Daan Pieren

98 Chapter 4 ABSTRACT Aging leads to alterations in the immune system resulting in diminished responsiveness against pathogens, collectively defined as immunosenescence. Features of immunosenescence accumulate with age prominently in the CD8 + T-cell population. Progression of immunosenescence is complex and not fully understood, but can be ascribed to differentiation of T cells during the course of life. We aimed to gain novel insight in the progression of immunosenescence in human CD8 + T cells. To this end, we identified novel markers of immunosenescence in CD8 + T-cells and assessed how these markers relate to the aging-related accumulation of highly differentiated CD27 - CD28 - cells that is often used as proxy for immunosenescence. We found that co-expression of the transcription factor Helios and the aging-related marker TIGIT marked CD8 + T cells that failed to proliferate and to express activation markers CD69 and CD25 in response to stimulation in vitro . In blood from healthy individuals (21-82 years), we found that TIGIT + Helios + T cells accumulate by aging-related increase of the highly differentiated CD27 - CD28 - population. Interestingly, TIGIT + Helios + T cells also accumulated with age among the less differentiated CD27 + CD28 - T cells before their transit into the highly differentiated CD27 - CD28 - stage. This finding suggests that T cells with immunosenescent features become prominent at old age within earlier differentiation states as well. Our findings show that co-expression of TIGIT and Helios refines the definition of immunosenescent CD8 + T cells and challenge the current dogma of late differentiation stage as proxy for T-cell immunosenescence, since immunosenescence may also accumulate by aging within an earlier differentiation stage.