Molecular Imaging of PD-1 Unveils Unknown Characteristics of PD-1 Itself by Visualizing "PD-1 Microclusters"

Programmed cell death-1 (PD-1) is one of the most famous coinhibitory receptors that are expressed on effector T cells to regulate their function. The PD-1 ligands, PD-L1 and PD-L2, are expressed by various cells throughout the body at steady state and their expression was further regulated within different pathological conditions such as tumor-bearing and chronic inflammatory diseases. In recent years, immune checkpoint inhibitor (ICI) therapies with anti-PD-1 or anti-PD-L1 has become a standard treatment for various malignancies and has shown remarkable antitumor effects. Since the discovery of PD-1 in 1992, a huge number of studies have been conducted to elucidate the function of PD-1. Herein, this paper provides an overview of PD-1 biological findings and sheds some light on the current technology for molecular imaging of PD-1.

Evaluation of therapeutic PD-1 antibodies by an advanced single-molecule imaging system detecting human PD-1 microclusters

With recent advances in immune checkpoint inhibitors (ICIs), immunotherapy has become the standard treatment for various malignant tumors. Their indications and dosages have been determined empirically, taking individually conducted clinical trials into consideration, but without a standard method to evaluate them. Here we establish an advanced imaging system to visualize human PD-1 microclusters, in which a minimal T cell receptor (TCR) signaling unit co-localizes with the inhibitory co-receptor PD-1 in vitro. In these microclusters PD-1 dephosphorylates both the TCR/CD3 complex and its downstream signaling molecules via the recruitment of a phosphatase, SHP2, upon stimulation with the ligand hPD-L1. In this system, blocking antibodies for hPD-1-hPD-L1 binding inhibits hPD-1 microcluster formation, and each therapeutic antibody (pembrolizumab, nivolumab, durvalumab and atezolizumab) is characterized by a proprietary optimal concentration and combinatorial efficiency enhancement. We propose that our imaging system could digitally evaluate PD-1-mediated T cell suppression to evaluate their clinical usefulness and to develop the most suitable combinations among ICIs or between ICIs and conventional cancer treatments.

Abstract 2144: Programmed cell death 2 forms coinhibitory microclusters that directly attenuate T cell receptor signaling by recruiting the phosphatase SHP2

Background: Anti-PD-1 antibodies have made tremendous therapeutic effects on advanced or recurrent non-small cell lung cancer. However, the expression level of PD-L1 in tumor tissue which is the biomarker in the clinical setting is not necessarily correlated with their efficacy. In recent reports, some kinds of tumors express PD-L2 which is another ligand of PD-1. It is possible that the binding between PD-L2 and PD-1 is contributing to this mechanism. Because we intended to analyze the microstructural basis of immunological synapse, we used an imaging system combined with a planar lipid-bilayer incorporating GPI-anchored major histocompatibility complexes (MHCs) and the other ligands all those are required for T cell activation. We then found the "Microcluster", which is a clustering of receptors and their downstream signaling molecules, functioning as a signalosome for T cell antigen recognition and activation. Here, we imaged the behavior of the PD-1-PD-L2 signalosome recruiting a phosphatase and illustrated the machineries how PD-L2 suppress T cell activation in competition with PD-L1 toward PD-1 binding.

Methods: We purified GPI-anchored murine MHC, adhesion molecule and PD-L2 (mPD-L2) and reconstituted them into lipid-bilayer on a coverslip. Densities of these ligands were finely adjusted as same as those on typical antigen presenting cells (APCs) or tumor cells. Primary CD4+ T cell isolated from moss cytochrome C-specific TCR-Tg mice in Rag2−/- Pdcd1−/− background or T cell hybridoma expressing the same TCR (2D12) were retrovirally transduced with GFP- or HaloTag-tagged proteins (e.g. PD-1 and phosphatases SHP1/2) and imaged these proteins on the lipid-bilayers by confocal microscopy.

Results: We showed PD-1 is translocated to TCRs together forming microclusters and then accumulates at the central region of the immunological synapse in the presence of PD-L2. We also confirmed the rapid (20 seconds after TCR stimulation) and transient recruitment of SHP2, not SHP1, to PD-1 microclusters. PD-1-PD-L2 binding ware biochemically and biologically demonstrated to dephosphorylate the TCR downstream signaling molecules and introduce the reduction of IL-2 production, respectively, correlating with PD-1-PD-L2 microcluster formation. In the presence of both PD-L1 and PD-L2 on a lipid-bilayer, PD-L2 accumulated toward PD-1 microclusters much more stronger than PD-L1 did, suggesting that PD-L2 possesses higher affinity against PD-1 than PD-L1 if they were compared as an each single molecule. We further confirmed more effective suppression of IL-2 production by PD-1-PD-L2 binding.

Conclusions: Our results indicate that the function of PD-L1 and PD-L2 is almost the same in the point of suppressing TCR signaling by the recruitment of SHP2 to PD-1 signalosomes during the initiation of T cell activation. Contrary, PD-L2 more potently occupies PD-1 microclusters than PD-L1. We are now evaluating the effects of anti-PD-1/L1/L2 antibody for TCR signaling.

Citation Format: Tomohiro Takehara, Ei Wakamatsu, Hiroaki Machiyama, Kenzo Soejima, Koichi Fukunaga, Tadashi Yokosuka. Programmed cell death 2 forms coinhibitory microclusters that directly attenuate T cell receptor signaling by recruiting the phosphatase SHP2 [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 2144.

Abstract LB-057: Programmed cell death 2 forms coinhibitory microclusters that directly attenuate T cell receptor signaling by recruiting phosphatase SHP2

Background: Anti-PD-1 antibodies have made tremendous therapeutic effects on advanced or recurrent non-small cell lung cancer. However, the expression level of PD-L1 in tumor tissue which is the biomarker in the clinical setting is not necessarily correlated with their efficacy. In recent reports, some kinds of tumors express PD-L2 which is another ligand of PD-1.It is possible that the binding between PD-L2 and PD-1 is contributing to this mechanism. Because we intended to analyze the microstructural basis of immunological synapse, we used a system of a planar bilayer incorporated with glycophosphatidylinositol (GPI)-anchored intercellular adhesion molecule 1 (ICAM-1) and major histocompatibility complex (MHC) class II (I-Ek) loaded with a moth cytochrome c (MCC) peptide. Using our unique imaging analysis, we found the "TCR microcluster", where TCR proximal signaling molecules are recruited. In addition, PD-1 colocalizes with TCR microclusters in the presence of PD-L1 and SHP2 recruiting to PD-1 suppresses T cell activity by dephosphorylating the activated signaling molecules at TCR microclusters. Here, we investigated the PD-1-PD-L2 pathway using this dynamicimaging technique to reveal the molecular behavior of PD-1 bounded by PD-L2, not PD-L1.

Methods: We established tumor cell line (BHK) highly expressing murine PD-L2 (mPD-L2)-GPI and purified mPD-L2-GPI by affinity column with anti-PD-L2. Primary CD4+ T cell isolated from AND-Tg mice in Rag2-/- Pdcd1-/-background and T cell hybridoma expressing AND-TCR (2D12) were infected with retroviral supernatants of PD-1-GFP or GFP-SHP2. We used planar bilayers of mPD-L2-GPI, mICAM-1 and I-Ekloaded with MCC peptideand observed PD-1-GFP-transfected CD4+ T cell and GFP-SHP2-transfected 2D12 by confocal microscopy. Each GPI protein was reconstituted into the planar bilayer as the same density of these ligands expressed by LPS-stimulated B cells.

Results: We showed PD-1 is translocated to TCR microclusters and then accumulates at the central region of the immunological synapse in the presence of PD-L2. We also confirmed SHP2 is immediately but transiently recruited to PD-1-PD-L2 microclusters.

Conclusions: Our results indicate that both PD-L1 and PD-L2 suppress TCR signaling by the recruitment of SHP2 at the same signaling clusters for the initiation of T cell activation. Further experiments are ongoing to elucidate the mechanism of the individual effect for cancer immunotherapy by PD-1-, PD-L1- and/or PD-L2 blockade and also of the in vivo effects of each blocking antibody.

Citation Format: Tomohiro Takehara, Ei Wakamatsu, Hiroaki Machiyama, Hiroyuki Yasuda, Kenzo Soejima, Tadashi Yokosuka. Programmed cell death 2 forms coinhibitory microclusters that directly attenuate T cell receptor signaling by recruiting phosphatase SHP2 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr LB-057.

Inhibition of T cell activation and function by the adaptor protein CIN85

T cell activation is initiated by signaling molecules downstream of the T cell receptor (TCR) that are organized by adaptor proteins. CIN85 (Cbl-interacting protein of 85 kDa) is one such adaptor protein. Here, we showed that CIN85 limited T cell responses to TCR stimulation. Compared to activated wild-type (WT) T cells, those that lacked CIN85 produced more IL-2 and exhibited greater proliferation. After stimulation of WT T cells with their cognate antigen, CIN85 was recruited to the TCR signaling complex. Early TCR signaling events, such as phosphorylation of ζ-chain-associated protein kinase 70 (Zap70), Src homology 2 (SH2) domain-containing leukocyte protein of 76 kDa (SLP76), and extracellular signal-regulated kinase (Erk), were enhanced in CIN85-deficient T cells. The inhibitory function of CIN85 required the SH3 and PR regions of the adaptor, which associated with the phosphatase suppressor of TCR signaling-2 (Sts-2) after TCR stimulation. Together, our data suggest that CIN85 is recruited to the TCR signaling complex and mediates inhibition of T cell activation through its association with Sts-2.

Blockage of Core Fucosylation Reduces Cell-Surface Expression of PD-1 and Promotes Anti-tumor Immune Responses of T Cells

Programmed cell death 1 (PD-1) is highly expressed on exhausted T cells and inhibits T cell activation. Antibodies that block the interaction between PD-1 and its ligand prevent this inhibitory signal and reverse T cell dysfunction, providing beneficial anti-tumor responses in a substantial number of patients. Mechanisms for the induction and maintenance of high PD-1 expression on exhausted T cells have not been fully understood. Utilizing a genome-wide loss-of-function screening method based on the CRISPR-Cas9 system, we identified genes involved in the core fucosylation pathway as positive regulators of cell-surface PD-1 expression. Inhibition of Fut8, a core fucosyltransferase, by genetic ablation or pharmacologic inhibition reduced cell-surface expression of PD-1 and enhanced T cell activation, leading to more efficient tumor eradication. Taken together, our findings suggest that blocking core fucosylation of PD-1 can be a promising strategy for improving anti-tumor immune responses.

Role of interleukin-25 in development of spontaneous arthritis in interleukin-1 receptor antagonist-deficient mice

Interleukin (IL)-25, which is a member of the IL-17 family of cytokines, induces production of such Th2 cytokines as IL-4, IL-5, IL-9 and/or IL-13 by various types of cells, including Th2 cells, Th9 cells and group 2 innate lymphoid cells (ILC2). On the other hand, IL-25 can suppress Th1- and Th17-associated immune responses by enhancing Th2-type immune responses. Supporting this, IL-25 is known to suppress development of experimental autoimmune encephalitis, which is an IL-17-mediated autoimmune disease in mice. However, the role of IL-25 in development of IL-17-mediated arthritis is not fully understood. Therefore, we investigated this using IL-1 receptor antagonist-deficient (IL-1Ra^-/-) mice, which spontaneously develop IL-17-dependent arthritis. However, development of spontaneous arthritis (incidence rate, disease severity, proliferation of synovial cells, infiltration of PMNs, and bone erosion in joints) and differentiation of Th17 cells in draining lymph nodes in IL-25^-/- IL-1Ra^-/- mice were similar to in control IL-25^+/+ IL-1Ra^-/- mice. These observations indicate that IL-25 does not exert any inhibitory and/or pathogenic effect on development of IL-17-mediated spontaneous arthritis in IL-1Ra^-/- mice.

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IL-25 is known to inhibit Th17 cell differentiation.

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IL-25 is known to suppress Th17-mediated autoimmune diseases in mice.

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IL-25 does not play any inhibitory and/or pathogenic role in IL-17-mediated arthritis.

Micro-adhesion rings surrounding TCR microclusters are essential for T cell activation

Saito et al. describe a ring of focal adhesion molecules that surrounds T cell receptor microclusters and is essential for early T cell activation.

The immunological synapse (IS) formed at the interface between T cells and antigen-presenting cells represents a hallmark of initiation of acquired immunity. T cell activation is initiated at T cell receptor (TCR) microclusters (MCs), in which TCRs and signaling molecules assemble at the interface before IS formation. We found that each TCR-MC was transiently bordered by a ring structure made of integrin and focal adhesion molecules in the early phase of activation, which is similar in structure to the IS in microscale. The micro–adhesion ring is composed of LFA-1, focal adhesion molecules paxillin and Pyk2, and myosin II (MyoII) and is supported by F-actin core and MyoII activity through LFA-1 outside-in signals. The formation of the micro–adhesion ring was transient but especially sustained upon weak TCR stimulation to recruit linker for activation of T cells (LAT) and SLP76. Perturbation of the micro–adhesion ring induced impairment of TCR-MC development and resulted in impaired cellular signaling and cell functions. Thus, the synapse-like structure composed of the core TCR-MC and surrounding micro–adhesion ring is a critical structure for initial T cell activation through integrin outside-in signals.

Protein kinase C-η controls CTLA-4-mediated regulatory T cell function

Regulatory T cells (T_reg cells), which maintain immune homeostasis and self-tolerance, form an immunological synapse (IS) with antigen-presenting cells (APCs). However, signaling events at the T_reg IS remain unknown. Here we show that protein kinase C-η (PKC-η) associated with CTLA-4 and was recruited to the T_reg IS. PKC-η-deficient T_reg cells displayed defective suppressive activity, including suppression of tumor immunity but not autoimmune colitis. Phosphoproteomic analysis revealed an association between CTLA-4-PKC-η and the GIT-PIX-PAK complex, an IS-localized focal adhesion complex. Defective activation of this complex in PKC-η-deficient T_reg cells was associated with reduced CD86 depletion from APCs by T_reg cells. These results reveal a novel CTLA-4-PKC-η signaling axis required for contact-dependent suppression, implicating this pathway as a potential cancer immunotherapy target.

The lymphoid lineage-specific actin-uncapping protein Rltpr is essential for costimulation via CD28 and the development of regulatory T cells

Although T cell activation can result from signaling via T cell antigen receptor (TCR) alone, physiological T cell responses require costimulation via the coreceptor CD28. Through the use of an N-ethyl-N-nitrosourea-mutagenesis screen, we identified a mutation in Rltpr. We found that Rltpr was a lymphoid cell-specific, actin-uncapping protein essential for costimulation via CD28 and the development of regulatory T cells. Engagement of TCR-CD28 at the immunological synapse resulted in the colocalization of CD28 with both wild-type and mutant Rltpr proteins. However, the connection between CD28 and protein kinase C-θ and Carma1, two key effectors of CD28 costimulation, was abrogated in T cells expressing mutant Rltpr, and CD28 costimulation did not occur in those cells. Our findings provide a more complete model of CD28 costimulation in which Rltpr has a key role.

Programmed cell death 1 forms negative costimulatory microclusters that directly inhibit T cell receptor signaling by recruiting phosphatase SHP2

After encounter with its ligand, PD-1 translocates into TCR microclusters, where it transiently recruits SHP2 and suppresses phosphorylation of TCR signaling components and TCR-driven stop signals.

Programmed cell death 1 (PD-1) is a negative costimulatory receptor critical for the suppression of T cell activation in vitro and in vivo. Single cell imaging elucidated a molecular mechanism of PD-1–mediated suppression. PD-1 becomes clustered with T cell receptors (TCRs) upon binding to its ligand PD-L1 and is transiently associated with the phosphatase SHP2 (Src homology 2 domain–containing tyrosine phosphatase 2). These negative costimulatory microclusters induce the dephosphorylation of the proximal TCR signaling molecules. This results in the suppression of T cell activation and blockade of the TCR-induced stop signal. In addition to PD-1 clustering, PD-1–TCR colocalization within microclusters is required for efficient PD-1–mediated suppression. This inhibitory mechanism also functions in PD-1^hi T cells generated in vivo and can be overridden by a neutralizing anti–PD-L1 antibody. Therefore, PD-1 microcluster formation is important for regulation of T cell activation.

A motif in the V3 domain of the kinase PKC-θ determines its localization in the immunological synapse and functions in T cells via association with CD28

Protein kinase C-θ (PKC-θ) translocates to the center of the immunological synapse, but the underlying mechanism and its importance in T cell activation are unknown. Here we found that the V3 domain of PKC-θ was necessary and sufficient for localization to the immunological synapse mediated by association with the coreceptor CD28 and dependent on the kinase Lck. We identified a conserved proline-rich motif in V3 required for association with CD28 and immunological synapse localization. We found association with CD28 to be essential for PKC-θ-mediated downstream signaling and the differentiation of T helper type 2 cells (T(H)2 cells) and interleukin 17-producing helper T cells (T(H)17 cells) but not of T helper type 1 cells (T(H)1 cells). Ectopic expression of V3 sequestered PKC-θ from the immunological synapse and interfered with its functions. Our results identify a unique mode of CD28 signaling, establish a molecular basis for the immunological synapse localization of PKC-θ and indicate V3-based 'decoys' may be therapeutic modalities for T cell-mediated inflammatory diseases.

Dynein-driven transport of T cell receptor microclusters regulates immune synapse formation and T cell activation

When T cells recognize a peptide-major histocompatibility complex on antigen-presenting cells (APCs), T cell receptor microclusters (TCR-MCs) are generated and move to the center of the T cell-APC interface to form the central supramolecular activation cluster (cSMAC). cSMAC formation depends on stimulation strength and regulates T cell activation. We demonstrate that the dynein motor complex colocalized and coimmunoprecipitated with the TCR complex and that TCR-MCs moved along microtubules (MTs) toward the center of the immune synapse in a dynein-dependent manner to form cSMAC. MTs are located in close proximity to the plasma membrane at the activation site. TCR-MC velocity and cSMAC formation were impaired by dynein or MT inhibitors or by ablation of dynein expression. T cells with impaired cSMAC formation exhibited enhanced cellular activation including protein phosphorylation and interleukin-2 production. These results indicate that cSMAC formation by TCR-MC movement depends on dynein and MTs, and the movement regulates T cell activation.

CIN85 drives B cell responses by linking BCR signals to the canonical NF-kappaB pathway

CIN85 transduces B cell receptor signals to IKK-β, and its expression in B cells is essential for T cell–independent type II antibody responses in mice.

CIN85, an adaptor protein which binds the C-terminal domain of tyrosine phosphorylated Cbl and Cbl-b, has been thought to be involved in the internalization and subsequent degradation of receptors. However, its physiological function remains unclear. To determine its role in B cells, we used Mb1-cre to generate mice with a B cell–specific deletion of CIN85. These mice had impaired T cell–independent type II antibody responses in vivo and diminished IKK-β activation and cellular responses to B cell receptor (BCR) cross-linking in vitro. Introduction of a constitutively active IKK-β construct corrected the defective antibody responses as well as cellular responses in the mutant mice. Together, our results suggest that CIN85 links the BCR to IKK-β activation, thereby contributing to T cell–independent immune responses.

Dynamic regulation of T cell activation and co-stimulation through TCR-microclusters

TCR-microclusters (MC) are generated upon TCR stimulation prior to the immune synapse formation independently of lipid rafts. TCR-MCs contain receptors, kinases and adaptors, and function as the signaling unit for T cell activation. The TCR complex, but not the signaling molecules, is transported to the center to form cSMAC. The co-stimulation receptor CD28 joins the signaling region of cSMAC and recruits PKCθ and Carma1. CTLA-4 accumulates in the same region and competes with CD28 for negative regulation of T cell activation. T cell activation is therefore mediated by two spatially distinct signaling compartments: TCR signaling by the peripheral TCR-MC and co-stimulation signal by the central signaling cSMAC.

Spatiotemporal basis of CTLA-4 costimulatory molecule-mediated negative regulation of T cell activation

T cell activation is positively and negatively regulated by a pair of costimulatory receptors, CD28 and CTLA-4, respectively. Because these receptors share common ligands, CD80 and CD86, the expression and behavior of CTLA-4 is critical for T cell costimulation regulation. However, in vivo blocking of CD28-mediated costimulation by CTLA-4 and its mechanisms still remain elusive. Here, we demonstrate the dynamic behavior of CTLA-4 in its real-time competition with CD28 at the central-supramolecular activation cluster (cSMAC), resulting in the dislocalization of protein kinase C-θ and CARMA1 scaffolding protein. CTLA-4 translocation to the T cell receptor microclusters and the cSMAC is tightly regulated by its ectodomain size, and its accumulation at the cSMAC is required for its inhibitory function. The CTLA-4-mediated suppression was demonstrated by the in vitro anergy induction in regulatory T cells constitutively expressing CTLA-4. These results show the dynamic mechanism of CTLA-4-mediated T cell suppression at the cSMAC.

CRTAM confers late-stage activation of CD8+ T cells to regulate retention within lymph node

In vivo immune response is triggered in the lymph node, where lymphocytes for entry into, retention at, and migration to effector sites are dynamically regulated. The molecular mechanism underlying retention regulation is the key to elucidating in vivo regulation of immune response. In this study, we describe the function of the adhesion molecule class I-restricted T cell-associated molecule (CRTAM) in regulating CD8+ T cell retention within the lymph node and eventually effector function. We previously identified CRTAM as a receptor predominantly expressed on activated CD8+ T cells, and nectin-like molecule-2 (Necl2) as its ligand. In vivo function of CRTAM-Necl2 interaction was analyzed by generating CRTAM(-/-) mice. CRTAM(-/-) mice exhibited reduced protective immunity against viral infection and impaired autoimmune diabetes induction in vivo. Although Ag-specific CRTAM(-/-) CD8+ T cells showed normal CTL functions in vitro, their number in the draining lymph node was reduced. Because CRTAM+ T cells bound efficiently to Necl2-expressing CD8+ dendritic cells (DCs) that reside in T cell area of lymph node, CRTAM may induce retention by binding to CD8+ DCs at the late stage of activation before proliferation. The CRTAM-mediated late interaction with DCs induced retention of activated CD8+ T cells in an Ag-independent fashion, and this possibly resulted in effective CTL development in the draining lymph node.

Dynamic regulation of T-cell costimulation through TCR-CD28 microclusters

SUMMARY

T-cell activation requires contact between T cells and antigen-presenting cells (APCs) to bring T-cell receptors (TCRs) and major histocompatibility complex peptide (MHCp) together to the same complex. These complexes rearrange to form a concentric circular structure, the immunological synapse (IS). After the discovery of the IS, dynamic imaging technologies have revealed the details of the IS and provided important insights for T-cell activation. We have redefined a minimal unit of T-cell activation, the 'TCR microcluster', which recognizes MHCp, triggers an assembly of assorted molecules downstream of the TCR, and induces effective signaling from TCRs. The relationship between TCR signaling and costimulatory signaling was analyzed in terms of the TCR microcluster. CD28, the most valuable costimulatory receptor, forms TCR-CD28 microclusters in cooperation with TCRs, associates with protein kinase C theta, and effectively induces initial T-cell activation. After mature IS formation, CD28 microclusters accumulate at a particular subregion of the IS, where they continuously assemble with the kinases and not TCRs, and generate sustained T-cell signaling. We propose here a 'TCR-CD28 microcluster' model in which TCR and costimulatory microclusters are spatiotemporally formed at the IS and exhibit fine-tuning of T-cell responses by assembling with specific players downstream of the TCR and CD28.

Spatiotemporal regulation of T cell costimulation by TCR-CD28 microclusters and protein kinase C theta translocation

T cell activation is mediated by microclusters (MCs) containing T cell receptors (TCRs), kinases, and adaptors. Although TCR MCs translocate to form a central supramolecular activation cluster (cSMAC) of the immunological synapse at the interface of a T cell and an antigen-presenting cell, the role of MC translocation in T cell signaling remains unclear. Here, we found that the accumulation of MCs at cSMAC was important for T cell costimulation. Costimulatory receptor CD28 was initially recruited coordinately with TCR to MCs, and its signals were mediated through the assembly with the kinase PKCtheta. The accumulation of MCs at the cSMAC was accompanied by the segregation of CD28 from the TCR, which resulted in the translocation of both CD28 and PKCtheta to a spatially unique subregion of cSMAC. Thus, costimulation is mediated by the generation of a unique costimulatory compartment in the cSMAC via the dynamic regulation of MC translocation.

LFA-1 decreases the antigen dose for T cell activation in vivo

Leukocyte adhesion molecule leukocyte function-associated antigen (LFA)-1 not only mediates intercellular binding but also delivers co-stimulatory signals in T cells. LFA-1 has been shown to decrease the threshold of TCR signal and an antigen dose required for T cell activation and proliferation in vitro. However, physiological significance of the role of LFA-1 in TCR signal has remained unclear. We examined whether LFA-1 decreased the antigen dose for T cell activation in vivo. We showed here that, although collagen-induced arthritis (CIA) could not be induced by immunization and challenge with a standard amount of type-II collagen in LFA-1-deficient mice, a higher dose of the antigen did induce CIA in the absence of LFA-1. We also showed that CD4+ T cells could be primed by immunization with a high, but not low, dose of ovalbumin antigen in LFA-1-deficient mice. These results suggest that LFA-1 decreases the threshold of TCR signal for T cell activation in vivo as well as in vitro. Further studies using TCR-transgenic LFA-1-deficient mice showed that LFA-1 cooperated with TCR in sustained Erk1/2 phosphorylation. Moreover, TCR could induce sustained Erk1/2 phosphorylation in the absence of LFA-1 when T cells were stimulated with a high, but not low, dose of antigen, suggesting that LFA-1 may cooperate with TCR in sustaining Erk1/2 phosphorylation.

Genetic analysis reveals an intrinsic property of the germinal center B cells to generate A:T mutations

The immunoglobulin genes undergo a high frequency of point mutations at both C:G and A:T pairs in the germinal center (GC) B cells. This hypermutation process is initiated by the activation-induced cytidine deaminase (AID), which converts cytosine to uracil and generates a U:G lesion. Replication of this lesion, or its repair intermediate the abasic site, could introduce C:G mutations but the mechanisms leading to mutations at non-damaged A:T pairs remain elusive. Using a lacZ-transgenic system in which endogenous genome mutations can be detected with high sensitivity, we found that GC B cells exhibited a much higher ratio of A:T mutations as compared to naïve B, non-GC B, and cells of other tissues. This property does not require AID or active transcription of the target gene, and is dependent on DNA polymerase eta. These in vivo results demonstrate that GC B cells are unique in having an intrinsic propensity to generate A:T mutations during repair of endogenous DNA damage. These findings have important implications in understanding how AID, which can only target C:G base pairs, is able to induce the entire spectrum of mutations observed in immunoglobulin variable region genes in GC B cells.

Cloning of B cell-specific membrane tetraspanning molecule BTS possessing B cell proliferation-inhibitory function

Lymphocyte proliferation is regulated by signals through antigen receptors, co-stimulatory receptors, and other positive and negative modulators. Several membrane tetraspanning molecules are also involved in the regulation of lymphocyte growth and death. We cloned a new B cell-specific tetraspanning (BTS) membrane molecule, which is similar to CD20 in terms of expression, structure and function. BTS is specifically expressed in the B cell line and its expression is increased after the pre-B cell stage. BTS is expressed in intracellular granules and on the cell surface. Overexpression of BTS in immature B cell lines induces growth retardation through inhibition of cell cycle progression and cell size increase without inducing apoptosis. This inhibitory function is mediated predominantly by the N terminus of BTS. The development of mature B cells is inhibited in transgenic mice expressing BTS, suggesting that BTS is involved in the in vivo regulation of B cells. These results indicate that BTS plays a role in the regulation of cell division and B cell growth.

T cell receptor-proximal signals are sustained in peripheral microclusters and terminated in the central supramolecular activation cluster

T cell receptor (TCR) signaling is initiated and sustained in microclusters; however, it's not known whether signaling also occurs in the TCR-rich central supramolecular activation cluster (cSMAC). We showed that the cSMAC formed by fusion of microclusters contained more CD45 than microclusters and is a site enriched in lysobisphosphatidic acid, a lipid involved in sorting ubiquitinated membrane proteins for degradation. Calcium signaling via TCR was blocked within 2 min by anti-MHCp treatment and 1 min by latrunculin-A treatment. TCR-MHCp interactions in the cSMAC survived these perturbations for 10 min and hence were not sufficient to sustain signaling. TCR microclusters were also resistant to disruption by anti-MHCp and latrunculin-A treatments. We propose that TCR signaling is sustained by stabilized microclusters and is terminated in the cSMAC, a structure from which TCR are sorted for degradation. Our studies reveal a role for F-actin in TCR signaling beyond microcluster formation.

Immunological synapse and microclusters: the site for recognition and activation of T cells

An immunological synapse (IS) is formed at the interface between antigen-presenting cells and T cells, and is believed to be the structure responsible for antigen recognition and T-cell activation. However, recent imaging analyses reveal that T-cell receptor microclusters (MCs) formed prior to IS are the site for antigen recognition and T-cell activation. MCs are continuously generated at the periphery of the interface, even after IS formation, and induce sustained activation signals. MC formation is not accompanied by lipid-raft clustering. Central supramolecular activation cluster is considered functional in recycling and degradation of T-cell receptors, directional secretion of cytokines and cytolytic granules, generation of sustained signals, or maintenance of the cell-cell conjugation.

A critical role for the innate immune signaling molecule IRAK-4 in T cell activation

IRAK-4 is a protein kinase that is pivotal in mediating signals for innate immune responses. Here, we report that IRAK-4 signaling is also essential for eliciting adaptive immune responses. Thus, in the absence of IRAK-4, in vivo T cell responses were significantly impaired. Upon T cell receptor stimulation, IRAK-4 is recruited to T cell lipid rafts, where it induces downstream signals, including protein kinase C activation through the association with Zap70. This signaling pathway was found to be required for optimal activation of nuclear factor kappaB. Our findings suggest that T cells use this critical regulator of innate immunity for the development of acquired immunity, suggesting that IRAK-4 may be involved in direct signal cross talk between the two systems.

Newly generated T cell receptor microclusters initiate and sustain T cell activation by recruitment of Zap70 and SLP-76

T cell receptor (TCR) activation and signaling precede immunological synapse formation and are sustained for hours after initiation. However, the precise physical sites of the initial and sustained TCR signaling are not definitively known. We report here that T cell activation was initiated and sustained in TCR-containing microclusters generated at the initial contact sites and the periphery of the mature immunological synapse. Microclusters containing TCRs, the tyrosine kinase Zap70 and the adaptor molecule SLP-76 were continuously generated at the periphery. TCR microclusters migrated toward the central supramolecular cluster, whereas Zap70 and SLP-76 dissociated from these microclusters before the microclusters coalesced with the TCR-rich central supramolecular cluster. Tyrosine phosphorylation and calcium influx were induced as microclusters formed at the initial contact sites. Inhibition of signaling prevented recruitment of Zap70 into the microclusters. These results indicated that TCR-rich microclusters initiate and sustain TCR signaling.

Heterotypic interaction of CRTAM with Necl2 induces cell adhesion on activated NK cells and CD8+ T cells

NK cells and CD8+ T cells exhibit cytotoxicity and cytokine production upon recognizing target cells through cell-cell interaction. We screened the molecules involved in the recognition and regulation of these cells using cDNA subtraction between naive and activated NK cells. We identified class I-restricted T cell-associated molecule (CRTAM), a two Ig domain-bearing surface receptor, as a molecule rapidly and transiently expressed on NK cells and CD8+ T cells upon activation. CRTAM is expressed as a dimer on the cell surface, and its expression is transcriptionally regulated. Using an expression-cloning system, we then further identified Nectin-like (Necl) molecule 2, a three Ig domain-containing receptor, as a ligand of CRTAM. While Necl2 mediates homotypic interaction, CRTAM interacts with Necl2 but not with CRTAM itself. The heterotypic CRTAM-Necl2 interaction has a higher affinity than the homotypic Necl2 interaction. Although there was no clear alteration in the cytotoxic function of the NK cells and CD8+ T cells against the Necl2-expressing target cells, T cells expressing CRTAM tightly bound to Necl2-expressing cells. CRTAM+ cells did not induce homotypic aggregation but they did exert strong heterotypic binding with Necl2+ cells, which was inhibited by the addition of the CRTAM-Ig fusion protein. These results suggest that the heterotypic interaction between CRTAM and Necl2 plays an important role in the adhesion, interaction or migration of NK cells and CD8+ T cells upon stimulation.

In vivo overexpression of CTLA-4 suppresses lymphoproliferative diseases and thymic negative selection

Cytotoxic T lymphocyte antigen-4 (CTLA-4) induces major inhibitory signals for T cell activation. From analyses of TCR-transgenic (Tg) CTLA-4-deficient mice, it has been believed that CTLA-4 does not affect thymocyte development. To focus upon the in vivo function of CTLA-4 in thymocyte development from a different aspect, we have established Tg mice expressing either full-length CTLA-4 (FL-Tg) or a mutant CTLA-4 lacking the cytoplasmic region (truncated, TR-Tg), and analyzed thymocyte development. TR-T cells express much higher CTLA-4 on the cell surface than FL-T cells, in which most CTLA-4 was localized in intracellular vesicles. While CTLA-4-/- mice exhibit lymphoproliferative disease, neither of the Tg mice with CTLA-4-/- background developed the disorder. Although the development of thymocytes appeared normal in both Tg mice, in vivo depletion of double-positive thymocytes by injection of anti-CD3 Ab as well as the elimination of minor lymphocyte-stimulating antigen-reactive thymocytes were impaired in FL-Tg mice but not in TR-Tg mice. Functionally, cross-linking of CTLA-4 on thymocytes from FL-Tg mice, but not from TR-Tg mice, inhibited proliferation. These results reveal a potential role of CTLA-4, through its cytoplasmic domain, in the negative selection of thymocytes and in the prevention of lymphoproliferative disease.

CD25(+)CD4(+) regulatory T cells exert in vitro suppressive activity independent of CTLA-4

Cytotoxic T lymphocyte antigen-4 (CTLA-4) is constitutively expressed on CD25(+)CD4(+) regulatory T cells (Treg) and is suggested to play a role in Treg-mediated suppression. However, the results of analysis with anti-CTLA-4 have been controversial. We addressed this issue by analyzing mice over-expressing or deficient in CTLA-4. For over-expression, CTLA-4 transgenic mice expressing a full-length (FL) or a truncated (TL) mutant of CTLA-4 were analyzed. FL T cells expressed similar levels of CTLA-4 to Treg, whereas TL T cells expressed much higher levels on the cell surface. The number of Treg in both mice was decreased, although Foxp3 expression was not altered. Treg from both mice exerted suppressive activity, whereas CD25(-) T cells from FL mice showed no suppression. Furthermore, CD25(+)CD4 thymocytes from young CTLA-4-deficient mice were analyzed and found to exhibit suppressive activity. These results indicate that Treg exert in vitro suppressive activity independent of CTLA-4 expression.

NFAM1, an immunoreceptor tyrosine-based activation motif-bearing molecule that regulates B cell development and signaling

A functional cDNA cloning system was developed by using a retrovirus library encoding CD8-chimeric proteins and a nuclear factor of activated T cells (NFAT)-GFP reporter cell line to identify molecules inducing NFAT activation. By using this strategy, NFAT activating molecule 1 (NFAM1) was cloned as an immunoreceptor tyrosine-based activation motif (ITAM)-bearing cell surface molecule belonging to the Ig superfamily and is predominantly expressed in spleen B and T cells. NFAM1 crosslinking induced ITAM phosphorylation, ZAP-70/Syk recruitment, NFAT activation, and cytokine production. In vivo overexpression of NFAM1 in bone marrow chimeras and transgenic mice induced severe impairment of early B cell development in an ITAM-dependent manner. In NFAM1-expressing B cells, B cell antigen receptor stimulation induced NFAM1 translocation to lipid raft, and NFAM1 co-crosslinking augmented B cell antigen receptor signaling. The results suggest that NFAM1 modulates B cell signaling through its ITAM, which regulates B cell development.

[Natural evolution of coronary artery ectasia after coronary artery bypass grafting]

Coronary artery aneurysm and ectasia, unusual angiographic findings, are considered as variant of atherosclerotic coronary artery disease. A 49-year-old man whose right coronary artery ectasia had progressed to large aneurysm, accompanied by advanced obstructive coronary artery disease, 6 years after the initial coronary artery bypass grafting. It was treated with ligation of aneurysm and distal bypass grafting under cardiopulmonary bypass. Fragile fresh clot was formed within the aneurysm irrespective of coumadin therapy as a standard regimen for the coronary artery ectasias. We will discuss the surgical management for the dilated coronary artery with significant coronary stenosis.

IgE-mediated activation of NK cells through Fc gamma RIII

NK cells express Fc gamma RIII (CD16), which is responsible for IgG-dependent cell cytotoxicity and for production of several cytokines and chemokines. Whereas Fc gamma RIII on NK cells is composed of both Fc gamma RIII alpha and FcR gamma chains, that on mast cells is distinct from NK cells and made of Fc gamma RIII alpha, FcR beta, and FcR gamma. Mast cells show degranulation and release several mediators, which cause anaphylactic responses upon cross-linking of Fc gamma RIII as well as Fc epsilon RI with aggregated IgE. In this paper, we examined whether IgE activates NK cells through Fc gamma RIII on their cell surface. We found that NK cells produce several cytokines and chemokines related to an allergic reaction upon IgE stimulation. Furthermore, NK cells exhibited cytotoxicity against IgE-coated target cells in an Fc gamma RIII-dependent manner. These effects of IgE through Fc gamma RIII were not observed in NK cells from FcR gamma-deficient mice lacking Fc gamma RIII expression. Collectively, these results demonstrate that NK cells can be activated with IgE through Fc gamma RIII and exhibit both cytokine/chemokine production and Ab-dependent cell cytotoxicity. These data imply that not only mast cells but also NK cells may contribute to IgE-mediated allergic responses.

Predominant role of T cell receptor (TCR)-alpha chain in forming preimmune TCR repertoire revealed by clonal TCR reconstitution system

The CDR3 regions of T cell receptor (TCR)-alpha and -beta chains play central roles in the recognition of antigen (Ag)-MHC complex. TCR repertoire is created on the basis of Ag recognition specificity by CDR3s. To analyze the potential spectrum of TCR-alpha and -beta to exhibit Ag specificity and generate TCR repertoire, we established hundreds of TCR transfectants bearing a single TCR-alpha or -beta chain derived from a cytotoxic T cell (CTL) clone, RT-1, specific for HIVgp160 peptide, and randomly picked up TCR-beta or -alpha chains. Surprisingly, one-third of such TCR-beta containing random CDR3 beta from naive T cells of normal mice could reconstitute the antigen-reactive TCR coupling with RT-1 TCR-alpha. A similar dominant function of TCR-alpha in forming Ag-specific TCR, though low-frequency, was obtained for lymphocytic choriomeningitis virus-specific TCR. Subsequently, we generated TCR-alpha and/or -beta transgenic (Tg) mice specific for HIVgp160 peptide, and analyzed the TCR repertoire of Ag-specific CTLs. Similar to the results from TCR reconstitution, TCR-alpha Tg generated CTLs with heterogeneous TCR-beta, whereas TCR-beta Tg-induced CTLs bearing a single TCR-alpha. These findings of Ag recognition with minimum involvement of CDR3 beta expand our understanding regarding the flexibility of the spectrum of TCR and suggest a predominant role of TCR-alpha chain in determining the preimmune repertoire of Ag-specific TCR.

Washing of the residual solution of cardiopulmonary bypass circuit after coronary artery bypass grafting in idiopathic thrombocytopenic purpura

A 76-year-old female with chronic idiopathic thrombocytopenic purpura required coronary bypass grafting. Preoperative treatment with high-dose intravenous immunoglobulin at a dose of 0.4 g/kg/day raised the platelet count from 57,000 to 110,000/microL. After termination of cardiopulmonary bypass (CPB) the residual blood in the CPB circuit was washed to reduce total immunoglobulin G (IgG) level, including platelet-associated immunoglobulin G (PA-IgG), and returned to the patient. Intraoperative platelet transfusion was used due to a drop in platelet count to the pretreatment level. The postoperative level of IgG and PA-IgG remained significantly lower than preoperatively. The postoperative course was uneventful and without bleeding complications. Perioperative management of a patient with idiopathic thrombocytopenic purpura undergoing open-heart surgery is discussed.

[Surgical treatment of chronic pulmonary thromboembolism caused by a cavernous hemangioma at the lower limb]

A 48-years old man complained of dyspnea and was admitted to the hospital. Chest enhanced CT confirmed the presence of the thrombus in the pulmonary artery. Cardiac catheterization showed severe pulmonary hypertension (mean PAP 75 mmHg). ATIII level, protein C and S antigen were within normal range. Anticardiolipin antibody and lupus anti-coagulant determination were negative. He was diagnosed as chronic pulmonary thromboembolism, and underwent pulmonary thromboendarterectomy via median sternotomy under deep hypothermic intermittent circulatory arrest. At the same time IVC filter was inserted. The origin of the thrombus was not detected before operation, but after surgery, MR angiography of total body showed a cavernous hemangioma at left lower limb. We speculated this lesion was the origin of pulmonary embolism.

Mediastinal irrigation with superoxidized water after open-heart surgery: the safety and pitfalls of cardiovascular surgical application

Postoperative mediastinal infection after open-heart surgery via median sternotomy is a devastating complication. In this paper, we describe a simple method of irrigating the mediastinum using superoxidized water to prevent perioperative contamination. After mediastinal hemostasis was done, warm superoxidized water of more than 21 was uninterruptedly irrigated for 5 min immediately prior to sternal closure. We have used this method in 25 patients undergoing cardiac surgery, and noted significant perioperative electrocardiographic changes, including ST elevation, without hemodynamic compromise in 15 of these patients.

[Pulmonary hyalinizing granuloma diagnosed by thoracoscopy]

A 36-year-old woman was examined by our hospital for pulmonary coin shadows in both lungs, as disclosed on chest X-ray films. She had no subjective complaints other than allergic rhinitis, but exhibited hypergam-maglobulinemia, particularly in IgE. Inhalative allergen tests were positive for three types of allergens, but no autoimmune disease was detected. Although transbronchial and percutaneous fine needle biopsies failed to obtain enough specimens because of the wandering shadow a thoracoscopic biopsy was effective because of the subpleural location of the target lesions. The histologic findings were consistent with pulmonary hyalinizing granuloma, with extensive, hyalinized lamellar collagen bundles arranged haphazardly in the central area. Infiltration by lymphocytes and plasma cells, together with the destruction of bronchiolar and vessel walls, were observed in the marginal areas. No amyloid deposits or lymphocytic monoclonality were observed in the lesion. Twenty-five months after the biopsy, the patient's clinical and radiographic data had not changed.

[Sasumata(U+I) shape skin incision for the surgery through median sternotomy]

Since April, 1994 we have tried median sternotomy in a new skin incision which resembles to "Sasumata", a Japanese historical weapon of police, for the surgery of eleven patients with thymic disease or lung cancer for cosmetic reasons. An U-shaped skin incision in the upper half portion of the breast and a vertical incision from the bottom of "U" to xyphoid process were made. A comparison was made in eight patients with former conventional method, a linear incision on the sternum. The new incision was useful not only for cosmetic reasons but also for the following reasons; enough operation area, good lymph node dissection from lower neck to upper mediastinum without additional collar incision, and good healing of the skin. There were no differences between the two incision groups on operation time, bleeding volume and hospital stay after surgery. In a postoperative research almost all patients have been satisfied with the new incision.

Plasma lipids and apolipoproteins as discriminators for presence and severity of angiographically defined coronary artery disease

The relationship of coronary artery disease to with plasma lipids and apolipoproteins was examined in 100 male patients aged 32-69 years undergoing coronary angiography for suspected myocardial infarction. Patients with angiographically defined coronary artery disease had significantly lower plasma levels of high density lipoprotein (HDL) cholesterol, apolipoproteins A-I and A-II, and significantly higher values of low density lipoprotein (LDL) cholesterol and apolipoprotein B than those in patients without coronary artery disease. The ratios derived from the measurements as LDL cholesterol/HDL cholesterol, apo B/apo A-I and apo B/apo A-II were highly significantly increased in the patients with coronary artery disease. Coronary score values, by which the severity of coronary artery disease was quantified, were not related to plasma levels of the HDL components, while they were positively correlated with those of the LDL components. These results suggest that, in single measurements, plasma levels of the HDL components, HDL-cholesterol and apo A-I, contribute strongly to the discrimination between patients with coronary artery disease and those without this disease, whereas the LDL components, LDL-cholesterol and apo B are more suitable parameters for the severity of the disease than are the HDL components. The ratios of LDL cholesterol/HDL cholesterol, apo B/apo A-I and apo B/apo A-II were powerful discriminators for either presence or severity of coronary artery disease.