Daan Pieren

142 Chapter 5 by these T EMRA cells [20] may actually be caused by the age-related increase in KIR + RA + T cells. The negative correlation of KIR + RA + T cells with CD8 + T-cell proliferation led us to hypothesize that KIR + RA + T cells may regulate CD8 + T-cell proliferation. Indeed, KIR + RA + CD8 + T cells appeared to express markers previously reported to be expressed by suppressive CD4 + and CD8 + Tregs such as TIGIT, Helios, and CD122 [35,40,46,51,52]. Moreover, we show that KIR + RA + T cells suppress proliferation of other CD8 + T cells, in agreement with the hypothesis of age- related accumulation of Tregs, which is thought to hamper protective T cell responses [53-56]. KIR + RA + T cells have until now been regarded to be part of the virtual memory-cell subset. It has been speculated that murine virtual memory cells may acquire tolerogenic mechanisms [57], whereas another report showed that murine virtual memory cells can mediate protective immunity by killing [22]. An explanation for these contrasting results may be the existence of different subsets among murine virtual memory T cells like the KIR + RA + and NKG2A + RA + subsets we report among human virtual memory T cells. A question that remains is by what mechanism KIR + RA + T cells suppress proliferation of other CD8 + T cells. First, we observed that KIR + RA + T cells highly express the co-inhibitory receptor TIGIT. As TIGIT expression has been reported to directly and indirectly contribute to the stability and suppressive capacity of CD4 + Tregs [39,40], these findings may indicate a similar role for TIGIT in suppressive KIR + RA + T cells we present here. Second, Helios expression has been shown to be required for the survival and suppressive capacity of CD8 + Tregs in mice [35]. As we observed that KIR + RA + T cells highly express Helios, targeting Helios may be an additional candidate for unraveling the suppressive mechanism of KIR + RA + T cells. Lastly, killing of other cells by release of granzymes and perforins has been described as one of the mechanisms by which Tregs may exert immune suppression [58,59]. This mechanism may also be involved in regulation by KIR + RA + T cells since they express CD107a, indicative of enhanced release of granules containing cytotoxic factors, and their transcriptome is enriched with transcripts for pathways involved in killing of lymphocytes. A study in mice has shown that virtual memory T cells act as bystander cells during infection [22]. The KIR + RA + T cells we describe here can be considered as a subset of the T cells defined as virtual memory cells. We here show that KIR + RA + CD8 + T cells present in influenza A-infected older adults are highly activated during acute infection, but not specific for the highly conserved and immunodominant influenza A epitope GILGFVFTL. This provides evidence in favor of a potential bystander role of these cells. Interestingly, we also found a large proportion of activated KIR + RA + T cells in adults suffering from a primary