of Mac-1 as well as F4/80. We also show that only CD11c+/high-AF cells strongly expressed the macrophage marker MOMA-2, while interestingly Mac-3 was expressed at high levels by CD11c+/high-AF and low-AF alike.

CONCLUSIONS

This study shows that the combination of CD11c-expression and autofluorescence is necessary and sufficient to accurately separate DCs from macrophage subpopulations in mouse lungs.

Induction of mast cell activation and CC chemokine responses in remodeling tracheal allografts

Activated mast cells release stored and newly synthesized mediators that influence the caliber and responsiveness of inflamed airways. In this work, we show that alloimmune-mediated mechanisms induce mast cell activation and expression of CC chemokines in remodeling rat tracheal allografts. Decreased expression of rat mast cell protease (RMCP) I and II, in concert with tryptase release in tracheal allografts, identified degranulation of stored serine proteases as an early mast cell response to allotransplantation. Transient upregulation of c-Kit expression occurred in a synchronous manner, suggesting that c-Kit receptor signaling controls mast cell responses. Increased expression of CC chemokine ligand (CCL) 2 and CCL3 by RMCP I-positive cells identified mast cells as epithelial and mesenchymal sources of chemoattractant chemokines in allograft airways. Cyclosporin A immunosuppression both attenuated and delayed these changes in mast cell phenotypes. Incubation of rat basophil leukemia 2H3 cells with CCL2 or CCL3 decreased surface c-Kit expression, an effect blocked by protease inhibitors. By controlling surface receptor availability, CC chemokines may regulate c-Kit signaling via a novel proteolytic mechanism. These data suggest that targeting alloimmune responses and restoring quiescence of mast cells may attenuate the development of fibroproliferative and obstructive distortions of bronchiolar architecture in lung allografts.

Why are dendritic cells important in allergic diseases of the respiratory tract?

Increasing evidence points to the role of antigen-presenting dendritic cells (DC) in regulating adaptive immune responses. DC are especially sensitive to signals derived from microbes, allergens, and the airway tissue microenvironment, can polarize naïve T-cells into either Th1 or Th2 effector cells, and are increasingly recognized as having a central role in the establishment of T-cell memory and tolerance to inhaled antigens. DC form a closely meshed network within the respiratory mucosa and are rapidly recruited from the circulation in response to a variety of proinflammatory stimuli. Studies using animal models have highlighted the role of DC in both initiation and maintenance of allergic airway inflammation. Increased numbers of airway mucosal DC are found in both allergic rhinitis and asthma, and an increasing number of investigators have highlighted important functional differences between DC from atopic and normal individuals. This article reviews recent information on the involvement of DC in the pathogenesis of allergic airway disease and the means by which DC could be exploited as targets for therapy in asthma and allergic rhinitis.

Interleukin-15 production during liver allograft rejection in humans

BACKGROUND

The activity of interleukin (IL)-15, a cytokine produced by macrophages, is similar to that of IL-2. We investigated whether IL-15 plays a role in liver allograft rejection.

METHODS

We evaluated plasma levels and intrahepatic expression of IL-15 in 35 patients after liver transplantation, and then analyzed in vitro the influence of anticalcineurin drugs or steroids on IL-15 production and secretion. Finally, we examined the effects of IL-15 on lymphocyte proliferation in mixed lymphocyte culture in the presence or absence of anticalcineurin drugs or steroids.

RESULTS

Plasma levels and in situ expression of IL-15 were enhanced during liver allograft rejection, particularly during steroid-resistant acute rejection and during chronic rejection. In vitro, IL-15 production and secretion were inhibited by neither anticalcineurin drugs nor steroids. Exogenous IL-15 enhanced cell-mediated immune response, and this effect was not inhibited by immunosuppressive drugs.

CONCLUSIONS

IL-15 can play a role in the initiation and outcome of acute and chronic rejection. Anti-IL-15 therapy in combination with classic immunosuppression therapy might thus be beneficial in the prevention of acute, and especially chronic, allograft rejection.

Pulmonary Langerhans cell histiocytosis: emerging concepts in pathobiology, radiology, and clinical evolution of disease

Pulmonary Langerhans cell histiocytosis (PLCH) is an uncommon disorder of adult smokers associated with a significant morbidity. Arising from the aberrant accumulation of Langerhans and other immune cells, PLCH tends to cause a relatively isolated pulmonary involvement as compared to other forms of Langerhans cell (LC) and histiocytic disorders. Increased knowledge of cytokine triggers, dendritic cell trafficking, and clonality of LC populations in PLCH have resulted in an improved understanding of the pathobiology of PLCH. High-resolution CT (HRCT) of the chest has led to better appreciation of nodular and cystic radiographic abnormalities characteristic of the disease. Correlation of HRCT abnormalities with lung pathologic changes has led to an improved comprehension of clinical evolution of PLCH. Current clinical predictors for PLCH outcomes remain poor, although long-term follow-up and radiologic monitoring may help to define disease progression. This review discusses advances in PLCH emphasizing the etiopathologic bases of the disease and currently available radiologic modalities for monitoring disease progression.

Hierarchical contributions of allorecognition pathways in chronic lung rejection

The role of allorecognition in initiating lung graft rejection is not clearly defined. Using the heterotopic tracheal transplantation model, we examined the contributions of the indirect and direct allorecognition pathways in chronic airway rejection. Fully mismatched, wild-type grafts were transplanted into major histocompatibility complex (MHC) II-/-, class II-like accessory molecule (H2-DMalpha)-/- using MHC I-/- and wild-type allorecipients as control subjects. Similarly, MHC I-/-, MHC II-/-, or MHC I/II-/- allografts were transplanted into wild-type mice with appropriate control subjects. Grafts from nonimmunosuppressed recipients were evaluated at Weeks 2, 4, and 6. Grafts transplanted into MHC II-/- and H2-DMalpha-/- allorecipients showed a more intact epithelium and reduced lumen obliteration compared with grafts transplanted into wild-type or MHC I-/- allorecipients (p < 0.05 for each). These grafts exhibited abundant CD4+ and CD8+ cell infiltrates similar to control allografts. MHC I-/- and MHC I/II-/- but not MHC II-/- allografts placed in wild-type animals demonstrated less severe rejection compared with allograft control subjects (p < 0.05 for each). Although the indirect allorecognition pathway has the strongest influence on rejection, the direct pathway is sufficient to ultimately cause chronic airway rejection. In addition, these results suggest that MHC class I molecules are the principal alloantigens in the mouse heterotopic tracheal model of obliterative bronchiolitis.

APCs in the liver and spleen recruit activated allogeneic CD8+ T cells to elicit hepatic graft-versus-host disease

Host APCs are required for initiating T cell-dependent acute graft-vs-host disease (GVHD), but the role of APCs in the effector phase of acute GVHD is not known. To measure the effect of tissue-resident APCs on the local development of acute GVHD, we selectively depleted host macrophages and DCs from the livers and spleens, but not from the skin, peripheral lymph nodes (PLN), or mesenteric lymph nodes (MLN), of C57BL/6 (B6) mice by i.v. administration of liposomal clodronate before allogeneic bone marrow transplantation. Depletion of host hepatic and splenic macrophages and DCs significantly inhibited the proliferation of donor C3H.SW CD8(+) T cells in the spleen, but not in the PLN or MLN, of B6 mice. Such organ-selective depletion of host tissue APCs also markedly reduced the trafficking of allogeneic CD8(+) T cells into the livers and spleens, but not PLN and MLN, of B6 recipients compared with that of the control mice. Acute hepatic, but not cutaneous, GVHD was inhibited as well, resulting in improved survival of liposomal clodronate-treated B6 recipients. When C3H.SW CD8(+) T cells were activated in normal B6 recipients, recovered, and adoptively transferred into secondary B6 recipients, activated donor CD8(+) T cells rapidly migrated into the livers and spleens of control B6 recipients but were markedly decreased in B6 mice that were depleted of hepatic and splenic macrophages and DCs. Thus, tissue-resident APCs control the local recruitment of allo-reactive donor T cells and the subsequent development of acute GVHD.

PG490-88, a derivative of triptolide, attenuates obliterative airway disease in a mouse heterotopic tracheal allograft model

The current treatment of obliterative bronchiolitis in lung transplant recipients is sub-optimal. Triptolide is a novel immunosuppressant that has a mechanism of action distinct from currently available immunosuppressants, including induction of T-cell apoptosis, blockade of fibroblast proliferation/maturation and inhibition of transforming growth factor-beta (TGF-beta) mRNA production. We hypothesized that triptolide may be helpful in blocking obliterative airway disease in lung transplant recipients. We investigated the effect of PG490-88, a water-soluble derivative of triptolide, in a mouse heterotopic tracheal allograft model of obliterative airway disease. We show that PG490-88 attenuates airway obliteration in this model and inhibits accumulation of inflammatory cells, and therefore may have preventive or therapeutic benefits for patients with obliterative airway disease (OAD) following lung transplantation.

Inflammation in lung transplantation for CF. Immunosuppression and modulation of inflammation

Lung transplantation is an accepted therapy for selected individuals with end-stage lung disease due to cystic fibrosis (CF). Recent data show that CF recipients of lung transplantation have survival as good as those of any other diagnostic group. After transplantation, CF patients confront the major threats to life and health of graft infection and rejection. Inflammation is the common mediator of injury to the lung in both these instances. Graft infection after lung transplantation involves the same micro-organisms as are typical with CF as well as opportunistic agents. Prophylactic strategies and aggressive diagnosis via bronchoscopy are both critical in the effective treatment of post-transplant lung infections. Graft rejection involves the detection and recognition of foreign antigen and the subsequent activation of specific T-lymphocyte clones leading to inflammatory injury to the donor organ. Immunosuppression is used to prevent and/or modulate host response to the donor organ and titrated to serum therapeutic drug monitoring and transbronchial biopsy findings. Precise clinical monitoring and aggressive diagnostic approaches are crucial to minimizing graft injury and enhancing life after transplantation. Although most CF lung transplant recipients experience both graft infection and rejection and the 5-yr survival rate remains at approx 50%, improvement in diagnosis and therapy continue over time. With the introduction of new approaches to antimicrobial therapy, new immunosuppressant agents and promising strategies to promote immune tolerance, survival after lung transplantation is likely to improve in the coming decades.

Clearance of mobilized porcine peripheral blood progenitor cells is delayed by depletion of the phagocytic reticuloendothelial system in baboons

INTRODUCTION

Attempts to achieve immunological tolerance to porcine tissues in nonhuman primates through establishment of mixed hematopoietic chimerism are hindered by the rapid clearance of mobilized porcine leukocytes, containing progenitor cells (pPBPCs), from the circulation. Eighteen hours after infusing 1-2 x 10(10) pPBPC/kg into baboons that had been depleted of circulating anti-alphaGal and complement, these cells are almost undetectable by flow cytometry. The aim of the present study was to identify mechanisms that contribute to rapid clearance of pPBPCs in the baboon. This was achieved by depleting, or blocking the Fc-receptors of, cells of the phagocytic reticuloendothelial system (RES) using medronate liposomes (MLs) or intravenous immunoglobulin (IVIg), respectively.

METHODS

Baboons (preliminary studies, n=4) were used in a dose-finding and toxicity study to assess the effect of MLs on macrophage depletion in vivo. In another study, baboons (n=9) received a nonmyeloablative conditioning regimen (NMCR) aimed at inducing immunological tolerance, including splenectomy, whole body irradiation (300 cGy) or cyclophosphamide (80 mg/kg), thymic irradiation (700 cGy), T-cell depletion, complement depletion with cobra venom factor, mycophenolate mofetil, anti-CD154 monoclonal antibody, and multiple extracorporeal immunoadsorptions of anti-alphaGal antibodies. The baboons were divided into three groups: Group 1 (n=5) NMCR+pPBPC transplantation; Group 2 (n=2) NMCR+ML+pPBPC transplantation; and Group 3 (n=2) NMCR+IVIg+pPBPC transplantation. Detection of pig cells in the blood was assessed by fluorescence-activated cell sorter and polymerase chain reaction (PCR).

RESULTS

PRELIMINARY STUDIES

ML effectively depleted macrophages from the circulation in a dose-dependent manner. Group 1: On average, 14% pig cells were detected 2 hr postinfusion of 1 x 10(10) pPBPC/kg. After 18 hr, there were generally less than 1.5% pig cells detectable. Group 2: Substantially higher levels of pig cell chimerism (55-78%) were detected 2 hr postinfusion, even when a smaller number (0.5-1 x 10(10)/kg) of pPBPCs had been infused, and these levels were better sustained 18 hr later (10-52%). Group 3: In one baboon, 4.4% pig cells were detected 2 hr after infusion of 1 x 10(10) pPBPC/kg. After 18 hr, however, 7.4% pig cells were detected. A second baboon died 2 hr after infusion of 4 x 10(10) pPBPC/kg, with a total white blood cell count of 90,000, of which 70% were pig cells. No differences in microchimerism could be detected between the groups as determined by PCR.

CONCLUSIONS

This is the first study to report an efficient decrease of phagocytic function by depletion of macrophages with MLs in a large-animal model. Depletion of macrophages with MLs led to initial higher chimerism and prolonged the survival of circulating pig cells in baboons. Blockade of macrophage function with IVIg had a more modest effect. Cells of the RES, therefore, play a major role in clearing pPBPCs from the circulation in baboons. Depletion or blockade of the RES may contribute to achieving mixed hematopoietic chimerism and induction of tolerance to a discordant xenograft.

Bronchoalveolar lavage macrophage and lymphocyte phenotypes in lung transplant recipients

Recent publications have demonstrated potentially pathologic changes in bronchoalveolar lavage (BAL) from clinically stable lung transplant recipients (SLTRs), but there are few available data on alveolar macrophages (AMs). We formulated the hypothesis that changes in BAL AM and lymphocyte phenotypes would be apparent even in SLTRs.A cross-sectional study using a standardized 3 x 60 ml BAL, investigating lymphocyte and AM phenotypes in 19 SLTRs, 5 subjects with bronchiolitis obliterans syndrome (BOS) and 18 normal control volunteers. BAL lymphocyte and AM markers were assessed using flow cytometry. We confirmed a significant elevation of neutrophils in all lung transplant recipients with a more marked elevation in the BOS subjects. Flow-cytometric analysis showed increased numbers of natural killer (NK; CD56/CD16-positive) cells, increased CD11b- and CD11c-positive CD3 lymphocytes, increased CD8-positive lymphocytes and increased HLA-DR expression in CD8 cells from the lung transplant recipients, when compared with normals (p <.005). In contrast, the expression of a number of AM surface markers, associated with a range of host defense functions against bacteria, fungi and viruses (CD11a, CD11b, CD11c, HLA-DR, CD14), was lower in both SLTRs and those with BOS (p <.05). These novel findings are consistent with complex lymphocyte and macrophage changes that may result from clinically silent infection, partially suppressed rejection, or both.

Cell-cell contacts with epithelial cells modulate the phenotype of human macrophages

Interactions of macrophages with epithelium represent one of the pathways involved in regulating local immune mechanisms. We studied the effect of cell-cell contact with an epithelial monolayer on the phenotype of macrophages. Human monocytes and THP-1 macrophages were co-cultured with monolayers of human bronchial epithelial cells (HBECs), the alveolar type II-like cell line A549, renal adenocarcinoma epithelial cells (RA), and the lung fibroblast strain HFL-1. The expression of CD11b, CD14, CD54, and HLA-DR was measured by immunocytochemistry and flow cytometry and showed epithelial cell induction of CD54 and HLA-DR in monocytes and of all antigens in THP-1 cells. Co-culture with fibroblasts did not change the phenotype of macrophages. Separation by a filter insert inhibited most of the effects. Culture supernatants did not induce prominent phenotypic changes. Cell-cell contacts with epithelium appear to be of importance in regulating the phenotype of macrophages.

Dexamethasone inhibits the antigen presentation of dendritic cells in MHC class II pathway

Glucocorticoids (GC) are physiological inhibitors of inflammatory responses and are widely used as anti-inflammatory and immunosuppressive agents in treatment of many autoimmune and allergic diseases. In the present study, we demonstrated that one of the mechanisms by which GC can suppress the immune responses is to inhibit the differentiation and antigen presentation of dendritic cells (DC). DC were differentiated from murine bone marrow hematopoietic progenitor cells by culture with GM-CSF and IL-4 with or without dexamethasone (Dex). Our data showed that Dex, in a dose dependent manner, down-regulated surface expression of CD86, CD40, CD54 and MHC class II molecules by DC, but the expression of MHC class I, CD80, CD95 and CD95L were not affected. In addition, Dex-treated DC showed an impaired function to activate alloreactive T cells and to secrete IL-Ibeta and IL-12p70. Moreover, Dex inhibited DC to present antigen by MHC class II pathway. However, the endocytotic activity of DC was not affected. The inhibitory effect of Dex on the expression of costimulatory molecules and the antigen-presenting capacity of DC could be blocked by the addition of RU486, a potent steroid hormone antagonist, suggesting the requirement of binding to cytosolic receptors in the above-described action of Dex. Since DC have the unique property to present antigen to responding naive T cells and are required in the induction of a primary response, the functional suppression of DC by Dex may be one of the mechanisms by which GC regulate immune responses in vivo.

Regulation of immunologic homeostasis in peripheral tissues by dendritic cells: the respiratory tract as a paradigm

Dendritic cells are now recognized as the gatekeepers of the immune response, possessing a unique potential for acquisition of antigens at extremely low exposure levels and for efficient presentation of these in an immunogenic form to the naive T-cell system. Dendritic cell populations throughout the body exhibit a wide range of features in common that are associated with their primary functions, and these are considered in the initial section of this review. In addition, it is becoming evident that the properties and functions of these cells are refined by microenvironmental factors unique to their tissues of residence, a prime example being mucosal microenvironments such as those in respiratory tract tissues, and the latter represents the focus of the second section of this review.