Phase 1 trial of 4-1BB-based adoptive T-cell therapy targeting human telomerase reverse transcriptase in patients with advanced refractory solid tumors.

BACKGROUND AIMS: Human telomerase reverse transcriptase (hTERT) is an attractive target for anti-cancer therapies. We developed an effective method for generating hTERT-specific CD8+ T cells (hTERT-induced natural T cells [TERTiNTs]) using peripheral blood mononuclear cells (PBMCs) from patients with solid cancers and investigated their feasibility and safety. METHODS: This was a single-center phase 1 trial using a 3 + 3 dose escalation design to evaluate six dose levels of TERTiNTs. PBMCs from each patient were screened using an hTERT peptide panel to select those that stimulated CD8+ T cells. The four most stimulatory peptides were used to produce autologous CD8+ T cells from patients refractory or intolerant to standard therapies. Eligible patients received a single intravenous infusion of TERTiNTs at different dose levels (4 × 108 cells/m2, 8 × 108 cells/m2 and 16 × 108 cells/m2). Pre-conditioning chemotherapy, including cyclophosphamide alone or in combination with fludarabine, was administered to induce lymphodepletion. RESULTS: From January 2014 to October 2019, a total of 24 patients with a median of three prior lines of therapy were enrolled. The most common adverse events were lymphopenia (79.2%), nausea (58.3%) and neutropenia (54.2%), mostly caused by pre-conditioning chemotherapy. The TERTiNT infusion was well tolerated, and dose-limiting toxicities were not observed. None of the patients showed objective responses. Seven patients (30.4%) achieved stable disease with a median progression-free survival of 3.9 months (range, 3.2-11.3). At the highest dose level (16 × 108 cells/m2), four of five patients showed disease stabilization. CONCLUSIONS: The generation of TERTiNTs was feasible and safe and provided an interesting disease control rate in heavily pre-treated cancer patients.

Cross-Activation of Regulatory T Cells by Self Antigens Limits Self-Reactive and Activated CD8+ T Cell Responses.

The interaction between regulatory T (Treg) cells and self-reactive T cells is a crucial mechanism for maintaining immune tolerance. In this study, we investigated the cross-activation of Treg cells by self-antigens and its impact on self-reactive CD8+ T cell responses, with a focus on the P53 signaling pathway. We discovered that major histocompatibility complex (MHC) I-restricted self-peptides not only activated CD8+ T cells but also induced the delayed proliferation of Treg cells. Following HLA-A*0201-restricted Melan-A-specific (pMelan) CD8+ T cells, we observed the direct expansion of Treg cells and concurrent suppression of pMelan+CD8+ T cell proliferation upon stimulation with Melan-A peptide. Transcriptome analysis revealed no significant alterations in specific signaling pathways in pMelan+CD8+ T cells that were co-cultured with activated Treg cells. However, there was a noticeable upregulation of genes involved in P53 accumulation, a critical regulator of cell survival and apoptosis. Consistent with such observation, the blockade of P53 induced a continuous proliferation of pMelan+CD8+ T cells. The concurrent stimulation of Treg cells through self-reactive TCRs by self-antigens provides insights into the immune system's ability to control activated self-reactive CD8+ T cells as part of peripheral tolerance, highlighting the intricate interplay between Treg cells and CD8+ T cells and implicating therapeutic interventions in autoimmune diseases and cancer immunotherapy.

Identification of regulatory functions for 4-1BB and 4-1BBL in myelopoiesis and the development of dendritic cells.

The costimulatory molecule 4-1BB and its ligand 4-1BBL can control adaptive immunity, but here we show that their interaction also suppressed myelopoiesis. We found that 4-1BBL was expressed on hematopoietic stem cells, differentiating common myeloid progenitors and granulocyte-macrophage progenitors, and 4-1BB was inducible on activated myeloid progenitors. Steady-state numbers of granulocyte-macrophage progenitors, myeloid-lineage cells and mature dendritic cells were higher in 4-1BB- and 4-1BBL-deficient mice, indicative of a negative function, and we confirmed that result with bone marrow chimeras and in vitro, where the absence of interactions between 4-1BB and 4-1BBL led to enhanced differentiation into dendritic cell lineages. The regulatory activity was mediated by 4-1BBL, with binding by 4-1BB inhibiting differentiation of myeloid progenitors. Thus, 4-1BB and 4-1BBL have a previously unknown function in limiting myelopoiesis and the development of dendritic cells.

RELT negatively regulates the early phase of the T-cell response in mice.

RELT (tumor necrosis factor receptor superfamily member 19-like, TNFRSF19L) is a TNFR superfamily member that is primarily expressed in immune cells and lymphoid tissues, but whose immunological function is not well-defined. Here, we show that RELT is expressed by naive T cells and DCs, and their activation or maturation decreases RELT expression. Using RELT knockout (RELT-/- ) mice, we demonstrate that RELT deficiency selectively promotes the homeostatic proliferation of CD4+ T cells but not CD8+ T cells, and enhances anti-tumor CD8+ T-cell responses. We also demonstrate, using an adoptive transfer model in which RELT is knocked-out in either the transferred transgenic CD8+ T cells or the recipient melanoma-bearing mice, that RELT on multiple immune cells limits the hyper-response of tumor-specific CD8+ T cells. Hyper-responsiveness of RELT-deficient T cells was induced by promoting their proliferation. Taken together, our findings suggest that RELT acts as a negative regulator that controls the early phase of T-cell activation probably by promoting T-cell apoptosis.

The hyaluronic acid-rich node and duct system is a structure organized for innate immunity and mediates the local inflammation.

The Hyaluronic Acid-rich Node and Duct System (HAR-NDS or NDS), Primo Vascular System (PVS) or Bonghan System (BHS), is thought to be a third circulatory system independent of the blood and lymphatic systems and a structure of connected nodes and ducts. Although it seems to be part of the immune system as it is enriched with cells of innate immunity, little is known about its immunological roles. We performed cellular profiling and secretome analysis of NDS in a steady state and under TLR2- or TLR4-mediated local inflammation, and found that the NDS is pre-dominantly enriched with the myeloid cells, selectively attracts the inflammatory macrophages and neutrophils, has a flexible structure just like the lymph node, and is structured with the fibroblastic reticular cells and reticular network. NDS dominantly harbored the myeloid cells in both steady and activated status, and secreted various types of inflammatory cytokines by proinflammatory stimuli. These results suggest that NDS is the lymphoid structure for the innate immunity and plays an intermediary role in the innate immune cell-mediated local inflammation.

In vivo 4-1BB deficiency in myeloid cells enhances peripheral T cell proliferation by increasing IL-15.

4-1BB signals are considered positive regulators of T cell responses against viruses and tumors, but recent studies suggest that they have more complex roles in modulating T cell responses. Although dual roles of 4-1BB signaling in T cell responses have been suggested, the underlying mechanisms are still not fully understood. In this study, we tested whether 4-1BB expression affected T cell responses differently when expressed in myeloid versus lymphoid cells in vivo. By assessing the proliferation of 4-1BB(+/+) and 4-1BB(-/-) T cells in lymphocyte-deficient RAG2(-/-) and RAG2(-/-)4-1BB(-/-) mice, we were able to compare the effects on T cell responses of 4-1BB expression on myeloid versus T cells. Surprisingly, adoptively transferred T cells were more responsive in tumor-bearing RAG2(-/-)4-1BB(-/-) mice than in RAG2(-/-) mice, and this enhanced T cell proliferation was further enhanced if the T cells were 4-1BB deficient. Dendritic cells (DCs) rather than NK or tissue cells were the myeloid lineage cells primarily responsible for the enhanced T cell proliferation. However, individual 4-1BB(-/-) DCs were less effective in T cell priming in vivo than 4-1BB(+/+) DCs; instead, more DCs in the secondary lymphoid organs of RAG2(-/-)4-1BB(-/-) mice appeared to induce the enhanced T cell proliferation by producing and transpresenting more IL-15. Therefore, we conclude that in vivo 4-1BB signaling of myeloid cells negatively regulates peripheral T cell responses by limiting the differentiation of DCs and their accumulation in secondary lymphoid organs.

Authentic GITR Signaling Fails To Induce Tumor Regression unless Foxp3+ Regulatory T Cells Are Depleted.

The glucocorticoid-induced TNFR family-related protein (GITR, TNFRSF18, CD357) is expressed on effector and regulatory T (Treg) cells. Previous studies demonstrated that GITR triggering by anti-GITR mAb enhanced T and B cell-mediated immune responses. GITR-deficient T cells, however, also proliferate more than normal T cells, and this effect is unexplained. Because the activities of mAbs are controlled by their Fc regions, the true effect of GITR signaling needs to be determined by examining its interaction with authentic ligand. Therefore, we generated a pentamerized form of the GITRL extracellular domain (pGITRL) for ligation to GITR and compared its effect on T cells with that of anti-GITR mAb. The pGITRL was more effective than anti-GITR mAb in enhancing the proliferation of effector and regulatory cells in vitro and in vivo. Nonetheless, the growth of MC38 adenocarcinoma cells in vivo was only suppressed for initial 15 d by pGITRL, whereas it was suppressed indefinitely by anti-GITR mAb. Detailed analysis revealed that pGITRL induced extensive proliferation of Foxp3(+)CD4(+) Treg cells and led to the accumulation of activated Treg cells in tumor tissue and draining lymph nodes. Because GITR signaling could not neutralize the suppressive activity of activated Treg cells, pGITRL seems to lose its adjuvant effect when sufficient activated Treg cells have accumulated in the lymph nodes and tumor tissue. Indeed, the antitumor effects of pGITRL were markedly enhanced by depleting CD4(+) cells. These results suggest that GITR signaling has stimulatory effects on effector T cells and inhibitory effects through Treg cells.

Immune evasion in cancer: Mechanistic basis and therapeutic strategies.

Cancer immune evasion is a major stumbling block in designing effective anticancer therapeutic strategies. Although considerable progress has been made in understanding how cancers evade destructive immunity, measures to counteract tumor escape have not kept pace. There are a number of factors that contribute to tumor persistence despite having a normal host immune system. Immune editing is one of the key aspects why tumors evade surveillance causing the tumors to lie dormant in patients for years through "equilibrium" and "senescence" before re-emerging. In addition, tumors exploit several immunological processes such as targeting the regulatory T cell function or their secretions, antigen presentation, modifying the production of immune suppressive mediators, tolerance and immune deviation. Besides these, tumor heterogeneity and metastasis also play a critical role in tumor growth. A number of potential targets like promoting Th1, NK cell, γδ T cell responses, inhibiting Treg functionality, induction of IL-12, use of drugs including phytochemicals have been designed to counter tumor progression with much success. Some natural agents and phytochemicals merit further study. For example, use of certain key polysaccharide components from mushrooms and plants have shown to possess therapeutic impact on tumor-imposed genetic instability, anti-growth signaling, replicative immortality, dysregulated metabolism etc. In this review, we will discuss the advances made toward understanding the basis of cancer immune evasion and summarize the efficacy of various therapeutic measures and targets that have been developed or are being investigated to enhance tumor rejection.

Establishment and characterization of metastatic gastric cancer cell lines from murine gastric adenocarcinoma lacking Smad4, p53, and E-cadherin.

There is a strong need for murine gastric cancer cell line models recapitulating human gastric cancers. Here, we describe two murine gastric cancer cell lines designated as NCC-S1 and NCC-S3. They were generated from gastric adenocarcinomas that formed in a Villin-cre, Smad4(F/F) , Trp53(F/F) , Cdh1(F/wt) mouse and a Pdx1-cre, Trp53(F/F) , Cdh1(F/F) mouse, respectively. Molecular profiles of both cell lines were very similar to human gastric cancer. NCC-S1M and NCC-S3M subpopulation clones were isolated from pulmonary metastasis of heterotopic allografts of NCC-S1 and NCC-S3 cells, respectively. NCC-S1M and NCC-S3M showed enhanced in vivo growth rates and metastatic potentials and exhibited epithelial-to-mesenchymal transition features. NCC-S1M cells developed orthotopic and heterotopic tumors in immunocompetent mice in predictable manner, and were useful for testing the efficacy of an immunotherapeutic agent, anti-4-1BB antibody. NCC-S1M and NCC-S3M cells demonstrated Wnt/ß-catenin pathway activation, and knockdown of Ctnnb1 reversed the metastatic phenotype of NCC-S1M. These results underscore the role of Wnt/ß-catenin pathway in metastatic phenotype of gastric cancer. Taken together, our novel metastatic gastric cancer cell lines are useful resources for drug development and metastasis research.

CD137-inducing factors from T cells and macrophages accelerate the destabilization of atherosclerotic plaques in hyperlipidemic mice.

CD137 (4-1BB), a member of the tumor necrosis factor receptor superfamily, has been reported to be expressed in atherosclerotic plaques, and to promote lesion formation. However, the role of CD137 in mediating atherosclerotic plaque stability and the possible underlying molecular and cellular mechanisms are poorly understood. Here, apolipoprotein E-deficient (ApoE(-/-)) and CD137-deficient ApoE(-/-) (ApoE(-/-)CD137(-/-)) mice fed a chow diet for 66 wk were used. CD137 induces plaque instability, which is characterized by increased plaque necrosis, decreased collagen content, decreased vascular smooth muscle cell (VSMC) content, and increased macrophage infiltration. CD137 also increases the infiltration of effector T (Teff) cells into plaque lesion sites, resulting in increased interferon-γ (IFN-γ) expression. Interestingly, Teff-cell-derived IFN-γ inhibits collagen synthesis in atherosclerotic plaques. Furthermore, CD137 activation increases the apoptosis of VSMCs, possibly by decreasing the antiapoptotic regulator, Bcl-2, and subsequently up-regulating cleaved caspase-3. In macrophages, activation of CD137 signaling boosted the oxidized low density lipoprotein-induced expression of matrix metalloproteinase 9 via the p38 mitogen-activated protein kinase and extracellular signal-regulated kinase1/2 signaling pathways. In summary, activation of CD137 signaling decreases the stability of advanced atherosclerotic plaques via its combined effects on Teff cells, VSMCs, and macrophages.

TLR2 signaling in tubular epithelial cells regulates NK cell recruitment in kidney ischemia-reperfusion injury.

Damage-associated molecular patterns released from damaged kidney cells initiate postischemic inflammation, an essential step in the progression of kidney ischemia-reperfusion injury (IRI). However, the mechanism that coordinates this highly specific process in ischemic kidneys remains to be clarified. Previously, we demonstrated that CD137 from NK cells specifically stimulates CD137 ligand (CD137L) on tubular epithelial cells (TECs) such that TECs produced the high CXCR2 chemokine levels required for neutrophil chemotaxis. We report in the present study that endogenous TLR2 ligands released from ischemic TECs induce CCR5 chemokine expression, which is critical to promoting NK cell recruitment. By implanting CD137L(-/-) TECs into the kidney capsule of TLR2(-/-) mice, we further showed that TLR2-mediated NK cell recruitment is an uncoupled event that can occur independently of CD137L signaling in TECs, which is responsible for recruiting neutrophils. Therefore, our findings identify TECs as both a target for kidney damage and also as a master regulator that actively modulates stepwise signaling, leading to the initiation and amplification of acute sterile inflammation that inflicts kidney IRI. Being clinically important, the signaling pathway of innate receptors in epithelial cells may therefore be a good target to block acute sterile inflammation resulting from tissue damage, including kidney IRI.

Reverse signaling through the costimulatory ligand CD137L in epithelial cells is essential for natural killer cell-mediated acute tissue inflammation.

Renal ischemia-reperfusion injury (IRI) after kidney transplantation is a major cause of delayed graft function. Even though IRI is recognized as a highly coordinated and specific process, the pathways and mechanisms through which the innate response is activated are poorly understood. In this study, we used a mouse model of acute kidney IRI to examine whether the interactions of costimulatory receptor CD137 and its ligand (CD137L) are involved in the early phase of acute kidney inflammation caused by IRI. We report here that the specific expressions of CD137 on natural killer cells and of CD137L on tubular epithelial cells (TECs) are required for acute kidney IRI. Reverse signaling through CD137L in TECs results in their production of the chemokine (C-X-C motif) receptor 2 ligands CXCL1 and CXCL2 and the subsequent induction of neutrophil recruitment, resulting in a cascade of proinflammatory events during kidney IRI. Our findings identify an innate pathogenic pathway for renal IRI involving the natural killer cell-TEC-neutrophil axis, whereby CD137-CD137L interactions provide the causal contribution of epithelial cell dysregulation to renal IRI. The CD137L reverse signaling pathway in epithelial cells therefore may represent a good target for blocking the initial stage of inflammatory diseases, including renal IRI.

4-1BB signal stimulates the activation, expansion, and effector functions of γδ T cells in mice and humans.

We show here that the expression of 4-1BB is rapidly induced in γδ T cells following antigenic stimulation in both mice and humans, and ligation of the newly acquired 4-1BB with an agonistic anti-4-1BB augments cell division and cytokine production. We further demonstrate that γδ rather than αß T cells protect mice from Listeria monocytogenes (LM) infection and 4-1BB stimulation enhances the γδ T-cell activities in the acute phase of LM infection. IFN-γ produced from γδ T cells was the major soluble factor regulating LM infection. Vγ1(+) T cells were expanded in LM-infected mice and 4-1BB signal triggered an exclusive expansion of Vγ1(+) T cells and induced IFN-γ in these Vγ1(+) T cells. Similarly, 4-1BB was induced on human γδ T cells and shown to be fully functional. Combination treatment with human γδ T cells and anti-hu4-1BB effectively protected against LM infection in human γδ T cell-transferred NOD-SCID mice. Taken together, these data provide evidence that the 4-1BB signal is an important regulator of γδ T cells and induces robust host defense against LM infection.

CRIg signals induce anti-intracellular bacterial phagosome activity in a chloride intracellular channel 3-dependent manner.

Macrophages provide a first line of defense against bacterial infection by engulfing and killing invading bacteria, but intracellular bacteria such as Listeria monocytogenes (LM) can survive in macrophages by various mechanisms of evasion. Complement receptor of the immunoglobulin (CRIg), a C3b receptor, binds to C3b on opsonized bacteria and facilitates clearance of the bacteria by promoting their uptake. We found that CRIg signaling induced by agonistic anti-CRIg mAb enhanced the killing of intracellular LM by macrophages, and that this occurred in LM-containing phagosomes. Chloride intra-cellular channel 3 CLIC3, an intracellular chloride channel protein, was essential for CRIg-mediated LM killing by directly interacting with the cytoplasmic domain of CRIg, and the two proteins colocalized on the membranes of LM-containing vacuoles. CLIC3(-/-) mice were as susceptible to LM as CRIg(-/-) mice. These findings identify a mechanism embedded in the process by which macrophages take up opsonized bacteria that prevents the bacteria from evading cell-mediated killing.

CD137 expressed on neutrophils plays dual roles in antibacterial responses against Gram-positive and Gram-negative bacterial infections.

Severe sepsis and septic shock caused mainly by bacterial infections are life-threatening conditions that urge the development of novel therapies. However, host responses to and pathophysiology of sepsis have not been clearly understood, which remains a major obstacle for the development of effective therapeutics. Recently, we have shown that stimulation of a costimulatory molecule, CD137, enhanced survival of mice infected with the Gram-positive (G(+)) intracellular bacterium Listeria monocytogenes but decreased survival in a polymicrobial sepsis model. Herein, we report that CD137 deficiency or blocking of CD137 signaling decreased antibacterial responses of mice infected with G(+) bacteria (Staphylococcus aureus, Streptococcus pneumoniae, and Enterococcus faecalis) but increased these responses in mice infected with Gram-negative (G(-)) bacteria (Escherichia coli, Pseudomonas aeruginosa, and Salmonella enterica serovar Typhimurium). Consistent with these findings, stimulation of CD137 by administration of agonistic antibody enhanced responses against G(+) bacteria, whereas it decreased these responses against G(-) bacteria. Neutrophils were responsible for CD137-mediated opposite roles in control of G(+) and G(-) bacterial infections. Stimulation of CD137 enhanced activities of neutrophils against S. aureus but decreased these activities against E. coli, while CD137 blocking produced opposite results with the stimulation of CD137 in vivo and in vitro. Furthermore, we found that combined signaling of CD137 and Toll-like receptor 2 (TLR2) induced synergistic production of tumor necrosis factor alpha (TNF-α) and interleukin-6 (IL-6) by neutrophils, but combined signaling of CD137 and TLR4 did not. Our data strongly suggest that CD137 may play a dual role in sepsis in association with TLRs.

4-1BB triggering ameliorates experimental autoimmune encephalomyelitis by modulating the balance between Th17 and regulatory T cells.

Agonistic anti-4-1BB Ab is known to ameliorate experimental autoimmune encephalomyelitis. 4-1BB triggering typically leads to the expansion of CD8(+) T cells, which produce abundant IFN-γ, and this in turn results in IDO-dependent suppression of autoimmune responses. However, because neutralization of IFN-γ or depletion of CD8(+) T cell only partially abrogates the effect of 4-1BB triggering, we sought to identify an additional mechanism of 4-1BB-triggered suppression of autoimmune responses using IFN-γ- or IFN-γR-deficient mice. 4-1BB triggering inhibited the generation of Th17 cells that is responsible for experimental autoimmune encephalomyelitis induction and progression, and increased Foxp3(+)CD4(+) regulatory T (Treg) cells, particularly among CD4(+) T cells. This was not due to a direct effect of 4-1BB signaling on CD4(+) T cell differentiation: 4-1BB signaling not only reduced Th17 cells and increased Treg cells in wild-type mice, which could be due to IFN-γ production by the CD8(+) T cells, but also did so in IFN-γ-deficient mice, in that case by downregulating IL-6 production. These results show that although secondary suppressive mechanisms evoked by 4-1BB triggering are usually masked by the strong effects of IFN-γ, 4-1BB signaling seems to modulate autoimmune responses by a number of mechanisms, and modulation of the Th17 versus Treg cell balance is one of those mechanisms.

Targeting TNF superfamily members for therapeutic intervention in rheumatoid arthritis.

Rheumatoid arthritis (RA) is an inflammatory disease is one of the most serious medical problems, affecting ∼1% of all people worldwide, irrespective of race. The disease is autoimmune in nature and characterized by chronic inflammation of the synovial tissues in multiple joints that leads to joint destruction. Although T cells are central players in RA development, B cells are required for full penetrance of disease largely via their production of autoantibodies against Fc domain of IgG rheumatoid factor (RF). Treatment options for RA are limited and if any, are inadequate due to associated side effects. Members of the tumor necrosis factor (TNF) superfamily play important roles in a number of autoimmune diseases, including RA. In this review, we briefly summarize key features of the superfamily, we will consider how the well-characterized members concerned with immune regulation are coordinated and their roles in rheumatoid arthritis.

Microscopic nodes and ducts inside lymphatics and on the surface of internal organs are rich in granulocytes and secretory granules.

The blood and lymphatic systems are the two well-established circulatory systems. The existence of a third circulatory system representing acupuncture meridians was claimed in the 1960s. The very existence and function of the system, however, remained uncertain. We have found that microscopic nodes and ducts inside lymphatics, as well as on the surface of internal organs of the rat. The nodes and ducts are covered by a layer of EMP-3-positive spindle-shaped epithelium with, below, a layer of vWF-positive but CD31-negative endothelium. The nodes contain a variety of immune cells, usually enriched with mast cells, eosinophils, neutrophils and histiocytes, as well as chromaffin cells, other granule-containing cells. Secretory granules originating from the mast cells in the nodes appear to pass along ductules, two or more of which make up a duct. Our results reveal a potential circulatory system whose anatomical structure and cellular content differ from the blood and lymph systems, and which may be involved in the transport of secretory granules.

PDCA expression by B lymphocytes reveals important functional attributes.

We have demonstrated in this study the existence of a PDCA-expressing functional B cell population (PDCA+ B lymphocytes), which differentiates from activated conventional B (PDCA-IgM+) lymphocytes. Stimulation with anti-micro, LPS, CpG oligodeoxynucleotide, HSV-1, or CTLA-4 Ig activates the PDCA+ B lymphocytes, leading to cell division and induction of type I IFNs and IDO. Notably, the PDCA+ B lymphocytes are capable of Ag-specific Ab production and Ig class switching, which is corroborated by transfer experiments in B- and PDCA+ B lymphocyte-deficient microMT mice. Importantly, in lupus-prone MRL-Fas(lpr) mice, PDCA+ B lymphocytes remain the principal source of autoantibodies. The PDCA+ B lymphocytes have phenotypes with plasmacytoid dendritic cells, but are a distinct cell population in that they develop from C-kit+B220+ pro-B precursors. Thus, our data suggest that not all PDCA+ cells are dendritic cell-derived plasmacytoid dendritic cells and that a significant majority is the PDCA+ B lymphocyte population having distinct phenotype and function.

Peripheral 4-1BB signaling negatively regulates NK cell development through IFN-gamma.

Stimulation of 4-1BB (CD137) was shown to produce strong anticancer effects in vivo. In contrast, 4-1BB-deficient (4-1BB(-/-)) B6 mice are remarkably resistant to tumor growth. We set out to determine the mechanisms involved in these seemingly contradictory observations. We found that the therapeutic effects of 4-1BB triggering were mainly dependent on CD8(+) T cells and partially on NK cells, whereas CD8(+) T and NK cells were equally needed to suppress tumor growth in 4-1BB(-/-) mice. Cellular analysis showed that the frequency and number of NK cells in the spleen and bone marrow were decreased by 4-1BB triggering but were increased in the absence of 4-1BB signaling in tumor-challenged mice. The 4-1BB-mediated downregulation of NK cell development was primarily dependent on IFN-gamma, which was produced by peripheral CD8(+) T and NK cells. The suppression of NK cell development by 4-1BB-mediated IFN-gamma production occurred in the bone marrow. As 4-1BB signaling increased in the periphery, more CD8(+) T cells but fewer NK cells contributed to the antitumor immunity. As 4-1BB signaling decreased, more NK cells participated in the antitumor immunity. We conclude that 4-1BB signaling results in a shift of the dominant type of immune cell in antitumor immunity from the innate NK cell to the adaptive CD8(+) T cell and that the level of IFN-gamma is critical for this 4-1BB-mediated shift.