CD28 and inducible costimulatory protein Src homology 2 binding domains show distinct regulation of phosphatidylinositol 3-kinase, Bcl-xL, and IL-2 expression in primary human CD4 T lymphocytes

Relation of clinical culture method to T-cell memory status and efficacy in xenograft models of adoptive immunotherapy

BACKGROUND AIMS

Cytotoxic T lymphocytes modified with chimeric antigen receptors (CARs) for adoptive immunotherapy of hematologic malignancies are effective in pre-clinical models, and this efficacy has translated to success in several clinical trials. Many early trials were disappointing in large part because of the lack of proliferation and subsequent persistence of transferred cells. Recent investigations have pointed to the importance of delivering highly proliferative cells, whether of naive or early memory phenotypes.

METHODS

We investigated the influence of two common cell culturing methods used in early trials and their relationship to T-cell phenotype and pre-clinical efficacy.

RESULTS

We observed that stimulation with soluble anti-CD3 antibody OKT-3 and high-dose interleukin-2 produces more effector memory-type T cells with shorter average telomeres when compared with cells generated with the use of CD3/CD28 beads. When used in xenograft models of leukemia, bead-stimulated cells proliferated earlier and to a higher degree than those generated with the use of OKT-3/IL2 and resulted in better disease control despite no difference in distribution or migration throughout the mouse. Inclusion of the known successful clinical 4-1BB endodomain in the CAR could not rescue the function of OKT-3/IL-2-cultured cells. T cells isolated from animals that survived long-term (>120 days) retained a central memory-like phenotype and demonstrated a memory response to a large re-challenge of CD19-positive leukemia.

CONCLUSIONS

In summary, we confirm that cells with a younger phenotype or higher proliferative capacity perform better in pre-clinical models and that cell culturing influences cell phenotype seemingly independent of the 4-1BB endodomain in the CAR structure.

Blood and beyond: properties of circulating and tissue-resident human virus-specific αβ CD8(+) T cells

CD8(+) αβ T-cell responses form an essential line of defence against viral infections. An important part of the mechanisms that control the generation and maintenance of these responses have been elucidated in experimental mouse models. In recent years it has become clear that CD8(+) T-cell responses in humans not only show similarities, but also display differences to those occurring in mice. Furthermore, while several viral infections occur primarily in specialised organ systems, for obvious reasons, most human CD8(+) T-cell investigations were performed on cells deriving from the circulation. Indeed, several lines of evidence now point to essential functional differences between virus-specific CD8(+) memory T cells found in the circulation and those providing protection in organ systems, such as the lungs. In this review, we will focus on summarising recent insights into human CD8(+) T-cell differentiation in response to several viruses and emphasise that for a complete understanding of anti-viral immunity, it is pivotal to scrutinize such responses in both blood and tissue.

T-cell costimulatory blockade in organ transplantation

Before it became possible to derive T-cell lines and clones, initial experimentation on the activation requirements of T lymphocytes was performed on transformed cell lines, such as Jurkat. These studies, although technically correct, proved misleading as most transformed T cells can be activated by stimulation of the clonotypic T-cell receptor (TCR) alone. In contrast, once it became possible to study nontransformed T cells, it quickly became clear that TCR stimulation by itself is insufficient for optimal activation of naïve T cells, but in fact, induces a state of anergy. It then became clear that functional activation of T cells requires not only recognition of major histocompatibility complex (MHC) and peptide by the TCR, but also requires ligation of costimulatory receptors expressed on the cell surface.

Evaluation of engraftment and immunological tolerance after reduced intensity conditioning in a rhesus hematopoietic stem cell gene therapy model

Reduced intensity conditioning (RIC) is desirable for hematopoietic stem cell (HSC) targeted gene therapy; however, RIC may be insufficient for efficient engraftment and inducing immunological tolerance to transgenes. We previously established long-term gene marking in our rhesus macaque autologous HSC transplantation model following 10 Gy total body irradiation (TBI). In this study, we evaluated RIC transplantation with 4 Gy TBI in two rhesus macaques that received equal parts of CD34(+) cells transduced with green fluorescent protein (GFP)-expressing lentiviral vector and empty vector not expressing transgenes. In both animals, equivalently low gene marking between GFP and empty vectors was observed 6 months post-transplantation, even with efficient transduction of CD34(+) cells in vitro. Autologous lymphocyte infusion with GFP marking resulted in an increase of gene marking in lymphocytes in a control animal with GFP tolerance, but not in the two RIC-transplanted animals. In vitro assays revealed strong cellular and humoral immune responses to GFP protein in the two RIC-transplanted animals, but this was not observed in controls. In summary, 4 Gy TBI is insufficient to permit engraftment of genetically modified HSCs and induce immunological tolerance to transgenes. Our findings should help in the design of conditioning regimens in gene therapy trials.

Clinical grade manufacturing of human alloantigen-reactive regulatory T cells for use in transplantation

Regulatory T cell (Treg) therapy has the potential to induce transplantation tolerance so that immunosuppression and associated morbidity can be minimized. Alloantigen-reactive Tregs (arTregs) are more effective at preventing graft rejection than polyclonally expanded Tregs (PolyTregs) in murine models. We have developed a manufacturing process to expand human arTregs in short-term cultures using good manufacturing practice-compliant reagents. This process uses CD40L-activated allogeneic B cells to selectively expand arTregs followed by polyclonal restimulation to increase yield. Tregs expanded 100- to 1600-fold were highly alloantigen reactive and expressed the phenotype of stable Tregs. The alloantigen-expanded Tregs had a diverse TCR repertoire. They were more potent than PolyTregs in vitro and more effective at controlling allograft injuries in vivo in a humanized mouse model.

Phosphatidylinositol 3-kinase-independent signaling pathways contribute to ICOS-mediated T cell costimulation in acute graft-versus-host disease in mice

We and others have previously shown that ICOS plays an important role in inducing acute graft-versus-host disease (GVHD) in murine models of allogeneic bone marrow transplantation. ICOS potentiates TCR-mediated PI3K activation and intracellular calcium mobilization. However, ICOS signal transduction pathways involved in GVHD remain unknown. In this study, we examined the contribution of ICOS-PI3K signaling in the pathogenic potential of T cells using a knock-in mouse strain, ICOS-YF, which selectively lost the ability to activate PI3K. We found that when total T cells were used as alloreactive T cells, ICOS-YF T cells caused less severe GVHD compared with ICOS wild-type T cells, but they induced much more aggressive disease than ICOS knockout T cells. This intermediate level of pathogenic capacity of ICOS-YF T cells was correlated with similar levels of IFN-γ-producing CD8 T cells that developed in the recipients of ICOS-WT or ICOS-YF T cells. We further evaluated the role of ICOS-PI3K signaling in CD4 versus CD8 T cell compartment using GVHD models that are exclusively driven by CD4 or CD8 T cells. Remarkably, ICOS-YF CD8 T cells caused disease similar to ICOS wild-type CD8 T cells, whereas ICOS-YF CD4 T cells behaved very similarly to their ICOS knockout counterparts. Consistent with their in vivo pathogenic potential, CD8 T cells responded to ICOS ligation in vitro by PI3K-independent calcium flux, T cell activation, and proliferation. Thus, in acute GVHD in mice, CD4 T cells heavily rely on ICOS-PI3K signaling pathways; in contrast, CD8 T cells can use PI3K-independent ICOS signaling pathways, possibly through calcium.

HHLA2 is a member of the B7 family and inhibits human CD4 and CD8 T-cell function

T-cell costimulation and coinhibition generated by engagement of the B7 family and their receptor CD28 family are of central importance in regulating the T-cell response, making these pathways very attractive therapeutic targets. Here we describe HERV-H LTR-associating protein 2 (HHLA2) as a member of the B7 family that shares 10-18% amino acid identity and 23-33% similarity to other human B7 proteins and phylogenetically forms a subfamily with B7x and B7-H3 within the family. HHLA2 is expressed in humans but not in mice, which is unique within the B7 and CD28 families. HHLA2 protein is constitutively expressed on the surface of human monocytes and is induced on B cells after stimulation with LPS and IFN-γ. HHLA2 does not interact with other known members of the CD28 family or the B7 family, but does bind a putative receptor that is constitutively expressed not only on resting and activated CD4 and CD8 T cells but also on antigen-presenting cells. HHLA2 inhibits proliferation of both CD4 and CD8 T cells in the presence of T-cell receptor signaling. In addition, HHLA2 significantly reduces cytokine production by T cells including IFN-γ, TNF-α, IL-5, IL-10, IL-13, IL-17A, and IL-22. Thus, we have identified a unique B7 pathway that is able to inhibit human CD4 and CD8 T-cell proliferation and cytokine production. This unique human T-cell coinhibitory pathway may afford unique strategies for the treatment of human cancers, autoimmune disorders, infection, and transplant rejection and may help to design better vaccines.

Follicular T-helper cell recruitment governed by bystander B cells and ICOS-driven motility

Germinal centres support antibody affinity maturation and memory formation. Follicular T-helper cells promote proliferation and differentiation of antigen-specific B cells inside the follicle. A genetic deficiency in the inducible co-stimulator (ICOS), a classic CD28 family co-stimulatory molecule highly expressed by follicular T-helper cells, causes profound germinal centre defects, leading to the view that ICOS specifically co-stimulates the follicular T-helper cell differentiation program. Here we show that ICOS directly controls follicular recruitment of activated T-helper cells in mice. This effect is independent from ICOS ligand (ICOSL)-mediated co-stimulation provided by antigen-presenting dendritic cells or cognate B cells, and does not rely on Bcl6-mediated programming as an intermediate step. Instead, it requires ICOSL expression by follicular bystander B cells, which do not present cognate antigen to T-helper cells but collectively form an ICOS-engaging field. Dynamic imaging reveals ICOS engagement drives coordinated pseudopod formation and promotes persistent T-cell migration at the border between the T-cell zone and the B-cell follicle in vivo. When follicular bystander B cells cannot express ICOSL, otherwise competent T-helper cells fail to develop into follicular T-helper cells normally, and fail to promote optimal germinal centre responses. These results demonstrate a co-stimulation-independent function of ICOS, uncover a key role for bystander B cells in promoting the development of follicular T-helper cells, and reveal unsuspected sophistication in dynamic T-cell positioning in vivo.

The human CD8β M-4 isoform dominant in effector memory T cells has distinct cytoplasmic motifs that confer unique properties

The CD8 co-receptor influences T cell recognition and responses in both anti-tumor and anti-viral immunity. During evolution in the ancestor of humans and chimpanzees, the CD8B gene acquired two additional exons. As a result, in humans, there are four CD8β splice variants (M1 to M4) that differ in their cytoplasmic tails. The M-1 isoform which is the equivalent of murine CD8β, is predominantly expressed in naïve T cells, whereas, the M-4 isoform is predominantly expressed in effector memory T cells. The characteristics of the M-4 isoform conferred by its unique 36 amino acid cytoplasmic tail are not known. In this study, we identified a dihydrophobic leucine-based receptor internalization motif in the cytoplasmic tail of M-4 that regulated its cell surface expression and downregulation after activation. Further the M-4 cytoplasmic tail was able to associate with ubiquitinated targets in 293T cells and mutations in the amino acids NPW, a potential EH domain binding site, either enhanced or inhibited the interaction. In addition, the M-4 tail was itself mono-ubiquitinated on a lysine residue in both 293T cells and a human T cell line. When peripheral blood human T cells expressed CD8αβ M-4, the frequency of MIP-1β secreting cells responding to antigen presenting cells was two-fold higher as compared to CD8αβ M-1 expressing T cells. Thus, the cytoplasmic tail of the CD8β M-4 isoform has unique characteristics, which likely contributed to its selective expression and function in human effector memory T cells.

Chimeric NKG2D CAR-expressing T cell-mediated attack of human ovarian cancer is enhanced by histone deacetylase inhibition

NKG2D ligands (NKG2DLs) are widely expressed on ovarian cancers to various degrees, making them attractive targets for immunotherapy. Here, we applied a chimeric antigen receptor (CAR) approach for the targeting of NKG2DLs expressed on human ovarian cancer cells and evaluated the impact of pharmacological upregulation of NKG2DLs on immune recognition. Various NKG2DLs, including MICA/B and ULBP-1, -2, -3, and -4, were expressed at various levels on the surface of all established ovarian cancer cell lines and primary ovarian cancer samples tested. To redirect human T cells against NKG2DLs, an NKG2DL-specific CAR was generated by fusing the extracellular domain of the NKG2D receptor to the 4-1BB costimulatory and CD3-ζ chain signaling domains. In vitro expansion of chimeric NKG2D CAR T cells was delayed compared with untransduced T cells and control CAR T cells; the likely result of fratricide among activated T cells expressing NKG2DLs. However, NKG2D CAR T cells did expand and were selectively enriched during prolonged culture. In coculture, CD4(+) and CD8(+) NKG2D CAR T cells specifically recognized and killed NKG2DL-expressing ovarian cancer cell lines but not NKG2DL-negative cells. Notably, pretreatment of ovarian cancer cells expressing moderate to low levels of NKG2DLs with the histone deacetylase inhibitor sodium valproate (VPA) upregulated NKG2DL cell surface expression and consequently enhanced their immune recognition by chimeric NKG2D CAR T cells. Our results demonstrate that VPA-induced upregulation of NKG2DL expression enhances the immune recognition of ovarian cancer cells by engineered NKG2D CAR T cells, and rationalizes the use of VPA in combination with NKG2DL-targeted immunotherapy in ovarian cancer.

Advances in targeting cell surface signalling molecules for immune modulation

The past decade has witnessed a surge in the development of immunomodulatory approaches to combat a broad range of human diseases, including cancer, viral infections, autoimmunity and inflammation as well as in the prevention of transplant rejection. Immunomodulatory approaches mostly involve the use of monoclonal antibodies or recombinant fusion proteins that target cell surface signalling molecules on immune cells to drive immune responses towards the desired direction. Advances in our understanding of the human immune system, along with valuable lessons learned from the first generation of therapeutic biologics, are aiding the design of the next generation of immunomodulatory biologics with better therapeutic efficacy, minimized adverse effects and long-lasting clinical benefit. The recent encouraging results from antibodies targeting programmed cell death protein 1 (PD1) and B7 homolog 1 (B7H1; also known as PDL1) for the treatment of various advanced human cancers show that immunomodulatory therapy has come of age.

Mitochondrial superoxide generation enhances P2X7R-mediated loss of cell surface CD62L on naive human CD4+ T lymphocytes

Migration of naive CD4(+) T lymphocytes into lymphoid tissue is essential for their activation and subsequent roles in adaptive immunity. The adhesion molecule L-selectin (CD62L), critical for this process, is highly expressed on naive CD4(+) T lymphocytes and is downregulated upon T lymphocyte activation. We demonstrate protein expression of P2X7R on naive CD4(+) T lymphocytes and show functional channel activity in whole-cell patch clamp recordings. CD62L downregulation occurs rapidly in response to extracellular ATP, a process that is blocked by selective antagonists of P2X7R. This loss of surface CD62L expression was not associated with externalization of phosphatidylserine. While investigating the mechanisms for this process, we revealed that pharmacological modulation of mitochondrial complex I or III, but not inhibition of NADPH oxidase, enhanced P2X7R-dependent CD62L downregulation by increasing ATP potency. Enhanced superoxide generation in the mitochondria of rotenone- and antimycin A-treated cells was observed and may contribute to the enhanced sensitivity of P2X7R to ATP. P2X7R-dependent exposure of phosphatidylserine was also revealed by preincubation with mitochondrial uncouplers prior to ATP treatment. This may present a novel mechanism whereby P2X7R-dependent phosphatidylserine exposure occurs only when cells have enhanced mitochondrial reactive oxygen species generation. The clearance of apoptotic cells may therefore be enhanced by this mechanism which requires functional P2X7R expression.

B7-H5 costimulates human T cells via CD28H

The B7/CD28 family has profound modulatory effects in immune responses and constitutes an important target for the development of novel therapeutic drugs against human diseases. Here we describe a new CD28 homologue (CD28H) that has unique functions in the regulation of the human immune response and is absent in mice. CD28H is constitutively expressed on all naive T cells. Repetitive antigenic exposure, however, induces a complete loss of CD28H on many T cells, and CD28H negative T cells have a phenotype of terminal differentiation and senescence. After extensive screening in a receptor array, a B7-like molecule, B7 homologue 5 (B7-H5), was identified as a specific ligand for CD28H. B7-H5 is constitutively found in macrophages and could be induced on dendritic cells. The B7-H5/CD28H interaction selectively costimulates human T-cell growth and cytokine production via an AKT-dependent signalling cascade. Our study identifies a novel costimulatory pathway regulating human T-cell responses.

Antigen-specific blocking of CD4-specific immunological synapse formation using BPI and current therapies for autoimmune diseases

In this review, we discuss T-cell activation, etiology, and the current therapies of autoimmune diseases (i.e., MS, T1D, and RA). T-cells are activated upon interaction with antigen-presenting cells (APC) followed by a "bull's eye"-like formation of the immunological synapse (IS) at the T-cell-APC interface. Although the various disease-modifying therapies developed so far have been shown to modulate the IS and thus help in the management of these diseases, they are also known to present some undesirable side effects. In this study, we describe a novel and selective way to suppress autoimmunity by using a bifunctional peptide inhibitor (BPI). BPI uses an intercellular adhesion molecule-1 (ICAM-1)-binding peptide to target antigenic peptides (e.g., proteolipid peptide, glutamic acid decarboxylase, and type II collagen) to the APC and therefore modulate the immune response. The central hypothesis is that BPI blocks the IS formation by simultaneously binding to major histocompatibility complex-II and ICAM-1 on the APC and selectively alters the activation of T cells from T(H)1 to T(reg) and/or T(H)2 phenotypes, leading to tolerance.

TCR affinity and specificity requirements for human regulatory T-cell function

We investigated whether TCRs restricted to the more ubiquitously expressed MHC class I molecules could be used to redirect human regulatory T cells (Tregs). Using a series of HLA-A2-restricted TCRs that recognize the same peptide-MHC class I complex (pMHC) with affinities varying up to 3500 fold, we observed that TCR affinity had no effect on the ability of the introduced TCRs to confer potent Ag-specific suppressive activity. Surprisingly, we found a naturally occurring, low-affinity MHC class I-restricted TCR specific for an NY-ESO-1 epitope that was unable to redirect a functional CD4 T-effector cell response could confer potent antigen-specific suppressive activity when expressed in Tregs and severely impair the expansion of highly functional HIV-1(GAG)-specific CD8 T cells expressing a high-affinity TCR. This suppressive activity was only observed when both Ags were presented by the same cell, and no suppression was observed when the target Ags were put in distinct cells. These studies underscore the clinical utility of using MHC class I-restricted TCRs to endow Tregs with specificity to control autoimmune disease and highlight the conditions in which this approach would have most therapeutic benefit.

CD27 costimulation augments the survival and antitumor activity of redirected human T cells in vivo

The costimulatory effects of CD27 on T lymphocyte effector function and memory formation has been confined to evaluations in mouse models, in vitro human cell culture systems, and clinical observations. Here, we tested whether CD27 costimulation actively enhances human T-cell function, expansion, and survival in vitro and in vivo. Human T cells transduced to express an antigen-specific chimeric antigen receptor (CAR-T) containing an intracellular CD3 zeta (CD3ζ) chain signaling module with the CD27 costimulatory motif in tandem exerted increased antigen-stimulated effector functions in vitro, including cytokine secretion and cytotoxicity, compared with CAR-T with CD3ζ alone. After antigen stimulation in vitro, CD27-bearing CAR-T cells also proliferated, up-regulated Bcl-X(L) protein expression, resisted apoptosis, and underwent increased numerical expansion. The greatest impact of CD27 was noted in vivo, where transferred CAR-T cells with CD27 demonstrated heightened persistence after infusion, facilitating improved regression of human cancer in a xenogeneic allograft model. This tumor regression was similar to that achieved with CD28- or 4-1BB-costimulated CARs, and heightened persistence was similar to 4-1BB but greater than CD28. Thus, CD27 costimulation enhances expansion, effector function, and survival of human CAR-T cells in vitro and augments human T-cell persistence and antitumor activity in vivo.

A screening assay to identify agents that enhance T-cell recognition of human melanomas

Although a series of melanoma differentiation antigens for immunotherapeutic targeting has been described, heterogeneous expression of antigens such as Melan-A/MART-1 and gp100 results from a loss of antigenic expression in many late stage tumors. Antigen loss can represent a means for tumor escape from immune recognition, and a barrier to immunotherapy. However, since antigen-negative tumor phenotypes frequently result from reversible gene regulatory events, antigen enhancement represents a potential therapeutic opportunity. Accordingly, we have developed a cell-based assay to screen for compounds with the ability to enhance T-cell recognition of melanoma cells. This assay is dependent on augmentation of MelanA/MART-1 antigen presentation by a melanoma cell line (MU89). T-cell recognition is detected as interleukin-2 production by a Jurkat T cell transduced to express a T-cell receptor specific for an HLA-A2 restricted epitope of the Melan-A/MART-1 protein. This cellular assay was used to perform a pilot screen by using 480 compounds of known biological activity. From the initial proof-of-principle primary screen, eight compounds were identified as positive hits. A panel of secondary screens, including orthogonal assays, was used to validate the primary hits and eliminate false positives, and also to measure the comparative efficacy of the identified compounds. This cell-based assay, thus, yields consistent results applicable to the screening of larger libraries of compounds that can potentially reveal novel molecules which allow better recognition of treated tumors by T cells.

Immunomodulation of inducible co-stimulator (ICOS) in human cytokine-induced killer cells against cholangiocarcinoma through ICOS/ICOS ligand interaction

OBJECTIVE

To evaluate the immunomodulation of inducible co-stimulator (ICOS) in cytokine-induced killer (CIK) cells against cholangiocarcinoma.

METHODS

CIK cells were generated from normal peripheral blood mononuclear cells. Methyl thiazolyl tetrazolium assay was performed to assess proliferation of CIK-ICOS and controlled CIK cells; ELISA was used to analyze the expression of cytokines. Reverse transcription-polymerase chain reaction and immunohistochemistry were performed to evaluate the expression of ICOS ligand (ICOSL) in CIK cells and human cholangiocarcinoma cell line QBC939 cells. The cytotoxicity of CIK cells was determined either by lactate dehydrogenase-releasing assay in vivo or alteration of tumor size prior to and after the treatment of CIK cells in vivo.

RESULTS

CIK-ICOS cells proliferated more and expressed higher secretion a level of interferon-γ than the controlled CIK. These cells exhibited higher cytotoxicity against cholangiocarcinoma cell lines at all efficacy: toxicity (E:T) ratios tested than the controlled CIK cells. More importantly, the anti-ICOSL antibody was able to attenuate the elevated cytotoxicity mediated by ICOS overexpression. When injected into cholangiocarcinoma xenografts in severe combined immunodeficiency mice, CIK-ICOS cells survived better than the controlled CIK cells around xenografts and significantly reduced the growth rate of cholangiocarcinoma, with least volume increase and more severe necrosis of the xenografts than controlled mice treated with saline, CIK or CIK-enhanced green fluorescent protein.

CONCLUSION

ICOS can enhance the cytotoxic effect of CIK cells against cholangiocarcinoma both in vitro and in vivo. This effect is mediated by ICOS-augmented cytokine secretion and cell proliferation, and in part through ICOS-ICOSL interaction.

Evidence implicating the Ras pathway in multiple CD28 costimulatory functions in CD4+ T cells

CD28 costimulation is a critical event in the full activation of CD4⁺ T cells that augments cytokine gene transcription, promotes cytokine mRNA stability, prevents induction of anergy, increases cellular metabolism, and increases cell survival. However, despite extensive biochemical analysis of the signaling events downstream of CD28, molecular pathways sufficient to functionally replace the diverse aspects of CD28-mediated costimulation in normal T cells have not been identified. Ras/MAPK signaling is a critical pathway downstream of T cell receptor stimulation, but its role in CD28-mediated costimulation has been controversial. We observed that physiologic CD28 costimulation caused a relocalization of the RasGEF RasGRP to the T cell-APC interface by confocal microscopy. In whole cell biochemical analysis, CD28 cross-linking with either anti-CD28 antibody or B7.1-Ig augmented TCR-induced Ras activation. To determine whether Ras signaling was sufficient to functionally mimic CD28 costimulation, we utilized an adenoviral vector encoding constitutively active H-Ras (61L) to transduce normal, Coxsackie-Adenovirus Receptor (CAR) transgenic CD4⁺ T cells. Like costimulation via CD28, active Ras induced AKT, JNK and ERK phosphorylation. In addition, constitutive Ras signaling mimicked the ability of CD28 to costimulate IL-2 protein secretion, prevent anergy induction, increase glucose uptake, and promote cell survival. Importantly, we also found that active Ras mimicked the mechanism by which CD28 costimulates IL-2 production: by increasing IL-2 gene transcription, and promoting IL-2 mRNA stability. Finally, active Ras was able to induce IL-2 production when combined with ionomycin stimulation in a MEK-1-dependent fashion. Our results are consistent with a central role for Ras signaling in CD28-mediated costimulation.

Biased binding of class IA phosphatidyl inositol 3-kinase subunits to inducible costimulator (CD278)

To better understand T lymphocyte costimulation by inducible costimulator (ICOS; H4; CD278), we analyzed proteins binding to ICOS peptides phosphorylated at the Y(191)MFM motif. Phosphorylated ICOS binds class IA phosphatidyl inositol 3-kinase (PI3-K) p85α, p50-55α and p85β regulatory subunits and p110α, p110δ and p110β catalytic subunits. Intriguingly, T cells expressed high levels of both p110α or p110δ catalytic subunits, yet ICOS peptides, cell surface ICOS or PI3-kinase class IA regulatory subunits preferentially coprecipitated p110α catalytic subunits. Silencing p110α or p110δ partially inhibited Akt/PKB activation induced by anti-CD3 plus anti-ICOS antibodies. However, silencing p110α enhanced and silencing p110δ inhibited Erk activation. Both p110α- and p110δ-specific inhibitors blocked cytokine secretion induced by TCR/CD3 activation with or without ICOS costimulus, but only p110α inhibitors blocked ICOS-induced cell elongation. Thus, p110α and p110δ are essential to optimal T cell activation, but their abundance and activity differentially tune up distinct ICOS signaling pathways.

Expression of a functional CCR2 receptor enhances tumor localization and tumor eradication by retargeted human T cells expressing a mesothelin-specific chimeric antibody receptor

PURPOSE

Adoptive T-cell immunotherapy with tumor infiltrating lymphocytes or genetically-modified T cells has yielded dramatic results in some cancers. However, T cells need to traffic properly into tumors to adequately exert therapeutic effects.

EXPERIMENTAL DESIGN

The chemokine CCL2 was highly secreted by malignant pleural mesotheliomas (MPM; a planned tumor target), but the corresponding chemokine receptor (CCR2) was minimally expressed on activated human T cells transduced with a chimeric antibody receptor (CAR) directed to the MPM tumor antigen mesothelin (mesoCAR T cells). The chemokine receptor CCR2b was thus transduced into mesoCAR T cells using a lentiviral vector, and the modified T cells were used to treat established mesothelin-expressing tumors.

RESULTS

CCR2b transduction led to CCL2-induced calcium flux and increased transmigration, as well as augmentation of in vitro T-cell killing ability. A single intravenous injection of 20 million mesoCAR + CCR2b T cells into immunodeficient mice bearing large, established tumors (without any adjunct therapy) resulted in a 12.5-fold increase in T-cell tumor infiltration by day 5 compared with mesoCAR T cells. This was associated with significantly increased antitumor activity.

CONCLUSIONS

CAR T cells bearing a functional chemokine receptor can overcome the inadequate tumor localization that limits conventional CAR targeting strategies and can significantly improve antitumor efficacy in vivo.

A novel association between filamin A and NF-κB inducing kinase couples CD28 to inhibitor of NF-κB kinase α and NF-κB activation

CD28 costimulatory molecule plays a critical role in the activation of NF-κB. Indeed, while stimulation of T cells with either professional APCs or anti-TCR plus anti-CD28 antibodies efficiently activates NF-κB, TCR alone fails to do that. Moreover, CD28 stimulation by B7 in the absence of TCR may activate IκB kinase α (IKKα) and a non-canonical NF-κB2-like pathway, in human primary CD4(+) T cells. Despite its functional relevance in NF-κB activation, the molecules connecting autonomous CD28-mediated signals to IKKα and NF-κB activation remain still unknown. In searching for specific upstream activators linking CD28 to the IKKα/NF-κB cascade, we identify a novel constitutive association between filamin A (FLNa) and the NF-κB inducing kinase (NIK), in both Jurkat and human primary T cells. Following CD28 engagement by B7, in the absence of TCR, FLNa-associated NIK is activated and induces IKKα kinase activity. Both proline (P(208)YAP(211)P(212)) and tyrosine residues (Y(206)QPY(209)APP) within the C-terminal proline-rich motif of CD28 are involved in the recruitment of FLNa/NIK complexes to the membrane as well as in the activation of NIK and IKKα.

B7-h2 is a costimulatory ligand for CD28 in human

CD28 and CTLA-4 are cell surface cosignaling molecules essential for the control of T cell activation upon the engagement of their ligands B7-1 and B7-2 from antigen-presenting cells. By employing a receptor array assay, we have demonstrated that B7-H2, best known as the ligand of inducible costimulator, was a ligand for CD28 and CTLA-4 in human, whereas these interactions were not conserved in mouse. B7-H2 and B7-1 or B7-2 interacted with CD28 through distinctive domains. B7-H2-CD28 interaction was essential for the costimulation of human T cells' primary responses to allogeneic antigens and memory recall responses. Similar to B7-1 and B7-2, B7-H2 costimulation via CD28 induced survival factor Bcl-xL, downregulated cell cycle inhibitor p27(kip1), and triggered signaling cascade of ERK and AKT kinase-dependent pathways. Our findings warrant re-evaluation of CD28 and CTLA-4's functions previously attributed exclusively to B7-1 and B7-2 and have important implications in therapeutic interventions against human diseases.

The inducible costimulator (ICOS) is critical for the development of human T(H)17 cells

Human T helper 17 (T(H)17) cells regulate host defense, autoimmunity, and tumor immunity. Although cytokines that control human T(H)17 cell development have been identified, the costimulatory molecules important for T(H)17 cell generation are unknown. Here, we found that the inducible costimulator (ICOS) was critical for the differentiation and expansion of human T(H)17 cells. Human cord blood contained a subset of CD161(+)CD4(+) T cells that were recent emigrants from the thymus, expressed ICOS constitutively, and were imprinted as T(H)17 cells through ICOS signaling. ICOS stimulation induced c-MAF, RORC2, and T-bet expression in these cells, leading to increased secretion of interleukin-21 (IL-21), IL-17, and interferon-γ (IFN-γ) compared with cells stimulated with CD28. Conversely, CD28 ligation abrogated ICOS costimulation, dampening RORC2 expression while promoting the expression of the aryl hydrocarbon receptor, which led to reduced secretion of IL-17 and enhanced production of IL-22 compared with cells stimulated with ICOS. Moreover, ICOS promoted the robust expansion of IL-17(+)IFN-γ(+) human T cells, and the antitumor activity of these cells after adoptive transfer into mice bearing large human tumors was superior to that of cells expanded with CD28. The therapeutic effectiveness of ICOS-expanded cells was associated with enhanced functionality and engraftment in vivo. These findings reveal a vital role for ICOS signaling in the generation and maintenance of human T(H)17 cells and suggest that components of this pathway could be therapeutically targeted to treat cancer or chronic infection and, conversely, that interruption of this pathway may have utility in multiple sclerosis and other autoimmune syndromes. These findings have provided the rationale for designing new clinical trials for tumor immunotherapy.

IFN-γ upregulates survivin and Ifi202 expression to induce survival and proliferation of tumor-specific T cells

BACKGROUND

A common procedure in human cytotoxic T lymphocyte (CTL) adoptive transfer immunotherapy is to expand tumor-specific CTLs ex vivo using CD3 mAb prior to transfer. One of the major obstacles of CTL adoptive immunotherapy is a lack of CTL persistence in the tumor-bearing host after transfer. The aim of this study is to elucidate the molecular mechanisms underlying the effects of stimulation conditions on proliferation and survival of tumor-specific CTLs.

METHODOLOGY/PRINCIPAL FINDINGS

Tumor-specific CTLs were stimulated with either CD3 mAb or cognate Ag and analyzed for their proliferation and survival ex vivo and persistence in tumor-bearing mice. Although both Ag and CD3 mAb effectively induced the cytotoxic effecter molecules of the CTLs, we observed that Ag stimulation is essential for sustained CTL proliferation and survival. Further analysis revealed that Ag stimulation leads to greater proliferation rates and less apoptosis than CD3 mAb stimulation. Re-stimulation of the CD3 mAb-stimulated CTLs with Ag resulted in restored CTL proliferative potential, suggesting that CD3 mAb-induced loss of proliferative potential is reversible. Using DNA microarray technology, we identified that survivin and ifi202, two genes with known functions in T cell apoptosis and proliferation, are differentially induced between Ag- and CD3 mAb-stimulated CTLs. Analysis of the IFN-γ signaling pathway activation revealed that Ag stimulation resulted in rapid phosphorylation of STAT1 (pSTAT1), whereas CD3 mAb stimulation failed to activate STAT1. Chromatin immunoprecipitation revealed that pSTAT1 is associated with the promoters of both survivin and ifi202 in T cells and electrophoresis mobility shift assay indicated that pSTAT1 directly binds to the gamma activation sequence element in the survivin and ifi202 promoters. Finally, silencing ifi202 expression significantly decreased T cell proliferation.

CONCLUSIONS/SIGNIFICANCE

Our findings delineate a new role of the IFN-γ signaling pathway in regulating T cell proliferation and apoptosis through upregulating survivin and ifi202 expression.

Topoisomerase inhibitors modulate expression of melanocytic antigens and enhance T cell recognition of tumor cells

While there are many obstacles to immune destruction of autologous tumors, there is mounting evidence that tumor antigen recognition does occur. Unfortunately, immune recognition rarely controls clinically significant tumors. Even the most effective immune response will fail if tumors fail to express target antigens. Importantly, reduced tumor antigen expression often results from changes in gene regulation rather than irrevocable loss of genetic information. Such perturbations are often reversible by specific compounds or biological mediators, prompting a search for agents with improved antigen-enhancing properties. Some recent findings have suggested that certain conventional chemotherapeutic agents may have beneficial properties for cancer treatment beyond their direct cytotoxicities against tumor cells. Accordingly, we screened an important subset of these agents, topoisomerase inhibitors, for their effects on antigen levels in tumor cells. Our analyses demonstrate upregulation of antigen expression in a variety of melanoma cell lines and gliomas in response to nanomolar levels of certain specific topoisomerase inhibitors. To demonstrate the ability of CD8+ T cells to recognize tumors, we assayed cytokine secretion in T cells transfected with T cell receptors directed against Melan-A/MART-1 antigen. Three days of daunorubicin treatment resulted in enhanced antigen expression by tumor cells, in turn inducing co-cultured antigen-specific T cells to secrete Interleukin-2 and Interferon-γ. These results demonstrate that specific topoisomerase inhibitors can augment melanoma antigen production, suggesting that a combination of chemotherapy and immunotherapy may be of potential value in the treatment of otherwise insensitive cancers.

Steric shielding of surface epitopes and impaired immune recognition induced by the ebola virus glycoprotein

Many viruses alter expression of proteins on the surface of infected cells including molecules important for immune recognition, such as the major histocompatibility complex (MHC) class I and II molecules. Virus-induced downregulation of surface proteins has been observed to occur by a variety of mechanisms including impaired transcription, blocks to synthesis, and increased turnover. Viral infection or transient expression of the Ebola virus (EBOV) glycoprotein (GP) was previously shown to result in loss of staining of various host cell surface proteins including MHC1 and β1 integrin; however, the mechanism responsible for this effect has not been delineated. In the present study we demonstrate that EBOV GP does not decrease surface levels of β1 integrin or MHC1, but rather impedes recognition by steric occlusion of these proteins on the cell surface. Furthermore, steric occlusion also occurs for epitopes on the EBOV glycoprotein itself. The occluded epitopes in host proteins and EBOV GP can be revealed by removal of the surface subunit of GP or by removal of surface N- and O- linked glycans, resulting in increased surface staining by flow cytometry. Importantly, expression of EBOV GP impairs CD8 T-cell recognition of MHC1 on antigen presenting cells. Glycan-mediated steric shielding of host cell surface proteins by EBOV GP represents a novel mechanism for a virus to affect host cell function, thereby escaping immune detection.

The Ebola virus (EBOV) is a highly pathogenic virus that infects humans and non-human primates, causing severe disease or death in the majority of these cases. The interaction of this virus with its host on a cellular level is only just beginning to be understood. EBOV, like many viruses, affects the expression or function of several cell surface proteins, including adhesion factors and protein complexes responsible for allowing the immune system to recognize infected cells. Our group and others have previously shown that expression of the main viral glycoprotein of EBOV in cultured cells is sufficient to cause this disruption. Here we have identified the mechanism by which this disruption occurs. Heavily glycosylated domains of the EBOV glycoprotein form a steric shield over proteins at the cell surface. This steric interference blocks the detection of affected surface proteins using antibody reagents, but also has the functional effect of abrogating cell adhesion and preventing interactions with CD8 T cells. The results from this study highlight a novel mechanism for viral disruption of host cell surface protein functions and give insight to interactions between the Ebola virus and its host.

Phosphoinositide 3-kinase activity in T cells regulates the magnitude of the germinal center reaction

The generation of high-affinity Abs is essential for immunity and requires collaboration between B and T cells within germinal centers (GCs). By using novel mouse models with a conditional deletion of the p110δ catalytic subunit of the PI3K pathway, we established that p110δ is required in T cells, but not in B cells, for the GC reaction. We found the formation of T follicular helper (T(FH)) cells to be critically dependent on p110δ in T cells. Furthermore, by deleting phosphatase and tensin homolog deleted on chromosome 10, which opposes p110δ in activated T cells, we found a positive correlation between increased numbers of T(FH) cells and GC B cells. These results are consistent with the hypothesis that T cell help is the limiting factor in the GC reaction. P110δ was not required for the expression of B cell lymphoma 6, the downregulation of CCR7, or T cell entry into primary follicles. Instead, p110δ was the critical catalytic subunit for ICOS downstream signaling and the production of key T(FH) cytokines and effector molecules. Our findings support a model in which the magnitude of the GC reaction is controlled by the activity of the PI3K pathway in T(FH) cells.

PI3Ks in lymphocyte signaling and development

Lymphocyte development and function are regulated by tyrosine kinase and G-protein coupled receptors. Each of these classes of receptors activates phosphoinositide 3-kinase (PI3K). In this chapter, we summarize current understanding of how PI3K contributes to key aspects of the adaptive immune system.

Analysis of CXCR3 and atypical variant expression and signalling in human T lymphocytes

Members of the chemokine (Chemotactic cytokines) superfamily and their receptors play a major role in trafficking of immune cells under homeostatic and inflammatory conditions. The chemokine receptor CXCR3 is expressed mainly on activated T lymphocytes and binds three pro-inflammatory, interferon-gamma-inducible chemokines: monokine induced by IFN-gamma (Mig/CXCL9), IFN-gamma-induced protein-10 (IP-10/CXCL10) and IFN-gamma-inducible T-cell alpha-chemoattractant (I-TAC/CXCL11). CXCR3 and its agonists are involved in a variety of inflammatory pathologies, making this receptor an attractive target for the design of new anti-inflammatory drugs. Interestingly, a growing body of evidence suggests the existence of at least two novel variants of CXCR3, namely CXCR3-B and CXCR3-alt, which present challenges in the design of new anti-inflammatory drugs targeting CXCR3. In this chapter, we describe the collection, isolation and activation of human peripheral blood-derived T lymphocytes and methods to examine the expression of CXCR3 and its atypical variants at both mRNA and protein levels, as well as protocols for exploring the biochemical and functional responses of T lymphocytes to all known CXCR3 agonists.

Inducible costimulator promotes helper T-cell differentiation through phosphoinositide 3-kinase

The T-cell costimulatory receptors, CD28 and the inducible costimulator (ICOS), are required for the generation of follicular B helper T cells (T(FH)) and germinal center (GC) reaction. A common signal transducer used by CD28 and ICOS is the phosphoinositide 3-kinase (PI3K). Although it is known that CD28-mediated PI3K activation is dispensable for GC reaction, the role of ICOS-driven PI3K signaling has not been defined. We show here that knock-in mice that selectively lost the ability to activate PI3K through ICOS had severe defects in T(FH) generation, GC reaction, antibody class switch, and antibody affinity maturation. In preactivated CD4(+) T cells, ICOS delivered a potent PI3K signal that was critical for the induction of the key T(FH) cytokines, IL-21 and IL-4. Under the same settings, CD28 was unable to activate PI3K but supported a robust secondary expansion of T cells. Thus, our results demonstrate a nonredundant function of ICOS-PI3K pathway in the generation of T(FH) and suggest that CD28 and ICOS play differential roles during a multistep process of T(FH) differentiation.

Long-term protection from syngeneic acute lymphoblastic leukemia by CpG ODN-mediated stimulation of innate and adaptive immune responses

Acute lymphoblastic leukemia (ALL) is the most common childhood cancer and remains a major cause of mortality in children with recurrent disease and in adults. Despite observed graft-versus-leukemia effects after stem cell transplantation, successful immune therapies for ALL have proven elusive. We previously reported immunostimulatory oligodeoxynucleotides containing CpG motifs (CpG ODN) enhance allogeneic T(h)1 responses and reduce leukemic burden of primary human ALL xenografts. To further the development of CpG ODN as a novel ALL therapy, we investigated the antileukemia activity induced by CpG ODN in a transplantable syngeneic pre-B ALL model. CpG ODN induced early killing of leukemia by innate immune effectors both in vitro and in vivo. Mice were treated with CpG ODN starting 7 days after injection with leukemia to mimic a minimal residual disease state and achieved T cell-dependent remissions of more than 6 months. In addition, mice in remission after CpG ODN treatment were protected from leukemia rechallenge, and adoptive transfer of T cells from mice in remission conferred protection against leukemia growth. To our knowledge, this is the first demonstration that CpG ODN induce a durable remission and ongoing immune-mediated protection in ALL, suggesting this treatment may have clinical utility in patients with minimal residual disease.

PD-1 signaling in primary T cells

SUMMARY

Programmed death-1 (PD-1) is a cell surface molecule that regulates the adaptive immune response. Engagement of PD-1 by its ligands PD-L1 or PD-L2 transduces a signal that inhibits T-cell proliferation, cytokine production, and cytolytic function. While a great deal is known concerning the biologic roles PD-1 plays in regulating the primary immune response and in T-cell exhaustion, comparatively little is known regarding how PD-1 ligation alters signaling pathways. PD-1 ligation is known to inhibit membrane-proximal T-cell signaling events, while ligation of the related inhibitory molecule cytotoxic T-lymphocyte antigen-4 appears to target more downstream signaling pathways. A major obstacle to an in-depth understanding of PD-1 signaling is the lack of physiologic models in which to study signal transduction. This review focuses on: (i) signaling pathways altered by PD-1 ligation, (ii) factors recruited upon PD-1 phosphorylation, and (iii) exploring the hypothesis that PD-1 ligation induces distinct signals during various stages of immune-cell differentiation. Lastly, we describe models to dissect the function of the PD-1 cytoplasmic tail using primary cells in the absence of agonist antibodies.

The role of costimulation in antibody deficiencies: ICOS and common variable immunodeficiency

SUMMARY

The identification of mutations in the inducible costimulator (ICOS) gene in nine patients with common variable immunodeficiency (CVID) was a major breakthrough. CVID is a complex, highly heterogeneous primary immunodeficiency disease, and the discovery of these mutations revealed a molecular basis. ICOS belongs to the CD28 family of costimulatory molecules and is expressed exclusively on activated T cells. It has at least three critical functions: germinal center formation, isotype class switching, and the development of memory B cells. The discovery of human ICOS deficiency showed that a monogenic disorder could account for the full spectrum of manifestations seen in childhood and adulthood-onset CVID, including autoimmune, inflammatory, and malignant disease complications, as well as recurrent infections. Moreover, this discovery showed that a disorder which had previously been perceived as a B-cell disease might in fact have its genetic origin in human T cells. In this article, we review the role of ICOS in the mammalian immune system and human disease, as well as the discovery and characteristics of patients with ICOS deficiency. Finally, we also discuss how these 'human knockouts' have contributed to our understanding of ICOS functions and have suggested potential avenues for using therapeutic ICOS manipulation to treat other diseases.

CD28 and ICOS play complementary non-overlapping roles in the development of Th2 immunity in vivo

Previous work has shown ICOS can function independently of CD28, but whether either molecule can compensate for the other in vivo is not known. Since ICOS is a potent inducer of Th2 cytokines and linked to allergy and elevated serum IgE in humans, we hypothesized that augmenting ICOS costimulation in murine allergic airway disease may overcome CD28 deficiency. While ICOS was expressed on T cells from CD28(-/-) mice, Th2-mediated airway inflammation was not induced in CD28(-/-) mice by increased ICOS costimulation. Further, we determined if augmenting CD28 costimulation could compensate for ICOS deficiency. ICOS(-/-) mice had a defect in airway eosinophilia that was not overcome by augmenting CD28 costimulation. CD28 costimulation also did not fully compensate for ICOS for antibody responses, germinal center formation or the development of follicular B helper T cells. CD28 and ICOS play complementary non-overlapping roles in the development of Th2 immunity in vivo.

Chimeric receptors containing CD137 signal transduction domains mediate enhanced survival of T cells and increased antileukemic efficacy in vivo

Persistence of T cells engineered with chimeric antigen receptors (CARs) has been a major barrier to use of these cells for molecularly targeted adoptive immunotherapy. To address this issue, we created a series of CARs that contain the T cell receptor-zeta (TCR-zeta) signal transduction domain with the CD28 and/or CD137 (4-1BB) intracellular domains in tandem. After short-term expansion, primary human T cells were subjected to lentiviral gene transfer, resulting in large numbers of cells with >85% CAR expression. In an immunodeficient mouse xenograft model of primary human pre-B-cell acute lymphoblastic leukemia, human T cells expressing anti-CD19 CARs containing CD137 exhibited the greatest antileukemic efficacy and prolonged (>6 months) survival in vivo, and were significantly more effective than cells expressing CARs containing TCR-zeta alone or CD28-zeta signaling receptors. We uncovered a previously unrecognized, antigen-independent effect of CARs expressing the CD137 cytoplasmic domain that likely contributes to the enhanced antileukemic efficacy and survival in tumor bearing mice. Furthermore, our studies revealed significant discrepancies between in vitro and in vivo surrogate measures of CAR efficacy. Together these results suggest that incorporation of the CD137 signaling domain in CARs should improve the persistence of CARs in the hematologic malignancies and hence maximize their antitumor activity.

Genetic engineering of T cells for adoptive immunotherapy

To be effective for the treatment of cancer and infectious diseases, T cell adoptive immunotherapy requires large numbers of cells with abundant proliferative reserves and intact effector functions. We are achieving these goals using a gene therapy strategy wherein the desired characteristics are introduced into a starting cell population, primarily by high efficiency lentiviral vector-mediated transduction. Modified cells are then expanded using ex vivo expansion protocols designed to minimally alter the desired cellular phenotype. In this article, we focus on strategies to (1) dissect the signals controlling T cell proliferation; (2) render CD4 T cells resistant to HIV-1 infection; and (3) redirect CD8 T cell antigen specificity.

Control of large, established tumor xenografts with genetically retargeted human T cells containing CD28 and CD137 domains

Mesothelin is a cell-surface molecule over-expressed on a large fraction of carcinomas, and thus is an attractive target of immunotherapy. A molecularly targeted therapy for these cancers was created by engineering T cells to express a chimeric receptor with high affinity for human mesothelin. Lentiviral vectors were used to express a single-chain variable fragment that binds mesothelin and that is fused to signaling domains derived from T-cell receptor zeta, CD28, and CD137 (4-1BB). When stimulated by mesothelin, lentivirally transduced T cells were induced to proliferate, express the antiapoptotic gene Bcl-X(L), and secrete multiple cytokines, all features characteristic of central memory T cells. When transferred intratumorally or intravenously into NOD/scid/IL2rgamma(-/-) mice engrafted with large pre-established tumors, the engineered T cells reduced the tumor burden, and in some cases resulted in complete eradication of the tumors at low effector-to-target ratios. Incorporation of the CD137 signaling domain specifically reprogrammed cells for multifunctional cytokine secretion and enhanced persistence of T cells. These findings have important implications for adoptive immunotherapy of cancer, especially in the context of poorly immunogenic tumors. Genetically redirected T cells have promise of targeting T lymphocytes to tumor antigens, confer resistance to the tumor microenvironment, and providing immunosurveillance.

Antigen-independent adhesion and cell spreading by inducible costimulator engagement inhibits T cell migration in a PI-3K-dependent manner

Engagement of the costimulatory protein ICOS activates effector/memory T cells in tissue by enhancing TCR-mediated proliferation and cytokine production. We now report that in an antigen-independent manner, ICOS also induces adhesion and spreading in human effector/memory T cells, consequently inhibiting cell migration. T cell spreading and elongation after ICOS ligation are accompanied by the formation of two types of actin-rich membrane protrusions: thin, finger-like structures similar to filopodia and short, discrete microspikes. Although filopodia/microspike formation occurs independently of the PI-3K signaling cascade, ICOS-mediated T cell elongation depends on PI-3K activity, which inhibits the accumulation of GTP-bound RhoA. Further inhibition of RhoA activation exacerbates the ICOS-mediated, elongated phenotype. We propose that in inflamed tissue, ICOS engagement by ICOS ligand on a professional or nonprofessional APC prevents the forward motility of the T cell by inhibiting RhoA-dependent uropod retraction. The resulting ICOS-induced T cell spreading and filopodia/microspike formation may promote antigen recognition by enhancing a T cell's scanning potential of an adherent APC surface.

The paracaspase MALT1 controls caspase-8 activation during lymphocyte proliferation

Caspase-8, an initiator caspase involved in lymphocyte apoptosis, is paradoxically required for lymphocyte proliferation. It is not understood how caspase-8 is controlled during antigenic signaling to allow for activation while averting the triggering of apoptosis. Here, we show that caspase-8 undergoes limited activation upon antigenic stimulation, and this activation is dependent on the paracaspase MALT1. The paracaspase domain of MALT1, in a protease-independent manner, induces caspase-8 activation through direct association. MALT1 diminishes the activation of apoptotic effector caspases, but it does not alter the activity of caspase-8 toward c-FLIP(L), which is required for antigenic signaling. Mutants of MALT1 that fail to activate caspase-8 and permit c-FLIP(L) cleavage cannot facilitate NF-kappaB activation or IL-2 induction. Our results reveal a mechanism that utilizes a protease potentially deadly to the cell for proliferative signaling and demonstrate a functional connection between the caspase and paracaspase families to enable nonapoptotic processes.

Mode of transmission affects the sensitivity of human immunodeficiency virus type 1 to restriction by rhesus TRIM5alpha

Rhesus TRIM5alpha (rhTRIM5alpha), but not human TRIM5alpha (huTRIM5alpha), potently inhibits human immunodeficiency virus (HIV) infection and is thus a potentially valuable therapeutic tool. Primary human CD4 T cells engineered to express rhTRIM5alpha were highly resistant to cell-free HIV type 1 (HIV-1) infection. However, when cocultured with unmodified T cells, rhTRIM5alpha-expressing cells became highly permissive to HIV-1 infection. Physical separation of rhTRIM5alpha-expressing cells and unmodified cells revealed that rhTRIM5alpha efficiently restricts cell-free but not cell-associated HIV transmission. Furthermore, we observed that HIV-infected human cells could infect rhesus CD4 T cells by cell-to-cell contact, but the infection was self-limiting. Subsequently, we noted that a spreading infection ensued when HIV-1-infected rhTRIM5alpha-expressing human cells were cultured with huTRIM5alpha- but not rhTRIM5alpha-expressing cells. Our results suggest that cell-associated HIV transmission in humans is blocked only when both donor and recipient cells express rhTRIM5alpha. These studies further define the role of rhTRIM5alpha in cell-free and cell-associated HIV transmission and delineate the utility of rhTRIM5alpha in anti-HIV therapy.

ICOS ligation recruits the p50alpha PI3K regulatory subunit to the immunological synapse

ICOS ligation in concert with TCR stimulation results in strong PI3K activation in T lymphocytes. The ICOS cytoplasmic tail contains an YMFM motif that binds the p85alpha subunit of class IA PI3K, similar to the YMNM motif of CD28, suggesting a redundant function of the two receptors in PI3K signaling. However, ICOS costimulation shows greater PI3K activity than CD28 in T cells. We show in this report that ICOS expression in activated T cells triggers the participation of p50alpha, one of the regulatory subunits of class IA PI3Ks. Using different T-APC cell conjugate systems, we report that p50alpha accumulates at the immunological synapse in activated but not in resting T cells. Our results demonstrate that ICOS membrane expression is involved in this process and that p50alpha plasma membrane accumulation requires a functional YMFM Src homology 2 domain-binding motif in ICOS. We also show that ICOS triggering with its ligand, ICOSL, induces the recruitment of p50alpha at the synapse of T cell/APC conjugates. In association with the p110 catalytic subunit, p50alpha is known to carry a stronger lipid kinase activity compared with p85alpha. Accordingly, we observed that ICOS engagement results in a stronger activation of PI3K. Together, these findings provide evidence that p50alpha is likely a determining factor in ICOS-mediated PI3K activity in T cells. These results also suggest that a differential recruitment and activity of class IA PI3K subunits represents a novel mechanism in the control of PI3K signaling by costimulatory molecules.

CD28 costimulation is essential for human T regulatory expansion and function

The costimulatory requirements required for peripheral blood T regulatory cells (Tregs) are unclear. Using cell-based artificial APCs we found that CD28 but not ICOS, OX40, 4-1BB, CD27, or CD40 ligand costimulation maintained high levels of Foxp3 expression and in vitro suppressive function. Only CD28 costimulation in the presence of rapamycin consistently generated Tregs that consistently suppressed xenogeneic graft-vs-host disease in immunodeficient mice. Restimulation of Tregs after 8-12 days of culture with CD28 costimulation in the presence of rapamycin resulted in >1000-fold expansion of Tregs in <3 wk. Next, we determined whether other costimulatory pathways could augment the replicative potential of CD28-costimulated Tregs. We observed that while OX40 costimulation augmented the proliferative capacity of CD28-costimulated Tregs, Foxp3 expression and suppressive function were diminished. These studies indicate that the costimulatory requirements for expanding Tregs differ from those for T effector cells and, furthermore, they extend findings from mouse Tregs to demonstrate that human postthymic Tregs require CD28 costimulation to expand and maintain potent suppressive function in vivo.

Ubiquitination mediated by inhibitor of apoptosis proteins

Inhibitor of apoptosis (IAP) proteins are a family of evolutionarily conserved proteins that regulate apoptosis as well as other cellular processes. The functions of many IAPs are defined by their RING domains, which possess E3 ubiquitin ligase activity and promote proteasomal degradation of an increasing number of target proteins. In this chapter, we describe the methods used in our laboratories to study the IAP's E3 activity.

Regulation of T-cell migration by co-stimulatory molecules

Migration of primed T-cells to the antigenic site is an essential event in the development of effective immunity. This process is tightly regulated in order to ensure efficient and specific responses. Most studies have focused on non-specific mediators of T-cell migration, including integrins and chemokines. However, recent studies have highlighted the key role of the T-cell receptor and co-stimulatory molecules in guiding T-cell access to antigenic tissue. Here, we review the experimental evidence for an essential contribution of co-stimulation-mediated molecular interactions regulating T-cell migration in the development of T-cell immunity and tolerance.

Cdc42-interacting protein-4 functionally links actin and microtubule networks at the cytolytic NK cell immunological synapse

An essential function of the immunological synapse (IS) is directed secretion. NK cells are especially adept at this activity, as they direct lytic granules to the synapse for secretion, which enables cytotoxicity and facilitates host defense. This initially requires rearrangement of the actin cytoskeleton and, subsequently, microtubule-dependent trafficking of the lytic granules. As these two steps are sequential, specific linkages between them are likely to serve as critical regulators of cytotoxicity. We studied Cdc42-interacting protein-4 (CIP4), which constitutively interacts with tubulin and microtubules but focuses to the microtubule organizing center (MTOC) after NK cell activation, when it is able to associate with Wiskott-Aldrich syndrome protein (WASp) and the actin filament-rich IS. WASp deficiency, overexpression of CIP4, or parts of CIP4 interfere with this union and block normal CIP4 localization, MTOC polarization to the IS, and cytotoxicity. Reduction of endogenous CIP4 expression using small interfering RNA similarly inhibits MTOC polarization and cytotoxic activity but does not impair actin filament accumulation at the IS, or Cdc42 activation. Thus, CIP4 is an important cytoskeletal adaptor that functions after filamentous actin accumulation and Cdc42 activation to enable MTOC polarization and NK cell cytotoxicity.