Daan Pieren

99 Co-expression of TIGIT and Helios by CD8+ T cells INTRODUCTION Viral respiratory diseases such as influenza and COVID-19 severely strike the elderly due to the debilitating changes aging can impose on the immune system [1-3]. Immunological dysfunctions that evolve during aging occur throughout the immune system and are collectively defined as immunosenescence [3]. This is illustrated for CD8 + T cells that are required to control viral infections and malignant cells, but lose proper responsiveness at old age, as is reflected by their reduced capacity to proliferate and become activated in response to stimulus at old age [4-7]. The concept of immunosenescence is approached from many different angles and immune markers, and it is difficult to achieve a clear view of how T cells change during aging. We aimed to improve insight into the progression of immunosenescence in CD8 + T cells by identifying novel markers of aging-related dysfunction and stratified these markers to T-cell differentiation as classically defined by expression of co-stimulatory markers CD27 and CD28 [8,9]. Differentiation of CD8 + T cells during the course of life results in gain of features that are associated with immunosenescence and the accumulation of highly differentiated T cells at old age [3,8,10,11]. Progression towards senescence in CD8 + T cells can be stratified by three successive differentiation states based on expression of CD27 and CD28 [8,9]. This approach describes a gradual path from the early differentiation state (CD27 + CD28 + ) via an intermediate differentiation state (CD27 + CD28 - ) into the late differentiation state (CD27 - CD28 - ) that expresses many features of immunosenescence, such as reduced capacity to proliferate [8]. Due to these features, the accumulation of CD27 - CD28 - cells is often used as a proxy for the level of immunosenescence in the CD8 + T-cell pool [8-10,12]. Although accumulation of late-differentiated T cells may be useful as surrogate marker of immunosenescence progression, it does not fully pinpoint functional changes in the CD8 + T-cell population that accumulate by aging, such as reduced proliferation and activation. To better understand progression of immunosenescence by aging, it would be helpful to define markers that more precisely pinpoint functionally immunosenescent cells and complement the current view on differentiation as surrogate marker of immunosenescence. The co-inhibitory receptor TIGIT has recently been identified as a potent marker of immunosenescence as it is expressed by dysfunctional T cells that accumulate at older age [13]. Its clear functional role was shown by blocking of TIGIT resulting in improved functionality of immunosenescent T cells [13-15]. The transcription factor Helios is a marker that may further define different functional subsets 4