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Whereas CD4 + T cells conventionally mediate antitumor immunity by providing help to CD8 + T cells, recent clinical studies have implied an important role for cytotoxic CD4 + T cells in cancer immunity. Using an orthotopic melanoma model, we provide a detailed account of antitumoral CD4 + T cell responses and their regulation by major histocompatibility complex class II (MHC II) in the skin. Intravital imaging revealed prominent interactions of CD4 + T cells with tumor debris-laden MHC II + host antigen-presenting cells that accumulated around tumor cell nests, although direct recognition of MHC II + melanoma cells alone could also promote CD4 + T cell control. CD4 + T cells stably suppressed or eradicated tumors even in the absence of other lymphocytes by using tumor necrosis factor-α and Fas ligand (FasL) but not perforin-mediated cytotoxicity. Interferon-γ was critical for protection, acting both directly on melanoma cells and via induction of nitric oxide synthase in myeloid cells. Our results illustrate multifaceted and context-specific aspects of MHC II-dependent CD4 + T cell immunity against cutaneous melanoma, emphasizing modulation of this axis as a potential avenue for immunotherapies.

Panel j was inadvertently labelled as panel k in the caption to Fig. 4. Similarly, 'Fig. 4k' should have been 'Fig. 4j' in the sentence beginning 'TNF-α-deficient gBT-I cells were…'. In addition, the surname of author Umaimainthan Palendira was misspelled 'Palendria'. These errors have been corrected online.

The immune system can suppress tumour development both by eliminating malignant cells and by preventing the outgrowth and spread of cancer cells that resist eradication 1 . Clinical and experimental data suggest that the latter mode of control-termed cancer-immune equilibrium 1 -can be maintained for prolonged periods of time, possibly up to several decades 2-4 . Although cancers most frequently originate in epithelial layers, the nature and spatiotemporal dynamics of immune responses that maintain cancer-immune equilibrium in these tissue compartments remain unclear. Here, using a mouse model of transplantable cutaneous melanoma 5 , we show that tissue-resident memory CD8 + T cells (T RM cells) promote a durable melanoma-immune equilibrium that is confined to the epidermal layer of the skin. A proportion of mice (~40%) transplanted with melanoma cells remained free of macroscopic skin lesions long after epicutaneous inoculation, and generation of tumour-specific epidermal CD69 + CD103 + T RM cells correlated with this spontaneous disease control. By contrast, mice deficient in T RM formation were more susceptible to tumour development. Despite being tumour - free at the macroscopic level, mice frequently harboured melanoma cells in the epidermal layer of the skin long after inoculation, and intravital imaging revealed that these cells were dynamically surveyed by T RM cells. Consistent with their role in melanoma surveillance, tumour-specific T RM cells that were generated before melanoma inoculation conferred profound protection from tumour development independently of recirculating T cells. Finally, depletion of T RM cells triggered tumour outgrowth in a proportion (~20%) of mice with occult melanomas, demonstrating that T RM cells can actively suppress cancer progression. Our results show that T RM cells have a fundamental role in the surveillance of subclinical melanomas in the skin by maintaining cancer-immune equilibrium. As such, they provide strong impetus for exploring these cells as targets of future anticancer immunotherapies.