Cell-Intrinsic Barriers of T Cell-Based Immunotherapy Hazem E. Ghoneim, Anthony E. Zamora, Paul G. Thomas, Ben A. Youngblood  Trends in Molecular Medicine  Volume 22, Issue 12, Pages 1000-1011 (December 2016) DOI: 10.1016/j.molmed.2016.10.002 Copyright © 2016 Elsevier Ltd Terms and Conditions

Figure 1 Key Figure: Epigenetic Barriers to Immune Checkpoint Blockade (ICB)-Mediated Rejuvenation of T Cells (A) The schematic depicts the repression in developmental plasticity acquired by antigen-specific T cells during clonal expansion in response to acute or chronic antigen exposure. Upon acute antigen exposure, naive CD8+ T cells clonally expand and differentiate into effector cells. During chronic high-level antigen exposure, persistent T cell receptor (TCR) stimulation can cause T cell exhaustion. Effector function is progressively repressed during the development of T cell exhaustion, thereby leading to a heterogeneous population of T cells at various levels of exhaustion. Partially exhausted T cells are phenotypically defined by sustained expression of a minimal number of immune inhibitory receptors (IRs) and are characterized by differential expression of T-bet and Eomes (T-bethighEomeslow T cells), whereas fully exhausted T cells are marked by coexpression of multiple IRs. The ICB obstructs inhibitory signals from immune IRs, resulting in rejuvenation of partially exhausted T cells. (B) The schematic depicts the effect of progressive epigenetic programming that reinforces silencing of genes involved in effector function, proliferation, and metabolic fitness of exhausted T cells. Partially exhausted T cells may retain a degree of epigenetic plasticity at exhaustion-specific silenced genes manifested by partially methylated DNA and the deposition of fewer repressive and more permissive histone marks. Terminally differentiated exhausted T cells may be more epigenetically restrained through fully methylated DNA, the deposition of more repressive histone marks, and the loss of permissive histone marks. (C) The diagram models the potential reprogramming of exhaustion-associated epigenetic programs that could remove cell-intrinsic barriers to achieving a better, and possibly durable, rejuvenation response of fully exhausted T cells following ICB therapy. Trends in Molecular Medicine 2016 22, 1000-1011DOI: (10.1016/j.molmed.2016.10.002) Copyright © 2016 Elsevier Ltd Terms and Conditions

Figure 2 Genetically Engineered T Cell Immunotherapeutic Approaches. The schematic depicts the multistep process of generating genetically engineered human T cells targeted against specific tumor antigens. (Step 1) Autologous T cells can be isolated from a patient's peripheral blood mononuclear cells or an excised tumor. (Step 2) T cell pools or specific subsets can be stimulated with (i) activating CD3 antibody (OKT3), (ii) anti-CD3/CD28-coated beads, or (iii) allogeneic feeder cell lines. (Step 3) Specific T cell subsets can be sorted or enriched according to their differentiation status to exploit proliferative and effector functions. (Step 4) Engineered T cells can be generated by cloning either T cell receptors (TCRs) or chimeric antigen receptors (CARs) and transducing a patient's T cells with retro- or lentiviruses, thus redirecting recognition toward tumor-associated antigens. (Step 5) Engineered T cells can then be expanded in vitro in the presence of conditioning cytokines (e.g., IL-2 or IL-15) to increase the frequency of tumor-specific T cells generated. (Step 6) Engineered T cells specific for an antigen (target) of interest are selected. (Step 7) Engineered T cells can then be reinfused into the patient (usually post-lymphodepletion). Abbreviations: LTR, long terminal repeat; TCM, central memory T cells; TEM, T effector memory cells; TN, naive T cells; TSCM, stem cell memory T cells; TM, transmembrane domain. Trends in Molecular Medicine 2016 22, 1000-1011DOI: (10.1016/j.molmed.2016.10.002) Copyright © 2016 Elsevier Ltd Terms and Conditions