Rachael A. Clark, MD, PhD
Associate Professor, Department of Dermatology, Harvard Medical School
Associate Dermatologist, Dermatology, Brigham And Women's Hospital
DF/HCC Program AffiliationCancer ImmunologyCutaneous Oncology and Melanoma
Squamous cell carcinomas of the skin (SCC) are a leading cause of death in organ transplant recipients and treatment of non-melanoma skin cancers, of which SCC is the second most frequent type, account for 4.5% of all Medicare cancer costs. The incidence and malignancy of SCC are greatly increased in patients with decreased T cell function, suggesting a role for the immune system in controlling these tumors. T cells are found within SCC but fail to control tumor growth. We have found that SCC evade the immune system at least in part by two newly identified mechanisms. First, by failing to express E-selectin on tumor vessels, SCC evade the population of antigen-experienced skin homing T cells most capable of recognizing the tumor. Second, by recruiting FOXP3+ regulatory T cells (Treg), SCC create a local environment of immune suppression around the tumor. Imiquimod, a TLR7 agonist effective in the treatment of skin cancers, neutralizes both of these defenses, inducing expression of E-selectin on tumor vessels, restoring the ability of CLA+ skin homing T cells to enter the tumor and reducing the % FOXP3+ Treg to levels found in normal skin. Our ongoing studies focus on determining how SCC inhibit vascular E-selectin and recruit Treg with the goal of developing novel agents for the treatment of SCC and their premalignant precursor lesions, actinic keratoses (AK). Additional studies involve determining the mechanisms by which imiquimod induces vascular E-selectin and restores T cell homing, with the goal of developing agents that can treat established SCC without the risk of widespread immune stimulation that is a concern with TLR agonists such as imiquimod. We are currently investigating the hypotheses that nitric oxide (NO) produced by dendritic cells (DC) within SCC inhibit vascular E-selectin and that recruitment of NO-producing DC and Treg both occur via CCR2. We are determining if inhibition of CCR2 function, induction of E-selectin on tumor vessels and inhibition of Treg recruitment can enhance the immunologic response to SCC. These questions are being investigated first in vitro and then in immunodeficient mice grafted with human SCC tumors. Lastly, studies will determine if aberrant homing and Treg recruitment also occur in AK. If so, therapies that induce E-selectin and inhibit recruitment of Treg may be effective in the treatment of these lesions, inducing their immunologic destruction before they progress to SCC. Novel therapies are being evaluated in vitro and in mice grafted with human AK. Our work focuses on the development of novel therapies for squamous cell carcinomas and their premalignant precursor lesions that are safe for use in normal and immunosuppressed individuals. The skin is an accessible tissue in which to study tumor immunity because immune reactions in the skin are visible, easily sampled and can be manipulated with topical medications. Because impaired T cell homing and recruitment of regulatory T cells occur in many human malignancies, findings and novel therapies arising from this work should be applicable to the treatment of other types of human cancer.
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