Detection of antibodies against Paracoccidioides brasiliensis melanin in in vitro and in vivo studies during infection

Radioimmunotherapy as a pathogen-agnostic treatment method for opportunistic mucormycosis infections

Invasive fungal infections (IFIs) such as mucormycosis are causing devastating morbidity and mortality in immunocompromised patients as anti-fungal agents do not work in the setting of a suppressed immune system. The coronavirus disease 2019 (COVID-19) pandemic has created a novel landscape for IFIs in post-pandemic patients, resulting from severe immune suppression caused by COVID-19 infection, comorbidities (diabetes, obesity) and immunosuppressive treatments such as steroids. The antigen-antibody interaction has been employed in radioimmunotherapy (RIT) to deliver lethal doses of ionizing radiation emitted by radionuclides to targeted cells and has demonstrated efficacy in several cancers. One of the advantages of RIT is its independence of the immune status of a host, which is crucial for immunosuppressed post-COVID-19 patients. In the present work we targeted the fungal pan-antigens 1,3-beta-glucan and melanin pigment, which are present in the majority of pathogenic fungi, with RIT, thus making such targeting pathogen-agnostic. We demonstrated in experimental murine mucormycosis in immunocompetent and immunocompromised mice that lutetium-177 (177Lu)-labelled antibodies to these two antigens effectively decreased the fungal burden in major organs, including the brain. These results are encouraging because they show the effectiveness of pathogen-agnostic RIT in significantly decreasing fungal burden in vivo, while they can also potentially be applied to treat the broad range of invasive fungal infections that express the pan-antigens 1,3-beta-glucan or melanin.

Fungal Melanin and the Mammalian Immune System

Melanins are ubiquitous complex polymers that are commonly known in humans to cause pigmentation of our skin. Melanins are also present in bacteria, fungi, and helminths. In this review, we will describe the diverse interactions of fungal melanin with the mammalian immune system. We will particularly focus on Cryptococcus neoformans and also discuss other major melanotic pathogenic fungi. Melanin interacts with the immune system through diverse pathways, reducing the effectiveness of phagocytic cells, binding effector molecules and antifungals, and modifying complement and antibody responses.

Melanin as a Virulence Factor in Different Species of Genus Paracoccidioides

Paracoccidioidomycosis (PCM) is a granulomatous systemic mycosis caused by the thermo-dimorphic fungi of the genus Paracoccidioides. Melanin production by fungi can affect their pathogenesis and virulence. This study evaluates the production of melanin by different isolates of genus Paracoccidioides and examines how the presence of this polymer affects yeast cell phagocytosis, as well as laccase enzyme production. The results obtained showed that the isolates of genus Paracoccidioides: P. lutzii (Pb01, Pb66, ED01, Pb1578, and Pb8334), P. restrepiensis (PS3-Pb60855), P. brasiliensis (S1-Pb18), and P. americana (PS2-Pbcão) produce melanin in the presence of L-3,4-dihydroxyphenylalanine (L-DOPA). Phagocytosis assays were carried out with peritoneal macrophages from C57Bl/6 mice that were challenged with Pb18, Pb60855, and Pb01. We observed that melanin interferes with phagocytosis in the presence or absence of complement or heat-inactivated serum. This article confirms that different species of the genus Paracoccidioides produce melanin in different magnitudes and that the polymer functions as a virulence factor.

The Role of Melanin in Fungal Pathogenesis for Animal Hosts

Melanins are a class of pigments that are ubiquitous throughout biology. They play incredibly diverse and important roles ranging from radiation protection to immune defense, camouflage, and virulence. Fungi have evolved to use melanin to be able to persist in the environment and within organisms. Fungal melanins are often located within the cell well and are able to neutralize reactive oxygen species and other radicals, defend against UV radiation, bind and sequester non-specific peptides and compounds, and produce a physical barrier that defends the cell. For this reason, melanized fungi are often well-suited to be human pathogens-melanin allows fungi to neutralize the microbicidal oxidative bursts of our innate immune system, bind and inactivate to antimicrobial peptides and enzymes, sequester antifungal pharmaceuticals, and create a shield to block immune recognition of the fungus. Due to the importance and pervasiveness of melanin in fungal virulence, mammalian immune systems have evolved antifungal strategies that involve directly detecting and binding to fungal melanins. Such strategies include the use of melanin-specific antibody responses and C-type lectins like the newly discovered melanin-specific MelLec receptor.

Comparative Radioimmunotherapy of Experimental Melanoma with Novel Humanized Antibody to Melanin Labeled with 213Bismuth and 177Lutetium

Melanoma is a cancer with increasing incidence and there is a need for alternatives to immunotherapy within effective approaches to treatment of metastatic melanoma. We performed comparative radioimmunotherapy (RIT) of experimental B16-F10 melanoma with novel humanized IgG to melanin h8C3 labeled with a beta emitter, 177Lu, and an alpha-emitter, 213Bi, as well as biodistribution, microSPECT/CT imaging, and mouse and human dosimetry calculations. microSPECT/CT imaging showed that a humanized antibody that targets "free" melanin in the tumor microenvironment had high tumor uptake in B16F10 murine melanoma in C57Bl/6 mice, with little to no uptake in naturally melanized tissues. Extrapolation of the mouse dosimetry data to an adult human demonstrated that doses delivered to major organs and the whole body by 177Lu-h8C3 would be approximately two times higher than those delivered by 213Bi-h8C3, while the doses to the tumor would be almost similar. RIT results indicated that 213Bi-h8C3 was more effective in slowing down the tumor growth than 177Lu-h8C3, while both radiolabeled antibodies did not produce significant hematologic or systemic side effects. We concluded that h8C3 antibody labeled with 213Bi is a promising reagent for translation into a clinical trial in patients with metastatic melanoma.

Structure-function analysis and therapeutic efficacy of antibodies to fungal melanin for melanoma radioimmunotherapy

Metastatic melanoma remains difficult to treat despite recent approvals of several new drugs. Recently we reported encouraging results of Phase I clinical trial of radiolabeled with 188Re murine monoclonal IgM 6D2 to melanin in patients with Stage III/IV melanoma. Subsequently we generated a novel murine IgG 8C3 to melanin. IgGs are more amenable to humanization and cGMP (current Good Manufacturing Practice) manufacturing than IgMs. We performed comparative structural analysis of melanin-binding IgM 6D2 and IgG 8C3. The therapeutic efficacy of 213Bi- and 188Re-labeled 8C3 and its comparison with anti-CTLA4 immunotherapy was performed in B16-F10 murine melanoma model. The primary structures of these antibodies revealed significant homology, with the CDRs containing a high percentage of positively charged amino acids. The 8C3 model has a negatively charged binding surface and significant number of aromatic residues in its H3 domain, suggesting that hydrophobic interactions contribute to the antibody-melanin interaction. Radiolabeled IgG 8C3 showed significant therapeutic efficacy in murine melanoma, safety towards healthy melanin-containing tissues and favorable comparison with the anti-CTLA4 antibody. We have demonstrated that antibody binding to melanin relies on both charge and hydrophobic interactions while the in vivo data supports further development of 8C3 IgG as radioimmunotherapy reagent for metastatic melanoma.

Paracoccidioides Spp.: Virulence Factors and Immune-Evasion Strategies

Paracoccidioides spp. are dimorphic fungal pathogens responsible for one of the most relevant systemic mycoses in Latin America, paracoccidioidomycosis (PCM). Their exact ecological niche remains unknown; however, they have been isolated from soil samples and armadillos (Dasypus novemcinctus), which have been proposed as animal reservoir for these fungi. Human infection occurs by inhalation of conidia or mycelia fragments and is mostly associated with immunocompetent hosts inhabiting and/or working in endemic rural areas. In this review focusing on the pathogen perspective, we will discuss some of the microbial attributes and molecular mechanisms that enable Paracoccidioides spp. to tolerate, adapt, and ultimately avoid the host immune response, establishing infection.

Melanins Protect Sporothrix brasiliensis and Sporothrix schenckii from the Antifungal Effects of Terbinafine

Terbinafine is a recommended therapeutic alternative for patients with sporotrichosis who cannot use itraconazole due to drug interactions or side effects. Melanins are involved in resistance to antifungal drugs and Sporothrix species produce three different types of melanin. Therefore, in this study we evaluated whether Sporothrix melanins impact the efficacy of antifungal drugs. Minimal inhibitory concentrations (MIC) and minimal fungicidal concentrations (MFC) of two Sporothrix brasiliensis and four Sporothrix schenckii strains grown in the presence of the melanin precursors L-DOPA and L-tyrosine were similar to the MIC determined by the CLSI standard protocol for S. schenckii susceptibility to amphotericin B, ketoconazole, itraconazole or terbinafine. When MICs were determined in the presence of inhibitors to three pathways of melanin synthesis, we observed, in four strains, an increase in terbinafine susceptibility in the presence of tricyclazole, a DHN-melanin inhibitor. In addition, one S. schenckii strain grown in the presence of L-DOPA had a higher MFC value when compared to the control. Growth curves in presence of 2×MIC concentrations of terbinafine showed that pyomelanin and, to a lesser extent, eumelanin were able to protect the fungi against the fungicidal effect of this antifungal drug. Our results suggest that melanin protects the major pathogenic species of the Sporothrix complex from the effects of terbinafine and that the development of new antifungal drugs targeting melanin synthesis may improve sporotrichosis therapies.

Fungal Melanin: What do We Know About Structure?

The production of melanin significantly enhances the virulence of many important human pathogenic fungi. Despite fungal melanin’s importance in human disease, as well as melanin’s contribution to the ability of fungi to survive in diverse hostile environments, the structure of melanin remains unsolved. Nevertheless, ongoing research efforts have progressively revealed several notable structural characteristics of this enigmatic pigment, which will be the focus of this review. These compositional and organizational insights could further our ability to develop novel therapeutic approaches to combat fungal disease and enhance our understanding of how melanin is inserted into the cell wall.

Occurrence of Ochroconis and Verruconis species in clinical specimens from the United States

Ochroconis is a dematiaceous fungus able to infect immunocompetent people. Recently, the taxonomy of the genus has been reevaluated, and the most relevant species, Ochroconis gallopava, was transferred to the new genus Verruconis. Due to the important clinical implications of these fungi and based on the recent classification, it was of interest to know the spectra of Ochroconis and Verruconis species in clinical samples received in a reference laboratory in the United States. A set of 51 isolates was identified morphologically and molecularly based on sequence analyses of the nuclear ribosomal RNA (nrRNA), actin, and β-tubulin genes. Verruconis gallopava was the most common species (68.6%), followed by Ochroconis mirabilis (21.5%). One isolate of Ochroconis cordanae was found, being reported for the first time in a clinical setting. The most common anatomical site of isolation was the lower respiratory tract (58.8%), followed by superficial and deep tissues at similar frequencies (21.6 and 19.6%, respectively). Interestingly, three new species were found, which are Ochroconis olivacea and Ochroconis ramosa from clinical specimens and Ochroconis icarus of an environmental origin. The in vitro antifungal susceptibilities of eight antifungal drugs against the Ochroconis isolates revealed that terbinafine and micafungin were the most active drugs.

Detection of antibodies against customized epitope: use of a coating antigen employing VEGF as fusion partner

Diagnosis of many infectious, autoimmune diseases and cancers depends on the detection of specific antibodies against peptide epitope by enzyme-linked immunosorbent assay (ELISA). However, small peptides are difficult to be coated on the plate surfaces. In this study, we selected GnRH as a model hapten to evaluate whether VEGF121 would be suitable as an irrelevant hapten-carrier to develop a universal platform for specific antibodies detection. Firstly, GnRH was fused to the C terminus of VEGF121 and the resultant fusion protein VEGF-GnRH expressed effectively as inclusion bodies in Escherichia coli. Thereafter, VEGF-GnRH was easily purified to near homogeneity with a yield of about 235 mg from 2.1 L induced culture. At last, VEGF-GnRH was used to perform ELISA and western blot, and our results suggested that VEGF-GnRH was capable of detecting anti-GnRH antibodies in sera both qualitatively and quantitatively. Indeed, previous studies of our laboratory had demonstrated that other fusion proteins such as VEGF-Aβ10, VEGF-GRP, VEGF-CETPC, and VEGF-βhCGCTP37 were able to detect their corresponding antibodies specifically. Therefore, VEGF121 may be a suitable irrelevant fusion partner of important diagnostic peptide markers. Our works would shed some light on the development of a universal platform for detection of specific antibodies.