Effects of DNA- and Mycobacterium bovis BCG-based delivery of the Flt3 ligand on protective immunity to Mycobacterium tuberculosis

Bridging the gaps to overcome major hurdles in the development of next-generation tuberculosis vaccines

Although tuberculosis (TB) remains one of the leading causes of death from an infectious disease worldwide, the development of vaccines more effective than bacille Calmette-Guérin (BCG), the only licensed TB vaccine, has progressed slowly even in the context of the tremendous global impact of TB. Most vaccine candidates have been developed to strongly induce interferon-γ (IFN-γ)-producing T-helper type 1 (Th1) cell responses; however, accumulating evidence has suggested that other immune factors are required for optimal protection against Mycobacterium tuberculosis (Mtb) infection. In this review, we briefly describe the five hurdles that must be overcome to develop more effective TB vaccines, including those with various purposes and tested in recent promising clinical trials. In addition, we discuss the current knowledge gaps between preclinical experiments and clinical studies regarding peripheral versus tissue-specific immune responses, different underlying conditions of individuals, and newly emerging immune correlates of protection. Moreover, we propose how recently discovered TB risk or susceptibility factors can be better utilized as novel biomarkers for the evaluation of vaccine-induced protection to suggest more practical ways to develop advanced TB vaccines. Vaccines are the most effective tools for reducing mortality and morbidity from infectious diseases, and more advanced technologies and a greater understanding of host-pathogen interactions will provide feasibility and rationale for novel vaccine design and development.

Pathological and protective roles of dendritic cells in Mycobacterium tuberculosis infection: Interaction between host immune responses and pathogen evasion

Dendritic cells (DCs) are principal defense components that play multifactorial roles in translating innate immune responses to adaptive immunity in Mycobacterium tuberculosis (Mtb) infections. The heterogeneous nature of DC subsets follows their altered functions by interacting with other immune cells, Mtb, and its products, enhancing host defense mechanisms or facilitating pathogen evasion. Thus, a better understanding of the immune responses initiated, promoted, and amplified or inhibited by DCs in Mtb infection is an essential step in developing anti-tuberculosis (TB) control measures, such as host-directed adjunctive therapy and anti-TB vaccines. This review summarizes the recent advances in salient DC subsets, including their phenotypic classification, cytokine profiles, functional alterations according to disease stages and environments, and consequent TB outcomes. A comprehensive overview of the role of DCs from various perspectives enables a deeper understanding of TB pathogenesis and could be useful in developing DC-based vaccines and immunotherapies.

Recombinant BCG to Enhance Its Immunomodulatory Activities

The bacillus Calmette-Guérin (BCG) is an attenuated Mycobacterium bovis derivative that has been widely used as a live vaccine against tuberculosis for a century. In addition to its use as a tuberculosis vaccine, BCG has also been found to have utility in the prevention or treatment of unrelated diseases, including cancer. However, the protective and therapeutic efficacy of BCG against tuberculosis and other diseases is not perfect. For three decades, it has been possible to genetically modify BCG in an attempt to improve its efficacy. Various immune-modulatory molecules have been produced in recombinant BCG strains and tested for protection against tuberculosis or treatment of several cancers or inflammatory diseases. These molecules include cytokines, bacterial toxins or toxin fragments, as well as other protein and non-protein immune-modulatory molecules. The deletion of genes responsible for the immune-suppressive properties of BCG has also been explored for their effect on BCG-induced innate and adaptive immune responses. Most studies limited their investigations to the description of T cell immune responses that were modified by the genetic modifications of BCG. Some studies also reported improved protection by recombinant BCG against tuberculosis or enhanced therapeutic efficacy against various cancer forms or allergies. However, so far, these investigations have been limited to mouse models, and the prophylactic or therapeutic potential of recombinant BCG strains has not yet been illustrated in other species, including humans, with the exception of a genetically modified BCG strain that is now in late-stage clinical development as a vaccine against tuberculosis. In this review, we provide an overview of the different molecular engineering strategies adopted over the last three decades in order to enhance the immune-modulatory potential of BCG.

Advancing Adjuvants for Mycobacterium tuberculosis Therapeutics

Tuberculosis (TB) remains one of the leading causes of death worldwide due to a single infectious disease agent. BCG, the only licensed vaccine against TB, offers limited protection against pulmonary disease in children and adults. TB vaccine research has recently been reinvigorated by new data suggesting alternative administration of BCG induces protection and a subunit/adjuvant vaccine that provides close to 50% protection. These results demonstrate the need for generating adjuvants in order to develop the next generation of TB vaccines. However, development of TB-targeted adjuvants is lacking. To help meet this need, NIAID convened a workshop in 2020 titled “Advancing Vaccine Adjuvants for Mycobacterium tuberculosis Therapeutics”. In this review, we present the four areas identified in the workshop as necessary for advancing TB adjuvants: 1) correlates of protective immunity, 2) targeting specific immune cells, 3) immune evasion mechanisms, and 4) animal models. We will discuss each of these four areas in detail and summarize what is known and what we can advance on in order to help develop more efficacious TB vaccines.

BCG-Prime and boost with Esx-5 secretion system deletion mutant leads to better protection against clinical strains of Mycobacterium tuberculosis

Although vaccination with BCG prevents disseminated forms of childhood tuberculosis (TB), it does not protect against pulmonary infection or Mycobacterium tuberculosis (Mtb) transmission. In this study, we generated a complete deletion mutant of the Mtb Esx-5 type VII secretion system (Mtb Δesx-5). Mtb Δesx-5 was highly attenuated and safe in immunocompromised mice. When tested as a vaccine candidate to boost BCG-primed immunity, Mtb Δesx-5 improved protection against highly virulent Mtb strains in the murine and guinea pig models of TB. Enhanced protection provided by heterologous BCG-prime plus Mtb Δesx-5 boost regimen was associated with increased pulmonary influx of central memory T cells (T_CM), follicular helper T cells (T_FH) and activated monocytes. Conversely, lower numbers of T cells expressing exhaustion markers were observed in vaccinated animals. Our results suggest that boosting BCG-primed immunity with Mtb Δesx-5 is a potential approach to improve protective immunity against Mtb. Further insight into the mechanism of action of this novel prime-boost approach is warranted.

Interplay between alveolar epithelial and dendritic cells and Mycobacterium tuberculosis

The innate response plays a crucial role in the protection against tuberculosis development. Moreover, the initial steps that drive the host-pathogen interaction following Mycobacterium tuberculosis infection are critical for the development of adaptive immune response. As alveolar Mϕs, airway epithelial cells, and dendritic cells can sense the presence of M. tuberculosis and are the first infected cells. These cells secrete mediators, which generate inflammatory signals that drive the differentiation and activation of the T lymphocytes necessary to clear the infection. Throughout this review article, we addressed the interaction between epithelial cells and M. tuberculosis, as well as the interaction between dendritic cells and M. tuberculosis. The understanding of the mechanisms that modulate those interactions is critical to have a complete view of the onset of an infection and may be useful for the development of dendritic cell-based vaccine or immunotherapies.

Recent advances in FLT3 inhibitors for acute myeloid leukemia

Fms-like tyrosine kinase-3 (FLT3) mutations occur in approximately 30% of acute myeloid leukemia (AML) cases, suggesting FLT3 as an attractive target for AML treatment. Early FLT3 inhibitors enhance antileukemia efficacy by inhibiting multiple targets, and thus had stronger off-target activity, increasing their toxicity. Recently, a number of potent and selective FLT3 inhibitors have been developed, many of which are effective against multiple mutations. This review outlines the evolution of AML-targeting FLT3 inhibitors by focusing on their chemotypes, selectivity and activity over FLT3 wild-type and FLT3 mutations as well as new techniques related to FLT3. Compounds that currently enter the late clinical stage or have entered the market are also briefly reported.

Nasal double DNA adjuvant induces salivary FimA-specific secretory IgA antibodies in young and aging mice and blocks Porphyromonas gingivalis binding to a salivary protein

BACKGROUND

We previously showed that nasal administration of a combination of dendritic cell (DC) targeted DNA plasmid expressing Flt3 ligand and CpG oligodeoxynucleotides 1826 as a mucosal adjuvant (double adjuvant, DA) provoked protective immunity in the upper respiratory tract of young adult and aging mice. Here, we investigated whether the nasal DA system induces secretory (S)IgA antibodies (Abs) toward recombinant fimbrillin (rFimA) of Porphyromonas gingivalis (P. gingivalis) in the saliva of young adult and aging mice. Further, we examined the functional applicability of rFimA-specific salivary SIgA Abs.

METHODS

BALB/c mice (8- or 48-week-old) were nasally immunized with rFimA plus DA three times at weekly intervals. Control mice were nasally administered rFimA alone. Saliva samples were collected 1 week after the final immunization, and were subjected to rFimA-specific ELISA. To examine the functional applicability of rFimA-specific SIgA Abs, IgA-enriched saliva samples were subjected to an inhibition assay in order to assess the numbers of P. gingivalis cells bound to the salivary protein statherin.

RESULTS

The 8- and 48-week-old mice administered nasal rFimA plus DA showed significantly increased levels of rFimA-specific SIgA Abs in saliva and elevated numbers of CD11c+ DCs in sublingual glands (SLGs), periglandular lymph nodes (PGLNs) and submandibular glands (SMGs) as well as nasopharyngeal-associated lymphoid tissues (NALT) compared to mice administered rFimA alone. Further, rFimA-specific SIgA Abs-containing saliva, in which IgG Abs of 8- and 48-week-old mice administered nasal rFimA plus DA were removed, significantly inhibited binding of P. gingivalis to the salivary protein.

CONCLUSIONS

These findings show that this DA system could be an effective nasal vaccine strategy for the enhancement of P. gingivalis-specific protective immunity in the oral cavity of adolescents and older individuals.

Enhanced delivery of Mycobacterium tuberculosis antigens to antigen presenting cells using RVG peptide

Among the various strategies to improve vaccines against infectious diseases, targeting of antigens to dendritic cells (DCs), which are professional antigen presenting cells (APCs), has received increased attention in recent years. Here, we investigated whether a synthetic peptide region named RVG, originated from Rabies Virus Glycoprotein that binds to the α-7 subunit of the nicotinic acetylcholine receptors (AchR-α7) of APCs, could be used for the delivery of Mycobacterium tuberculosis (Mtb) peptide antigens to DCs and macrophages. Mouse bone marrow derived DCs (BMDCs) and human THP-1 macrophages stimulated with RVG fused peptide epitopes 85B₂₄₁ and 85B₉₆ (represent Ag85B_241-256 and Ag85B_96-111, respectively) from antigen 85B (Ag85B) of Mtb showed enhanced antigen presentation as compared to unfused peptide epitopes and BCG. Further, BMDCs stimulated with RVG fused 85B₂₄₁ showed higher levels of IL-12 positive cells. Consistent with in vitro data, splenocytes of mice immunized with RVG-85B₂₄₁ showed increased number of antigen specific IFN-γ, IL-2, and TNF-α producing cells in relation to splenocytes from mice immunized with 85B₂₄₁ alone. These results suggest that RVG may be a promising tool to develop effective alternate vaccines against tuberculosis (TB).

Immune Responses to Bacillus Calmette-Guérin Vaccination: Why Do They Fail to Protect against Mycobacterium tuberculosis?

Mycobacterium tuberculosis (M.tb), the causative agent of tuberculosis (TB), is the current leading cause of death due to a single infectious organism. Although curable, the broad emergence of multi-, extensive-, extreme-, and total-drug resistant strains of M.tb has hindered eradication efforts of this pathogen. Furthermore, computational models predict a quarter of the world’s population is infected with M.tb in a latent state, effectively serving as the largest reservoir for any human pathogen with the ability to cause significant morbidity and mortality. The World Health Organization has prioritized new strategies for improved vaccination programs; however, the lack of understanding of mycobacterial immunity has made it difficult to develop new successful vaccines. Currently, Mycobacterium bovis bacillus Calmette–Guérin (BCG) is the only vaccine approved for use to prevent TB. BCG is highly efficacious at preventing meningeal and miliary TB, but is at best 60% effective against the development of pulmonary TB in adults and wanes as we age. In this review, we provide a detailed summary on the innate immune response of macrophages, dendritic cells, and neutrophils in response to BCG vaccination. Additionally, we discuss adaptive immune responses generated by BCG vaccination, emphasizing their specific contributions to mycobacterial immunity. The success of future vaccines against TB will directly depend on our understanding of mycobacterial immunity.

FMS-like tyrosine kinase 3 ligand treatment of mice aggravates acute lung injury in response to Streptococcus pneumoniae: role of pneumolysin

FMS-like tyrosine kinase-3 ligand (Flt3L) is a dendritic cell (DC) growth and differentiation factor with potential in antitumor therapies and antibacterial immunization strategies. However, the effect of systemic Flt3L treatment on lung-protective immunity against bacterial infection is incompletely defined. Here, we examined the impact of deficient (in Flt3L knockout [KO] mice), normal (in wild-type [WT] mice), or increased Flt3L availability (in WT mice pretreated with Flt3L for 3, 5, or 7 days) on lung DC subset profiles and lung-protective immunity against the major lung-tropic pathogen, Streptococcus pneumoniae. Although in Flt3L-deficient mice the numbers of DCs positive for CD11b (CD11b(pos) DCs) and for CD103 (CD103(pos) DCs) were diminished, lung permeability, a marker of injury, was unaltered in response to S. pneumoniae. In contrast, WT mice pretreated with Flt3L particularly responded with increased numbers of CD11b(pos) DCs and with less pronounced numbers of CD103(pos) DCs and impaired bacterial clearance and with increased lung permeability following S. pneumoniae challenge. Notably, infection of Flt3L-pretreated mice with S. pneumoniae lacking the pore-forming toxin, pneumolysin (PLY), resulted in substantially less lung CD11b(pos) DCs activation and reduced lung permeability. Collectively, this study establishes that Flt3L treatment enhances the accumulation of proinflammatory activated lung CD11b(pos) DCs which contribute to acute lung injury in response to PLY released by S. pneumoniae.

BCG vaccination induces different cytokine profiles following infant BCG vaccination in the UK and Malawi

Background. BCG vaccination of infants is thought to provide good protection in all settings. This study investigated whether Malawian infants made weaker responses across a cytokine panel after BCG vaccination, compared with UK infants.

Methods. Diluted whole-blood samples were cultured with Mycobacterium tuberculosis purified protein derivative for 6 days from BCG-vaccinated infants 3 months (n = 40 Malawi, 28 UK) and 12 months (n = 34 Malawi, 26 UK) after vaccination, and also from UK unvaccinated infants (n = 9 at 3 months, n = 10 at 12 months). Forty-two cytokines were measured in supernatants using a multiplex bead array assay. Principal component analysis was used to summarize the overall patterns in cytokine responses.

Results. We found differences in median responses in 27 of the 42 cytokines: 7 higher in the UK and 20 higher in Malawi. The cytokines with higher responses in the UK were all T helper 1 related. The cytokines with higher responses in Malawi included innate proinflammatory cytokines, regulatory cytokines, interleukin 17, T helper 2 cytokines, chemokines, and growth factors. Principal component analysis separated the BCG-vaccinated infants from Malawi from the UK vaccinated infants and from the unvaccinated infants.

Conclusions. Malawian infants make cytokine responses following BCG vaccination, but the cytokine profile is different from that in the UK. The different biosignatures following BCG vaccination in the 2 settings may indicate variability in the protective efficacy of infant BCG vaccination.

Mtb32 is a promising tuberculosis antigen for DNA vaccination in pre- and post-exposure mouse models

Identification of antigens that provide protective immunity via prophylactic and therapeutic vaccination against Mycobacterium tuberculosis is critical for the development of subunit vaccines for tuberculosis (TB). In this study, we performed a head-to-head comparison of seven well-known TB antigens delivered by DNA vaccine, and evaluated their respective immunogenicities and protective efficacies in pre- and post-exposure mouse models. All TB antigens were designed as a chimeric fusion with Flt3-L to enhance antigen-specific T-cell immunity upon vaccination. Prophylactic vaccination with the Flt3L (F)-Mtb32 DNA vaccine elicited significant protection in both the spleen and lungs against M. tuberculosis challenge, comparable to the Bacillus Calmette-Guerin vaccine. F-Ag85A and F-Mtb32 DNA vaccines, in combination with chemotherapy, reduced the bacterial burden to undetectable levels in the lungs of all mice infected with M. tuberculosis. These data collectively indicate that the F-Mtb32 DNA vaccine confers the most efficient protective immunity that suppresses bacterial growth in the active or latent status of M. tuberculosis.

Cytokine adjuvants for vaccine therapy of neoplastic and infectious disease

Vaccination, the revolutionary prophylactic immunotherapy developed in the eighteenth century, has become the most successful and cost-effective of medical remedies available to modern society. Due to the remarkable accomplishments of the past century, the number of diseases and pathogens for which a traditional vaccine approach might reasonably be employed has dwindled to unprecedented levels. While this happy scenario bodes well for the future of public health, modern immunologists and vaccinologists face significant challenges if we are to address the scourge of recalcitrant pathogens like HIV and HCV and well as the significant obstacles to immunotherapy imposed by neoplastic self. Here, the authors review the clinical and preclinical literature to highlight the manner by which the host immune system can be successfully manipulated by cytokine adjuvants, thereby significantly enhancing the efficacy of a wide variety of vaccination platforms.

The nasal dendritic cell-targeting Flt3 ligand as a safe adjuvant elicits effective protection against fatal pneumococcal pneumonia

We have previously shown that a pneumococcal surface protein A (PspA)-based vaccine containing DNA plasmid encoding the Flt3 ligand (FL) gene (pFL) as a nasal adjuvant prevented nasal carriage of Streptococcus pneumoniae. In this study, we further investigated the safety and efficacy of this nasal vaccine for the induction of PspA-specific antibody (Ab) responses against lung infection with S. pneumoniae. C57BL/6 mice were nasally immunized with recombinant PspA/Rx1 (rPspA) plus pFL three times at weekly intervals. When dynamic translocation of pFL was initially examined, nasal pFL was taken up by nasal dendritic cells (DCs) and epithelial cells (nECs) but not in the central nervous systems, including olfactory nerve and epithelium. Of importance, nasal pFL induced FL protein synthesis with minimum levels of inflammatory cytokines in the nasal washes (NWs) and bronchoalveolar lavage fluid (BALF). NWs and BALF as well as plasma of mice given nasal rPspA plus pFL contained increased levels of rPspA-specific secretory IgA and IgG Ab responses that were correlated with elevated numbers of CD8(+) and CD11b(+) DCs and interleukin 2 (IL-2)- and IL-4-producing CD4(+) T cells in the nasal mucosa-associated lymphoid tissues (NALT) and cervical lymph nodes (CLNs). The in vivo protection by rPspA-specific Abs was evident in markedly reduced numbers of CFU in the lungs, airway secretions, and blood when mice were nasally challenged with Streptococcus pneumoniae WU2. Our findings show that nasal pFL is a safe and effective mucosal adjuvant for the enhancement of bacterial antigen (Ag) (rPspA)-specific protective immunity through DC-induced Th2-type and IL-2 cytokine responses.

Mycobacterial cell-wall skeleton as a universal vaccine vehicle for antigen conjugation

Mycobacterial cell-wall skeleton (CWS) is an immunoactive and biodegradable particulate adjuvant and has been used for immunotherapy in patients with cancer. The CWS of Mycobacterium bovis bacillus Calmette-Guérin (BCG-CWS) was studied as a universal vaccine vehicle for antigen conjugation, to develop potentially effective and safe vaccines. Here, we describe experiments in which protein antigens, such as keyhole limpet haemocyanin (KLH), ovalbumin (OVA) and bovine serum albumin (BSA) were highly efficiently coupled to 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide and N-hydroxysuccinimide (EDC/NHS)-activated carboxyl groups of BCG-CWS, and tested the immunogenicity of OVA-conjugated BCG-CWS vaccine. We found that a strong immune response was induced in mice immunised with OVA-conjugated BCG-CWS, which was similar to the enhancement of the immune responses in mice immunised with OVA and complete Freund's adjuvant. Covalent conjugation of OVA to BCG-CWS was essential for Th1-skewed immune responses, with prominent expression of IFN-γ. Furthermore, antigen-conjugated BCG-CWS vaccine is simple to manufacture, safe, and easy to use. Our results suggest that mycobacterial CWS as a universal vaccine vehicle for conjugation of a wide variety of antigens constitutes a breakthrough for development of the most promising vaccines for infections, allergic diseases, and cancer.

Recombinant BCG as a vaccine vehicle to protect against tuberculosis

Mycobacterium bovis Bacille Calmette Guérin (BCG) was first administered to humans in 1921 and has subsequently been delivered to an estimated 3 billion individuals, with a low incidence of serious complications. The vaccine is immunogenic and is stable and cheap to produce. Additionally, the vaccine can be engineered to express foreign molecules in a functional form, and this has driven the development of BCG as a recombinant vector to protect against infectious diseases and malignancies such as cancer. However, it is now clear that the existing BCG vaccine has proved insufficient to control the spread of tuberculosis, and a major focus of tuberculosis vaccine development programs is the construction and testing of modified forms of BCG. This review summarizes the strategies employed to develop recombinant forms of BCG and describes the potential of these vaccines to stimulate protective immunity and protect against Mycobacterium tuberculosis infection.

The role of dendritic cells in mycobacterium-induced granulomas

The presence of dendritic cells (DCs) in mycobacterium-containing granulomas, as well as in other granuloma-inducing diseases, is beginning to be appreciated. This review will summarize what is known about DCs with regards to the granuloma and discuss the potential roles DCs may be playing during mycobacterial infection. Potential functions may include mycobacterial dissemination from lesions or sampling of granuloma-containing mycobacterial antigens and migration to the draining lymph nodes to maintain continuous T cell priming. Additionally, the review will discuss the potential outcomes of DC-T cell cross-talk within the granuloma and whether it results in boosting the effector functions of newly arrived T cells or anergizing systemic T cells locally. Understanding the DCs complex and changing role during this critical stage may help explain how latency is achieved and maintained. Such knowledge might also lead to improved vaccination strategies.

Fms-like tyrosine kinase 3 ligand regulates migratory pattern and antigen uptake of lung dendritic cell subsets in a murine model of allergic airway inflammation

Fms-like tyrosine kinase 3 ligand (Flt3L) reverses the features of allergic airway inflammation and increases a Th2-suppressive regulatory lung CD11c(high)CD11b(low) dendritic cell (DC) subset in a mouse model. We examined the migratory pattern and Ag uptake efficiency of lung DC subsets in the therapeutic effect of Flt3L. Lung CD11c(high)CD11b(low) and CD11c(low)CD11b(high) DCs from PBS-treated, OVA-sensitized, and Flt3L-treated/OVA-sensitized BALB/c mice were sorted using MACS and FACS for phenotype analysis. Lymphatic chemokine expression in thoracic lymph nodes was determined by immunohistochemistry. Migration of two lung DC subsets to lymphatic chemokines was examined in vitro using a Transwell chemotaxis assay. Labeled Ag was intranasally delivered into mouse lung to track the migration and Ag uptake of lung DCs. The in vitro cytokine secretion of mediastinal lymph node cells was determined using ELISA. CD11c(low)CD11b(high) DCs have higher expression of CCR5, CCR6, and CCR7, but lower expression of CCR2 than CD11c(high)CD11b(low) DCs. CD11c(low)CD11b(high) DCs in Flt3L-treated/OVA-sensitized mice demonstrated a less mature phenotype, inefficiency in Ag uptake, and impaired migration in vitro to lymphatic chemokine than those in OVA-sensitized mice. Administration of Flt3L decreased the expression of CCR5 and CCR7 in CD11c(low)CD11b(high) DCs in OVA-sensitized mice. Fewer Ag-carrying cells were detected in the lungs and lymph nodes in Flt3L-treated/OVA-sensitized mice than OVA-sensitized mice with a greater decrease in CD11c(low)CD11b(high) DCs. Mediastinal lymph node cells from Flt3L-treated mice secreted higher levels of Th1 cytokines and IL-10 than OVA-sensitized mice in vitro. In conclusion, Flt3L-generated lung immunogenic CD11c(low)CD11b(high) DCs have a less mature phenotype, impaired Ag uptake, and impaired migration to draining lymph nodes.

T cells in mycobacterial infection and disease

There has been an increase in our understanding of the complexity of the T cell response to mycobacterial infection recently. Improved tools have allowed the determination of the location and kinetics of naïve T cell activation in the mouse as well the variety of function of mycobacteria-specific cells in humans. There is also an increased appreciation of the balance required during mycobacterial infection between anti-bacterial activity and control of the immunopathologic response. The integration of the T cell functional data with the consequences of infection should improve rational vaccine design.

Fms-like tyrosine kinase 3 ligand increases a lung DC subset with regulatory properties in allergic airway inflammation

BACKGROUND

Dendritic cell (DC) subsets display different functional roles in regulating immune responses and lead to various outcomes, including T(H)1 versus T(H)2 or regulatory versus immunologic responses. Administration of Fms-like tyrosine kinase 3 (Flt3) ligand prevents and reverses allergic airway inflammation and airway hyperresponsiveness in a mouse model. However, the underlying mechanisms are unclear.

OBJECTIVE

We characterized and examined the role of lung DC subsets in the therapeutic effect of Flt3 ligand.

METHODS

DCs were isolated from the lungs of ovalbumin (OVA)-sensitized and OVA-challenged mice treated with recombinant human Flt3 ligand. Two populations of CD11c+ cells labeled with fluorochrome-conjugated antibodies were sorted. The ability of the purified cells to stimulate T-cell proliferation and cytokine secretion patterns by different DC subsets was examined. Also, DCs were adoptively transferred in mice to examine their effect on pulmonary function.

RESULTS

Two DC populations, CD11c(high)CD11b(low) and CD11c(low)CD11b(high), were identified in the lungs of naive and OVA-sensitized and OVA-challenged mice with and without treatment with Flt3 ligand. The expression levels of CD8alpha, B220, CD19, F4/80, MHC II, CCR7, CD40, programmed death ligand 1, programmed death ligand 2, CD80, and CD86 were distinctly different between the 2 DC populations, which supports the notion that CD11c(high)CD11b(low) and CD11c(low)CD11b(high) DCs potentially have regulatory and immunogenic properties, respectively. Administration of Flt3 ligand increased the DCs with regulatory potential in the lungs of antigen-sensitized mice, and CD11c(high)CD11b(low) DCs acquired a maximum degree of regulatory capacity after Flt3 ligand treatment.

CONCLUSION

These data suggest that Flt3 ligand reverses airway hyperresponsiveness by regulating the function of lung DCs in a mouse model of allergic airway inflammation.