GM-CSF restores innate, but not adaptive, immune responses in glucocorticoid-immunosuppressed human blood in vitro

Integrated multi-omics analyses reveal the altered transcriptomic characteristics of pulmonary macrophages in immunocompromised hosts with Pneumocystis pneumonia

Introduction

With the extensive use of immunosuppressants, immunosuppression-associated pneumonitis including Pneumocystis jirovecii pneumonia (PCP) has received increasing attention. Though aberrant adaptive immunity has been considered as a key reason for opportunistic infections, the characteristics of innate immunity in these immunocompromised hosts remain unclear.

Methods

In this study, wild type C57BL/6 mice or dexamethasone-treated mice were injected with or without Pneumocystis. Bronchoalveolar lavage fluids (BALFs) were harvested for the multiplex cytokine and metabolomics analysis. The single-cell RNA sequencing (scRNA-seq) of indicated lung tissues or BALFs was performed to decipher the macrophages heterogeneity. Mice lung tissues were further analyzed via quantitative polymerase chain reaction (qPCR) or immunohistochemical staining.

Results

We found that the secretion of both pro-inflammatory cytokines and metabolites in the Pneumocystis-infected mice are impaired by glucocorticoids. By scRNA-seq, we identified seven subpopulations of macrophages in mice lung tissues. Among them, a group of Mmp12⁺ macrophages is enriched in the immunocompetent mice with Pneumocystis infection. Pseudotime trajectory showed that these Mmp12⁺ macrophages are differentiated from Ly6c⁺ classical monocytes, and highly express pro-inflammatory cytokines elevated in BALFs of Pneumocystis-infected mice. In vitro, we confirmed that dexamethasone impairs the expression of Lif, Il1b, Il6 and Tnf, as well as the fungal killing capacity of alveolar macrophage (AM)-like cells. Moreover, in patients with PCP, we found a group of macrophages resembled the aforementioned Mmp12⁺ macrophages, and these macrophages are inhibited in the patient receiving glucocorticoid treatment. Additionally, dexamethasone simultaneously impaired the functional integrity of resident AMs and downregulated the level of lysophosphatidylcholine, leading to the suppressed antifungal capacities.

Conclusion

We reported a group of Mmp12⁺ macrophages conferring protection during Pneumocystis infection, which can be dampened by glucocorticoids. This study provides multiple resources for understanding the heterogeneity and metabolic changes of innate immunity in immunocompromised hosts, and also suggests that the loss of Mmp12⁺ macrophages population contributes to the pathogenesis of immunosuppression-associated pneumonitis.

Inhaled Antibiotics for Mycobacterial Lung Disease

Mycobacterial lung diseases are an increasing global health concern. Tuberculosis and nontuberculous mycobacteria differ in disease severity, epidemiology, and treatment strategies, but there are also a number of similarities. Pathophysiology and disease progression appear to be relatively similar between these two clinical diagnoses, and as a result these difficult to treat pulmonary infections often require similarly extensive treatment durations of multiple systemic drugs. In an effort to improve treatment outcomes for all mycobacterial lung diseases, a significant body of research has investigated the use of inhaled antibiotics. This review discusses previous research into inhaled development programs, as well as ongoing research of inhaled therapies for both nontuberculous mycobacterial lung disease, and tuberculosis. Due to the similarities between the causative agents, this review will also discuss the potential cross-fertilization of development programs between these similar-yet-different diseases. Finally, we will discuss some of the perceived difficulties in developing a clinically utilized inhaled antibiotic for mycobacterial diseases, and potential arguments in favor of the approach.

The Effects of Ex Vivo Administration of Granulocyte-Macrophage Colony-Stimulating Factor and Endotoxin on Cytokine Release of Whole Blood Are Determined by Priming Conditions

Background

Lipopolysaccharide- (LPS-) induced tumour necrosis factor alpha (TNFα) secretion in critically ill patients can be considered as a measure of immune responsiveness. It can be enhanced by granulocyte-macrophage colony stimulating factor (GM-CSF). We investigated the effect of GM-CSF on ex vivo stimulated cytokine production using various preincubation regimens in healthy donors and patients with sepsis.

Results

The maxima for the stimuli occurred 3 hours after stimulation. In donors, there was an increase (p < 0.001) of LPS-induced TNFα levels following incubation with GM-CSF. The simultaneous incubation with GM-CSF and LPS caused an inhibition of TNFα production (p < 0.001). Postincubation with GM-CSF did not yield any difference. In patients, preincubation with GM-CSF yielded an enhanced ex vivo TNFα-response when TNFα levels were low. Patients with increased TNFα concentrations did not show a GM-CSF stimulation effect. The GM-CSF preincubation yielded an increase of IL-8 production in patients and donors.

Conclusions

This study demonstrates the immune-modulating properties of GM-CSF depending on the absence or presence of LPS or systemic TNFα. The timing of GM-CSF administration may be relevant for the modulation of the immune system in sepsis. The lack of stimulation in patients with high TNFα may represent endotoxin tolerance.

Chitosan oligosaccharides block LPS-induced O-GlcNAcylation of NF-κB and endothelial inflammatory response

It is known that chitosan oligosaccharides (COS) suppress LPS-induced vascular endothelial inflammatory response by mechanism involving NF-κB blockade. It remains unknown how COS inhibit NF-κB. We provided evidence both in cultured endothelial cells and mouse model supporting a new mechanism. Regardless of the endothelial cell types, the LPS-induced NF-κB-dependent inflammatory gene expression was suppressed by COS, which was associated with reduced NF-κB nucleus translocation. LPS enhanced O-GlcNAc modification of NF-κB/p65 and activated NF-κB pathway, which could be prevented either by siRNA knockdown of O-GlcNAc transferase (OGT) or pretreatment with COS. Inhibition of either mitogen-activated protein kinase or superoxide generation abolishes LPS-induced NF-κB O-GlcNAcylation. Consistently, aortic tissues from LPS-treated mice presented enhanced NF-κB/p65 O-GlcNAcylation in association with upregulated gene expression of inflammatory cytokines in vascular tissues; however, pre-administration of COS prevented these responses. In conclusion, COS decreased OGT-dependent O-GlcNAcylation of NF-κB and thereby attenuated LPS-induced vascular endothelial inflammatory response.

Effects of small intestinal submucosa (SIS) on the murine innate immune microenvironment induced by heat-killed Staphylococcus aureus

The use of biological scaffold materials for wound healing and tissue remodeling has profoundly impacted regenerative medicine and tissue engineering. The porcine-derived small intestinal submucosa (SIS) is a licensed bioscaffold material regularly used in wound and tissue repair, often in contaminated surgical fields. Complications and failures due to infection of this biomaterial have therefore been a major concern and challenge. SIS can be colonized and infected by wound-associated bacteria, particularly Staphylococcus aureus. In order to address this concern and develop novel intervention strategies, the immune microenvironment orchestrated by the combined action of S. aureus and SIS should be critically evaluated. Since the outcome of tissue remodeling is largely controlled by the local immune microenvironment, we assessed the innate immune profile in terms of cytokine/chemokine microenvironment and inflammasome-responsive genes. BALB/c mice were injected intra-peritoneally with heat-killed S. aureus in the presence or absence of SIS. Analyses of cytokines, chemokines and microarray profiling of inflammasome-related genes were done using peritoneal lavages collected 24 hours after injection. Results showed that unlike SIS, the S. aureus-SIS interactome was characterized by a Th1-biased immune profile with increased expressions of IFN-γ, IL-12 and decreased expressions of IL-4, IL-13, IL-33 and IL-6. Such modulation of the Th1/Th2 axis can greatly facilitate graft rejections. The S. aureus-SIS exposure also augmented the expressions of pro-inflammatory cytokines like IL-1β, Tnf-α, CD30L, Eotaxin and Fractalkine. This heightened inflammatory response caused by S. aureus contamination could enormously affect the biocompatibility of SIS. However, the mRNA expressions of many inflammasome-related genes like Nlrp3, Aim2, Card6 and Pycard were down-regulated by heat-killed S. aureus with or without SIS. In summary, our study explored the innate immune microenvironment induced by the combined exposure of SIS and S. aureus. These results have practical implications in developing strategies to contain infection and promote successful tissue repair.

Negative regulation of HIV-1 transcription by a heterodimeric NF-κB1/p50 and C-terminally truncated STAT5 complex

Signal transducers and activator of transcription (STAT) proteins are often constitutively activated in leukocytes of HIV-1(+) individuals, which frequently show a dominant expression of a C-terminally truncated isoform of STAT5 (STAT5Δ). STAT5Δ can act as a negative regulator of human immunodeficiency virus type 1 (HIV-1) expression in both CD8-depleted primary leukocytes and chronically infected promonocytic U1 cells stimulated with granulocyte-macrophage colony-stimulating factor (GM-CSF). Activated STAT5Δ can directly bind to two consensus sequences in the HIV-1 long terminal repeat (LTR) promoter; binding impairs recruitment of RNA polymerase II (Crotti, A., Lusic, M., Lupo, R., Lievens, P. M., Liboi, E., Della Chiara, G., et al. (2007). Naturally occurring C-terminally truncated STAT5 is a negative regulator of HIV-1 expression. Blood, 109, 5380-5389). One of the STAT consensus sequences overlaps with one nuclear factor κB (NF-κB) binding site; interestingly, NF-κB1/p50 homodimers, frequently detected in monocytic cells, are negative regulators of HIV transcription. Here, we show that GM-CSF stimulation of U1 cells, while not inducing NF-κB activation, leads to STAT5Δ phosphorylation and binding to the NF-κB/STAT target sequence in the HIV LTR promoter, which already associates with p50 under unstimulated conditions. STAT5Δ was found to associate with p50, but not with RelA/p65, in both U1 cells expressing endogenous proteins and 293T cells overexpressing these factors. Furthermore, GM-CSF stimulation promoted concurrent binding of STAT5Δ and p50 at the HIV LTR promoter in U1 cells. Immunoprecipitation of chromatin from GM-CSF-stimulated U1 cells confirmed in vivo binding of p50 to the viral promoter together with STAT5Δ. Thus, cytokine-activated STAT5Δ/p50 complexes can contribute to the maintenance of HIV-1 latency in monocytic cells.

A novel TNF receptor-associated factor 6 binding domain mediates NF-kappa B signaling by the common cytokine receptor beta subunit

GM-CSF, IL-3, and IL-5 are proinflammatory cytokines that control the production and function of myeloid and lymphoid cells. Their receptors are composed of a ligand-specific alpha subunit and a shared common signal-transducing beta subunit (beta common receptor or GM-CSFR beta [beta(c)]). The pleiotropic nature of biologic outcomes mediated by beta(c) and the presence of large, uncharacterized regions of its cytoplasmic domain suggest that much remains to be learned about its downstream signaling pathways. Although some previous work has attempted to link beta(c) with NF-kappaB activation, a definitive mechanism that mediates this pathway has not been described and, to date, it has not been clear whether the receptor can directly activate NF-kappaB. We demonstrate that NF-kappaB activation by beta(c) is dependent on TNFR-associated factor 6 (TRAF6) and that association of TRAF6 with beta(c) requires a consensus-binding motif found in other molecules known to interact with TRAF6. Furthermore, point mutation of this motif abrogated the ability of beta(c) to mediate NF-kappaB activation and reduced the viability of an IL-3-dependent hematopoietic cell line. Because this receptor plays a key role in hematopoiesis and the beta(c) cytoplasmic domain identified in this work mediates hematopoietic cell viability, this new pathway is likely to contribute to immune cell biology. This work is significant because it is the first description of a TRAF6-dependent signaling pathway associated with a type I cytokine receptor. It also suggests that TRAF6, a mediator of TNFR and TLR signaling, may be a common signaling intermediate in diverse cytokine receptor systems.

Growth hormone mitigates against lethal irradiation and enhances hematologic and immune recovery in mice and nonhuman primates

Medications that can mitigate against radiation injury are limited. In this study, we investigated the ability of recombinant human growth hormone (rhGH) to mitigate against radiation injury in mice and nonhuman primates. BALB/c mice were irradiated with 7.5 Gy and treated post-irradiation with rhGH intravenously at a once daily dose of 20 microg/dose for 35 days. rhGH protected 17 out of 28 mice (60.7%) from lethal irradiation while only 3 out of 28 mice (10.7%) survived in the saline control group. A shorter course of 5 days of rhGH post-irradiation produced similar results. Compared with the saline control group, treatment with rhGH on irradiated BALB/c mice significantly accelerated overall hematopoietic recovery. Specifically, the recovery of total white cells, CD4 and CD8 T cell subsets, B cells, NK cells and especially platelets post radiation exposure were significantly accelerated in the rhGH-treated mice. Moreover, treatment with rhGH increased the frequency of hematopoietic stem/progenitor cells as measured by flow cytometry and colony forming unit assays in bone marrow harvested at day 14 after irradiation, suggesting the effects of rhGH are at the hematopoietic stem/progenitor level. rhGH mediated the hematopoietic effects primarily through their niches. Similar data with rhGH were also observed following 2 Gy sublethal irradiation of nonhuman primates. Our data demonstrate that rhGH promotes hematopoietic engraftment and immune recovery post the exposure of ionizing radiation and mitigates against the mortality from lethal irradiation even when administered after exposure.

Granulocyte colony-stimulating factor, granulocyte-macrophage colony-stimulating factor, and other immunomodulatory therapies for the treatment of infectious diseases in solid organ transplant recipients

PURPOSE OF REVIEW

Infections continue to cause significant morbidity and mortality in SOT recipients despite major advances in immunosuppressive and antimicrobial regimens. Immunomodulatory cytokines provide a potential means to augment the host immune response to infection. This review will focus on cytokine therapy for the prophylaxis and treatment of infections in solid organ transplant recipients, and will speculate on the potential for further advances in the field.

RECENT FINDINGS

In kidney and liver transplant recipients, granulocyte colony-stimulating factor (G-CSF) has been used successfully to reverse ganciclovir-induced neutropenia or cytomegalovirus-induced neutropenia. Although G-CSF also reversed corticosteroid-induced suppression of the neutrophil respiratory burst in vitro, prophylactic G-CSF failed to reduce infections or mortality in nonneutropenic solid organ transplant recipients. Published clinical experience with granulocyte-macrophage colony-stimulating factor (GM-CSF) in this population has been limited to case reports and a small case series, whereas the use of macrophage colony-stimulating factor (M-CSF) or interferon-gamma (IFN-gamma) has not been systematically investigated in controlled clinical trials.

SUMMARY

Despite encouraging results in vitro and in preclinical models, immunomodulatory cytokines have not met expectations when administered to SOT recipients. Nonetheless, the principle of selective enhancement of innate immunity for the prevention and treatment of infections in this patient population has promise and warrants further study.

Modulatory effects of 6-carboxymethylthiopurine on activated murine macrophages

The immunological activity of macrophages against pathogens in hosts includes the phagocytosis and the production of nitric oxide. We report herein the investigation of the effect of 6-carboxymethylthiopurine on nitric oxide production by murine macrophages as well as its effect on the cell viability and proliferation after stimulus with Mycobacterium bovis bacille Calmette-Guérin, interferon-gamma or a combination of both. J774A.1 macrophages stimulated or not by bacille Calmette-Guérin (20 microg/mL), interferon-gamma or both, were cultured in the presence of 6-carboxymethylthiopurine (125, 250 and 500 microm). Nitric oxide production was measured by the Griess method and cell viability/proliferation by the diphenyltetrazolium assay [3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide]. We observed an increase of J774A.1 cell proliferation after stimulus with bacille Calmette-Guérin at 125, 250 and 500 microm (69.1, 124.0 and 89.7%, respectively) and with interferon-gamma at 125 and 250 microm (64.8% and 61.7%, respectively) (p < 0.05). In all cultures treated with 6-carboxymethylthiopurine, interferon-gamma-activated nitric oxide production by J774A.1 cells decreased as well as when subjected to interferon-gamma plus bacille Calmette-Guérin stimuli at 500 microm (p < 0.05). Altogether these data point to an anti-inflammatory effect of 6-carboxymethylthiopurine on stimulated macrophages.

TNF-alpha drives human CD14+ monocytes to differentiate into CD70+ dendritic cells evoking Th1 and Th17 responses

Many mechanisms involving TNF-alpha, Th1 responses, and Th17 responses are implicated in chronic inflammatory autoimmune disease. Recently, the clinical impact of anti-TNF therapy on disease progression has resulted in re-evaluation of the central role of this cytokine and engendered novel concept of TNF-dependent immunity. However, the overall relationship of TNF-alpha to pathogenesis is unclear. Here, we demonstrate a TNF-dependent differentiation pathway of dendritic cells (DC) evoking Th1 and Th17 responses. CD14(+) monocytes cultured in the presence of TNF-alpha and GM-CSF converted to CD14(+) CD1a(low) adherent cells with little capacity to stimulate T cells. On stimulation by LPS, however, they produced high levels of TNF-alpha, matrix metalloproteinase (MMP)-9, and IL-23 and differentiated either into mature DC or activated macrophages (M phi). The mature DC (CD83(+) CD70(+) HLA-DR (high) CD14(low)) expressed high levels of mRNA for IL-6, IL-15, and IL-23, induced naive CD4 T cells to produce IFN-gamma and TNF-alpha, and stimulated resting CD4 T cells to secret IL-17. Intriguingly, TNF-alpha added to the monocyte culture medium determined the magnitude of LPS-induced maturation and the functions of the derived DC. In contrast, the M phi (CD14(high)CD70(+)CD83(-)HLA-DR(-)) produced large amounts of MMP-9 and TNF-alpha without exogenous TNF stimulation. These results suggest that the TNF priming of monocytes controls Th1 and Th17 responses induced by mature DC, but not inflammation induced by activated M phi. Therefore, additional stimulation of monocytes with TNF-alpha may facilitate TNF-dependent adaptive immunity together with GM-CSF-stimulated M phi-mediated innate immunity.

GM-CSF and the impaired pulmonary innate immune response following hyperoxic stress

We have previously demonstrated that mice exposed to sublethal hyperoxia (an atmosphere of >95% oxygen for 4 days, followed by return to room air) have significantly impaired pulmonary innate immune response. Alveolar macrophages (AM) from hyperoxia-exposed mice exhibit significantly diminished antimicrobial activity and markedly reduced production of inflammatory cytokines in response to stimulation with LPS compared with AM from control mice in normoxia. As a consequence of these defects, mice exposed to sublethal hyperoxia are more susceptible to lethal pneumonia with Klebsiella pneumoniae than control mice. Granulocyte/macrophage colony-stimulating factor (GM-CSF) is a growth factor produced by normal pulmonary alveolar epithelial cells that is critically involved in maintenance of normal AM function. We now report that sublethal hyperoxia in vivo leads to greatly reduced alveolar epithelial cell GM-CSF expression. Systemic treatment of mice with recombinant murine GM-CSF during hyperoxia exposure preserved AM function, as indicated by cell surface Toll-like receptor 4 expression and by inflammatory cytokine secretion following stimulation with LPS ex vivo. Treatment of hyperoxic mice with GM-CSF significantly reduced lung bacterial burden following intratracheal inoculation with K. pneumoniae, returning lung bacterial colony-forming units to the level of normoxic controls. These data point to a critical role for continuous GM-CSF activity in the lung in maintenance of normal AM function and demonstrate that lung injury due to hyperoxic stress results in significant impairment in pulmonary innate immunity through suppression of alveolar epithelial cell GM-CSF expression.

GM-CSF priming of human monocytes is dependent on ERK1/2 activation

The ability to augment monocyte functions such as TNF-alpha-producing capacities confers a high immunostimulating potential to GM-CSF. In the present investigation, the mechanism of the GMCSF-mediated enhancement of monocyte cytokine production was analysed with regard to the involvement of intracellular signalling pathways. GM-CSF primes human monocytes dose- and time-dependently for enhanced LPS-stimulated TNF-alpha synthesis. Pre-incubation with 10 ng/ml GM-CSF for 6 h before LPS stimulation (10 ng/ml) caused a 3.4 +/- 1.9-fold increase in TNF-alpha release compared to unprimed controls. This was associated with increased phosphorylation of IkappaBalpha and elevated nuclear levels of the NF-kappaB components p50 and p65 and NF-kappaB binding to DNA. LPS-induced AP-1 binding to DNA was also enhanced in GM-CSF-pre-incubated cells. GMCSF treatment also caused a slight increase in TLR4 expression on monocytes while CD14 expression remained unchanged. GM-CSF-priming was unaffected by inhibitors of p38 MAPK (SB203580) and lipoxygenase (NDGA). In contrast, the broad-spectrum tyrosine kinase inhibitor genistein and the MEK-1 inhibitor (PD98059) abrogated GM-CSF priming of TNF-alpha release and activation of both NF-kappaB and AP-1. It is concluded that a tyrosine kinase of the GM-CSF-triggered ERK1/2 pathway augments the LPS-induced NF-kappaB and AP-1 activation.

Inflammation--a new therapeutic target in pneumonia

Inflammation is a hallmark of pneumonia. Therefore, managing inflammation is an attractive adjunct to targeted antibiotic therapy, mainly in severe pneumonia. Recent investigations indicate that glucocorticoids given in physiological doses (from 10-fold to 100-fold less than doses administered in the past) could be of benefit. We could also manage inflammation by administering or influencing cytokines. A major concern is that drugs designed to target a single cytokine or receptor could prove ineffective due to the redundancy of signaling pathways involved. This may require selection of drugs with broad activity or the targeting of molecules common to inflammatory signaling pathways. Drugs affecting multiple molecules or key inflammatory pathway intermediates could be more effective, but their use will need to be weighed against the risk of impairing innate immunity. Indirect approaches to manage inflammation, such as neutralizing cytotoxic substances in the lung (e.g., inhibiting, neutralizing and eliminating endotoxin), could be used in combination with other approaches. Ideally, potential treatment of life-threatening bacterial pneumonia will combine immunoadjuvant and conventional antibiotic therapy, although intense clinical research with immunotherapy has not yet yielded a successful treatment adjunct. We believe that compounds capable of stimulating early host defense and microbial clearance, but not the later phases of inflammatory tissue injury associated with sepsis, may be advantageous.

PGE/cAMP and GM-CSF synergise to induce a pro-tolerance cytokine profile in monocytic cell lines

This study demonstrates a synergistic action of prostaglandin E and GM-CSF which causes the release of pro-tolerant cytokines in two monocyte cell lines: U937 and ML-1. The prostaglandin effect is cyclic AMP dependent since stimulators of adenyl cyclase such as forskolin (fsk) can replace PGE. Fsk and GM-CSF combinations raised messenger RNA for IL-10, interleukin-1 receptor antagonist (IL-1ra), and CD14 as well as the released proteins. Effective levels of interleukin 12 are reduced. In these respects, the monocyte cells resemble the alternatively activated or tumour associated macrophages. A differential pattern in co-stimulatory molecule expression is seen; CD80 is unchanged but CD86 is markedly elevated and such a change is not seen in the alternatively activated macrophage but has been previously reported in monocytes resident in the non-inflamed gut. Control of leukocyte responses by two agents acting in synergy could be effective in critical situations such as discrimination between pathogens and commensal bacteria, etc. Monocytes modified in such a way could provide a pro-tolerant environment (high IL-10, low IL-12) for antigen presentation by dendritic cells and thus may contribute to a normally permissive milieu, e.g., for food absorption.

Interaction between conidia, lung macrophages, immunosuppressants, proinflammatory cytokines and transcriptional regulation

The immunosuppressive effect of dexamethasone (DEX) on macrophage killing activity and cytokine production in response to Aspergillus fumigatus conidia is antagonized by granulocyte-macrophage colony-stimulating factor (GM-CSF). However, the intersection of signaling pathways and the molecular mechanism of this antagonism remain to be defined. We postulated that DEX inhibition of NF-kappaB was opposed by induction of IkappaB kinases (IKK) by GM-CSF + conidia stimulation, degradation of IkappaB, and release of nuclear factor kappa B (NF-kappaB). This hypothesis was tested using resident peritoneal macrophages from CD-1 mice and the murine macrophage RAW 264.7 cell line. Macrophages were unstimulated or stimulated with A. fumigatus conidia and simultaneously treated with DEX, GM-CSF or DEX + GM-CSF for 2 4 hours. IkappaB degradation in cytoplasmic extracts and translocation of NF-kappaB in nuclear extracts was measured by Western blot analysis. This showed GM-CSF reverses the immunosuppressive effect of DEX by enhancing the degradation of IkappaB and promoting the translocation of NF-kappaB to the nucleus. This would allow the production of proinflammatory cytokines by macrophages, facilitating resistance to A. fumigatus.

The potential of GM-CSF to improve resistance against infections in organ transplantation

Immunosuppressed patients retain transplants but become more susceptible to opportunistic infections, which is a major complication in organ transplantation. Life-long immunosuppression for such patients could be reduced by creating immune tolerance, although this might be associated with an increased risk for infections and malignancies. An alternative therapeutic concept could consist of boosting the innate immune response against infections while continuing to suppress the adaptive immune response to prevent graft rejection. We propose granulocyte-macrophage colony-stimulating factor (GM-CSF) as a novel candidate to achieve this goal, based on recent studies in which beneficial effects were demonstrated in immunosuppressed mice with skin allografts and in dexamethasone-suppressed blood from healthy volunteers and blood from liver transplant recipients undergoing immunosuppressive therapy. Such data suggest that GM-CSF or other endogenous factors with similar properties should be examined in clinical trials.