In this study, we found that the CD80 levels were maintained by Ag-specific CD8
+ T cells during the effector and memory phases. The maintenance of CD80 in the memory phase was caused by the extrinsic acquisition from the lymphoid organs. Previous reports have shown that the CD80 acquisition by human CD4
+ T cells was observed during the early stages of activation (
12). In agreement with this finding, we confirmed that CD80 was detected during the early stages of activation (
Fig. 1B and C). However, to our knowledge, the present study is the first to demonstrate the maintenance of CD80 levels by Ag-specific CD8
+ T cells in lymphoid organs after virus infection and for a long period, suggesting that CD80 extrinsically acquired by Ag-specific CD8
+ T cells in lymphoid organs could define the characteristics of Ag-experienced CD8
+ T cells. More importantly, we newly demonstrated that CD80 present on memory CD8
+ T cells was involved in limiting their ability of expansion and IL-2 production, which suggested the
in vivo physiological role of CD80 in the firing of recall responses by memory CD8
+ T cells.
Importantly, the extrinsic acquisition of CD80 by T cells persisted at the later stages of infection after Ag clearance. One possible explanation is that memory CD8
+ T cells interact more efficiently with APCs, such as DCs and macrophages, than naïve CD8
+ T cells and thus, might acquire or trogocytose CD80 from APCs via the interaction period, even though these APCs do not load specific Ag peptides. Given that the Ag-independent DCs can interact with T cells at the immunological synapse and induce T cell activation (
13), APCs that are physically close to memory T cells might be associated with maintaining high levels of CD80 on memory CD8
+ T cells in an Ag-independent manner. Another explanation for CD80 longevity may be the different stabilities of intrinsic expression of CD80 and extrinsic acquisition of CD80 on T cells. While intrinsically expressed CD80 molecules progressively disappeared on Ag-specific CD8
+ T cells during the effector T cells differentiation, which began at day 30 post-infection, extrinsically acquired CD80 molecules were maintained even after day 80 post-infection (
Fig. 2C and D,
Fig. 3E and F). The importance of CD80 trogocytosed by T cells can be suggested based on the previous reports. Firstly, the extrinsic acquisition of CD80 on T cells may be required for the induction and maintenance of Ag-specific memory T cells, since the differentiation and function of those T cells were impaired in primary immune response of CD80/CD86-KO mice (
1415) and in recall responses (
15). As the previous study did not identify the levels of extrinsic acquisition of CD80 by T cells during recall response, it is not known exactly whether this effect is derived from the acquisition of CD80 and CD86 or other mechanisms. In the current study, we established the
in vivo system to examine the role of extrinsically acquired CD80 by Ag-specific CD8
+ T cells. CD80 acquisition during T cell differentiation did not seem to change the frequency and function of Ag-specific CD8
+ T cells during their differentiation (
Fig. 4). CD80 present on memory CD8
+ T cells rather inhibited cell expansion and IL-2 production during recall response (
Fig. 5). It should be noted that CD80, extrinsically acquired by CD8
+ T cells, plays a role in regulating recall CD8
+ T cell response, which is quite different from the immunological role of CD80 intrinsically expressed by APCs. Secondly, based on previous reports that the
in vitro CD80 acquisition by T cells from APCs allows T cells to become APCs (
612), T cells that acquired CD80 may act as APCs by allowing T cell-T cell interactions (
1617). Based on our
in vivo effect of CD80, extrinsically acquired by CD8
+ T, cells on their recall response (
Fig. 5), the possible role of CD80 on CD8
+ T cells does not seem to have a substantial impact on delivery of stimulation signals to overcome its regulatory function, at least during recall response. However, the dimension of the impact of CD80 acquired by CD8
+ T cells in priming CD8
+ T cells during the primary response should be more carefully reconsidered
in vivo. Thirdly, CD80 molecules on T cells might inhibit T cell immune responses by interacting with PD-L1, because it has been reported that PD-L1 can deliver negative signals to CD8
+ T cells via CD80 engagement (
9). This possibility is also supported by some studies indicating that the interaction between PD-L1 and CD80 is required for the induction and maintenance of peripheral T cells tolerance, the regulation of memory T cells homeostasis, and the control of the effector T cells response via inducing apoptosis (
181920). Indeed, the current study clearly demonstrated that CD80, extrinsically acquired by Ag-specific CD8
+ T cells, significantly inhibited Ag-dependent expansion of memory CD8
+ T cells and their IL-2 production under
in vivo re-exposure to the same Ag (
Fig. 5), which supports the negative regulatory role of CD80 present on CD8
+ T cells. According to a recent study showing that CD80 on APCs interact with PD-L1 on the same cells (
2122), CD80 acquired by CD8
+ T cells might also interact with PD-L1 expressed on the same CD8
+ T cells to activate a recall immune response.
Our data showed that CD80 molecules are extrinsically acquired by Ag-specific CD8+ T cells in the lymphoid organs and further maintained continuously. This continuous existence of CD80 on memory CD8+ T cells appeared to be important in promoting an appropriate recall immune response, dampening excessive CD8+ T cell recall responses. Our study provides information about a novel characteristic of Ag-experienced CD8+ T cells in lymphoid organs. More importantly, the physiological role of CD80 present on memory CD8+ T cells could be applied to generate optimal recall immune responses against repeated infection with pathogens.