Dendritic cell-based immunization induces Coccidioides Ag2/PRA-specific immune response

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.

Vaccine development for pathogenic fungi: current status and future directions

INTRODUCTION

Fungal infections are caused by a broad range of pathogenic fungi that are found worldwide with different geographic distributions, incidences, and mortality rates. Considering that there are relatively few approved medications available for combating fungal diseases and no vaccine formulation commercially available, multiple groups are searching for new antifungal drugs, examining drugs for repurposing and developing antifungal vaccines, in order to control deaths, sequels, and the spread of these complex infections.

AREAS COVERED

This review provides a summary of advances in fungal vaccine studies and the different approaches under development, such as subunit vaccines, whole organism vaccines, and DNA vaccines, as well as studies that optimize the use of adjuvants. We conducted a literature search of the PubMed with terms: fungal vaccines and genus of fungal pathogens (Cryptococcus spp. Candida spp. Coccidioides spp. Aspergillus spp. Sporothrix spp. Histoplasma spp. Paracoccidioides spp. Pneumocystis spp. and the Mucorales order), a total of 177 articles were collected from database.

EXPERT OPINION

Problems regarding the immune response development in an immunocompromised organism, the similarity between fungal and mammalian cells, and the lack of attention by health organizations to fungal infections are closely related to the fact that, at present, there are no fungal vaccines available for clinical use.

Macrophage and Dendritic Cell Activation and Polarization in Response to Coccidioidesposadasii Infection

Coccidioidomycosis is a fungal, respiratory disease caused by Coccidioides immitis and Coccidioides posadasii. The host immune responses that define disease outcome during infection are largely unknown, although T helper responses are required. Adaptive immunity is influenced by innate immunity as antigen-presenting cells activate and educate adaptive responses. Macrophage and dendritic cell (DC) recognition of pathogen surface molecules are critical for Coccidioides clearance. We characterize the broad innate immune responses to Coccidioides by analyzing macrophage and dendritic cell responses to Coccidioides arthroconidia using avirulent, vaccine Coccidioides strain NR-166 (Δcts2/Δard1/Δcts3), developed from parental virulent strain C735. We developed a novel flow cytometry-based method to analyze macrophage phagocytosis to complement traditional image-scoring methods. Our study found that macrophage polarization is blocked at M0 phase and activation reduced, while DCs polarize into proinflammatory DC1s, but not anti-inflammatory DC2, following interaction with Coccidioides. However, DCs exhibit a contact-dependent reduced activation to Coccidioides as defined by co-expression of MHC-II and CD86. In vivo, only modest DC1/DC2 recruitment and activation was observed with avirulent Coccidioides infection. In conclusion, the vaccine Coccidioides strain recruited a mixed DC population in vivo, while in vitro data suggest active innate immune cell inhibition by Coccidioides.

The Known Unknowns of the Immune Response to Coccidioides

Coccidioidomycosis, otherwise known as Valley Fever, is caused by the dimorphic fungi Coccidioides immitis and C. posadasii. While most clinical cases present with self-limiting pulmonary infection, dissemination of Coccidioides spp. results in prolonged treatment and portends higher mortality rates. While the structure, genome, and niches for Coccidioides have provided some insight into the pathogenesis of disease, the underlying immunological mechanisms of clearance or inability to contain the infection in the lung are poorly understood. This review focuses on the known innate and adaptive immune responses to Coccidioides and highlights three important areas of uncertainty and potential approaches to address them. Closing these gaps in knowledge may enable new preventative and therapeutic strategies to be pursued.

The Role of the Interleukin-17 Axis and Neutrophils in the Pathogenesis of Endemic and Systemic Mycoses

Systemic and endemic mycoses are considered life-threatening respiratory diseases which are caused by a group of dimorphic fungal pathogens belonging to the genera Histoplasma, Coccidioides, Blastomyces, Paracoccidioides, Talaromyces, and the newly described pathogen Emergomyces. T-cell mediated immunity, mainly T helper (Th)1 and Th17 responses, are essential for protection against these dimorphic fungi; thus, IL-17 production is associated with neutrophil and macrophage recruitment at the site of infection accompanied by chemokines and proinflammatory cytokines production, a mechanism that is mediated by some pattern recognition receptors (PRRs), including Dectin-1, Dectine-2, TLRs, Mannose receptor (MR), Galectin-3 and NLPR3, and the adaptor molecules caspase adaptor recruitment domain family member 9 (Card9), and myeloid differentiation factor 88 (MyD88). However, these PRRs play distinctly different roles for each pathogen. Furthermore, neutrophils have been confirmed as a source of IL-17, and different neutrophil subsets and neutrophil extracellular traps (NETs) have also been described as participating in the inflammatory process in these fungal infections. However, both the Th17/IL-17 axis and neutrophils appear to play different roles, being beneficial mediating fungal controls or detrimental promoting disease pathologies depending on the fungal agent. This review will focus on highlighting the role of the IL-17 axis and neutrophils in the main endemic and systemic mycoses: histoplasmosis, coccidioidomycosis, blastomycosis, and paracoccidioidomycosis.

Host Response to Coccidioides Infection: Fungal Immunity

Coccidioidomycosis is a fungal, respiratory disease caused by Coccidioides immitis and Coccidioides posadasii. This emerging infectious disease ranges from asymptomatic to pulmonary disease and disseminated infection. Most infections are cleared with little to no medical intervention whereas chronic disease often requires life-long medication with severe impairment in quality of life. It is unclear what differentiates hosts immunity resulting in disease resolution versus chronic infection. Current understanding in mycology-immunology suggests that chronic infection could be due to maladaptive immune responses. Immunosuppressed patients develop more severe disease and mouse studies show adaptive Th1 and Th17 responses are required for clearance. This is supported by heightened immunosuppressive regulatory responses and lowered anti-fungal T helper responses in chronic Coccidioides patients. Diagnosis and prognosis is difficult as symptoms are broad and overlapping with community acquired pneumonia, often resulting in misdiagnosis and delayed treatment. Furthermore, we lack clear biomarkers of disease severity which could aid prognosis for more effective healthcare. As the endemic region grows and population increases in endemic areas, the need to understand Coccidioides infection is becoming urgent. There is a growing effort to identify fungal virulence factors and host immune components that influence fungal immunity and relate these to patient disease outcome and treatment. This review compiles the known immune responses to Coccidioides spp. infection and various related fungal pathogens to provide speculation on Coccidioides immunity.

Advances in Fungal Peptide Vaccines

Vaccination is one of the greatest public health achievements in the past century, protecting and improving the quality of life of the population worldwide. However, a safe and effective vaccine for therapeutic or prophylactic treatment of fungal infections is not yet available. The lack of a vaccine for fungi is a problem of increasing importance as the incidence of diverse species, including Paracoccidioides, Aspergillus, Candida, Sporothrix, and Coccidioides, has increased in recent decades and new drug-resistant pathogenic fungi are emerging. In fact, our antifungal armamentarium too frequently fails to effectively control or cure mycoses, leading to high rates of mortality and morbidity. With this in mind, many groups are working towards identifying effective and safe vaccines for fungal pathogens, with a particular focus of generating vaccines that will work in individuals with compromised immunity who bear the major burden of infections from these microbes. In this review, we detail advances in the development of vaccines for pathogenic fungi, and highlight new methodologies using immunoproteomic techniques and bioinformatic tools that have led to new vaccine formulations, like peptide-based vaccines.

Lung and general health effects of Toll-like receptor-4 (TLR4)-interacting SPA4 peptide

Background

A surfactant protein-A-derived peptide, which we call SPA4 peptide (amino acids: GDFRYSDGTPVNYTNWYRGE), alleviates lung infection and inflammation. This study investigated the effects of intratracheally administered SPA4 peptide on systemic, lung, and health parameters in an outbred mouse strain, and in an intratracheal lipopolysaccharide (LPS) challenge model.

Methods

The outbred CD-1 mice were intratracheally administered with incremental doses of SPA4 peptide (0.625–10 μg/g body weight) once every 24 h, for 3 days. Mice left untreated and those treated with vehicle were included as controls. Mice were euthanized after 24 h of last administration of SPA4 peptide. In order to assess the biological activity of SPA4 peptide, C57BL6 mice were intratracheally challenged with 5 μg LPS/g body weight and treated with 50 μg SPA4 peptide via intratracheal route 1 h post LPS-challenge. Mice were euthanized after 4 h of LPS challenge. Signs of sickness and body weights were regularly monitored. At the time of necropsy, blood and major organs were harvested. Blood gas and electrolytes, serum biochemical profiles and SPA4 peptide-specific immunoglobulin G (IgG) antibody levels, and common lung injury markers (levels of total protein, albumin, and lactate, lactate dehydrogenase activity, and lung wet/dry weight ratios) were determined. Lung, liver, spleen, kidney, heart, and intestine were examined histologically. Differences in measured parameters were analyzed among study groups by analysis of variance test.

Results

The results demonstrated no signs of sickness or changes in body weight over 3 days of treatment with various doses of SPA4 peptide. It did not induce any major toxicity or IgG antibody response to SPA4 peptide. The SPA4 peptide treatment also did not affect blood gas, electrolytes, or serum biochemistry. There was no evidence of injury to the tissues and organs. However, the SPA4 peptide suppressed the LPS-induced lung inflammation.

Conclusions

These findings provide an initial toxicity profile of SPA4 peptide. Intratracheal administration of escalating doses of SPA4 peptide does not induce any significant toxicity at tissue and organ levels. However, treatment with a dose of 50 μg SPA4 peptide, comparable to 2.5 μg/g body weight, alleviates LPS-induced lung inflammation.

Early Events in Coccidioidomycosis

Since its description nearly 130 years ago, hundreds of studies have deepened our understanding of coccidioidomycosis, also known as valley fever (VF), and provided useful diagnostic tests and treatments for the disease caused by the dimorphic fungi Coccidioides spp. In general, most of the literature has addressed well-established infections and has described patients who have experienced major complications. In contrast, little attention has been given to the earliest consequences of the pathogen-host interaction and its implications for disease manifestation, progression, and resolution. The purpose of this review is to highlight published studies on early coccidioidomycosis, identify gaps in our knowledge, and suggest new or former research areas that might be or remain fertile ground for insight into the early stages of this invasive fungal disease.

The Rise of Coccidioides: Forces Against the Dust Devil Unleashed

Coccidioidomycosis (Valley fever) is a fungal disease caused by the inhalation of Coccidioides posadasii or C. immitis. This neglected disease occurs in the desert areas of the western United States, most notably in California and Arizona, where infections continue to rise. Clinically, coccidioidomycosis ranges from asymptomatic to severe pulmonary disease and can disseminate to the brain, skin, bones, and elsewhere. New estimates suggest as many as 350,000 new cases of coccidioidomycosis occur in the United States each year. Thus, there is an urgent need for the development of a vaccine and new therapeutic drugs against Coccidioides infection. In this review, we discuss the battle against Coccidioides including the development of potential vaccines, the quest for new therapeutic drugs, and our current understanding of the protective host immune response to Coccidioides infection.