The fate of effector CD8 T cells in vivo is controlled by the duration of antigen stimulation

Multimodal stimulation screens reveal unique and shared genes limiting T cell fitness

Genes limiting T cell antitumor activity may serve as therapeutic targets. It has not been systematically studied whether there are regulators that uniquely or broadly contribute to T cell fitness. We perform genome-scale CRISPR-Cas9 knockout screens in primary CD8 T cells to uncover genes negatively impacting fitness upon three modes of stimulation: (1) intense, triggering activation-induced cell death (AICD); (2) acute, triggering expansion; (3) chronic, causing dysfunction. Besides established regulators, we uncover genes controlling T cell fitness either specifically or commonly upon differential stimulation. Dap5 ablation, ranking highly in all three screens, increases translation while enhancing tumor killing. Loss of Icam1-mediated homotypic T cell clustering amplifies cell expansion and effector functions after both acute and intense stimulation. Lastly, Ctbp1 inactivation induces functional T cell persistence exclusively upon chronic stimulation. Our results functionally annotate fitness regulators based on their unique or shared contribution to traits limiting T cell antitumor activity.

T-cell response to checkpoint blockade immunotherapies: from fundamental mechanisms to treatment signatures

Immune checkpoint immunotherapies act to block inhibitory receptors on the surface of T cells and other cells of the immune system. This can increase activation of immune cells and promote tumour clearance. Whilst this is very effective in some types of cancer, significant proportions of patients do not respond to single-agent immunotherapy. To improve patient outcomes, we must first mechanistically understand what drives therapy resistance. Many studies have utilised genetic, transcriptional, and histological signatures to find correlates of effective responses to treatment. It is key that we understand pretreatment predictors of response, but also to understand how the immune system becomes treatment resistant during therapy. Here, we review our understanding of the T-cell signatures that are critical for response, how these immune signatures change during treatment, and how this information can be used to rationally design therapeutic strategies. We highlight how chronic antigen recognition drives heterogeneous T-cell exhaustion and the role of T-cell receptor (TCR) signal strength in exhausted T-cell differentiation and molecular response to therapy. We explore how dynamic changes in negative feedback pathways can promote resistance to single-agent therapy. We speculate that this resistance may be circumvented in the future through identifying the most effective combinations of immunotherapies to promote sustained and durable antitumour responses.

Tumor reductive therapies and antitumor immunity

Tumor reductive therapy is to reduce tumor burden through direct killing of tumor cells. So far, there is no report on the connection between antitumor immunity and tumor reductive therapies. In the last few years, a new category of cancer treatment, immunotherapy, emerged and they are categorized separately from classic cytotoxic treatments (chemo and radiation therapy). The most prominent examples include cellular therapies (LAK and CAR-T) and immune checkpoint inhibitors (anti-PD-1 and CTLA-4). Recent advances in clinical immunotherapy and our understanding of the mechanism behind them revealed that these therapies have a closer relationship with classic cancer treatments than we thought. In many cases, the effectiveness of classic therapies is heavily influenced by the status of the underlying antitumor-immunity. On the other hand, immunotherapies have shown better outcome when combined with tumor reductive therapies, not only due to the combined effects of tumor killing by each therapy but also because of a synergy between the two. Many clinical observations can be explained once we start to look at these classic therapies from an immunity standpoint. We have seen their direct effect on tumor antigen in vivo that they impact antitumor immunity more than we have realized. In turn, antitumor immunity contributes to tumor control and destruction as well. This review will take the immunological view of the classic therapies and summarize historical as well as recent findings in animal and clinical studies to make the argument that most of the cancer treatments exert their ultimate efficacy through antitumor immunity.

Splenic Dendritic Cells Involved in Cross-Tolerance of Tumor Antigens Can Play a Stimulatory Role in Adoptive T-Cell Therapy

Circulating antigens released from tumor cells can drain into the spleen and be acquired by resident antigen-presenting cells (APCs). Here, we examined the ability of splenic dendritic cells to cross-present tumor antigens to CD8+ T cells and investigated the effects that this has on T-cell therapy in a murine model of lymphoma. In the presence of established lymphoma, langerin (CD207)-expressing CD8α+ dendritic cells acquired, processed, and cross-presented tumor antigens to naive CD8+ T cells. Although this resulted in initial T-cell proliferation, the T-cell population failed to expand measurably over the following days, and tumor-free survival was actually improved when langerin-expressing cells were depleted. In contrast, following adoptive T-cell therapy with in vitro-activated CD8+ T cells, marked antitumor activity was observed and associated with accumulation of activated antigen-specific CD8+ T cells in the spleen and blood, whereas tumor protection and T-cell accumulation were significantly reduced in animals depleted of langerin-expressing cells. Therefore, although resident APCs that acquire tumor antigens may induce tolerance in naive cells in the absence of further stimuli, they can play an important role in promoting antitumor immunity during the course of T-cell therapy. It is possible that further therapeutic benefit will result from improving the activation status of these APCs.

In situ vaccine, immunological memory and cancer cure

As surgery is able to remove primary tumors and limit metastases, the major challenge in cancer management is the prevention of post-resection recurrence and metastases. From the immune point of view, tumor resection removes the supply of tumor antigens that maintain an active concomitant antitumor immunity elicited by the primary tumor, and may also signal for deposition of immunological memory against future metastases. However, the natural course of this antitumor immunity in many cancer patients following complete tumor resection may not be favorable because protection is often lost after 1-3 years. Recent studies suggest that chemotherapy is able to activate this pre-existing antitumor immunity, and tumor resection following immune activation may lead to higher levels of immunological memory against future tumor antigens (in the form of metastases). Interleukin-12 added to chemotherapy mimics the function of a vaccine adjuvant in that it helps to enhance the antitumor immunity activated by chemotherapy and leaves a much stronger antitumor immune memory. This finding, when applied to cancer management, may help to maintain a strong and long lasting antitumor immunity following complete tumor resection, thus eliminating post-surgery recurrence and metastases.

Allospecific CD4(+) effector memory T cells do not induce graft-versus-host disease in mice

We studied whether allospecific CD4(+) effector memory T cells (T(EM)) could induce graft-versus-host disease (GVHD) using a novel GVHD model induced solely by CD4(+) T cell receptor transgenic TEa cells. Allospecific T(EM) generated in a lymphopenic host bore a typical memory phenotype. Moreover, these cells were able to elicit a faster and more effective proliferative response on challenge with alloantigen in vitro and to mediate "second-set" skin graft rejection in vivo. However, these allospecific T(EM) were unable to induce GVHD. Allospecific T(EM) recipients became tolerant to alloantigen as a result of clonal deletion. Even though allospecific T(EM) were able to respond to alloantigen initially, the expansion of these cells and inflammatory cytokine production during GVHD were dramatically decreased. The inability of allospecific T(EM) to sustain the alloresponse may be a result of enhanced activation-induced cell death. These observations provide insight into how allospecific CD4(+) T(EM) respond to alloantigen during GVHD and underscore the fundamental differences in alloresponses mediated by allospecific T(EM) in graft rejection and GVHD settings.

CD62L- memory T cells enhance T-cell regeneration after allogeneic stem cell transplantation by eliminating host resistance in mice

A major challenge in allogeneic hematopoietic cell transplantation is how to transfer T-cell immunity without causing graft-versus-host disease (GVHD). Effector memory T cells (CD62L(-)) are a cell subset that can potentially address this challenge because they do not induce GVHD. Here, we investigated how CD62L(-) T cells contributed to phenotypic and functional T-cell reconstitution after transplantation. On transfer into allogeneic recipients, CD62L(-) T cells were activated and expressed multiple cytokines and cytotoxic molecules. CD62L(-) T cells were able to deplete host radioresistant T cells and facilitate hematopoietic engraftment, resulting in enhanced de novo T-cell regeneration. Enhanced functional immune reconstitution was demonstrated in CD62L(-) T-cell recipients using a tumor and an influenza virus challenge model. Even though CD62L(-) T cells are able to respond to alloantigens and deplete host radioresistant immune cells in GVHD recipients, alloreactive CD62L(-) T cells lost the reactivity over time and were eventually tolerant to alloantigens as a result of prolonged antigen exposure, suggesting a mechanism by which CD62L(-) T cells were able to eliminate host resistance without causing GVHD. These data further highlight the unique characteristics of CD62L(-) T cells and their potential applications in clinical hematopoietic cell transplantation.

Intrinsic IL-21 signaling is critical for CD8 T cell survival and memory formation in response to vaccinia viral infection

CD4 T cell help plays an important role in promoting CD8 T cell immunity to pathogens. In models of infection with vaccinia virus (VV) and Listeria monocytogenes, CD4 T cell help is critical for the survival of activated CD8 T cells during both the primary and memory recall responses. Still unclear, however, is how CD4 T cell help promotes CD8 T cell survival. In this study, we first showed that CD4 T cell help for the CD8 T cell response to VV infection was mediated by IL-21, a cytokine produced predominantly by activated CD4 T cells, and that direct action of IL-21 on CD8 T cells was critical for the VV-specific CD8 T cell response in vivo. We next demonstrated that this intrinsic IL-21 signaling was essential for the survival of activated CD8 T cells and the generation of long-lived memory cells. We further revealed that IL-21 promoted CD8 T cell survival in a mechanism dependent on activation of the STAT1 and STAT3 pathways and subsequent upregulation of the prosurvival molecules Bcl-2 and Bcl-x(L). These results identify a critical role for intrinsic IL-21 signaling in CD8 T cell responses to an acute viral infection in vivo and may help design effective vaccine strategies.

Induction of type I IFN is required for overcoming tumor-specific T-cell tolerance after stem cell transplantation

Tumor-specific T-cell tolerance represents one major mechanism of tumor-induced immune evasion. Myeloablative chemotherapy with stem cell transplantation may offer the best chance of achieving a state of minimal residual disease and, thus, minimize tumor-induced immune evasion. However, studies have shown that tumor-specific T-cell tolerance persists after transplantation. Here, we showed that CD4(+)CD25(+) regulatory T (T(Reg)) cells play a critical role in tumor-specific CD8(+) T-cell tolerance after transplantation. Removal of T(Reg) cells from the donor lymphocyte graft did not overcome this tolerance because of rapid conversion of donor CD4(+)CD25(-) T cells into CD4(+)CD25(+)Foxp3(+) T(Reg) cells in recipients after transplantation, and depletion of T(Reg) cells in recipients was necessary for the reversal of tumor-specific tolerance. These results suggest that strategies capable of overcoming T-cell tolerance in recipients are required to promote antitumor immunity after transplantation. Toward this goal, we showed that dendritic cell (DC) vaccines coadministered with the TLR9 ligand, CpG could effectively overcome tumor-specific tolerance, leading to significant prolongation of tumor-free survival after transplantation. We further showed that CpG-induced type I interferon was critical for the reversal of tumor-specific tolerance in vivo. Collectively, these results may suggest effective immunotherapeutic strategies for treating cancer after stem cell transplantation.

STAT1 signaling in CD8 T cells is required for their clonal expansion and memory formation following viral infection in vivo

Recent advances have shown that direct type I IFN signaling on T cells is required for their efficient expansion in response to viral infections in vivo. It is not clear which intracellular signaling molecule is responsible for this effect. Although STAT1 has been shown to mediate many of the type I IFN-dependent biological effects, its role in T cells remains uncertain in vivo. In this study, we demonstrated that STAT1 signaling in CD8 T cells was required for their efficient expansion by promoting the survival of activated CD8 T cells upon vaccinia viral infection in vivo, suggesting that the direct effect of type I IFNs on CD8 T cells is mediated by STAT1. Furthermore, effector CD8 T cells that lack STAT1 signaling did not survive the contraction phase to differentiate into long-lived memory cells. These results identify a critical role for type I IFN-STAT1 signaling in multiple stages of CD8 T cell response in vivo and suggest that strategies to activate type I IFN-STAT1 signaling pathway may enhance vaccine potency.

CD4 T cells are required for CD8 T cell survival during both primary and memory recall responses

The role of CD4 T cell help in primary and secondary CD8 T cell responses to infectious pathogens remains incompletely defined. The primary CD8 T response to infections was initially thought to be largely independent of CD4 T cells, but it is not clear why some primary, pathogen-specific CD8 T cell responses are CD4 T cell dependent. Furthermore, although the generation of functional memory CD8 T cells is CD4 T cell help dependent, it remains controversial when the "help" is needed. In this study, we demonstrated that CD4 T cell help was not needed for the activation and effector differentiation of CD8 T cells during the primary response to vaccinia virus infection. However, the activated CD8 T cells showed poor survival without CD4 T cell help, leading to a reduction in clonal expansion and a diminished, but stable CD8 memory pool. In addition, we observed that CD4 T cell help provided during both the primary and secondary responses was required for the survival of memory CD8 T cells during recall expansion. Our study indicates that CD4 T cells play a crucial role in multiple stages of CD8 T cell response to vaccinia virus infection and may help to design effective vaccine strategies.

Extent of stimulation controls the formation of memory CD8 T cells

Only a small fraction of effector CD8 T cells survives to become long-lived memory cells, whereas the majority of them die after an acute infection. What controls the formation of memory CD8 T cells remains mostly unknown. In this study, we showed CD8 T cells primed earlier during vaccinia viral infection received stronger stimulation, divided more extensively, and survived better than those primed later, leading to generation of a larger memory pool. Despite differentiation into effectors, the late-primed CD8 T cells lacked full cell division, displayed increased apoptosis, and failed to develop into memory cells, suggesting that the extent of stimulation influences the survival of effector CD8 T cells. We further demonstrated that the extent of stimulation, which included both the duration and the levels of antigenic stimulation/costimulation, during priming determined the formation of memory CD8 T cells via controlling the extent of Akt activation, and functional suppression of Akt led to defective CD8 memory formation in vivo. Collectively, our data suggest that the extent of stimulation controls CD8 memory formation via activation of Akt and may provide important insights into the design of effective vaccines.