PDCD1 and melanoma: Some authors such as Machiraju and co-authors suggested that the increased presence of sPD1 in the blood may directly limit the efficacy of the therapeutic anti-PD1 antibody by competing with its binding to membrane-bound PD1 on immune cells and that increased sPD1 concentrations in melanoma patients indicated resistance to ipilimumab plus nivolumab combined treatment but not to anti-PD1 monotherapy [41].