Trichosporon hyphae should be carefully differentiated from Aspergillus hyphae in tissue: A case report

Fungal infections have increased in recent decades because of the increasing number of immunocompromised patients receiving aggressive chemotherapy and immunosuppressive treatment. Trichosporon is one of the emerging fungal pathogens causing fatal infection.1 Its morphology in tissue has been claimed to resemble that of Candida because of the combination of yeast cells and hyphal forms produced by both fungi.2 However, the resemblance of Trichosporon hyphae to those of Aspergillus has been less considered. We present an autopsied case of disseminated trichosporonosis that demonstrates the Aspergillus-like hyphae in cardiac fungal vegetation. A Japanese man in his thirties was treated with cytotoxic chemotherapy and peripheral blood stem cell transplantation for acute lymphoblastic leukemia. Although treatment initially appeared to be successful, his leukemia relapsed on day 83 post-transplant (PT). Corticosteroid therapy was initiated with oral antibiotics and the anti-fungal agent itraconazole. On day 139 PT, intensive antineoplastic chemotherapy was initiated and his blood cell count reached its lowest on day 146 PT; profound neutropenia (<100 cells/μL) continued for 13 days. His white blood cell count returned to 1,700 cells/μL without leukemic cells on day 164 PT. However, serum beta-D-glucan increased to 344 pg/mL (normal: <11.0 pg/mL) on day 167 PT, indicating fungal infection. Treatment with an additional anti-fungal agent (micafungin) was initiated on day 173 PT. Trichosporon sp. was cultured from the patient's central venous catheter blood on day 195 PT and was identified at the genus level. His general condition further deteriorated, and he died of multiorgan failure on day 231 PT. Autopsy was performed two hours after his death. At autopsy, red-spotted cream-white vegetations 20 × 30 × 10 and 15 × 10 × 10 mm in size were attached to the mitral and tricuspid valves, respectively. Additional macroscopic findings were 1–3 mm-sized whitish nodules in the myocardium, kidneys, liver, spleen, thyroid gland, and brain. Histologically, cardiac vegetations were composed of tangled fungal hyphae with purulent inflammatory cells (Figure 1a,b). Most hyphae were septate and regularly wide (2.2–4.2 µm) showing parallel contour with acute-angled dichotomous branching (Figure 1c,d), reminiscent of Aspergillus hyphae. Some hyphae were articulated, producing rectangular barrel-shaped arthroconidia 3.5–6.5 × 2.0–3.8 µm in size (Figure 1e). Pleomorphic yeast cells were seen close to the arthroconidia. Whitish nodules in the myocardium and other organs were identified as nodular necrosis or abscesses with the same fungal cells; most hyphae in these areas were irregularly wide and twisted, and appeared different from Aspergillus hyphae. Bone marrow was hypoplastic with no evidence of leukemia relapse. Immunohistochemistry was performed on paraffin sections of fungal-infected organs following the protocol described by Fukuzawa et al.3 using rabbit polyclonal anti-Trichospron, anti-Candida, or anti-Aspergillus antibodies. Fungi tested positive with anti-Trichosporon antibody and negative with the other antibodies (Figure 1f). Fungal DNA extracted from formalin-fixed paraffin-embedded tissue of autopsied infected organs was analyzed using the method described by Sugita et al.4 The intergenic spacer 1 region of the ribosomal RNA gene from the heart, kidney, spleen, and thyroid gland was found to be 100% identical to the corresponding sequences of Trichosporon asahii (T. asahii) genotype 1 in NCBI BLAST database (GenBank accession no. AB0663869) (https://blast.ncbi.nlm.nih.gov/Blast.cgi). Trichosporon species are yeast-like fungi that are widely distributed in nature, including water, soil, and decomposing wood; they are also occasionally found in human skin and alimentary tract flora.1 The main causative agent of deep-seated trichosporonosis is T. asahii.1 This organism has seven genotypes; genotype 1, detected in the present case, is most commonly found in clinical specimens in Japan.4 Trichosporon is the second most common cause of disseminated yeast infections in patients with hematological cancers, following Candida.1 This multi-drug-resistant fungus has emerged as a significant pathogen responsible for breakthrough infections during antifungal prophylaxis.1 In the present case, breakthrough trichosporonosis is suspected, as the patient was receiving itraconazole and micafungin prior to the isolation of Trichosporon. As Trichosporon can proliferate as both yeast cells and true hyphae in tissue, similar to Candida, caution has long been advised to avoid mistaking Trichosporon for Candida.2 However, the resemblance between Trichosporon hyphae and those of Aspergillus has rarely been noted.5 In the present case, the cardiac valve vegetation exhibited septate regularly wide hyphae with acute-angled dichotomous branching, appearing typical of those of Aspergillus. Given that trichosporonosis is much less common than aspergillosis, these hyphae were initially misidentified as those of Aspergillus. A subsequent more through histological examination, however, revealed arthroconidia, leading to the correct identification of the causative fungus. Arthroconidia are rectangular barrel-shaped fungal cells formed through hyperseptation and disarticulation of hyphal segments and serve as distinguishing features of Trichosporon.2 Although some other fungi can produce both true hyphae and arthroconidia in tissue, immunohistochemical and molecular analyses confirmed the diagnosis of disseminated trichosporonosis in the present case. In summary, Trichosporon should be considered as a potential etiological agent in fatal fungal infections and should be differentiated not only from Candida but also from Aspergillus in tissue. Masatomo Kimura designed the study and drafted the article. Masatomo Kimura, Fumiaki Ono and Takaaki Chikugo acquired the clinical data and performed the histopathological study. Takashi Sugita performed the molecular work. All authors read and approved the manuscript before submission. We would like to thank Dr. Masao Hotchi, Department of Pathology, Shinshu University School of Medicine, for performing the immunostaining for fungi. We thank Lisa Oberding, MSc, from Edanz (https://jp.edanz.com/ac) for editing a draft of this manuscript. None declared. Our institution does not require ethics committee approval for Case Reports. The investigation was conducted in accordance with the Declaration of Helsinki. The data that support the findings of this study are available from the corresponding author upon reasonable request.