The application relates to methods of assessing whether an individual has an exhausted CD8+ T cell or lack of CD4+ T cell costimulation phenotype, and the use of such methods in determining an individual's risk of autoimmune disease progression, progression of a chronic infection, not responding to a treatment for a chronic infection, not mounting an effective immune response to vaccination, infection-associated immunopathology, transplant rejection, or cancer progression. The application also relates to in vitro methods for assessing whether CD8+ and CD4+ T cells in a sample have an exhausted CD8+ T cell or lack of CD4+ T cell costimulation phenotype, and for identifying a substance capable of inducing an exhausted CD8+ T cell or lack of CD4+ T cell costimulation phenotype in an individual, as well as a kit for assessing whether an individual has an exhausted CD8+ T cell or lack of CD4+ T cell costimulation phenotype or whether an exhausted CD8+ T cell or lack of CD4+ T cell costimulation phenotype is present in a sample of CD8+ and CD4+ T cells.
