Cooperative stimulation of dendritic cells by Cryptococcus neoformans mannoproteins and CpG oligodeoxynucleotides

Dendritic Cells: Multifunctional Roles in Host Defenses to Cryptococcus Infections

Fungal infections are an increasingly growing public health concern, and Cryptococcus is one of the most problematic fungal organisms causing substantial mortality and morbidity worldwide. Clinically, this high incidence of cryptococcosis is most commonly seen in immunocompromised patients, especially those who lack an adaptive T cell response, such as HIV/AIDS patients. However, patients with other underlying immunodeficiencies are also at an increased risk for cryptococcosis. The adaptive immune response, in particular the Th1/Th17 T-cell-mediated responses, to pulmonary Cryptococcus infections are required for host protection. Dendritic cells (DCs), encompassing multiple subsets identified to date, are recognized as the major professional antigen-presenting cell (APC) subset essential for the initiation and execution of T-cell immunity. Apart from their prominent role in orchestration of the adaptive arm of the immune defenses, DCs are fully armed cells from the innate immune system capable of the recognition, uptake, and killing of the fungal cells. Thus, DCs serve as a critical point for the endpoint outcomes of either fungal control or unrestrained fungal infection. Multiple studies have shown that DCs are required for anti-cryptococcal defense in the lungs. In addition, the role of DCs in Cryptococcus gattii infections is just starting to be elucidated. C. gattii has recently risen to prominence with multiple outbreaks in the US and Canada, demonstrating increased virulence in non-immunocompromised individuals. C. gattii infection fails to generate an inflammatory immune response or a protective Th1/Th17 T cell response, at least in part, through a lack of proper DC function. Here we summarize the multiple roles of DCs, including subsets of DCs in both mouse and human models, the roles of DCs during cryptococcal infection, and mechanisms by cryptococcal cells to attempt to undermine these host defenses.

Effects of altered N-glycan structures of Cryptococcus neoformans mannoproteins, MP98 (Cda2) and MP84 (Cda3), on interaction with host cells

Cryptococcus neoformans is an opportunistic human fungal pathogen causing lethal meningoencephalitis. It has several cell wall mannoproteins (MPs) identified as immunoreactive antigens. To investigate the structure and function of N-glycans assembled on cryptococcal cell wall MPs in host cell interactions, we purified MP98 (Cda2) and MP84 (Cda3) expressed in wild-type (WT) and N-glycosylation-defective alg3 mutant (alg3Δ) strains. HPLC and MALDI-TOF analysis of the MP proteins from the WT revealed protein-specific glycan structures with different extents of hypermannosylation and xylose/xylose phosphate addition. In alg3Δ, MP98 and MP84 had truncated core N-glycans, containing mostly five and seven mannoses (M5 and M7 forms), respectively. In vitro adhesion and uptake assays indicated that the altered core N-glycans did not affect adhesion affinities to host cells although the capacity to induce the immune response of bone-marrow derived dendritic cells (BMDCs) decreased. Intriguingly, the removal of all N-glycosylation sites on MP84 increased adhesion to host cells and enhanced the induction of cytokine secretion from BMDCs compared with that on MP84 carrying WT N-glycans. Therefore, the structure-dependent effects of N-glycans suggested their complex roles in modulating the interaction of MPs with host cells to avoid nonspecific adherence to host cells and host immune response hyperactivation.

Interactions between the NLRP3-Dependent IL-1β and the Type I Interferon Pathways in Human Plasmacytoid Dendritic Cells

Generally, a reciprocal antagonistic interaction exists between the antiviral type I interferon (IFN) and the antibacterial nucleotide-binding oligomerization domain (NOD)-like receptor pyrin domain containing 3 (NLRP3)-dependent IL-1β pathways that can significantly shape immune responses. Plasmacytoid dendritic cells (pDCs), as professional type I IFN-producing cells, are the major coordinators of antiviral immunity; however, their NLRP3-dependent IL-1β secretory pathway is poorly studied. Our aim was to determine the functional activity of the IL-1β pathway and its possible interaction with the type I IFN pathway in pDCs. We found that potent nuclear factor-kappa B (NF-κB) inducers promote higher levels of pro-IL-1β during priming compared to those activation signals, which mainly trigger interferon regulatory factor (IRF)-mediated type I IFN production. The generation of cleaved IL-1β requires certain secondary signals in pDCs and IFN-α or type I IFN-inducing viruses inhibit IL-1β production of pDCs, presumably by promoting the expression of various NLRP3 pathway inhibitors. In line with that, we detected significantly lower IL-1β production in pDCs of psoriasis patients with elevated IFN-α levels. Collectively, our results show that the NLRP3-dependent IL-1β secretory pathway is inducible in pDCs; however, it may only prevail under inflammatory conditions, in which the type I IFN pathway is not dominant.

Type I Interferon Production of Plasmacytoid Dendritic Cells under Control

One of the most powerful and multifaceted cytokines produced by immune cells are type I interferons (IFNs), the basal secretion of which contributes to the maintenance of immune homeostasis, while their activation-induced production is essential to effective immune responses. Although, each cell is capable of producing type I IFNs, plasmacytoid dendritic cells (pDCs) possess a unique ability to rapidly produce large amounts of them. Importantly, type I IFNs have a prominent role in the pathomechanism of various pDC-associated diseases. Deficiency in type I IFN production increases the risk of more severe viral infections and the development of certain allergic reactions, and supports tumor resistance; nevertheless, its overproduction promotes autoimmune reactions. Therefore, the tight regulation of type I IFN responses of pDCs is essential to maintain an adequate level of immune response without causing adverse effects. Here, our goal was to summarize those endogenous factors that can influence the type I IFN responses of pDCs, and thus might serve as possible therapeutic targets in pDC-associated diseases. Furthermore, we briefly discuss the current therapeutic approaches targeting the pDC-type I IFN axis in viral infections, cancer, autoimmunity, and allergy, together with their limitations defined by the Janus-faced nature of pDC-derived type I IFNs.

Immunomodulatory Role of Capsular Polysaccharides Constituents of Cryptococcus neoformans

Cryptococcosis is a systemic fungal infection caused by Cryptococcus neoformans. In immunocompetent patients, cryptococcal infection is often confined to the lungs. In immunocompromised individuals, C. neoformans may cause life-threatening illness, either from novel exposure or through reactivation of a previously acquired latent infection. For example, cryptococcal meningitis is a severe clinical disease that can manifest in people that are immunocompromised due to AIDS. The major constituents of the Cryptococcus polysaccharide capsule, glucuronoxylomannan (GXM), and galactoxylomannan (GalXM), also known as glucuronoxylomanogalactan (GXMGal), are considered the primary virulence factors of Cryptococcus. Despite the predominance of GXM in the polysaccharide capsule, GalXM has more robust immunomodulatory effects on host cellular immunity. This review summarizes current knowledge regarding host-Crytococcus neoformans interactions and the role of capsular polysaccharides in host immunomodulation. Future studies will likely facilitate a better understanding of the mechanisms involved in antigenic recognition and host immune response to C. neoformans and lead to the development of new therapeutic pathways for cryptococcal infection.

Peeling the onion: the outer layers of Cryptococcus neoformans

Cryptococcus neoformans is an opportunistic fungal pathogen that is ubiquitous in the environment. It causes a deadly meningitis that is responsible for over 180,000 deaths worldwide each year, including 15% of all AIDS-related deaths. The high mortality rates for this infection, even with treatment, suggest a need for improved therapy. Unique characteristics of C. neoformans may suggest directions for drug discovery. These include features of three structures that surround the cell: the plasma membrane, the cell wall around it, and the outermost polysaccharide capsule. We review current knowledge of the fundamental biology of these fascinating structures and highlight open questions in the field, with the goal of stimulating further investigation that will advance basic knowledge and human health.

A Call to Arms: Quest for a Cryptococcal Vaccine

Cryptococcosis remains a significant cause of morbidity and mortality world-wide, particularly among AIDS patients. Yet, to date, there are no licensed vaccines clinically available to treat or prevent cryptococcosis. In this review, we provide a rationale to support continued investment in Cryptococcus vaccine research, potential challenges that must be overcome along the way, and a literature review of the current progress underway towards developing a vaccine to prevent cryptococcosis.

Immunology of Cryptococcal Infections: Developing a Rational Approach to Patient Therapy

Cryptococcal meningoencephalitis is responsible for upwards of 15% of HIV-related deaths worldwide and is currently the most common cause of non-viral meningitis in the US, affecting both previously healthy and people with immune suppression caused by cancer chemotherapy, transplantation, and biologic therapies. Despite a continued 30-50% attributable mortality, recommended therapeutic strategies have remained largely unchanged since the 1950s. Recent murine models and human studies examining the role of the immune system in both susceptibility to the infection as well as host damage have begun to influence patient care decisions. The Damage Framework Response, originally proposed in 1999, was recently used to discuss dichotomous etiologies of host damage in cryptococcal disease. These include patients suffering microbiological damage with low host immunity (especially those immunosuppressed with HIV) and those having low (live) microbiological burden but high immune-mediated damage (HIV-related immune reconstitution syndrome and non-HIV-related postinfectious inflammatory response syndrome). Cryptococcal disease in previously healthy hosts, albeit rare, has been known for a long time. Immunophenotyping and dendritic cell-T cell signaling studies on cerebral spinal fluid of these rare patients reveal immune capacity for recognition and T-cell activation pathways including increased levels of HLA-DR and CD56. However, despite effective T-cell signals, brain biopsy and autopsy specimens demonstrated an M2 alternative macrophage polarization and poor phagocytosis of fungal cells. These studies expand the paradigm for cryptococcal disease susceptibility to include a prominent role for immune-mediated damage and suggest a need for careful individual consideration of immune activation during therapy of cryptococcal disease in diverse hosts.

Interactions of Cryptococcus with Dendritic Cells

The fungal pathogens Cryptococcus neoformans and Cryptococcus gattii can cause life-threatening infections in immune compromised and immune competent hosts. These pathogens enter the host via inhalation, and respiratory tract innate immune cells such as dendritic cells (DCs) are one of the first host cells they encounter. The interactions between Cryptococcus and innate immune cells play a critical role in the progression of disease in the host. This review will focus specifically on the interactions between Cryptococcus and dendritic cells (DCs), including recognition/processing by DCs, effects of immune mediators on DC recruitment and activity, and the potential for DC vaccination against cryptococcosis.

Innate Immunity against Cryptococcus, from Recognition to Elimination

Cryptococcus species, the etiological agents of cryptococcosis, are encapsulated fungal yeasts that predominantly cause disease in immunocompromised individuals, and are responsible for 15% of AIDS-related deaths worldwide. Exposure follows the inhalation of the yeast into the lung alveoli, making it incumbent upon the pattern recognition receptors (PRRs) of pulmonary phagocytes to recognize highly conserved pathogen-associated molecular patterns (PAMPS) of fungi. The main challenges impeding the ability of pulmonary phagocytes to effectively recognize Cryptococcus include the presence of the yeast's large polysaccharide capsule, as well as other cryptococcal virulence factors that mask fungal PAMPs and help Cryptococcus evade detection and subsequent activation of the immune system. This review will highlight key phagocyte cell populations and the arsenal of PRRs present on these cells, such as the Toll-like receptors (TLRs), C-type lectin receptors, NOD-like receptors (NLRs), and soluble receptors. Additionally, we will highlight critical cryptococcal PAMPs involved in the recognition of Cryptococcus. The question remains as to which PRR-ligand interaction is necessary for the recognition, phagocytosis, and subsequent killing of Cryptococcus.

The enhanced antitumor-specific immune response with mannose- and CpG-ODN-coated liposomes delivering TRP2 peptide

PURPOSE

Dendritic cell (DC)-based cancer vaccines is a newly emerging and potent form of immune therapy. As for any new technology, there are still considerable challenges that need to be addressed. Here, we investigate the antitumor potential of a novel liposomal vaccine, M/CpG-ODN-TRP2-Lipo.

METHODS

We developed a vaccination strategy by assembling the DC-targeting mannose and immune adjuvant CpG-ODN on the surface of liposomes, which were loaded with melanoma-specific TRP2_180-188 peptide as liposomal vaccine. M/CpG-ODN-TRP2-Lipo treatment was used to intendedly induce activation of DCs and antitumor- specific immune response in vivo.

RESULTS

Our results demonstrated in vitro that the prepared liposomal particles were efficiently taken up by DCs. This uptake led to an enhanced activation of DCs, as measured by the upregulation of MHC II, CD80, and CD86. Furthermore, M/CpG-ODN-TRP2-Lipo effectively inhibited the growth of implanted B16 melanoma and prolonged the survival of mice. This therapy significantly reduced the number of myeloid-derived suppressor cells (MDSCs) and regulatory T cells, while simultaneously increasing the number of activated T cells, tumor antigen-specific CD8⁺ cytotoxic T cells, and interferon-γ-producing cells. At the same time, it was found to suppress tumor angiogenesis and tumor cell proliferation, as well as up-regulate their apoptosis. Interestingly, MyD88-knockout mice had significantly shorter median survival times compared to wild-type mice following the administration of M/CpG-ODN-TRP2-Lipo.

CONCLUSIONS

The results suggested that the antitumor activities of the vaccine partially rely on the Myd88 signaling pathway. Interestingly, compared to whole tumor cell lysate-based vaccine, M/CpG-ODN-TRP2-Lipo, tumor specific antigen peptide-based vaccine, improved survival of tumor-bearing mice as well as enhanced their antitumor responses. All in all, we describe a novel vaccine formulation, M/CpG-ODN-TRP2-Lipo, with the aim of improving antitumor responses by alleviating the immunosuppressive environment in tumors.

The Overlooked Glycan Components of the Cryptococcus Capsule

Pathogenic species of Cryptococcus kill approximately 200,000 people each year. The most important virulence mechanism of C. neoformans and C. gattii, the causative agents of human and animal cryptococcosis, is the ability to form a polysaccharide capsule. Acapsular mutants of C. neoformans are avirulent in mice models of infection, and extracellularly released capsular polysaccharides are deleterious to the immune system. The principal capsular component in the Cryptococcus genus is a complex mannan substituted with xylosyl and glucuronyl units, namely glucuronoxylomannan (GXM). The second most abundant component of the cryptococcal capsule is a galactan with multiple glucuronyl, xylosyl, and mannosyl substitutions, namely glucuronoxylomannogalactan (GXMGal). The literature about the structure and functions of these two polysaccharides is rich, and a number of comprehensive reviews on this topic are available. Here, we focus our discussion on the less explored glycan components associated with the cryptococcal capsule, including mannoproteins and chitin-derived molecules. These glycans were selected for discussion on the basis that i) they have been consistently detected not only in the cell wall but also within the cryptococcal capsular network and ii) they have functions that impact immunological and/or pathogenic mechanisms in the Cryptococcus genus. The reported functions of these molecules strongly indicate that the biological roles of the cryptococcal capsule go far beyond the well-known properties of GXM and GXMGal.

Host Control of Fungal Infections: Lessons from Basic Studies and Human Cohorts

In the last few decades, the AIDS pandemic and the significant advances in the medical management of individuals with neoplastic and inflammatory conditions have resulted in a dramatic increase in the population of immunosuppressed patients with opportunistic, life-threatening fungal infections. The parallel development of clinically relevant mouse models of fungal disease and the discovery and characterization of several inborn errors of immune-related genes that underlie inherited human susceptibility to opportunistic mycoses have significantly expanded our understanding of the innate and adaptive immune mechanisms that protect against ubiquitous fungal exposures. This review synthesizes immunological knowledge derived from basic mouse studies and from human cohorts and provides an overview of mammalian antifungal host defenses that show promise for informing therapeutic and vaccination strategies for vulnerable patients.

Antifungal Activity of Plasmacytoid Dendritic Cells and the Impact of Chronic HIV Infection

Due to the effectiveness of combined antiretroviral therapy, people living with HIV can control viral replication and live longer lifespans than ever. However, HIV-positive individuals still face challenges to their health and well-being, including dysregulation of the immune system resulting from years of chronic immune activation, as well as opportunistic infections from pathogenic fungi. This review focuses on one of the key players in HIV immunology, the plasmacytoid dendritic cell (pDC), which links the innate and adaptive immune response and is notable for being the body’s most potent producer of type-I interferons (IFNs). During chronic HIV infection, the pDC compartment is greatly dysregulated, experiencing a substantial depletion in number and compromise in function. This immune dysregulation may leave patients further susceptible to opportunistic infections. This is especially important when considering a new role for pDCs currently emerging in the literature: in addition to their role in antiviral immunity, recent studies suggest that pDCs also play an important role in antifungal immunity. Supporting this new role, pDCs express C-type lectin receptors including dectin-1, dectin-2, dectin-3, and mannose receptor, and toll-like receptors-4 and -9 that are involved in recognition, signaling, and response to a wide variety of fungal pathogens, including Aspergillus fumigatus, Cryptococcus neoformans, Candida albicans, and Pneumocystis jirovecii. Accordingly, pDCs have been demonstrated to recognize and respond to certain pathogenic fungi, measured via activation, cytokine production, and fungistatic activity in vitro, while in vivo mouse models indicated a strikingly vital role for pDCs in survival against pulmonary Aspergillus challenge. Here, we discuss the role of the pDC compartment and the dysregulation it undergoes during chronic HIV infection, as well as what is known so far about the role and mechanisms of pDC antifungal activity.

Innate Immune Responses to Cryptococcus

Cryptococcus species are encapsulated fungi found in the environment that predominantly cause disease in immunocompromised hosts after inhalation into the lungs. Even with contemporary antifungal regimens, patients with cryptococcosis continue to have high morbidity and mortality rates. The development of more effective therapies may depend on our understanding of the cellular and molecular mechanisms by which the host promotes sterilizing immunity against the fungus. This review will highlight our current knowledge of how Cryptococcus, primarily the species C. neoformans, is sensed by the mammalian host and how subsequent signaling pathways direct the anti-cryptococcal response by effector cells of the innate immune system.

Role of dendritic cell-pathogen interactions in the immune response to pulmonary cryptococcal infection

This review discusses the unique contributions of dendritic cells (DCs) to T-cell priming and the generation of effective host defenses against Cryptococcus neoformans (C.neo) infection. We highlight DC subsets involved in the early and later stages of anticryptococcal immune responses, interactions between C.neo pathogen-associated molecular patterns and pattern recognition receptors expressed by DC, and the influence of DC on adaptive immunity. We emphasize recent studies in mouse models of cryptococcosis that illustrate the importance of DC-derived cytokines and costimulatory molecules and the potential role of DC epigenetic modifications that support maintenance of these signals throughout the immune response to C.neo. Lastly, we stipulate where these advances can be developed into new, immune-based therapeutics for treatment of this global pathogen.

Innate host defenses against Cryptococcus neoformans

Cryptococcus neoformans, the predominant etiological agent of cryptococcosis, can cause life-threatening infections of the central nervous system in immunocompromised and immunocompetent individuals. Cryptococcal meningoencephalitis is the most common disseminated fungal infection in AIDS patients, and remains the third most common invasive fungal infection among organ transplant recipients. The administration of highly active antiretroviral therapy (HAART) has resulted in a decrease in the number of cases of AIDS-related cryptococcosis in developed countries, but in developing countries where HAART is not readily available, Cryptococcus is still a major concern. Therefore, there is an urgent need for the development of novel therapies and/or vaccines to combat cryptococcosis. Understanding the protective immune responses against Cryptococcus is critical for development of vaccines and immunotherapies to combat cryptococcosis. Consequently, this review focuses on our current knowledge of protective immune responses to C. neoformans, with an emphasis on innate immune responses.

Cryptococcus and Phagocytes: Complex Interactions that Influence Disease Outcome

Cryptococcus neoformans and C. gattii are fungal pathogens that cause life-threatening disease. These fungi commonly enter their host via inhalation into the lungs where they encounter resident phagocytes, including macrophages and dendritic cells, whose response has a pronounced impact on the outcome of disease. Cryptococcus has complex interactions with the resident and infiltrating innate immune cells that, ideally, result in destruction of the yeast. These phagocytic cells have pattern recognition receptors that allow recognition of specific cryptococcal cell wall and capsule components. However, Cryptococcus possesses several virulence factors including a polysaccharide capsule, melanin production and secretion of various enzymes that aid in evasion of the immune system or enhance its ability to thrive within the phagocyte. This review focuses on the intricate interactions between the cryptococci and innate phagocytic cells including discussion of manipulation and evasion strategies used by Cryptococcus, anti-cryptococcal responses by the phagocytes and approaches for targeting phagocytes for the development of novel immunotherapeutics.

Nanocarriers for DNA Vaccines: Co-Delivery of TLR-9 and NLR-2 Ligands Leads to Synergistic Enhancement of Proinflammatory Cytokine Release

Adjuvants enhance immunogenicity of vaccines through either targeted antigen delivery or stimulation of immune receptors. Three cationic nanoparticle formulations were evaluated for their potential as carriers for a DNA vaccine, and muramyl dipeptide (MDP) as immunostimulatory agent, to induce and increase immunogenicity of Mycobacterium tuberculosis antigen encoding plasmid DNA (pDNA). The formulations included (1) trimethyl chitosan (TMC) nanoparticles, (2) a squalene-in-water nanoemulsion, and (3) a mineral oil-in-water nanoemulsion. The adjuvant effect of the pDNA-nanocomplexes was evaluated by serum antibody analysis in immunized mice. All three carriers display a strong adjuvant effect, however, only TMC nanoparticles were capable to bias immune responses towards Th1. pDNA naturally contains immunostimulatory unmethylated CpG motifs that are recognized by Toll-like receptor 9 (TLR-9). In mechanistic in vitro studies, activation of TLR-9 and the ability to enhance immunogenicity by simultaneously targeting TLR-9 and NOD-like receptor 2 (NLR-2) was determined by proinflammatory cytokine release in RAW264.7 macrophages. pDNA in combination with MDP was shown to significantly increase proinflammatory cytokine release in a synergistic manner, dependent on NLR-2 activation. In summary, novel pDNA-Ag85A loaded nanoparticle formulations, which induce antigen specific immune responses in mice were developed, taking advantage of the synergistic combinations of TLR and NLR agonists to increase the adjuvanticity of the carriers used.

Masking the Pathogen: Evolutionary Strategies of Fungi and Their Bacterial Counterparts

Pathogens reduce immune recognition of their cell surfaces using a variety of inert structural polysaccharides. For example, capsular polysaccharides play critical roles in microbial survival strategies. Capsules are widely distributed among bacterial species, but relatively rare in eukaryotic microorganisms, where they have evolved considerable complexity in structure and regulation and are exemplified by that of the HIV/AIDS-related fungus Cryptococcus neoformans. Endemic fungi that affect normal hosts such as Histoplasma capsulatum and Blastomyces dermatitidis have also evolved protective polysaccharide coverings in the form of immunologically inert α-(1,3)-glucan polysaccharides to protect their more immunogenic β-(1,3)-glucan-containing cell walls. In this review we provide a comparative update on bacterial and fungal capsular structures and immunogenic properties as well as the polysaccharide masking strategies of endemic fungal pathogens.

CpG DNA assists the whole inactivated H9N2 influenza virus in crossing the intestinal epithelial barriers via transepithelial uptake of dendritic cell dendrites

Intestinal mucosa remains a pivotal barrier for the oral vaccine absorption of H9N2 whole inactivated influenza virus (WIV). However, CpG DNA, as an adjuvant, can effectively improve relevant mucosal and systemic immunity. The downstream mechanism is well confirmed, yet the evidence of CpG DNA assisting H9N2 WIV in transepithelial delivery is lacking. Here, we reported both in vitro and in vivo that CpG DNA combined with H9N2 WIV was capable of recruiting additional dendritic cells (DCs) to the intestinal epithelial cells (ECs) to form transepithelial dendrites (TEDs) for luminal viral uptake. Both CD103(+) and CD103(-) DCs participated in this process. The engagement of the chemokine CCL20 from the apical ECs and the DCs drove DC recruitment and TED formation. Virus-loaded CD103(+) but not CD103(-) DCs also quickly migrated into mesenteric lymph nodes within 2 h. Moreover, the mechanism of CpG DNA was independent of epithelial transcytosis and disruption of the epithelial barriers. Finally, the subsequent phenotypic and functional maturation of DCs was also enhanced. Our findings indicated that CpG DNA improved the delivery of H9N2 WIV via TEDs of intestinal DCs, and this may be an important mechanism for downstream effective antigen-specific immune responses.

Opportunistic yeast pathogens: reservoirs, virulence mechanisms, and therapeutic strategies

Life-threatening invasive fungal infections are becoming increasingly common, at least in part due to the prevalence of medical interventions resulting in immunosuppression. Opportunistic fungal pathogens of humans exploit hosts that are immunocompromised, whether by immunosuppression or genetic predisposition, with infections originating from either commensal or environmental sources. Fungal pathogens are armed with an arsenal of traits that promote pathogenesis, including the ability to survive host physiological conditions and to switch between different morphological states. Despite the profound impact of fungal pathogens on human health worldwide, diagnostic strategies remain crude and treatment options are limited, with resistance to antifungal drugs on the rise. This review will focus on the global burden of fungal infections, the reservoirs of these pathogens, the traits of opportunistic yeast that lead to pathogenesis, host genetic susceptibilities, and the challenges that must be overcome to combat antifungal drug resistance and improve clinical outcome.

Immunity to Cryptococcus neoformans and C. gattii during cryptococcosis

The vast majority of infection with cryptococcal species occurs with Cryptococcus neoformans in the severely immunocompromised. A significant exception to this is the infections of those with apparently normal immune systems by Cryptococcus gattii. Susceptibility to cryptococcosis can be broadly categorised as a defect in adaptive immune responses, especially in T cell immunity. However, innate immune cells such as macrophages play a key role and are likely the primary effector cell in the killing and ultimate clearance of cryptococcal infection. In this review we discuss the current state of our understanding of how the immune system responds to cryptococcal infection in health and disease, with reference to the work communicated at the 9th International Conference on Cryptococcus and Cryptococcosis (ICCC9). We have focussed on cell mediated responses, particularly early in infection, but with the aim of presenting a broad overview of our understanding of immunity to cryptococcal infection, highlighting some recent advances and offering some perspectives on future directions.

Exploiting fungal cell wall components in vaccines

Innate recognition of fungi leads to strong adaptive immunity. Investigators are trying to exploit this observation in vaccine development by combining antigens with evolutionarily conserved fungal cell wall carbohydrates to induce protective responses. Best studied is β-1,3-glucan, a glycan that activates complement and is recognized by dectin-1. Administration of antigens in association with β-1,3-glucan, either by direct conjugation or complexed in glucan particles, results in robust humoral and cellular immune responses. While the host has a host of mannose receptors, responses to fungal mannoproteins generally are amplified if cells are cooperatively stimulated with an additional danger signal such as a toll-like receptor agonist. Chitosan, a polycationic homopolymer of glucosamine manufactured by the deacetylation of chitin, is being studied as an adjuvant in DNA and protein-based vaccines. It appears particularly promising in mucosal vaccines. Finally, universal and organism-specific fungal vaccines have been formulated by conjugating fungal cell wall glycans to carrier proteins. A major challenge will be to advance these experimental findings so that at risk patients can be protected.

Cryptococcus antigens and immune responses: implications for a vaccine

Cryptococcosis is a fungal disease primarily occurring in immunocompromised individuals, such as AIDS patients, and is associated with high morbidity and mortality. However, cryptococcosis can occur within immunocompetent populations as observed during an outbreak in Vancouver Island, British Columbia, Canada, the Pacific Northwest and other regions of the USA and in Mediterranean Europe. Mortality rates due to cryptococcosis have significantly declined in economically developed countries since the widespread implementation of highly active antiretroviral therapy. However, the incidence and mortality of this disease remains high in economically undeveloped areas in Africa and Asia where HIV infections are high and availability of HAART is limited. The continuing AIDS epidemic coupled with the increased usage of immunosuppressive drugs to prevent organ transplant rejection or to treat autoimmune diseases has resulted in an increase in individuals at risk for developing cryptococcosis. The purpose of this review is to discuss the need, challenges and potential for developing vaccines against cryptococcosis.

Type-I interferon secretion in the acute phase promotes Cryptococcus neoformans infection-induced Th17 cell polarization in vitro

Cryptococcosis is a potentially fatal fungal disease commonly identified in patients with acquired immunodeficiency syndrome. Cryptococcus infection induces strong pro-inflammatory cytokine secretion, i.e. type-I interferon (IFN-I) via the Toll-like receptor signaling pathway. However, innate immune responses are insufficient in host defense against fungi infection and the clearance of Cryptococcus is dependent on the T helper (Th)17 cell-mediated mucosal immune response. In this study, IFN-I was identified as the early response cytokine to Cryptococcus neoformans infection via quantitative PCR (qPCR) and IFN-I was demonstrated to be crucial for interleukin (IL)-17A secretion in T cells, but not in innate immune cells. In addition, blockade of IFN-I reduced the protein expression levels of IL-22 and IL-23 in Th17 cells in vitro. These results suggest additional functions of IFN-I in immune regulation, which may be pivotal for the development of clinical immune therapy.

The role of dendritic cells in the innate recognition of pathogenic fungi (A. fumigatus, C. neoformans and C. albicans)

Dendritic cells (DCs) are the bridge between the innate and adaptive immune system. DCs are responsible for sensing and patrolling the environment, initiating a host response and instructing the proper adaptive immune response against pathogens. Recent advances in medical treatments have led to increased use of immunosuppressive drugs, leading to the emergence of fungal species that cause life-threatening infections in humans. Three of these opportunistic fungal pathogens: Aspergillus fumigatus, Candida albicans and Cryptococcus neoformans pose the biggest concern for the immune-compromised host. Here we will review the interactions between DCs and these fungal pathogens, the receptors expressed on DCs that mediate these responses and the signaling mechanisms that shape the adaptive host response.

Pattern recognition receptors: sentinels in innate immunity and targets of new vaccine adjuvants

The innate immune system plays an essential role in the host's first line of defense against microbial invasion, and involves the recognition of distinct pathogen-associated molecular patterns by pattern recognition receptors (PRRs). Activation of PRRs triggers cell signaling leading to the production of proinflammatory cytokines, chemokines and Type 1 interferons, and the induction of antimicrobial and inflammatory responses. These innate responses are also responsible for instructing the development of an appropriate pathogen-specific adaptive immune response. In this review, the focus is on different classes of PRRs that have been identified, including Toll-like receptors, nucleotide-binding oligomerization domain-like receptors, and the retinoic acid-inducible gene-I-like receptors, and their importance in host defense against infection. The role of PRR cooperation in generating optimal immune responses required for protective immunity and the potential of targeting PRRs in the development of a new generation of vaccine adjuvants is also discussed.

Early induction of CCL7 downstream of TLR9 signaling promotes the development of robust immunity to cryptococcal infection

We investigated mechanisms by which TLR9 signaling promoted the development of the protective response to Cryptococcus neoformans in mice with cryptococcal pneumonia. The afferent (week 1) and efferent (week 3) phase immune parameters were analyzed in the infected wild-type (TLR9(+/+)) and TLR-deficient (TLR9(-/-)) mice. TLR9 deletion diminished 1) accumulation and activation of CD11b(+) dendritic cells (DCs), 2) the induction of IFN-γ and CCR2 chemokines CCL7, CCL12, but not CCL2, at week 1, and 3) pulmonary accumulation and activation of the major effector cells CD4(+) and CD8(+) T cells, CD11b(+) lung DCs, and exudate macrophages at week 3. The significance of CCL7 induction downstream of TLR9 signaling was investigated by determining whether CCL7 reconstitution would improve immunological parameters in C. neoformans-infected TLR9(-/-) mice. Early reconstitution with CCL7 1) improved accumulation and activation of CD11b(+) DCs at week 1, 2) restored early IFN-γ production in the lungs, and 3) restored the accumulation of major effector cell subsets. CCL7 administration abolished the difference in lung fungal burdens between TLR9(+/+) and TLR9(-/-) mice at week 3; however, significant reduction of fungal burdens between PBS- and CCL7-treated mice has not been observed, suggesting that additional mechanism(s) apart from early CCL7 induction contribute to optimal fungal clearance in TLR9(+/+) mice. Collectively, we show that TLR9 signaling during the afferent phase contributes to the development of protective immunity by promoting the early induction of CCL7 and IFN-γ and the subsequent early recruitment and activation of DCs and additional effector cells in mice with cryptococcal pneumonia.

Adaptive immunity to fungi

Only a handful of the more than 100,000 fungal species on our planet cause disease in humans, yet the number of life-threatening fungal infections in patients has recently skyrocketed as a result of advances in medical care that often suppress immunity intensely. This emerging crisis has created pressing needs to clarify immune defense mechanisms against fungi, with the ultimate goal of therapeutic applications. Herein, we describe recent insights in understanding the mammalian immune defenses deployed against pathogenic fungi. The review focuses on adaptive immune responses to the major medically important fungi and emphasizes how dendritic cells and subsets in various anatomic compartments respond to fungi, recognize their molecular patterns, and signal responses that nurture and shape the differentiation of T cell subsets and B cells. Also emphasized is how the latter deploy effector and regulatory mechanisms that eliminate these nasty invaders while also constraining collateral damage to vital tissue.

Systems biology of infectious diseases: a focus on fungal infections

The study of infectious disease concerns the interaction between the host species and a pathogen organism. The analysis of such complex systems is improving with the evolution of high-throughput technologies and advanced computational resources. This article reviews integrative, systems-oriented approaches to understanding mechanisms underlying infection, immune response and inflammation to find biomarkers of disease and design new drugs. We focus on the systems biology process, especially the data gathering and analysis techniques rather than the experimental technologies or latest computational resources.

Immunity to fungal infections

Fungal diseases represent an important paradigm in immunology, as they can result from either a lack of recognition by the immune system or overactivation of the inflammatory response. Research in this field is entering an exciting period of transition from studying the molecular and cellular bases of fungal virulence to determining the cellular and molecular mechanisms that maintain immune homeostasis with fungi. The fine line between these two research areas is central to our understanding of tissue homeostasis and its possible breakdown in fungal infections and diseases. Recent insights into immune responses to fungi suggest that functionally distinct mechanisms have evolved to achieve optimal host-fungus interactions in mammals.

A novel tumor-associated pancreatic glycoprotein is internalized by human dendritic cells and induces their maturation

Aberrant glycosylation or overexpression of cell-surface glycosylated tumor-associated Ags (TAA) distinguish neoplastic from normal cells. Interactions of TAA MUC1 and HER2/neu with dendritic cells (DC) preclude efficient processing, which impairs immune responses. It is thus important to define the mechanisms of interactions between DC and glycosylated TAA and their trafficking and processing for further T cell activation. In this work, we study interactions between DC and the oncofetal fucose-rich glycovariants of bile salt-dependent lipase (BSDL), expressed in pancreatic cancer tissues and referred to as pathological BSDL carrying the fucosylated J28 glycotope (pBSDL-J28) because it is characterized by the mAb J28. The expression of pBSDL-J28 was assessed by immunohistochemistry and quantified by confocal microscopy. Nontumoral pancreatic tissues and cells do not express pBSDL-J28. Using multidisciplinary approaches and functional studies, we provide the first evidence, to our knowledge, that this tumoral glycoprotein is rapidly internalized by human DC through macropinocytosis and endocytosis via mannose receptors and then transported to late endosomes for processing. Interestingly, pBSDL-J28 per se induced DC maturation with increased expression of costimulatory and CD83 molecules associated with cytokine secretion (IL-8 and IL-6). Surprisingly, DC retained their full ability to internalize Ags, making this maturation atypical. Finally, the allogeneic pBSDL-J28-treated DC stimulated lymphocyte proliferation. Besides, pulsing DC with pBSDL-J28 C-terminal glycopolypeptide and maturation with CD40L triggered CD4(+) and CD8(+) T cell proliferation. Therefore, interactions of pBSDL-J28, expressed on tumoral pancreatic tissue, with DC may lead to adequate Ag trafficking and processing and result in T cell activation.

Mechanisms of cryptococcal virulence and persistence

Cryptococcus neoformans is an environmental yeast that is a leading cause of fatal mycosis in AIDS patients and a major cause of meningoencephalitis and CNS-related mortality around the globe. Although C. neoformans infection is mostly a manifestation of immune deficiency, up to 25% of cases reported in the USA occur in patients without recognizable immune defects, indicating that C. neoformans can develop mechanisms that allow it to evade immune defenses and persist in noncompromised hosts. This article discusses mechanisms and routes of infection and the most important elements of host response as well as the mechanisms that promote cryptococcal survival within the host. Metabolic adaptation to physiological host conditions and the mechanisms limiting immune recognition, interfering with phagocytosis and extending intracellular survival of C. neoformans are highlighted. We describe the mechanisms by which C. neoformans can alter adaptive host responses, especially cell-mediated immunity, which is required for clearance of this microbe. We also review cryptococcal strategies of survival in the CNS and briefly discuss adaptations developing in response to medical treatment.

Glucuronoxylomannan, galactoxylomannan, and mannoprotein occupy spatially separate and discrete regions in the capsule of Cryptococcus neoformans

The capsular polysaccharides of Cryptococcus neoformans have historically been divided into three components namely, glucuronoxylomannan (GXM), galactoxylomannan (GalXM), and mannoprotein (MP) but their relative spatial-geographical relationship in the capsule is unknown.  To explore this problem would require the capacity for visualizing these components in the capsule.  Prior studies have reported serological reagents to GXM and GalXM but no antibodies are available against MPs.  Consequently, we immunized Balb/c mice with C. neoformans recombinant mannoprotein 98 and recovered twelve monoclonal antibodies (mAbs) of which one, an IgG2a designated 18F2, bound to intact cells by immunofluorescence. mAb 18F2 bound to the cell wall surface in acapsular and encapsulated cells.  Using mAb 18F2 and previously generated antibodies to GXM and GalXM we have established the localization of three capsular components GXM, GalXM and one type of mannoprotein, MP98 on the C. neoformans cell.  The results show that MP98, like GalXM, is found near the cell wall and this information allows us to begin to discern the geography of the cryptococcal capsule.

TLR9 signaling is required for generation of the adaptive immune protection in Cryptococcus neoformans-infected lungs

To determine whether TLR9 signaling contributes to the development of the adaptive immune response to cryptococcal infection, wild-type (TLR9+/+) and TLR9 knockout (TLR9-/-) BALB/c mice were infected intratracheally with 10(4) C. neoformans 52D. We evaluated 1) organ microbial burdens, 2) pulmonary leukocyte recruitment, 3) pulmonary and systemic cytokine induction, and 4) macrophage activation profiles. TLR9 deletion did not affect pulmonary growth during the innate phase, but profoundly impaired pulmonary clearance during the adaptive phase of the immune response (a 1000-fold difference at week 6). The impaired clearance in TLR9-/- mice was associated with: 1) significantly reduced CD4(+), CD8+ T cell, and CD19+ B cell recruitment into the lungs; 2) defects in Th polarization indicated by altered cytokine responses in the lungs, lymphonodes, and spleen; and 3) diminished macrophage accumulation and altered activation profile, including robust up-regulation of Arg1 and FIZZ1 (indicators of alternative activation) and diminished induction of inducible nitric oxide synthase (an indicator of classical activation). Histological analysis revealed defects in granuloma formation and increased numbers of intracellular yeast residing within macrophages in the lungs of TLR9-/- mice. We conclude that TLR9 signaling plays an important role in the development of robust protective immunity, proper recruitment and function of effector cells (lymphocytes and macrophages), and, ultimately, effective cryptococcal clearance from the infected lungs.

Advances in the design and delivery of peptide subunit vaccines with a focus on toll-like receptor agonists

Considerable success has been made with many peptide antigen formulations, and peptide-based vaccines are emerging as the next generation of prophylactic and remedial immunotherapy. However, finding an optimal platform balancing all of the requirements for an effective, specific and safe immune response remains a major challenge for many infectious and chronic diseases. This review outlines how peptide immunogenicity is influenced by the way in which peptides are presented to the immune system, underscoring the need for multifunctional delivery systems that couple antigen and adjuvant into a single construct. Particular attention is given to the ability of Toll-like receptor agonists to act as adjuvants. A survey of recent approaches to developing peptide antigen delivery systems is given, many of which incorporate Toll-like receptor agonists into the design.

Immunostimulatory activity of potential probiotic yeast strains in the dorsal air pouch system and the gut mucosa

AIMS

To determine the immunostimulatory activity of 15 presumptive probiotic yeast strains in the dorsal air pouch system in comparison with their activity in the gut mucosa.

METHODS AND RESULTS

Presumptive probiotic yeast strains previously isolated from human gastrointestinal tract and Feta cheese were further characterized genotypically and biochemically. The Saccharomyces cerevisiae 982, Saccharomyces boulardii KK1 and Kluyveromyces lactis 630 strains exhibited in the air pouch increased polymorphonuclear cell influx and phagocytic activity as well as cytokine production with similar potency as the probiotics Ultra levure S. boulardii and Lactobacillus acidophilus NCFB 1748. Oral administration of these strains in mice results in differential activation of small intestine immune responses concerning IgA and cytokine production as well as Toll-like receptor expression.

CONCLUSION

Besides the Saccharomyces strains 982 and KK1, the K. lactis 630 strain could also be considered as a candidate probiotic.

SIGNIFICANCE AND IMPACT OF THE STUDY

The air pouch model may be used as an alternative and rapid method for the discrimination and selection of potential probiotic yeast strains.

Cryptococcosis: an emerging respiratory mycosis

Cryptococcosis occurs in immunocompromised and, in special cases, immunocompetent individuals. There have been a number of important advances in the field, but, despite current treatment, patients continue to die of the infection. This article reviews cryptococcosis epidemiology, clinical features, and management. Current knowledge is incomplete, however, so this article also discusses some of the gaps in the present understanding of cryptococcosis. The hope is that current research striving to understand the mechanisms of host evasion of Cryptococcus will result in improved treatment regimens that decrease both the mortality and morbidity of cryptococcosis.

The capsule of the fungal pathogen Cryptococcus neoformans

The capsule of the fungal pathogen Cryptococcus neoformans has been studied extensively in recent decades and a large body of information is now available to the scientific community. Well-known aspects of the capsule include its structure, antigenic properties and its function as a virulence factor. The capsule is composed primarily of two polysaccharides, glucuronoxylomannan (GXM) and galactoxylomannan (GalXM), in addition to a smaller proportion of mannoproteins (MPs). Most of the studies on the composition of the capsule have focused on GXM, which comprises more than 90% of the capsule's polysaccharide mass. It is GalXM, however, that is of particular scientific interest because of its immunological properties. The molecular structure of these polysaccharides is very complex and has not yet been fully elucidated. Both GXM and GalXM are high molecular mass polymers with the mass of GXM equaling roughly 10 times that of GalXM. Recent findings suggest, however, that the actual molecular weight might be different to what it has traditionally been thought to be. In addition to their structural roles in the polysaccharide capsule, these molecules have been associated with many deleterious effects on the immune response. Capsular components are therefore considered key virulence determinants in C. neoformans, which has motivated their use in vaccines and made them targets for monoclonal antibody treatments. In this review, we will provide an update on the current knowledge of the C. neoformans capsule, covering aspects related to its structure, synthesis and particularly, its role as a virulence factor.

Toll-like receptors in liver ischemia reperfusion injury: a novel target for therapeutic modulation?

BACKGROUND

There is increasing evidence that Toll-like receptors (TLRs) sense host tissue damage by engaging with endogenous ligands. TLRs are considered to be involved in many primarily non-immune-related diseases. Hepatic ischemia reperfusion injury (IRI) represents one of these disorders.

OBJECTIVE

To present the latest findings supporting the involvement of TLRs in liver IRI and to explore their role as potential targets for therapeutic intervention.

METHODS

A review of the literature summarizing the latest advances in TLR signaling, the role of TLRs in each hepatic cell population and the involvement of TLRs in the pathophysiology of hepatic IRI. The potential role of TLR-targeting treatment strategies in liver IRI is discussed.

CONCLUSIONS

Recent experimental evidence suggests that TLR activation on Kupffer cells provides the triggering signal for pro-inflammatory responses that lead to liver IRI. Modulating TLR signaling could have a beneficial effect in patients with liver IRI.

Virulence in Cryptococcus species

Cryptococcus neoformans and Cryptococcus gattii are the cause of life-threatening meningoencephalitis in immunocompromised and immunocompetent individuals respectively. The increasing incidence of cryptococcal infection as a result of the AIDS epidemic, the recent emergence of a hypervirulent cryptococcal strain in Canada and the fact that mortality from cryptococcal disease remains high have stimulated intensive research into this organism. Here we outline recent advances in our understanding of C. neoformans and C. gattii, including intraspecific complexity, virulence factors, and key signaling pathways. We discuss the molecular basis of cryptococcal virulence and the interaction between these pathogens and the host immune system. Finally, we discuss future challenges in the study and treatment of cryptococcosis.

Distinct patterns of dendritic cell cytokine release stimulated by fungal beta-glucans and toll-like receptor agonists

beta-Glucans derived from fungal cell walls have potential uses as immunomodulating agents and vaccine adjuvants. Yeast glucan particles (YGPs) are highly purified Saccharomyces cerevisiae cell walls composed of beta1,6-branched beta1,3-d-glucan and free of mannans. YGPs stimulated secretion of the proinflammatory cytokine tumor necrosis factor alpha (TNF-alpha) in wild-type murine bone marrow-derived myeloid dendritic cells (BMDCs) but did not stimulate interleukin-12p70 (IL-12p70) production. A purified soluble beta1,6-branched beta1,3-d-glucan, scleroglucan, also stimulated TNF-alpha in BMDCs. These two beta-glucans failed to stimulate TNF-alpha in Dectin-1 (beta-glucan receptor) knockout BMDCs. Costimulation of wild-type BMDCs with beta-glucans and specific Toll-like receptor (TLR) ligands resulted in greatly enhanced TNF-alpha production but decreased IL-12p70 production compared with TLR agonists alone. The upregulation of TNF-alpha and downregulation of IL-12p70 required Dectin-1, but not IL-10. Gamma interferon (IFN-gamma) priming did not overcome IL-12p70 reduction by beta-glucans. Similar patterns of cytokine regulation were observed in human monocyte-derived dendritic cells (DCs) costimulated with YGPs and the TLR4 ligand lipopolysaccharide. Finally, costimulation of BMDCs with YGPs and either the TLR9 ligand, CpG, or the TLR2/1 ligand, Pam(3)CSK(4), resulted in upregulated secretion of IL-1alpha and IL-10 and downregulated secretion of IL-1beta, IL-6, and IFN-gamma-inducible protein 10 but had no significant effects on IL-12p40, keratinocyte-derived chemokine, monocyte chemotactic protein 1, or macrophage inflammatory protein alpha, compared with the TLR ligand alone. Thus, beta-glucans have distinct effects on cytokine responses following DC stimulation with different TLR agonists. These patterns of response might contribute to the skewing of immune responses during mycotic infections and have implications for the design of immunomodulators and vaccines containing beta-glucans.

Influence of ligand valency on the targeting of immature human dendritic cells by mannosylated liposomes

An important challenge for the development of new generations of vaccines is the efficient delivery of antigens to antigen presenting cells such as dendritic cells. In the present study we compare the interaction of plain and targeted liposomes, containing mono-, di-, and tetraantennary mannosyl lipid derivatives, with human monocyte-derived immature dendritic cells (iDCs). Whereas efficient mannose receptor-mediated endocytosis by iDCs was observed for the mannosylated liposomes, in contrast, only nonspecific interaction with little uptake was observed with plain liposomes. In accordance with the clustering effect, liposomes prepared with multibranched mannosylated lipids displayed higher binding affinity for the mannose receptor than vesicles containing the monomannosylated analogs. Importantly, we have found that dimannosylated ligands present at the surface of the liposomes were as efficient as tetramannosylated ones to engage in multidentate interactions with the mannose receptor of iDCs, resulting in both cases in an effective uptake/endocytosis. This result will greatly facilitate, from a practical standpoint, the design of mannose-targeted vaccination constructs. Moreover, we showed that mannose-mediated uptake of liposomes did not result in an activation of iDCs. Altogether, our results suggest that antigen-associated targeted liposomes containing diantennary mannosylated lipids could be effective vectors for vaccines when combined with additional DC activation signals.

Toll-like receptor modulation: a novel therapeutic strategy in cardiovascular disease?

BACKGROUND

Toll-like receptors (TLRs) have been recently recognised as primary receptors in the innate immune system. Apart from initiating a prompt immune response against invading pathogens, TLRs are also considered to be an important link between innate immunity, inflammation and a variety of clinical disorders, including cardiovascular diseases. TLR signalling manipulation with novel drugs could offer important opportunities for cardiovascular disease modification.

OBJECTIVE

To present the latest knowledge supporting the involvement of TLRs in the pathogenesis and progress of cardiovascular diseases and explore the role of TLRs as potential targets for therapeutic intervention in cardiovascular territory.

METHODS

A review of the literature documenting implication of TLR signalling in cardiovascular disorders. Current progress in TLR-targeting drug development and the potential role of such a treatment strategy in cardiovascular disorders are discussed.

CONCLUSIONS

A growing body of evidence supports a role for TLRs in cardiovascular disease initiation and progression. Altering TLR signalling with novel drugs could be a beneficial therapeutic strategy for patients with cardiovascular disorders.

Role of the mannose receptor in a murine model of Cryptococcus neoformans infection

Cryptococcus neoformans is an encapsulated fungal pathogen with a predilection to infect persons with suppressed T-cell function. Cryptococcal mannoproteins (MP) are highly mannosylated antigens which elicit T-cell responses in infected mice and in convalescent patients. Key to the immunogenicity of MP is its capacity to bind to the conserved mannose receptor (MR), CD206, on dendritic cells (DCs). To test the role of the MR in the immune response to C. neoformans, wild-type and MR knockout (MR KO) mice were compared by using in vivo and ex vivo models of cryptococcosis. Following a pulmonary challenge with C. neoformans, MR KO mice died significantly faster than wild-type mice and had higher lung fungal burdens after 4 weeks of infection. Uptake of MP was similar when DCs obtained from wild-type and MR KO mice were compared. Additionally, MP did not upregulate the maturation markers major histocompatibility complex class II, CD86, and CD40 in either wild-type or MR KO DCs. However, MP stimulated lymphoproliferation in CD4(+) T cells obtained from the peripheral lymph nodes of infected wild-type but not MR KO mice. These studies demonstrate a nonredundant role for the MR in the development of CD4(+) T-cell responses to MP and protection from C. neoformans.