Downregulation of PIK3IP1/TrIP on T cells is controlled by TCR signal strength, PKC, and metalloprotease-mediated cleavage.

The protein known as PI3K-interacting protein (PIK3IP1), or transmembrane inhibitor of PI3K (TrIP), is highly expressed by T cells and can modulate PI3K activity in these cells. Several studies have also revealed that TrIP is rapidly downregulated following T cell activation. However, it is unclear as to how this downregulation is controlled. Using a novel monoclonal antibody that robustly stains cell-surface TrIP, we demonstrate that TrIP is lost from the surface of activated T cells in a manner dependent on the strength of signaling through the T cell receptor (TCR) and specific downstream signaling pathways. In addition, TrIP expression returns after 24 hours, suggesting that it may play a role in resetting TCR signaling at later time points. Finally, by expressing truncated forms of TrIP in cells, we identify the region in the extracellular stalk domain of TrIP that is targeted for proteolytic cleavage by metalloprotease ADAM17.

Tim-3 Is Not Required for Establishment of CD8+ T Cell Memory to Lymphocytic Choriomeningitis Virus.

Tim-3 is a transmembrane protein that is best known for being highly expressed on terminally exhausted CD8+ T cells associated with chronic infection and tumors, although its expression is not limited to those settings. Tim-3 is also expressed by CD8+ T cells during acute infection and by multiple other immune cell types, including CD4+ Th1 and regulatory T cells, dendritic cells, and mast cells. In this study, we investigated the role of Tim-3 signaling on CD8+ T cell memory using a Tim-3 conditional knockout mouse model and mice lacking the signaling portion of the Tim-3 cytoplasmic domain. Together, our results indicate that Tim-3 has at most a modest effect on the formation and function of CD8+ memory T cells.

IFNγ-induction of TH1-like regulatory T cells controls antiviral responses.

Regulatory T (Treg) cells are an immunosuppressive population that are required to maintain peripheral tolerance and prevent tissue damage from immunopathology, via anti-inflammatory cytokines, inhibitor receptors and metabolic disruption. Here we show that Treg cells acquire an effector-like state, yet remain stable and functional, when exposed to interferon gamma (IFNγ) during infection with lymphocytic choriomeningitis and influenza A virus. Treg cell-restricted deletion of the IFNγ receptor (encoded by Ifngr1), but not the interleukin 12 (IL12) receptor (encoded by Il12rb2), prevented TH1-like polarization (decreased expression of T-bet, CXC motif chemokine receptor 3 and IFNγ) and promoted TH2-like polarization (increased expression of GATA-3, CCR4 and IL4). TH1-like Treg cells limited CD8+ T cell effector function, proliferation and memory formation during acute and chronic infection. These findings provide fundamental insights into how Treg cells sense inflammatory cues from the environment (such as IFNγ) during viral infection to provide guidance to the effector immune response. This regulatory circuit prevents prolonged immunoinflammatory responses and shapes the quality and quantity of the memory T cell response.

Expression of Tim-3 drives phenotypic and functional changes in Treg cells in secondary lymphoid organs and the tumor microenvironment.

Regulatory T cells (Treg cells) are critical mediators of self-tolerance, but they can also limit effective anti-tumor immunity. Although under homeostasis a small fraction of Treg cells in lymphoid organs express the putative checkpoint molecule Tim-3, this protein is expressed by a much larger proportion of tumor-infiltrating Treg cells. Using a mouse model that drives cell-type-specific inducible Tim-3 expression, we show that expression of Tim-3 by Treg cells is sufficient to drive Treg cells to a more effector-like phenotype, resulting in increases in suppressive activity, effector T cell exhaustion, and tumor growth. We also show that T-reg-cell-specific inducible deletion of Tim-3 enhances anti-tumor immunity. Enhancement of Treg cell function by Tim-3 is strongly correlated with increased expression of interleukin-10 (IL-10) and a shift to a more glycolytic metabolic phenotype. Our data demonstrate that Tim-3+ Treg cells may be a relevant therapeutic target cell type for the treatment of cancer.

A Cre-driven allele-conditioning line to interrogate CD4+ conventional T cells.

CD4+ T cells share common developmental pathways with CD8+ T cells, and upon maturation, CD4+ T conventional T (Tconv) cells lack phenotypic markers that distinguish these cells from FoxP3+ T regulatory cells. We developed a tamoxifen-inducible ThPOKCreERT2.hCD2 line with Frt sites inserted on either side of the CreERT2-hCD2 cassette, and a Foxp3Ametrine-FlpO strain, expressing Ametrine and FlpO in Foxp3+ cells. Breeding these mice resulted in a CD4conviCreERT2-hCD2 line that allows for the specific manipulation of a gene in CD4+ Tconv cells. As FlpO removes the CreERT2-hCD2 cassette, CD4+ Treg cells are spared from Cre activity, which we refer to as allele conditioning. Comparison with an E8IiCreERT2.GFP mouse that enables inducible targeting of CD8+ T cells, and deletion of two inhibitory receptors, PD-1 and LAG-3, in a melanoma model, support the fidelity of these lines. These engineered mouse strains present a resource for the temporal manipulation of genes in CD4+ T cells and CD4+ Tconv cells.

The costimulatory activity of Tim-3 requires Akt and MAPK signaling and its recruitment to the immune synapse.

Expression of the transmembrane protein Tim-3 is increased on dysregulated T cells undergoing chronic activation, including during chronic infection and in solid tumors. Thus, Tim-3 is generally thought of as an inhibitory protein. We and others previously reported that under some circumstances, Tim-3 exerts paradoxical costimulatory activity in T cells (and other cells), including enhancement of the phosphorylation of ribosomal S6 protein. Here, we examined the upstream signaling pathways that control Tim-3-mediated increases in phosphorylated S6 in T cells. We also defined the localization of Tim-3 relative to the T cell immune synapse and its effects on downstream signaling. Recruitment of Tim-3 to the immune synapse was mediated exclusively by the transmembrane domain, replacement of which impaired the ability of Tim-3 to costimulate T cell receptor (TCR)-dependent S6 phosphorylation. Furthermore, enforced localization of the Tim-3 cytoplasmic domain to the immune synapse in a chimeric antigen receptor still enabled T cell activation. Together, our findings are consistent with a model whereby Tim-3 enhances TCR-proximal signaling under acute conditions.

Neuropilin-1 is a T cell memory checkpoint limiting long-term antitumor immunity.

Robust CD8+ T cell memory is essential for long-term protective immunity but is often compromised in cancer, where T cell exhaustion leads to loss of memory precursors. Immunotherapy via checkpoint blockade may not effectively reverse this defect, potentially underlying disease relapse. Here we report that mice with a CD8+ T cell-restricted neuropilin-1 (NRP1) deletion exhibited substantially enhanced protection from tumor rechallenge and sensitivity to anti-PD1 immunotherapy, despite unchanged primary tumor growth. Mechanistically, NRP1 cell-intrinsically limited the self-renewal of the CD44+PD1+TCF1+TIM3- progenitor exhausted T cells, which was associated with their reduced ability to induce c-Jun/AP-1 expression on T cell receptor restimulation, a mechanism that may contribute to terminal T cell exhaustion at the cost of memory differentiation in wild-type tumor-bearing hosts. These data indicate that blockade of NRP1, a unique 'immune memory checkpoint', may promote the development of long-lived tumor-specific Tmem that are essential for durable antitumor immunity.

Resistance to PD1 blockade in the absence of metalloprotease-mediated LAG3 shedding.

Mechanisms of resistance to cancer immunotherapy remain poorly understood. Lymphocyte activation gene-3 (LAG3) signaling is regulated by a disintegrin and metalloprotease domain-containing protein-10 (ADAM10)- and ADAM17-mediated cell surface shedding. Here, we show that mice expressing a metalloprotease-resistant, noncleavable LAG3 mutant (LAG3NC) are resistant to PD1 blockade and fail to mount an effective antitumor immune response. Expression of LAG3NC intrinsically perturbs CD4+ T conventional cells (Tconvs), limiting their capacity to provide CD8+ T cell help. Furthermore, the translational relevance for these observations is highlighted with an inverse correlation between high LAG3 and low ADAM10 expression on CD4+ Tconvs in the peripheral blood of patients with head and neck squamous cell carcinoma, which corresponded with poor prognosis. This correlation was also observed in a cohort of patients with skin cancers and was associated with increased disease progression after standard-of-care immunotherapy. These data suggest that subtle changes in LAG3 inhibitory receptor signaling can act as a resistance mechanism with a substantive effect on patient responsiveness to immunotherapy.

Treg-Cell-Derived IL-35-Coated Extracellular Vesicles Promote Infectious Tolerance.

Interleukin-35 (IL-35) is an immunosuppressive cytokine composed of Epstein-Barr-virus-induced protein 3 (Ebi3) and IL-12α chain (p35) subunits, yet the forms that IL-35 assume and its role in peripheral tolerance remain elusive. We induce CBA-specific, IL-35-producing T regulatory (Treg) cells in TregEbi3WT C57BL/6 reporter mice and identify IL-35 producers by expression of Ebi3TdTom gene reporter plus Ebi3 and p35 proteins. Curiously, both subunits of IL-35 are displayed on the surface of tolerogen-specific Foxp3+ and Foxp3neg (iTr35) T cells. Furthermore, IL-35 producers, although rare, secrete Ebi3 and p35 on extracellular vesicles (EVs) targeting a 25- to 100-fold higher number of T and B lymphocytes, causing them to acquire surface IL-35. This surface IL-35 is absent when EV production is inhibited or if Ebi3 is genetically deleted in Treg cells. The unique ability of EVs to coat bystander lymphocytes with IL-35, promoting exhaustion in, and secondary suppression by, non-Treg cells identifies a novel mechanism of infectious tolerance.

Tim-3 co-stimulation promotes short-lived effector T cells, restricts memory precursors, and is dispensable for T cell exhaustion.

Tim-3 is highly expressed on a subset of T cells during T cell exhaustion in settings of chronic viral infection and tumors. Using lymphocytic choriomeningitis virus (LCMV) Clone 13, a model for chronic infection, we found that Tim-3 was neither necessary nor sufficient for the development of T cell exhaustion. Nonetheless, expression of Tim-3 was sufficient to drive resistance to PD-L1 blockade therapy during chronic infection. Strikingly, expression of Tim-3 promoted the development of short-lived effector T cells, at the expense of memory precursor development, after acute LCMV infection. These effects were accompanied by increased Akt/mTOR signaling in T cells expressing endogenous or ectopic Tim-3. Conversely, Akt/mTOR signaling was reduced in effector T cells from Tim-3-deficient mice. Thus, Tim-3 is essential for optimal effector T cell responses, and may also contribute to exhaustion by restricting the development of long-lived memory T cells. Taken together, our results suggest that Tim-3 is actually more similar to costimulatory receptors that are up-regulated after T cell activation than to a dominant inhibitory protein like PD-1. These findings have significant implications for the development of anti-Tim-3 antibodies as therapeutic agents.

LAG3 limits regulatory T cell proliferation and function in autoimmune diabetes.

Inhibitory receptors (IRs) are pivotal in controlling T cell homeostasis because of their intrinsic regulation of conventional effector T (Tconv) cell proliferation, viability, and function. However, the role of IRs on regulatory T cells (Tregs) remains obscure because they could be required for suppressive activity and/or limit Treg function. We evaluated the role of lymphocyte activation gene 3 (LAG3; CD223) on Tregs by generating mice in which LAG3 is absent on the cell surface of Tregs in a murine model of type 1 diabetes. Unexpectedly, mice that lacked LAG3 expression on Tregs exhibited reduced autoimmune diabetes, consistent with enhanced Treg proliferation and function. Whereas the transcriptional landscape of peripheral wild-type (WT) and Lag3-deficient Tregs was largely comparable, substantial differences between intra-islet Tregs were evident and involved a subset of genes and pathways that promote Treg maintenance and function. Consistent with these observations, Lag3-deficient Tregs outcompeted WT Tregs in the islets but not in the periphery in cotransfer experiments because of enhanced interleukin-2-signal transducer and activator of transcription 5 signaling and increased Eos expression. Our study suggests that LAG3 intrinsically limits Treg proliferation and function at inflammatory sites, promotes autoimmunity in a chronic autoimmune-prone environment, and may contribute to Treg insufficiency in autoimmune disease.

Interleukin-35 Limits Anti-Tumor Immunity.

Regulatory T (Treg) cells pose a major barrier to effective anti-tumor immunity. Although Treg cell depletion enhances tumor rejection, the ensuing autoimmune sequelae limits its utility in the clinic and highlights the need for limiting Treg cell activity within the tumor microenvironment. Interleukin-35 (IL-35) is a Treg cell-secreted cytokine that inhibits T cell proliferation and function. Using an IL-35 reporter mouse, we observed substantial enrichment of IL-35(+) Treg cells in tumors. Neutralization with an IL-35-specific antibody or Treg cell-restricted deletion of IL-35 production limited tumor growth in multiple murine models of human cancer. Limiting intratumoral IL-35 enhanced T cell proliferation, effector function, antigen-specific responses, and long-term T cell memory. Treg cell-derived IL-35 promoted the expression of multiple inhibitory receptors (PD1, TIM3, LAG3), thereby facilitating intratumoral T cell exhaustion. These findings reveal previously unappreciated roles for IL-35 in limiting anti-tumor immunity and contributing to T cell dysfunction in the tumor microenvironment.

Design and construction of 2A peptide-linked multicistronic vectors.

The need for reliable, multicistronic vectors for multigene delivery is at the forefront of biomedical technology. This article describes the design and construction of 2A peptide-linked multicistronic vectors, which can be used to express multiple proteins from a single open reading frame (ORF). The small 2A peptide sequences, when cloned between genes, allow for efficient, stoichiometric production of discrete protein products within a single vector through a novel "cleavage" event within the 2A peptide sequence. Expression of more than two genes using conventional approaches has several limitations, most notably imbalanced protein expression and large size. The use of 2A peptide sequences alleviates these concerns. They are small (18-22 amino acids) and have divergent amino-terminal sequences, which minimizes the chance for homologous recombination and allows for multiple, different 2A peptide sequences to be used within a single vector. Importantly, separation of genes placed between 2A peptide sequences is nearly 100%, which allows for stoichiometric and concordant expression of the genes, regardless of the order of placement within the vector.

Generation of 2A-linked multicistronic cassettes by recombinant PCR.

The need for reliable, multicistronic vectors for multigene delivery is at the forefront of biomedical technology. It is now possible to express multiple proteins from a single open reading frame (ORF) using 2A peptide-linked multicistronic vectors. These small sequences, when cloned between genes, allow for efficient, stoichiometric production of discrete protein products within a single vector through a novel "cleavage" event within the 2A peptide sequence. Expression of more than two genes using conventional approaches has several limitations, most notably imbalanced protein expression and large size. The use of 2A peptide sequences alleviates these concerns. They are small (18-22 amino acids) and have divergent amino-terminal sequences, which minimizes the chance for homologous recombination and allows for multiple, different 2A peptide sequences to be used within a single vector. Importantly, separation of genes placed between 2A peptide sequences is nearly 100%, which allows for stoichiometric and concordant expression of the genes, regardless of the order of placement within the vector. This protocol describes the use of recombinant polymerase chain reaction (PCR) to connect multiple 2A-linked protein sequences. The final construct is subcloned into an expression vector.

Verification of 2A peptide cleavage.

The need for reliable, multicistronic vectors for multigene delivery is at the forefront of biomedical technology. It is now possible to express multiple proteins from a single open reading frame (ORF) using 2A peptide-linked multicistronic vectors. These small sequences, when cloned between genes, allow for efficient, stoichiometric production of discrete protein products within a single vector through a novel "cleavage" event within the 2A peptide sequence. The easiest and most effective way to assess 2A cleavage is to perform transient transfection of 293T cells (human embryonic kidney cells) followed by western blot analysis, as described in this protocol. 293T cells are easy to grow and can be efficiently transfected with a variety of vectors. Cleavage can be assessed by detection with antibodies against the target proteins or anti-2A serum.

The plasticity of regulatory T cell function.

Regulatory T cells (T(regs)) can suppress a wide variety of cell types, in diverse organ sites and inflammatory conditions. Whereas T(regs) possess multiple suppressive mechanisms, the number required for maximal function is unclear. Furthermore, whether any interrelationship or cross-regulatory mechanisms exist to orchestrate and control their utilization is unknown. In this study, we assessed the functional capacity of T(regs) lacking the ability to secrete both IL-10 and IL-35, which individually are required for maximal T(reg) activity. Surprisingly, IL-10/IL-35 double-deficient T(regs) were fully functional in vitro and in vivo. Loss of IL-10 and IL-35 was compensated for by a concurrent increase in cathepsin E (Ctse) expression, enhanced TRAIL (Tnfsf10) expression, and soluble TRAIL release, rendering IL-10/IL-35 double-deficient T(regs) functionally dependent on TRAIL in vitro and in vivo. Lastly, whereas C57BL/6 T(regs) are normally IL-10/IL-35 dependent, BALB/c T(regs), which express high levels of cathepsin E and enhanced TRAIL expression, are partially TRAIL dependent by default. These data reveal that cross-regulatory pathways exist that control the utilization of suppressive mechanisms, thereby providing T(reg) functional plasticity.

Cutting edge: regulatory T cells do not mediate suppression via programmed cell death pathways.

Regulatory T cells (Tregs) play a critical role in the immune system to regulate peripheral tolerance and prevent autoimmunity. However, the relative importance of different mechanisms of Treg function remains obscure. In this article, we reveal a limited role for programmed cell death pathways in mediating Treg suppression of conventional T cells. We show that Tregs are able to suppress the proliferation of conventional T cells that are resistant to apoptosis (Bim(-/-), Bim(-/-)Puma(-/-), Bcl-2 transgenic) or receptor-interacting serine-threonine kinase-dependent necrosis (also referred to as regulated necrosis or necroptosis) (Ripk3(-/-)) in several in vitro and in vivo assays. These data suggest that programmed cell death pathways, such as apoptosis and receptor-interacting serine-threonine kinase-dependent necrosis, are not required for Treg-mediated suppression.

Cutting edge: accelerated autoimmune diabetes in the absence of LAG-3.

Lymphocyte activation gene-3 (LAG-3; CD223) is a CD4 homolog that is required for maximal regulatory T cell function and for the control of CD4(+) and CD8(+) T cell homeostasis. Lag3(-)(/)(-) NOD mice developed substantially accelerated diabetes with 100% incidence. Adoptive transfer experiments revealed that LAG-3 was primarily responsible for limiting the pathogenic potential of CD4(+) T cells and, to a lesser extent, CD8(+) T cells. Lag3(-)(/)(-) mice exhibited accelerated, invasive insulitis, corresponding to increased CD4(+) and CD8(+) T cell islet infiltration and intraislet proliferation. The frequencies of islet Ag-reactive chromogranin A-specific CD4(+) T cells and islet specific glucose-6-phosphatase-specific CD8(+) T cells were significantly increased in the islets of Lag3(-)(/)(-) mice, suggesting an early expansion of pathogenic clones that is normally restrained by LAG-3. We conclude that LAG-3 is necessary for regulating CD4(+) and CD8(+) T cell function during autoimmune diabetes, and thus may contribute to limiting autoimmunity in disease-prone environments.

Cutting edge: regulatory T cells do not require stimulation through their TCR to suppress.

The mechanism and stimulatory requirements of regulatory T cell (Treg)-mediated suppression are still unclear. To assess the requirement for Treg stimulation by cognate peptide:MHC, we used T cells from OTII and AND TCR transgenic mice that are specific for and restricted by distinct, noncrossreactive peptide:MHC combinations. This allowed us to independently activate Tregs and their conventional T cell (Tconv) targets. Surprisingly, we found that suppression can occur in the absence of peptide:MHC-mediated stimulation of Tregs. This suppression was Treg dependent and not due to cold target inhibition. Using Rag1(-/-) TCR transgenic T cells, we show that regulation of Tconv proliferation by heterogeneous Tregs is not due to alloreactivity or crossreactivity. Finally, using anti-TCR-Vbeta8-coated microbeads and Vbeta8(-) Tregs, we show that TCR stimulation-independent suppression can occur in the absence of APCs. These data suggest that Tregs may possess constitutive regulatory activity that can be mediated in the absence of cognate peptide:MHC-TCR stimulation.

The development and function of regulatory T cells.

Regulatory T cells (Tregs) are a critical subset of T cells that mediate peripheral tolerance. There are two types of Tregs: natural Tregs, which develop in the thymus, and induced Tregs, which are derived from naive CD4(+) T cells in the periphery. Tregs utilize a variety of mechanisms to suppress the immune response. While Tregs are critical for the peripheral maintenance of potential autoreactive T cells, they can also be detrimental by preventing effective anti-tumor responses and sterilizing immunity against pathogens. In this review, we will discuss the development of natural and induced Tregs as well as the role of Tregs in a variety of disease settings and the mechanisms they utilize for suppression.