Abstract: 【Abstract】 Objective To investigate the effect of macrophage-derived exosomes on the morphological transformation of Candida albicans （CA）, and to explore the underlying mechanisms. Methods In vitro cultured human acute monocytic leukemia cell line THP-1 was induced and differentiated into M0 macrophages using the phorbol ester PMA. CA was activated and prepared as the fungal suspension. M0 macrophages were infected with the CA suspension, and the process of cell phagocytosis was observed under a high-content imaging analysis system. M0 macrophage-derived exosomes （exosome group） and CA-infected M0 macrophage-derived exosomes （CA exosome group） were extracted by differential centrifugation; transmission electron microscopy, nanoparticle tracking analysis, and Western blot analysis were performed to identify and compare exosomes in the two groups. The exosomes from the two groups were separately co-cultured with CA （exosome-treated group and CA exosome-treated group）, and independently cultured CA served as the blank control group; the morphological changes of CA were observed under an inverted microscope, the intracellular cyclic adenosine monophosphate （cAMP） contents were detected by the enzyme-linked immunosorbent assay （ELISA）, and the expression levels of cAMP-related genes, RAS1 and CDC35 （also known as Cyr1）, were detected by real-time quantitative PCR （RT-qPCR）. Results Western blot analysis showed that exosomes from the exosome group and CA exosome group both expressed the tumor susceptibility gene 101 protein （TSG101, an exosome marker）, and did not express calnexin （a negative marker）; transmission electron microscopy and nanoparticle tracking analysis showed no significant differences in the morphology or size of the exosomes between the two groups. Compared with the blank control group, the exosome-treated group and CA exosome-treated group both showed obvious inhibition of the yeast-to-mycelial phase transition of CA, with a noticeable reduction in the length of the hyphae under the inverted microscope. ELISA revealed that the intracellular cAMP content in CA significantly decreased in the exosome-treated group and CA exosome-treated group （16.70 ± 0.84 pmol/ml, 16.82 ± 0.87 pmol/ml, respectively） compared with the blank control group （21.82 ± 1.08 pmol/ml; t = 6.45, 6.23, respectively, both P = 0.003）. RT-qPCR revealed that the expression of the cAMP-related genes, RAS1 and CDC35, was down-regulated in the exosome-treated group and CA exosome-treated group compared with the blank control group （all P < 0.01）, and the RAS1 mRNA expression was significantly lower in the CA exosome-treated group than in the exosome-treated group （t = 7.43, P = 0.002）. Conclusion Both M0 macrophage-derived exosomes and CA-infected M0 macrophage-derived exosomes could effectively inhibit the mycelial growth of CA, and the latter one exhibited a stronger inhibitory effect, possibly by down-regulating cAMP in the cAMP/protein kinase A pathway.

Key words: Candida albicans, Macrophages, Exosomes, Morphological transformation, cAMP, Mycelial phase