PD1 pathway is critical in determining the response to CAR T cell therapy. Emerging data suggested that Inhibition of PD1 could enhance the efficacy of CAR T cell therapy. iPD1 CD19 eCAR T cells is an enhanced version of the classical 2nd generation anti-CD19 4-1BB-costimulatory chimeric antigen receptor engineered T cells with cell-intrinsic PD1 inhibition by incorporation of a PD1 shRNA-expressing cassette in the CAR lentivector. This design will enhance the anti-tumor activities of CAR T cells by inhibiting PD1 induction after CAR T cell activation. This pilot, single arm, one center, dose-escalation, open label study is to determine the safety and efficacy of iPD1 CD19 eCAR T cells in relapsed or refractory CD19 positive lymphoma.