Human skin contains multiple memory T cell subtypes which are crucial in protective cutaneous immunity and in mediating inflammatory dermatoses, but their roles in cutaneous squamous cell carcinoma (cSCC) are unclear. In this study, phenotypic and functional characterisation of memory T cells in human cSCCs and patient-matched normal skin and peripheral blood was performed. Flow cytometry of lymphocytes from cSCCs (n=53) showed 77.2% of tumoral T cells were CCR7-CD45RA- effector memory T cells, with fewer CCR7+CD45RA- central memory, CCR7-CD45RA+ effector memory RA, CCR7+CD45RA+ naive and CCR7+L-selectin- migratory memory T cells. Higher frequencies of CD8+CD69+CD103+ resident memory T cells (TRMs) were present in cSCC than normal skin (p=0.0026) and blood (p<0.001). Immunofluorescence microscopy confirmed that CD103 was mainly expressed on CD3+ and CD8+ cells in cSCC. Whilst the different cSCC TRM subgroups showed similar levels of IFNγ, TNF-α and IL-2 following PMA/ionomycin stimulation, IL-10 was more highly expressed in CD8+CD103+ TRMs compared with CD8+CD69- T cells and CD8+CD69+CD103- TRMs  (p<0.05). In addition, CD39, CTLA-4 and PD-1 were upregulated on CD8+CD103+ TRMs compared with CD8+CD69- T cells and CD8+CD69+CD103- TRMs in cSCC (p<0.05). Immunohistochemistry showed increased CD103+ cell frequencies in primary cSCCs which metastasised (n=38) compared with cSCCs which did not metastasise after at least 5 years of follow-up (n= 44, p<0.0001) and high CD103 expression was associated with earlier metastasis (p=0.0003). These results indicate that CD8+CD103+ TRMs are an important memory T cell subgroup in cSCCs which lead to dysfunctional anti-tumour immunity and poorer clinical outcomes in this cancer.

Cutaneous squamous cell carcinomas are infiltrated with CD8+CD103+ resident memory T cells which express inhibitory markers and are associated with metastasis