Matthew L. Bettini

Immune Inhibitory Molecules LAG-3 and PD-1 Synergistically Regulate T-cell Function to Promote Tumoral Immune Escape

Inhibitory receptors on immune cells are pivotal regulators of immune escape in cancer. Among these inhibitory receptors, CTLA-4 (targeted clinically by ipilimumab) serves as a dominant off-switch while other receptors such as PD-1 and LAG-3 seem to serve more subtle rheostat functions. However, the extent of synergy and cooperative interactions between inhibitory pathways in cancer remain largely unexplored. Here, we reveal extensive coexpression of PD-1 and LAG-3 on tumor-infiltrating CD4(+) and CD8(+) T cells in three distinct transplantable tumors. Dual anti-LAG-3/anti-PD-1 antibody treatment cured most mice of established tumors that were largely resistant to single antibody treatment. Despite minimal immunopathologic sequelae in PD-1 and LAG-3 single knockout mice, dual knockout mice abrogated self-tolerance with resultant autoimmune infiltrates in multiple organs, leading to eventual lethality. However, Lag3(-/-)Pdcd1(-/-) mice showed markedly increased survival from and clearance of multiple transplantable tumors. Together, these results define a strong synergy between the PD-1 and LAG-3 inhibitory pathways in tolerance to both self and tumor antigens. In addition, they argue strongly that dual blockade of these molecules represents a promising combinatorial strategy for cancer.

Stability and function of regulatory T cells is maintained by a neuropilin-1–semaphorin-4a axis

Regulatory T cells (Treg cells) have a crucial role in the immune system by preventing autoimmunity, limiting immunopathology, and maintaining immune homeostasis. However, they also represent a major barrier to effective anti-tumour immunity and sterilizing immunity to chronic viral infections. The transcription factor Foxp3 has a major role in the development and programming of Treg cells. The relative stability of Treg cells at inflammatory disease sites has been a highly contentious subject. There is considerable interest in identifying pathways that control the stability of Treg cells as many immune-mediated diseases are characterized by either exacerbated or limited Treg-cell function. Here we show that the immune-cell-expressed ligand semaphorin-4a (Sema4a) and the Treg-cell-expressed receptor neuropilin-1 (Nrp1) interact both in vitro, to potentiate Treg-cell function and survival, and in vivo, at inflammatory sites. Using mice with a Treg-cell-restricted deletion of Nrp1, we show that Nrp1 is dispensable for suppression of autoimmunity and maintenance of immune homeostasis, but is required by Treg cells to limit anti-tumour immune responses and to cure established inflammatory colitis. Sema4a ligation of Nrp1 restrained Akt phosphorylation cellularly and at the immunologic synapse by phosphatase and tensin homologue (PTEN), which increased nuclear localization of the transcription factor Foxo3a. The Nrp1-induced transcriptome promoted Treg-cell stability by enhancing quiescence and survival factors while inhibiting programs that promote differentiation. Importantly, this Nrp1-dependent molecular program is evident in intra-tumoral Treg cells. Our data support a model in which Treg-cell stability can be subverted in certain inflammatory sites, but is maintained by a Sema4a–Nrp1 axis, highlighting this pathway as a potential therapeutic target that could limit Treg-cell-mediated tumour-induced tolerance without inducing autoimmunity.

Distinct TCR signaling pathways drive proliferation and cytokine production in T cells

The physiological basis and mechanistic requirements for a large number of functional immunoreceptor tyrosine-based activation motifs (ITAMs; high ITAM multiplicity) in the complex of the T cell antigen receptor (TCR) and the invariant signaling protein CD3 remain obscure. Here we found that whereas a low multiplicity of TCR-CD3 ITAMs was sufficient to engage canonical TCR-induced signaling events that led to cytokine secretion, a high multiplicity of TCR-CD3 ITAMs was required for TCR-driven proliferation. This was dependent on the formation of compact immunological synapses, interaction of the adaptor Vav1 with phosphorylated CD3 ITAMs to mediate the recruitment and activation of the oncogenic transcription factor Notch1 and, ultimately, proliferation induced by the cell-cycle regulator c-Myc. Analogous mechanistic events were also needed to drive proliferation in response to weak peptide agonists. Thus, the TCR-driven pathways that initiate cytokine secretion and proliferation are separable and are coordinated by the multiplicity of phosphorylated ITAMs in TCR-CD3.

Thymocyte Development in Early Growth Response Gene 1-Deficient Mice

Early growth response gene 1 (Egr1) codes for a transcriptional regulator that contains a zinc-finger DNA binding domain. Egr1 expression is induced by a variety of extracellular stimuli including TCR-ligand interactions. Its pattern of expression in the thymus and dependence on ERK activation have led to speculation that it has a role in T cell development, but the exact nature of this role has been undefined. To more clearly define the role of Egr1 in thymocyte development, we have analyzed thymocytes from Egr1-deficient mice. We find that thymuses from Egr1-deficient mice contain twice as many cells as age-matched controls, and the increase in thymocyte number is apparent at the early CD4/CD8 double negative stage of development. Subsequent maturation to the CD4/CD8 double positive stage and survival of the double positive cells both appear normal in Egr1-deficient animals. We also find that Egr1 promotes positive selection of both CD4 and CD8 single positive cells without playing a major role in negative selection. Egr1 influences positive selection by enhancing expression of the helix-loop-helix inhibitor Id3 and the anti-apoptosis molecule bcl-2. Thus, Egr1 translates developmental signals into appropriate changes in gene expression at multiple stages of thymocyte development.

Tonic ubiquitylation controls T-cell receptor:CD3 complex expression during T-cell development

Expression of the T‐cell receptor (TCR):CD3 complex is tightly regulated during T‐cell development. The mechanism and physiological role of this regulation are unclear. Here, we show that the TCR:CD3 complex is constitutively ubiquitylated in immature double positive (DP) thymocytes, but not mature single positive (SP) thymocytes or splenic T cells. This steady state, tonic CD3 monoubiquitylation is mediated by the CD3ε proline‐rich sequence, Lck, c‐Cbl, and SLAP, which collectively trigger the dynamin‐dependent downmodulation, lysosomal sequestration and degradation of surface TCR:CD3 complexes. Blocking this tonic ubiquitylation by mutating all the lysines in the CD3 cytoplasmic tails significantly upregulates TCR levels on DP thymocytes. Mimicking monoubiquitylation by expression of a CD3ζ‐monoubiquitin (monoUb) fusion molecule significantly reduces TCR levels on immature thymocytes. Moreover, modulating CD3 ubiquitylation alters immunological synapse (IS) formation and Erk phosphorylation, thereby shifting the signalling threshold for positive and negative selection, and regulatory T‐cell development. Thus, tonic TCR:CD3 ubiquitylation results in precise regulation of TCR expression on immature T cells, which is required to maintain the fidelity of T‐cell development.

Development of thymically derived natural regulatory T cells

Natural regulatory T cells (nTregs) are defined by their inherent ability to establish and maintain peripheral self‐tolerance. In recent years, the development of nTregs has come under close examination with the advent of Forkhead Box P3 protein (FOXP3)‐green fluorescent protein reporter mice that pinpointed the initiation of FOXP3 expression within the thymus. The mechanism and pathway of nTreg development has only recently been studied in detail and to a large degree remains unclear. In this review, we will discuss our current understanding of nTreg lineage choice and development from a cellular and intracellular standpoint.

MAP kinase phosphatase activity sets the threshold for thymocyte positive selection

Phosphorylation of MAP kinases is important for proper translation of T cell antigen receptor (TCR) signals into thymocyte cell fates, but the role of MAP kinase phosphatase (MKP) activity in thymocyte development has not been characterized. To explore the role of MKP in thymocytes, we constructed a double mutant MKP-3 (DM-MKP3) that acts as a dominant-negative inhibitor of ERK- and JNK-specific MKP. Thymocytes developing in the presence of DM-MKP3 have enhanced frequencies of both CD4 and CD8 mature, single-positive cells and no increase in apoptosis. Expression of DM-MKP3 also results in an increased proportion of thymocytes with high levels of both CD69 and TCRβ, suggesting that the increased proportion of mature thymocytes is the result of an increased probability that CD4+CD8+ cells will be positively selected. Thus, MKP activity controls thymocyte cell fate by regulating the threshold of TCR signaling that is able to induce positive selection.

Membrane Association of the CD3ε Signaling Domain Is Required for Optimal T Cell Development and Function

The TCR:CD3 complex transduces signals that are critical for optimal T cell development and adaptive immunity. In resting T cells, the CD3ε cytoplasmic tail associates with the plasma membrane via a proximal basic-rich stretch (BRS). In this study, we show that mice lacking a functional CD3ε-BRS exhibited substantial reductions in thymic cellularity and limited CD4–CD8– double-negative (DN) 3 to DN4 thymocyte transition, because of enhanced DN4 TCR signaling resulting in increased cell death and TCR downregulation in all subsequent populations. Furthermore, positive, but not negative, T cell selection was affected in mice lacking a functional CD3ε-BRS, which led to limited peripheral T cell function and substantially reduced responsiveness to influenza infection. Collectively, these results indicate that membrane association of the CD3ε signaling domain is required for optimal thymocyte development and peripheral T cell function.

Generation of T cell receptor-retrogenic mice: improved retroviral-mediated stem cell gene transfer.

The use of retrogenic mice offers a rapid and flexible approach to T cell receptor (TCR)-transgenic mice. By transducing bone marrow progenitor cells with a retrovirus that encodes a given TCR-α/β subunit, TCR-retrogenic mice can be generated in as few as 4–6 weeks, whereas conventional TCR transgenics can take 6 months or longer. In this updated protocol, we have increased the efficiency of the bone marrow transduction and bone marrow reconstitution compared with our previously published protocol. The main departure from the previous protocol is the implementation of spin transduction with the viral supernatant instead of coculture with the viral producer cell line. The changes in this protocol improve bone marrow viability, increase consistency of the bone marrow transduction and bone marrow engraftment, and they reduce the ratio of bone marrow donor mice to bone marrow recipients.

T-cell receptor retrogenic mice: a rapid, flexible alternative to T-cell receptor transgenic mice

The T‐cell receptor (TCR) is unique in its complexity. It determines not only positive (life) and negative (death) selection in the thymus, but also mediates proliferation, anergy, differentiation, cytotoxicity and cytokine production in the periphery. Through its association with six CD3 signalling chains (εγ, δε and ζζ), the TCR is capable of recognizing an extensive variety of antigenic peptides, from both pathogens and self‐antigens, and translating these interactions into multiple signalling pathways that mediate diverse T‐cell developmental and functional responses. The analysis of TCR biology has been revolutionized by the development of TCR transgenic mice, which express a single clonotypic T‐cell population, with diverse specificities and genetic backgrounds. However, they are time consuming to generate and characterize, limiting the analysis of large numbers of TCR over a short period of time in multiple genetic backgrounds. The recent development of TCR retrogenic technology resolves these limitations and could in time have a similarly important impact on our understanding of T‐cell development and function. In this review, we will discuss the advantages and limitations of retrogenic technology compared with the generation and use of TCR transgenic mice for studying all aspects of T‐cell biology.