In agreement with a recent report [46] , we also observed a reduction in the peripheral blood memory B cell compartment of metastatic melanoma patients. We measured a reduction of the CD27+ subset of memory B cells in patients with both metastatic and non-metastatic melanoma compared to healthy volunteers ( Figure S2 ). Despite the reduction of circulating memory B cells in our cohort, patient-derived B cells were capable of secreting high amounts of IgG antibodies when activated in vitro with a TLR 9 agonist, with comparable antibody production to B cells from healthy individuals, and a high percentage of patient-and healthy volunteer-derived B cells expressed IgG antibodies within a few days in culture (80% of B cells from three patients with melanoma, Figure S2 ). Thus, while a reduced memory B compartment has been reported in cancer patients, we show that a melanomareactive portion of this compartment remains in our patient cohort.