The present invention relates to a method of assessing, predicting or monitoring the sensitivity of a subject having a tumor or cancer to an immunotherapeutic molecule acting on the subject's T cells, to a method of selecting an appropriate treatment of cancer, to a method of screening or identifying a compound suitable for improving the treatment of a cancer, and to the use of corresponding kits. The method of predicting or monitoring the sensitivity of a subject having a tumor to an immunotherapeutic molecule acting on the subject's T cells typically comprises a step a) of determining, in a biological sample from said subject, the expression level of soluble CD25 (sCD25) and, when the expression level is determined, a step b) of comparing at least said expression level to a reference expression level, thereby assessing whether the subject having a tumor is responsive or resistant to the immunotherapeutic molecule.
