CTCL is a cancer of skin homing T cells with variants that include leukemic CTCL (L-CTCL), a malignancy of central memory T cells (TCM), and mycosis fungoides (MF), a malignancy of skin resident effector memory T cells (TEM). We report that low-dose alemtuzumab (αCD52) effectively treated patients with refractory L-CTCL but not MF. Alemtuzumab depleted all T cells in blood and depleted both benign and malignant TCM from skin, but a diverse population of skin resident TEM remained in skin after therapy. T-cell depletion with alemtuzumab required the presence of neutrophils, a cell type frequent in blood but rare in normal skin. These data suggest that TCM were depleted because they recirculate between the blood and skin whereas skin resident TEM were spared because they are sessile and non-recirculating. After alemtuzumab treatment, skin T cells produced lower amounts of IL-4 and higher amounts of IFNγ. Moreover, there was a marked lack of infections in alemtuzumab-treated L-CTCL patients despite the complete absence of T cells in blood, suggesting that skin resident TEM can protect the skin from pathogens even in the absence of T cell recruitment from the circulation. Together, these data suggest that alemtuzumab may treat refractory L-CTCL without severely compromising the immune response to infection by depleting circulating TCM but sparing the skin resident TEM that provide local immune protection of the skin.

Purpose In leukemic CTCL (L-CTCL) malignant T cells accumulate in the blood and give rise to widespread skin inflammation. Patients have intense pruritus, increased IgE, decreased Th1 responses and most die from infection. Depleting malignant T cells while preserving normal immunity is a clinical challenge. L-CTCL has been variably described as a malignancy of regulatory, Th2 and Th17 cells. Experimental design We analyzed phenotype and cytokine production in malignant and benign L-CTCL T cells, characterized the effects of malignant T cells on healthy T cells and studied the immunomodulatory effects of treatment modalities in L-CTCL patients. Results 12/12 L-CTCL patients overproduced Th2 cytokines. Remaining benign T cells were also strongly Th2 biased, suggesting a global Th2 skewing of the T cell repertoire. Culture of benign T cells away from the malignant clone reduced Th2 and enhanced Th1 responses but separate culture had no effect on malignant T cells. Co-culture of healthy T cells with L-CTCL T cells reduced IFNγ production and neutralizing antibodies to IL-4 and IL-13 restored Th1 responses. In patients, enhanced Th1 responses were observed following a variety of treatment modalities that reduced malignant T cell burden. Conclusions A global Th2 bias exists in both benign and malignant T cells in L-CTCL and may underlie the infectious susceptibility of patients. Th2 cytokines from malignant cells strongly inhibited Th1 responses. Our results suggest therapies that inhibit Th2 cytokine activity, by virtue of their ability to improve Th1 responses, may have the potential to enhance both anti-cancer and anti-pathogen responses.