Abstract: 【Abstract】 Objective To investigate the distribution and expression of CD103 in skin lesions of mycosis fungoides （MF） across different clinical stages and pathological subtypes. Methods This retrospective case series study analyzed the clinical and pathological data from 36 patients with MF who visited the Department of Dermatology, Hangzhou Third People's Hospital from January 2017 to January 2021. Single-cell RNA sequencing was performed on skin biopsy samples from 2 patients with tumor-stage MF. Immunohistochemical study was conducted to assess the expression of CD103 and CD45RO in skin lesions of all the 36 patients. Normally distributed data were compared among multiple groups using one-way analysis of variance, followed by Tukey′s test for multiple comparisons. Results Among the 36 MF patients, there were 20 with classic MF （9 in the patch stage, 8 in the plaque stage, 3 in the tumor stage）, including 10 males and 10 females, aged 48.90 ± 17.51 years; 8 patients had folliculotropic MF （FMF）, including 4 males and 4 females, aged 35.13 ± 24.54 years; 8 had hypopigmented MF （HMF）, all of whom were males, aged 14.50 ± 9.41 years. Single-cell RNA sequencing and clustering analysis identified 16 T cell subsets （C01 - C16）, among which, 4 clusters （C01, C03, C11, C13） exhibited high copy number variation, indicating their malignant nature, and all the 4 clusters showed specifically high CD103 mRNA expression. Immunohistochemical study showed that CD103? tissue-resident memory T （TRM） cells were characteristically arranged linearly along the basal layer of the epidermis in patch-stage classic MF; in plaque-stage MF, CD103? TRM cells showed prominent epidermotropism with the presence of Pautrier microabscesses; in tumor-stage MF, CD103? TRM cells lost epidermotropism, and were scattered in the dermis and epidermis. All HMF cases were negative for CD103 staining. In FMF cases, CD103? TRM cells mainly infiltrated around hair follicles and small blood vessels in the dermis. The number of CD103? TRM cells （counted in 5 high-power fields） did not significantly differ among patients with patch, plaque, and tumor stages of classic MF （F = 3.06, P = 0.073）. However, CD103? TRM counts significantly differed among HMF, classic MF, and FMF groups （HMF: 8.91 ± 6.82; classic MF: 124.13 ± 122.39; FMF: 110.24 ± 109.15; F = 3.50, P = 0.042）, with the HMF group showing significantly lower counts compared with the classic MF group （P < 0.001） and FMF group （P = 0.005）. The CD103/CD45RO ratio varied significantly among different stages of classic MF （patch stage: 0.12 ± 0.08; plaque stage: 0.41 ± 0.20; tumor stage: 0.30 ± 0.35; F = 5.24, P = 0.017）, with a lower ratio in the patch stage than in the plaque stage （P = 0.013）; significant differences were also observed in the CD103/CD45RO ratio among different MF subtypes （HMF: 0.03 ± 0.02; classic MF: 0.26 ± 0.22; FMF: 0.34 ± 0.21; F = 5.87, P = 0.007）, which was significantly lower in the HMF group than in the classic MF group （P = 0.002） and FMF group （P < 0.001）. Conclusion The expression profile of CD103 facilitates the early diagnosis and subtype differentiation of MF, and may serve as a potential molecular marker for auxiliary diagnosis and disease stage assessment.

Key words: Mycosis fungoides, CD103, CD45RO, Tissue resident memory T cell, Pathology, molecular, Immunohistochemistry