GM-CSF+ Tc17 cells are required to bolster vaccine immunity against lethal fungal pneumonia without causing overt pathology

Fungal immunization potentiates CD4+ T cell-independent cDC2 responses for cross-presentation

The incidence rates of fungal infections are increasing, especially in immunocompromised individuals without an FDA-approved vaccine. Accumulating evidence suggests that T cells are instrumental in providing fungal immunity. An apt stimulation and responses of dendritic cells are pivotal in inducing T-cell responses and vaccine success. Using a mouse model of fungal vaccination, we explored the dynamics, kinetics, activation, and antigen presentation of dendritic cell subsets to unravel the features of dendritic cell responses in the absence of CD4+ T cell help. The subcutaneous fungal vaccination induced more robust cDC2 responses than the cDC1 subset in draining lymph nodes. A single immunization with Blastomyces yeasts bolstered DC responses that peaked around day 5 before reverting to basal levels by day 15. The migratory cDC2 was the dominant DC subset, with higher numbers than all other DC subsets combined. Fungal vaccination augmented costimulatory molecules CD80 and CD86 without altering the levels of MHC molecules. Despite the higher fungal antigen uptake with migratory cDC2, the mean cross-presentation ability of all DC subsets was similar. Counterintuitively, deleting CD4+ T cells enhanced the DC responses, and CD4+ T cells were dispensable for conventional cross-presenting cDC1 responses. Collectively, our study shows that fungal vaccination selectively augmented cDC2 responses, and CD4+ T cells were dispensable for DC activation, antigen uptake, expression of costimulatory molecules, and cross-presentation. Our study provides novel insights into DC responses to an effective fungal vaccine for designing efficacious vaccines tailored for immunocompromised hosts.

GM-CSF-mediated epithelial-immune cell cross-talk orchestrates pulmonary immunity to Aspergillus fumigatus

Aspergillus fumigatus causes life-threatening mold pneumonia in immunocompromised patients, particularly in those with quantitative or qualitative defects in neutrophils. Whereas innate immune cell cross-talk licenses neutrophil antifungal activity in the lung, the role of epithelial cells in this process is unknown. Here, we find that surfactant protein C (SPC)-expressing lung epithelial cells integrate infection-induced interleukin-1 and type III interferon signaling to produce granulocyte-macrophage colony-stimulating factor (GM-CSF) preferentially at local sites of fungal infection and neutrophil influx. Using in vivo models that distinguish the role of GM-CSF during acute infection from its homeostatic function in alveolar macrophage survival and surfactant catabolism, we demonstrate that epithelial-derived GM-CSF increases the accumulation and fungicidal activity of GM-CSF-responsive neutrophils, which is essential for host survival. Our findings establish SPC+ epithelial cells as a central player in regulating the quality and strength of neutrophil-dependent immunity against inhaled mold pathogens.

Mouse CD8+ T cell subsets differentially generate IL-17-expressing cells in the colon epithelium and lamina propria

Colon-resident CD8+ T cells actively contribute to gut homeostasis and the pathogenesis of inflammatory bowel disease. However, their heterogeneity in generating IL-17-expressing CD8+ T cells, i.e. Tc17 cells, has not been thoroughly revealed. This study aims to characterize the abilities of mouse colonic intraepithelial (IE) and lamina propria (LP) CD8+ T cell subsets to differentiate into Tc17 cells. Using flow cytometry, we found that normal TCRβ+CD4-CD8αα+ cells (CD8αα T cells) and TCRβ+CD4-CD8αβ T cells, (CD8αβ T cells), either IE or LP, expressed abundant granzymes and IFN-γ but minute IL-17A. Under the in vitro Tc17-inducing condition, IE CD8αα T cells showed the weakest Tc17 differentiation ability and LP CD8αβ T cells exhibited the strongest Tc17 differentiation ability, whereas IE CD8αβ T cells and LP CD8αα T cells demonstrated moderate Tc17 differentiation abilities. The expression of IL-6 receptor, TGF-β receptor, TCR signaling indicators, CD161, and IL-23 receptor was low in IE CD8αα T cells, median in IE CD8αβ T cells and LP CD8αα T cells, but high in LP CD8αβ T cells. IE CD8αα T cells weakly induced the expression of chemokines, cytokines, and host defense mediators in colonic epithelial cells while LP CD8αβ T cells showed a robust up-regulatory effect. Furthermore, these colonic CD8+ T cell subsets also exhibited different abilities to generate Tc17 cells in inflamed colons. Collectively, LP CD8αβ T cells have the strongest Tc17 differentiation ability and might play a more significant role than the other subsets in Tc17-mediated immunity or inflammation in the colon.

Fungal vaccines and adjuvants: a tool to reveal the interaction between host and fungi

Fungal infections are incurring high risks in a range from superficial mucosal discomforts (such as oropharyngeal candidiasis and vulvovaginal candidiasis) to disseminated life-threatening diseases (such as invasive pulmonary aspergillosis and cryptococcal meningitis) and becoming a global health problem in especially immunodeficient population. The major obstacle to conquer fungal harassment lies in the presence of increasing resistance to conventional antifungal agents used in newly clinically isolated strains. Although recombinant cytokines and mono-/poly-clonal antibodies are added into antifungal armamentarium, more effective antimycotic drugs are exceedingly demanded. It is comforting that the development of fungal vaccines and adjuvants opens up a window to brighten the prospective way in the diagnosis, prevention and treatment of fungal assaults. In this review, we focus on the progression of several major fungal vaccines devised for the control of Candida spp., Aspergillus spp., Cryptococcus spp., Coccidioides spp., Paracoccidioides spp., Blastomyces spp., Histoplasma spp., Pneumocystis spp. as well as the adjuvants adopted. We then expound the interaction between fungal vaccines/adjuvants and host innate (macrophages, dendritic cells, neutrophils), humoral (IgG, IgM and IgA) and cellular (Th1, Th2, Th17 and Tc17) immune responses which generally experience immune recognition of pattern recognition receptors, activation of immune cells, and clearance of invaded fungi. Furthermore, we anticipate an in-depth understanding of immunomodulatory properties of univalent and multivalent vaccines against diverse opportunistic fungi, providing helpful information in the design of novel fungal vaccines and adjuvants.

Mucosal vaccination: onward and upward

INTRODUCTION

The unique mucosal immune system allows the generation of robust protective immune responses at the front line of pathogen encounters. The needle-free delivery route and cold chain-free logistic requirements also provide additional advantages in ease and economy. However, the development of mucosal vaccines faces several challenges, and only a handful of mucosal vaccines are currently licensed. These vaccines are all in the form of live attenuated or inactivated whole organisms, whereas no subunit-based mucosal vaccine is available.

AREAS COVERED

The selection of antigen, delivery vehicle, route and adjuvants for mucosal vaccination are highly important. This is particularly crucial for subunit vaccines, as they often fail to elicit strong immune responses. Emerging research is providing new insights into the biological and immunological uniqueness of mucosal tissues. However, many aspects of the mucosal immunology still await to be investigated.

EXPERT OPINION

This article provides an overview of the current understanding of mucosal vaccination and discusses the remaining knowledge gaps. We emphasize that because of the potential benefits mucosal vaccines can bring from the biomedical, social and economic standpoints, the unmet goal to achieve mucosal vaccine success is worth the effort.