A PP2A inhibitor for use in a method of: treating a subject afflicted with cancer comprising administering to the subject an effective amount of the PP2A inhibitor in combination with an effective amount of a checkpoint inhibitor, wherein the amounts when taken together are effective to treat the subject; or: treating a subject afflicted with cancer and receiving a checkpoint inhibitor comprising administering to the subject an amount of the PP2A inhibitor effective to enhance treatment relative to the checkpoint inhibitor alone; or: treating a tumor or cancer in a subject comprising administering to the subject an effective amount of the PP2A inhibitor in combination with an effective amount of a checkpoint inhibitor, wherein the amounts when taken together are effective to treat the tumor or cancer; or: increasing a T-cell response to cancer cells in a subject afflicted with cancer comprising administering to the subject an amount of the PP2A inhibitor in combination with an effective amount of a checkpoint inhibitor effective to increase the T-cell response to the cancer cells; or: increasing T cell activation in a subject afflicted with cancer comprising administering to the subject an effective amount of the PP2A inhibitor in combination with an effective amount of a checkpoint inhibitor so as to thereby increase the T cell activation; wherein the checkpoint inhibitor is atezolizumab, nivolumab, pembrolizumab, ipilimumab or tremelimumab; and wherein the PP2A inhibitor is a compound having the structure: wherein bond α is present or absent; R 1 and R 2 together are =O; R 3 is OH, O - or OR 9, wherein R 9 is alkyl, alkenyl, alkynyl or aryl; R 4 is where X is O, S, NR 10, N + HR 10 or N + R 10 R 10, where each R 10 is independently H, alkyl, alkenyl, alkynyl, aryl, R 5 and R 6 taken together are =O; R 7 and R 8 are each H; or a salt, zwitterion or ester thereof. treating a subject afflicted with cancer comprising administering to the subject an effective amount of the PP2A inhibitor in combination with an effective amount of a checkpoint inhibitor, wherein the amounts when taken together are effective to treat the subject; or: treating a subject afflicted with cancer and receiving a checkpoint inhibitor comprising administering to the subject an amount of the PP2A inhibitor effective to enhance treatment relative to the checkpoint inhibitor alone; or: treating a tumor or cancer in a subject comprising administering to the subject an effective amount of the PP2A inhibitor in combination with an effective amount of a checkpoint inhibitor, wherein the amounts when taken together are effective to treat the tumor or cancer; or: increasing a T-cell response to cancer cells in a subject afflicted with cancer comprising administering to the subject an amount of the PP2A inhibitor in combination with an effective amount of a checkpoint inhibitor effective to increase the T-cell response to the cancer cells; or: increasing T cell activation in a subject afflicted with cancer comprising administering to the subject an effective amount of the PP2A inhibitor in combination with an effective amount of a checkpoint inhibitor so as to thereby increase the T cell activation; wherein the checkpoint inhibitor is atezolizumab, nivolumab, pembrolizumab, ipilimumab or tremelimumab; and wherein the PP2A inhibitor is a compound having the structure: wherein bond α is present or absent; R 1 and R 2 together are =O; R 3 is OH, O - or OR 9, wherein R 9 is alkyl, alkenyl, alkynyl or aryl; R 4 is where X is O, S, NR 10, N + HR 10 or N + R 10 R 10, where each R 10 is independently H, alkyl, alkenyl, alkynyl, aryl, where X is O, S, NR 10, N + HR 10 or N + R 10 R 10, where each R 10 is independently H, alkyl, alkenyl, alkynyl, aryl, R 5 and R 6 taken together are =O; R 7 and R 8 are each H; or a salt, zwitterion or ester thereof.