Transfer of macrophage-derived mycobacterial antigens to dendritic cells can induce naïve T-cell activation

Antigen transfer and its effect on vaccine-induced immune amplification and tolerance

Antigen transfer refers to the process of intercellular information exchange, where antigenic components including nucleic acids, antigen proteins/peptides and peptide-major histocompatibility complexes (p-MHCs) are transmitted from donor cells to recipient cells at the thymus, secondary lymphoid organs (SLOs), intestine, allergic sites, allografts, pathological lesions and vaccine injection sites via trogocytosis, gap junctions, tunnel nanotubes (TNTs), or extracellular vesicles (EVs). In the context of vaccine inoculation, antigen transfer is manipulated by the vaccine type and administration route, which consequently influences, even alters the immunological outcome, i.e., immune amplification and tolerance. Mainly focused on dendritic cells (DCs)-based antigen receptors, this review systematically introduces the biological process, molecular basis and clinical manifestation of antigen transfer.

Role and contribution of pulmonary CD103+ dendritic cells in the adaptive immune response to Mycobacterium tuberculosis

Despite international control programmes, the global burden of tuberculosis remains enormous. Efforts to discover novel drugs have largely focused on targeting the bacterium directly. Alternatively, manipulating the host immune response may represent a valuable approach to enhance immunological clearance of the bacilli, but necessitates a deeper understanding of the immune mechanisms associated with protection against Mycobacterium tuberculosis infection. Here, we examined the various dendritic cells (DC) subsets present in the lung and draining lymph nodes (LN) from mice intra-tracheally infected with M. tuberculosis. We showed that although limited in number, pulmonary CD103⁺ DCs appeared to be involved in the initial transport of mycobacteria to the draining mediastinal LN and subsequent activation of T cells. Using CLEC9A-DTR transgenic mice enabling the inducible depletion of CD103⁺ DCs, we established that this DC subset contributes to the control of mycobacterial burden and plays a role in the early activation of T cells, in particular CD8⁺ T cells. Our findings thus support a previously unidentified role for pulmonary CD103⁺ DCs in the rapid mobilization of mycobacteria from the lungs to the draining LN soon after exposure to M. tuberculosis, which is a critical step for the development of the host adaptive immune response.

Transfer of antigen from human B cells to dendritic cells

The cooperation of B lymphocytes with other antigen presenting cells (APCs) is often necessary in the efficient processing and presentation of antigen. Herein, we describe a mechanism by which B cells physically interact with dendritic cells (DCs) resulting in the transfer of B cell receptor (BCR)-enriched antigen to these APCs. Antigen transfer involves direct contact between the two cells followed by the capture of B cell derived membrane and intracellular components. Strikingly, DCs acquire greater amounts of antigen by transfer from B cells than by endocytosis of free antigen. Blocking scavenger receptor A, a DC surface receptor involved in membrane acquisition, abrogates these events. We propose that antigen transfer from B cells to DCs results in a more focused immunologic response due to the selective editing of Ag by the BCR.

Contrasting roles of macrophages and dendritic cells in controlling initial pulmonary Brucella infection

Control of pulmonary pathogens constitutes a challenging task as successful immune responses need to be mounted without damaging the lung parenchyma. Using immunofluorescence microscopy and flow cytometry, we analyzed in the mouse the initial innate immune response that follows intranasal inoculation of Brucella abortus. Bacteria were absent from parenchymal dendritic cells (DC) but present in alveolar macrophages in which they replicated. When the number of alveolar macrophages was reduced prior to Brucella infection, small numbers of pulmonary DC were infected and a massive recruitment of TNF-α- and iNOS-producing DC ensued. Coincidentally, Brucella disseminated to the lung-draining mediastinal lymph nodes (LN) where they replicated in both migratory DC and migratory alveolar macrophages. Together, these results demonstrate that alveolar macrophages are critical regulators of the initial innate immune response against Brucella within the lungs and show that pulmonary DC and alveolar macrophages play rather distinct roles in the control of microbial burden.

Role of dendritic cells in the lung: in vitro models, animal models and human studies

Dendritic cells (DCs) are the most potent antigen-presenting cells in the human lung and are now recognized as crucial initiators of immune responses in general. They are arranged as sentinels in a dense surveillance network inside and below the epithelium of the airways and alveoli, where they are ideally situated to sample inhaled antigen. DCs are known to play a pivotal role in maintaining the balance between tolerance and active immune response in the respiratory system. It is no surprise that the lungs became a main focus of DC-related investigations as this organ provides a large interface for interactions of inhaled antigens with the human body. During recent years there has been a constantly growing body of lung DC-related publications that draw their data from in vitro models, animal models and human studies. This review focuses on the biology and functions of different DC populations in the lung and highlights the advantages and drawbacks of different models with which to study the role of lung DCs. Furthermore, we present a number of up-to-date visualization techniques to characterize DC-related cell interactions in vitro and/or in vivo.

T cell and APC dynamics in situ control the outcome of vaccination

The factors controlling the progression of an immune response to generation of protective memory are poorly understood. We compared the in situ and ex vivo characteristics of CD8 T cells responding to different forms of the same immunogen. Immunization with live Listeria monocytogenes, irradiated L. monocytogenes (IRL), or heat-killed L. monocytogenes (HKL) induced rapid activation of CD8 T cells. However, only IRL and live L. monocytogenes inoculation induced sustained proliferation and supported memory development. Gene and protein expression analysis revealed that the three forms of immunization led to three distinct transcriptional and translational programs. Prior to cell division, CD8 T cell-dendritic cell clusters formed in the spleen after live L. monocytogenes and IRL but not after HKL immunization. Furthermore, HKL immunization induced rapid remodeling of splenic architecture, including loss of marginal zone macrophages, which resulted in impaired bacterial clearance. These results identify initial characteristics of a protective T cell response that have implications for the development of more effective vaccination strategies.

Modulation of Lymphocyte Migration to the Murine Spleen after Marginal Zone Macrophage Phagocytosis of Blood-Borne Particulate Material

Our previous studies identified a role for MZM in the movement of lymphocytes into the splenic white pulp. Here we show that phagocytosis of colloidal carbon by marginal zone macrophages results in a splenic influx of B lymphocytes, and T lymphocytes of memory/activated phenotype, with concomitant upregulation of B Lymphocyte Chemoattractant (BLC, CXCL13) mRNA, a chemokine acting on B and memory/activated T lymphocytes. The recruitment of B cells and activated T cells to the spleen after phagocytic uptake would allow an immune response against blood-borne pathogens to be quickly and effectively mounted by bringing together the two key cell types responsible for generating humoral immunity.

Dendritic Cells and Macrophages Form a Transepithelial Network against Foreign Particulate Antigens

Fine particles (0.1-2.5 microm in diameter) may cause increased pulmonary morbidity and mortality. We demonstrate with a cell culture model of the human epithelial airway wall that dendritic cells extend processes between epithelial cells through the tight junctions to collect particles in the "luminal space" and to transport them through cytoplasmic processes between epithelial cells across the epithelium or to transmigrate through the epithelium to take up particles on the epithelial surface. Furthermore, dendritic cells interacted with particle-loaded macrophages on top of the epithelium and with other dendritic cells within or beneath the epithelium to take over particles. By comparing the cellular interplay of dendritic cells and macrophages across epithelial monolayers of different transepithelial electrical resistance, we found that more dendritic cells were involved in particle uptake in A549 cultures showing a low transepithelial electrical resistance compared with dendritic cells in16HBE14o cultures showing a high transepithelial electrical resistance 10 min (23.9% versus 9.5%) and 4 h (42.1% versus 14.6%) after particle exposition. In contrast, the macrophages in A549 co-cultures showed a significantly lower involvement in particle uptake compared with 16HBE14o co-cultures 10 min (12.8% versus 42.8%) and 4 h (57.4% versus 82.7%) after particle exposition. Hence we postulate that the epithelial integrity influences the particle uptake by dendritic cells, and that these two cell types collaborate as sentinels against foreign particulate antigen by building a transepithelial interacting cellular network.

Environmental strains of Mycobacterium avium interfere with immune responses associated with Mycobacterium bovis BCG vaccination

Prior exposure of a vaccinee to certain species of environmental mycobacteria can prime the immune system against common mycobacterial antigens, which can in turn reduce the subsequent efficacy of live attenuated mycobacterial vaccines (such as Mycobacterium bovis BCG), in both human and livestock vaccination programs. In this study, two strains of Mycobacterium avium, both isolated from New Zealand livestock, were investigated to determine their growth characteristics and effects on the immune system in murine models. Markedly different effects on the immune system were observed; an IS901-negative strain (WAg 207) induced significant up-regulation of cell surface activation markers (major histocompatibility complex II, CD80, and CD86) on in vitro-derived dendritic cells and induced the release of proinflammatory monokines (interleukin-1beta [IL-1beta], IL-6, and tumor necrosis factor alpha) in dendritic cell-macrophage cocultures following direct in vitro contact of cells with bacteria. In contrast, an IS901-positive strain (WAg 206) had none of these effects. When mice were exposed to M. avium via oral infection prior to BCG parenteral immunization, both strains were shown to be capable of decreasing subsequent antigen-stimulated gamma interferon secretion by splenic lymphocytes, although this effect was more significant for strain WAg 206. Both strains also induced a mycobacterial antigen-specific serological response in M. avium-sensitized and BCG-immunized mice; this response was greater in WAg 206-sensitized mice, and there was a predominance of immunoglobulin G1 antibody. The down-regulation of IFN-gamma responses and the up-regulation of antibody responses are characteristic of a switch to a type 2 immune response. The different results may be linked to the inherent growth characteristics of the two strains, since WAg 206 was shown to grow slowly in murine macrophages in vitro and to cause a persistent systemic infection following infection in vivo, while WAg 207 grew fast and did not persist in mice. The implications of these findings for BCG vaccination protocols are discussed.

Binding of the adhesion and pathogen receptor DC-SIGN by monocytes is regulated by the density of Lewis X molecules

Soluble DC-SIGN (CD209) bind unsialylated Lewis X epitopes that are abundantly expressed on neutrophils. Due to the low expression of unsialylated Lewis X epitopes on monocytes, no binding of soluble DC-SIGN molecules was seen. In contrast, beads coated with multiple DC-SIGN molecules show a high percentage of binding to monocytes. The increased number of DC-SIGN molecules present on the beads enable multivalent interactions between the DC-SIGN molecules and the scarce Lewis X epitopes present on monocytes. Increased expression of unsialylated Lewis X epitopes on monocytes after neuraminidase treatment coincided with enhanced binding to soluble DC-SIGN. Multiple unsialylated Lewis X epitopes in close proximity of each other are now able to interact multivalently to soluble DC-SIGN. From these findings, we conclude that firm interactions between DC-SIGN and monocytes can be established by either increasing the density of DC-SIGN molecules at the cell surface or by increasing the number of Lewis X epitopes. Regulating the number of ligands endows monocytes with the capacity to modulate binding to DC-SIGN. This may result in a bi-directional cross-talk between DC and monocytes, to modulate innate and/or adaptive immune responses.

In Vivo Depletion of CD11c+Cells Delays the CD4+T Cell Response toMycobacterium tuberculosisand Exacerbates the Outcome of Infection

Although dendritic cells (DC) are potent APC that prime T cells against many pathogens, there is no direct evidence that DC are required for immunity to Mycobacterium tuberculosis (Mtb) infection. The requirement for DC to prime the CD4+ T cell response following Mtb infection was investigated using pCD11c-diptheria toxin receptor/GFP transgenic mice, in which DC can be transiently ablated in vivo. We show a critical role for DC in initiation of the CD4+ T cell response to the mycobacterial Ag early secretory Ag of tuberculosis 6. The delay in initiating the Ag-specific T cell response led to impaired control of Mtb replication. Interestingly, DC were not required for the secondary CD4+ T cell response following Mtb infection in peptide-vaccinated mice. Thus, this study shows that DC are essential for the initiation of the adaptive T cell response to the human pathogen Mtb.

Virus-specific CD4+ and CD8+ cytotoxic T-cell responses and long-term T-cell memory in individuals vaccinated against polio

The presence of poliovirus (PV)-specific CD4(+) T cells in individuals vaccinated against polio has been shown, but CD8(+) T-cell responses have not been described. Here, we functionally characterize the CD4(+) T-cell response and show for the first time that dendritic cells and macrophages can stimulate PV-specific CD8(+) T-cell responses in vitro from vaccinees. Both CD4(+) T and CD8(+) T cells secrete gamma interferon in response to PV antigens and are cytotoxic via the perforin/granzyme B-mediated pathway. Furthermore, the T cells also recognize and kill Sabin 1 vaccine-infected targets. The macrophage-stimulated CD4(+) T and CD8(+) T cells most likely represent memory T cells that persist for long periods in vaccinated individuals. Thus, immunity to PV vaccination involves not only an effective neutralizing antibody titer but also long-term CD4(+) and CD8(+) cytotoxic T-cell responses.

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.