hACE2 upregulation and participation of macrophages and clear cells in the immune response of epididymis to SARS-CoV-2 in K18-hACE2 mice.

Background: The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virus caused the coronavirus disease 2019 pandemic, and the prevalence of deaths among men is higher than among women. The epididymis, divided into caput, corpus, and cauda, shows a region-specific immunity. The K18-hACE2 mouse expresses human angiotensin-converting enzyme 2 (hACE2), the receptor that allows SARS-CoV-2 infection. However, studies using this transgenic mouse to evaluate the impact of this viral infection in epididymis have not yet been performed.

Objectives: We evaluated the expression of hACE2 in the epididymis of SARS-CoV-2-infected K18-hACE2 mice, and assessed the epididymal immune response, focusing on F4/80⁺ mononuclear phagocytes and tumor necrosis factor-alpha expression.

Materials and methods: The following analyses were performed in the epididymal sections of infected mice: epithelial height and duct diameter, birefringent collagen, Terminal deoxynucleotidyl Transferase-mediated dUTP Nick End Labelling, immunoreactions for detection of hACE2, spike, FGF, V-ATPase, F4/80, tumor necrosis factor-alpha, and iNOS. Viral particles were identified under electron microscopy. hACE2, Rigi, Tgfb1 and Tnfa expression were also evaluated by real-time quantitative polymerase chain reaction.

Results: All epididymal regions expressed hACE2, which increased in all epididymal regions in the infected mice. However, the caput appeared to be the most infected region. Despite this, the caput region showed minimal changes while the cauda showed significant epithelial changes associated with increased iNOS immunoexpression. The F4/80⁺ mononuclear phagocyte area increased significantly in both stroma and epithelium. In addition to the epithelial and stromal mononuclear phagocytes, tumor necrosis factor-alpha was also detected in clear cells, whose cytoplasm showed a significant increase of this cytokine in the infected animals.

Discussion and conclusion: The K18-hACE2 mouse is a useful model for evaluating the impact of SARS-CoV-2 infection in the epididymis. The infection induced hACE2 upregulation, favoring the virulence in the epididymis. The epididymal regions responded differentially to infection, and the activation of F4/80⁺ mononuclear phagocytes associated with the increased tumor necrosis factor-alpha immunolabeling in clear cells indicates a role of clear cells/mononuclear phagocytes immunoregulatory mechanisms in the epididymal immune response to SARS-CoV-2 infection.