Endemic Mycoses in Solid Organ Transplant Recipients

Genotypic diversity, virulence, and molecular genetic tools in Histoplasma

SUMMARYHistoplasmosis is arguably the most common fungal respiratory infection worldwide, with hundreds of thousands of new infections occurring annually in the United States alone. The infection can progress in the lung or disseminate to visceral organs and can be difficult to treat with antifungal drugs. Histoplasma, the causative agent of the disease, is a pathogenic fungus that causes life-threatening lung infections and is globally distributed. The fungus has the ability to germinate from conidia into either hyphal (mold) or yeast form, depending on the environmental temperature. This transition also regulates virulence. Histoplasma and histoplasmosis have been classified as being of emergent importance, and in 2022, the World Health Organization included Histoplasma as 1 of the 19 most concerning human fungal pathogens. In this review, we synthesize the current understanding of the ecological niche, evolutionary history, and virulence strategies of Histoplasma. We also describe general patterns of the symptomatology and epidemiology of histoplasmosis. We underscore areas where research is sorely needed and highlight research avenues that have been productive.

The epidemiology of invasive fungal infections in transplant recipients

Transplant patients, including solid-organ transplant (SOT) and hematopoietic stem cell transplant (HSCT) recipients, are exposed to various types of complications, particularly rejection. To prevent these outcomes, transplant recipients commonly receive long-term immunosuppressive regimens that in turn make them more susceptible to a wide array of infectious diseases, notably those caused by opportunistic pathogens. Among these, invasive fungal infections (IFIs) remain a major cause of mortality and morbidity in both SOT and HSCT recipients. Despite the continuing improvement in early diagnostics and treatments of IFIs, the management of these infections in transplant patients is still complicated. Here, we provide an overview concerning the most recent trends in the epidemiology of IFIs in SOT and HSCT recipients by describing the prominent yeast and mold species involved, the timing of post-transplant IFIs and the risk factors associated with their occurrence in these particularly weak populations. We also give special emphasis into basic research advances in the field that recently suggested a role of the global and long-term prophylactic regimen in orchestrating various biological disturbances in the organism and conditioning the emergence of the most adapted fungal strains to the particular physiological profiles of transplant patients.

Epidemiology and Management of Infections in Liver Transplant Recipients

Background: Improvements in liver transplant (LT) outcomes are attributed to advances in surgical techniques, use of potent immunosuppressants, and rigorous pre-LT testing. Despite these improvements, post-LT infections remain the most common complication in this population. Bacteria constitute the most common infectious agents, while fungal and viral infections are also frequently encountered. Multi-drug-resistant bacterial infections develop because of polymicrobial overuse and prolonged hospital stays. Immediate post-LT infections are commonly caused by viruses. Conclusions: Appropriate vaccination, screening of both donor and recipients before LT and antiviral prophylaxis in high-risk individuals are recommended. Antimicrobial drug resistance is common in high-risk LT and associated with poor outcomes; epidemiology and management of these cases is discussed. Additionally, we also discuss the effect of coronavirus disease 2019 (COVID-19) infection and monkeypox in the LT population.

Controlling infections in hospitalized pretransplant candidates

PURPOSE OF REVIEW

Infections in hospitalized patients awaiting solid organ transplantation can pose complicated diagnostic and therapeutic challenges. Goals of management include stabilizing the patient, treating or controlling infections, and decreasing the risk of reactivation of infection after transplant.

RECENT FINDINGS

Groups such as The Organ Procurement and Transplantation Network, American Society of Transplantation Infectious Diseases Community of Practice and the European Society of Clinical Microbiology and Infectious Diseases have updated their guidelines on screening and treatment of infection in transplant candidates. There are also recent developments in therapeutic options for tuberculosis, COVID-19, Clostridioides difficile colitis, bloodstream infections, and other common infections.

SUMMARY

Ideally, antimicrobial therapy should be complete prior to transplantation. In situations in which completion of therapy prior to transplant is not feasible, therapy may need to be prolonged or modified. In most situations, infections can be managed similarly to the general population, although some infections, particularly fungal and mycobacterial, require a different management approach. We review disease- and organ-specific management.

Tackling Histoplasmosis Infection in People Living with HIV from Latin America: From Diagnostic Strategy to Public Health Solutions

Histoplasmosis, caused by the thermally dimorphic fungus Histoplasma spp., is a disease with a broad clinical spectrum, presenting from asymptomatic/flu-like symptoms to progressive disseminated disease in people with immunosuppression. In recent years, the concept of histoplasmosis as a disease restricted to the American continent has changed, as now histoplasmosis is reported in many regions around the world. In Latin America, histoplasmosis represents a threat, especially in people with advanced HIV disease (AHD). Diagnosis of histoplasmosis in people living with HIV (PLHIV) is challenging due to the low index of suspicion of the disease, non-specificity of signs and symptoms, and limited access to specific laboratory testing, while the diagnostic delay is significantly associated with mortality. In the last decade, novel diagnostic tests have been developed for the rapid detection of histoplasmosis, such as commercial kits for antigen detection. Furthermore, advocacy groups were created that presented histoplasmosis as a public health problem, with emphasis on patients at risk of progressive disseminated disease. This review aims to discuss the impact of histoplasmosis associated with AHD in Latin America and the strategies employed to tackle histoplasmosis, from the implementation of laboratory testing to disease advocacy and public health interventions.

Infections after organ transplantation and immune response

Organ transplantation has provided another chance of survival for end-stage organ failure patients. Yet, transplant rejection is still a main challenging factor. Immunosuppressive drugs have been used to avoid rejection and suppress the immune response against allografts. Thus, immunosuppressants increase the risk of infection in immunocompromised organ transplant recipients. The infection risk reflects the relationship between the nature and severity of immunosuppression and infectious diseases. Furthermore, immunosuppressants show an immunological impact on the genetics of innate and adaptive immune responses. This effect usually reactivates the post-transplant infection in the donor and recipient tissues since T-cell activation has a substantial role in allograft rejection. Meanwhile, different infections have been found to activate the T-cells into CD₄⁺ helper T-cell subset and CD₈⁺ cytotoxic T-lymphocyte that affect the infection and the allograft. Therefore, the best management and preventive strategies of immunosuppression, antimicrobial prophylaxis, and intensive medical care are required for successful organ transplantation. This review addresses the activation of immune responses against different infections in immunocompromised individuals after organ transplantation.

Prevention, diagnosis, and management of donor derived infections in pediatric kidney transplant recipients

Donor derived infections (DDIs) in pediatric kidney transplant recipients remain challenging to diagnose and can result in serious morbidity and mortality. This review summarizes the current guidelines and recommendations for prevention, diagnosis, and treatment of unexpected DDIs in pediatric kidney transplant recipients. We provide a contemporary overview of DDI terminology, surveillance, epidemiology, and recommended approaches for assessing these rare events with an emphasis on the pediatric recipient. To address prevention and risk mitigation, important aspects of donor and pediatric candidate evaluations are reviewed, including current Organ Procurement and Transplantation Network (OPTN) and American Society of Transplantation (AST) recommendations. Common unexpected DDI encountered by pediatric transplant teams including multi-drug resistant organisms, tuberculosis, syphilis, West Nile Virus, toxoplasmosis, Chagas disease, strongyloidiasis, candidiasis, histoplasmosis, coccidioidomycosis, and emerging infections such as COVID-19 are discussed in detail. Finally, we consider the general challenges with management of DDIs and share our experience with a novel application of next generation sequencing (NGS) of microbial cell-free DNA that will likely define a future direction in this field.

Prosthetic Joint Infections Due to Histoplasma capsulatum: A Report of 3 Cases

Histoplasma capsulatum causes pneumonia and multisystemic disease in humans. Musculoskeletal involvement in histoplasmosis is most often tenosynovitis and rarely septic arthritis. Even more uncommon is the involvement of prosthetic joints. Here, we report a series of 3 cases of prosthetic joint failures caused by infection due to H capsulatum. Together with a review of 4 previously reported cases, we summarize host characteristics, clinical presentation, surgical approaches, antifungal management, and outcomes of this rare orthopedic joint infection.

An interferon gamma release assay specific for Histoplasma capsulatum to detect asymptomatic infected individuals: A proof of concept study

Histoplasmosis is the most common endemic mycosis in the Americas. Currently, there is no laboratory test capable to detect subclinical or latent infections by Histoplasma capsulatum (Hc), which might develop as severe infections in immunocompromised individuals. For the first time to our knowledge, we explore the suitability of an interferon gamma release assay (IGRA) to detect latent Hc infection in asymptomatic individuals. A cohort of 126 volunteers was enrolled in the study, 13 of which underwent a Hc infection in the past, and 93 of them showing risk factors for this infection. The remaining 20 participants did not refer any risk factors of Hc infection, but eight of them showed evidences of infection with Mycobacterium tuberculosis. All participants were recruited in Medellin, Colombia, between January 2014 and December 2017. Whole blood samples were cultured with four different Hc crude antigens and phytohemaglutinin as positive control. The interferon (IFN)-γ released by T lymphocytes upon antigen stimulation was quantified by ELISA. A defined cutoff value of 20 pg/ml for the IFN-γ concentration allowed us to distinguish between the group with documented past infections and the group of noninfected individuals with high sensitivity (70-92%) and specificity (85-95%), for the four tested antigens. Positive 82-95% and negative 77-92% predictive values were also very high, comparable to those reported for commercially available IGRAs. The new test constitutes a promising screening method to detect individuals with latent Hc infection, even decades after the primary infection, as evidenced in this study.

Immunodiagnostic Assays for the Investigation of Fungal Outbreaks

Fungal pathogens can affect humans, animals, and plants, and they can be found in the environment or as part of the host microbiome. Fungal diseases present a broad clinical spectrum, ranging from superficial to invasive infections, and can cause outbreaks. During an outbreak investigation, the laboratory plays an essential role in verifying the diagnosis and helping to confirm the source of the outbreak. Immunodiagnostic assays are important tools and often relied upon for the diagnosis of fungal infections, since the gold standard assays of culture and histopathology are time-consuming and often require invasive procedures. Immunodiagnostic assays range from complement fixation and immunodiffusion to enzyme immunoassays and, most recently, to point-of-care lateral flow devices. In general, these assays provide results faster and offer good analytical performance. These characteristics make immunodiagnostic assays good laboratory tools for outbreak investigations. The aim of this review is to describe the principles, advantages, limitations, and availability of immunodiagnostics assays in outbreak investigations, based on the experience of a reference laboratory.

Coccidioidomycosis in solid organ transplant recipients

PURPOSE OF REVIEW

The purpose of the review is an update of diagnosis and treatment of coccidioidomycosis infection in solid organ transplant (SOT) patients. Endemic fungal infections continue to be a cause of serious morbidity and mortality in transplant recipients.

RECENT FINDINGS

In transplant patients there are recommendations regarding screening in areas that are endemic for coccidioidomycosis. This screening involves serologic testing and chest imaging. In endemic areas pretransplant seropositivity varies from 1.4 to 5.6%. In immunocompromised patients with elevated complement fixation titers, evaluation of cerebrospinal fluid is recommended even in the absence of symptoms. Although coccidioidomycosis can be a self-limited disease in immunocompotent patients, all SOT patients should be treated regardless of severity. This may include intravenous amphotericin B in severe cases and fluconazole therapy in milder episodes. In those SOT recipients with evidence of prior coccidioidomycosis, lifelong secondary prophylaxis with fluconazole given risk of recurrent disease.

SUMMARY

Coccidioidomycosis continues to be a cause of serious morbidity and mortality in transplant recipients but with proper screening and treatment can be successfully managed.

Identifying molecularly defined antigens for a Histoplasma capsulatum-specific interferon gamma release assay

BACKGROUND

In a previous work we showed the feasibility of an interferon gamma release assay (IGRA) for detecting latent infection by Histoplasma capsulatum. While in that proof-of-concept study we used crude fungal extracts as antigens, the newest IGRAs developed for other infections are based on molecularly defined antigens, mostly on mixtures of immunogenic peptides.

AIMS

To identify proteins in H. capsulatum that might serve as molecularly defined antigens for an IGRA test.

METHODS

We surveyed the literature looking for known H. capsulatum-immunogenic proteins and assayed two of them as antigens in an IGRA test, in a study that involved 80 volunteers. Furthermore, we used several bioinformatics tools to identify specific H. capsulatum proteins and to analyze possible strategies for the design of H. capsulatum-specific immunogenic peptides.

RESULTS

Seven H. capsulatum-immunogenic proteins were retrieved from the literature. IGRA tests using either the heat shock protein 60 or the M antigen showed high sensitivities but low specificities, most likely due to the high sequence similarity with the corresponding orthologs in other pathogenic microorganisms. We identified around 2000 H. capsulatum-specific proteins, most of which remain unannotated. Class II T-cell epitope predictions for a small number of these proteins showed a great variability among different alleles, prompting for a "brute force" approach for peptide design.

CONCLUSIONS

The H. capsulatum genome encodes a large number of distinctive proteins, which represent a valuable source of potential specific antigens for an IGRA test. Among them, the Cfp4 protein stands out as a very attractive candidate.

Differential Behavior of Non-albicans Candida Species in the Central Nervous System of Immunocompetent and Immunosuppressed Mice

The genus Candida includes commensal fungi that can cause local and systemic infections, frequently involving vital organs as the central nervous system (CNS). Candida spp. occupy the fourth place among infections that affect the CNS. Although the incidence of Candida albicans is decreasing among patients under immunosuppressive therapies, the incidence of non-albicans Candida is increasing. In this context, the objective of this work was to evaluate the ability of non-albicans Candida species to spread to the CNS of immunocompetent and immunosuppressed mice. Adult female C57BL/6 mice were treated with prednisolone, intravenously infected with Candida glabrata, Candida krusei and Candida parapsilosis yeasts and then evaluated at the 3rd and 14th days after infection. All Candida species disseminated to the brain from immunocompetent animals and induced local inflammation at the third day post-infection. The immunosuppression resulted in body weight loss, leukopenia and reduced IL-2 production by spleen cell cultures. Higher fungal loads were recovered from the CNS of immunosuppressed mice. Inflammatory infiltration associated to a Th1 subset profile was higher in brain samples from C. krusei immunosuppressed mice compared with immunocompetent ones. Additionally, C. krusei was able to transform into pseudohypha inside microglia in vitro infected cells and also to induce elevated nitric oxide production. Altogether, these results indicate that C. glabrata, C. krusei and C. parapsilosis are able to disseminate to the CNS and promote local inflammation in both immunocompetent and immunosuppressed mice. C. krusei displayed a distinct behavior at the CNS triggering a local Th1 profile. The possible contribution of these non-albicans Candida species to other CNS pathologies as multiple sclerosis, Parkinson’s and Alzheimer’s diseases deserves further attention.