Additional Study Information: The main purpose of this research study is to determine if the experimental product, KTE-C19, when administered after a 3 day course of chemotherapy, is safe and effective in treating relapsed or refractory (r/r) indolent non-Hodgkin B-cell lymphoma (iNHL). KTE-C19 is made from white blood cells that are removed from the individual. A virus (retrovirus) is used to introduce a gene that creates a protein (called a chimeric antigen receptor or CAR) on the surface of T cells, a type of blood cell that fights infection and eliminates cancer cells. The hope is that the CAR on the T cells will bind to and kill cells that express CD19, a protein that is found on B-cell lymphomas.