The present disclosure provides methods for expanding tumor-infiltrating lymphocytes (TILs), such as tumor-infiltrating T cells, utilizing an agonist of PGC1α in vivo, ex vivo, or both. Exhausted T cells present in the TIL population fail to effectively proliferate, produce cytokines, or kill target cells. The present disclosure provides methods to correct these defects through the use of pharmacologic agents to reprogram the metabolism of the exhausted intratumoral T cells. Exemplary agonists of PGC1α include proliferator-activated receptor (PPAR)-gamma agonists (e.g., a thiazolidinedione (TZD), aleglitazar, farglitazar, muraglitazar, or tesaglitazar), AMPK activators (e.g., 5-aminoimidazole-4-carboxamide ribonucleotide, AICAR), and sirtuin activators (e.g., resveratrol, SRT1720, SRT2104, SRT2183, SRT1460). Also provided are kits can compositions that can be used with such methods.