T cells are well known for their roles in combating cancer and infection, but chronic exposure to antigens and inflammation can cause T cells to enter a state of “exhaustion.” Exhausted T cells lose critical effector functions including cytokine production, the ability to proliferate and memory T cell differentiation is also compromised. Exhausted T cells also express inhibitory receptors and become unresponsive to IL-7 and/or IL-15-driven self-renewal. This progression toward T cell exhaustion results in diminished control of chronic infection or cancer. Exhaustion can occur in both CD4+ and CD8+ T cell populations and the phenotypes of these subsets is somewhat heterogeneous. Nonetheless, T cell exhaustion is reversible and various immuno-oncology interventions have been examined or are currently being evaluated in order to improve outcomes in cancer and chronic infection.
Using Flow Cytometry to Monitor Exhaustion
Customized flow cytometry panels can be created to monitor CD4+ or CD8+ T cell exhaustion, and the level of complexity of these panels is only limited by the type of cytometer available for use. Exhaustion of CD4+ T cells is typically associated with co-expression of multiple inhibitory receptors, including PD-1, CTLA-4 and LAG-3, as well as decreased production of cytokines such as TNF-alpha and IFN-gamma, and decreased proliferative and cytotoxic functionality in vitro. Alterations in transcription factor expression, including EOMES and GATA-3, can also be measured. CD8+ T cells also show increased co-expression of inhibitory receptors, impaired production of cytokines and diminished proliferative and cytolytic function. In addition, CD8+ T cells are less responsive to IL-7 and IL-15, which leads to decreased memory T cell proliferation. Flow cytometry can be used to measure all of these parameters in these T cell subsets.
Additional proteome and exome sequences can accompany flow cytometry analysis to identify other markers of exhaustion, but for most pre-clinical and clinical applications, flow cytometry analysis is a powerful, flexible and relevant tool for monitoring this immune state.
 Wherry EJ, Kurachi M. Molecular and cellular insights into T cell exhaustion. Nat. Rev. Immunol.. 2015;15(8):486-499.
 Nguyen LT, Ohashi PS. Clinical blockade of PD1 and LAG3 — potential mechanisms of action. Nat. Rev. Immunol. 2014;15:45–56.