The invention relates to methods for diagnosing cancer and determining the responsiveness to chemotherapyin a subject based on the detection of HSP70-expressing exosomes in a bodily fluid sample obtained from said subject. The invention also relates to methods for treating cancer and in particular methods for restoring or enhancing the anti-tumor immune response in a patient in need thereof by inhibiting the activation MDSC as well as methods for inhibiting or reducing HSP70-expressing exosomes-mediated tumor resistance against a chemotherapeutic agent and therefore methods for restoring or enhancing the efficacy of said chemotherapeutic agent.