Workshop Report: Modulation of Antitumor Immune Responses by Dietary and Microbial Metabolites

The human microbiota maintains an enormous and diverse capacity to produce a diet-dependent metabolome that impacts both host tissue and microbial community homeostasis. Recent discoveries support a growing appreciation that microbial metabolites derived from bioactive foods are also important regulators of host immune and metabolic functions. To gain a better understanding of the current evidence for the roles of dietary and microbial metabolites in tumor immunity, the Division of Cancer Biology and the Division of Cancer Prevention, National Cancer Institute, cosponsored a workshop on August 31 and September 1, 2016, in Bethesda, Maryland. Workshop participants examined several lines of converging science that link nutrition, microbiology, and tumor immunology and identified key concepts and research opportunities that will accelerate our understanding of these interactions. In addition, the participants identified some of the critical gaps and research challenges that could be addressed through interdisciplinary collaborations, including future opportunities for translating new information into novel cancer prevention and treatment strategies based on targeting host immune functions that are altered by metabolite sensing pathways.

Understanding the tumor immune microenvironment (TIME) for effective therapy

The clinical successes in immunotherapy have been both astounding and at the same time unsatisfactory. Countless patients with varied tumor types have seen pronounced clinical response with immunotherapeutic intervention; however, many more patients have experienced minimal or no clinical benefit when provided the same treatment. As technology has advanced, so has the understanding of the complexity and diversity of the immune context of the tumor microenvironment and its influence on response to therapy. It has been possible to identify different subclasses of immune environment that have an influence on tumor initiation and response and therapy; by parsing the unique classes and subclasses of tumor immune microenvironment (TIME) that exist within a patient's tumor, the ability to predict and guide immunotherapeutic responsiveness will improve, and new therapeutic targets will be revealed.

Advances, challenges, and opportunities in extracellular RNA biology: insights from the NIH exRNA Strategic Workshop

Extracellular RNA (exRNA) has emerged as an important transducer of intercellular communication. Advancing exRNA research promises to revolutionize biology and transform clinical practice. Recent efforts have led to cutting-edge research and expanded knowledge of this new paradigm in cell-to-cell crosstalk; however, gaps in our understanding of EV heterogeneity and exRNA diversity pose significant challenges for continued development of exRNA diagnostics and therapeutics. To unravel this complexity, the NIH convened expert teams to discuss the current state of the science, define the significant bottlenecks, and brainstorm potential solutions across the entire exRNA research field. The NIH Strategic Workshop on Extracellular RNA Transport helped identify mechanistic and clinical research opportunities for exRNA biology and provided recommendations on high priority areas of research that will advance the exRNA field.

RNA editing, epitranscriptomics, and processing in cancer progression

The transcriptome is extensively and dynamically regulated by a network of RNA modifying factors. RNA editing enzymes APOBEC (apolipoprotein B mRNA editing enzyme, catalytic polypeptide-like) and ADAR (adenosine deaminase, RNA-specific) irreversibly recode primary RNA sequences, whereas newly described methylases (writers) and de-methylases (erasers) dynamically alter RNA molecules in response to environmental conditions. RNA modifications can affect RNA splicing, nuclear-cytoplasmic transport, translation, and regulation of gene expression by RNA interference. In addition, tRNA base modifications, processing, and regulated cleavage have been shown to alter global patterns of mRNA translation in response to cellular stress pathways. Recent studies, some of which were discussed at this workshop, have rekindled interest in the emerging roles of RNA modifications in health and disease. On September 10th, 2014, the Division of Cancer Biology, NCI sponsored a workshop to explore the role of epitranscriptomic RNA modifications and tRNA processing in cancer progression. The workshop attendees spanned a scientific range including chemists, virologists, and RNA and cancer biologists. The goal of the workshop was to explore the interrelationships between RNA editing, epitranscriptomics, and RNA processing and the enzymatic pathways that regulate these activities in cancer initiation and progression. At the conclusion of the workshop, a general discussion focused on defining the major challenges and opportunities in this field, as well as identifying the tools, technologies, resources and community efforts required to accelerate research in this emerging area.

The NIH Extracellular RNA Communication Consortium

The Extracellular RNA (exRNA) Communication Consortium, funded as an initiative of the NIH Common Fund, represents a consortium of investigators assembled to address the critical issues in the exRNA research arena. The overarching goal is to generate a multi-component community resource for sharing fundamental scientific discoveries, protocols, and innovative tools and technologies. The key initiatives include (a) generating a reference catalogue of exRNAs present in body fluids of normal healthy individuals that would facilitate disease diagnosis and therapies, (b) defining the fundamental principles of exRNA biogenesis, distribution, uptake, and function, as well as development of molecular tools, technologies, and imaging modalities to enable these studies,

Advances in geroscience: impact on healthspan and chronic disease

Population aging is unprecedented, without parallel in human history, and the 21st century will witness even more rapid aging than did the century just past. Improvements in public health and medicine are having a profound effect on population demographics worldwide. By 2017, there will be more people over the age of 65 than under age 5, and by 2050, two billion of the estimated nine billion people on Earth will be older than 60 (http://unfpa.org/ageingreport/). Although we can reasonably expect to live longer today than past generations did, the age-related disease burden we will have to confront has not changed. With the proportion of older people among the global population being now higher than at any time in history and still expanding, maintaining health into old age (or healthspan) has become a new and urgent frontier for modern medicine. Geroscience is a cross-disciplinary field focused on understanding the relationships between the processes of aging and age-related chronic diseases. On October 30-31, 2013, the trans-National Institutes of Health GeroScience Interest Group hosted a Summit to promote collaborations between the aging and chronic disease research communities with the goal of developing innovative strategies to improve healthspan and reduce the burden of chronic disease.

The role of inflammation in age-related disease

The National Institutes of Health (NIH) Geroscience Interest Group (GSIG) sponsored workshop, The Role of Inflammation in Age-Related Disease, was held September 6th-7th, 2012 in Bethesda, MD. It is now recognized that a mild pro-inflammatory state is correlated with the major degenerative diseases of the elderly. The focus of the workshop was to better understand the origins and consequences of this low level chronic inflammation in order to design appropriate interventional studies aimed at improving healthspan. Four sessions explored the intrinsic, environmental exposures and immune pathways by which chronic inflammation are generated, sustained, and lead to age-associated diseases. At the conclusion of the workshop recommendations to accelerate progress toward understanding the mechanistic bases of chronic disease were identified.

Metabolic reprogramming of the immune response in the tumor microenvironment

A Division of Cancer Biology, NCI sponsored workshop, Metabolic Reprogramming of the Immune Response in the Tumor Microenvironment, was held October 2nd in Bethesda, MD. The purpose of the workshop was to bring together cancer cell biologists and immunologists to explore the mechanistic relationships between the metabolic pathways used by cancer cells and anti-tumor immune cells and how this information could be used to improve cancer immunotherapy. At the conclusion of the workshop a general discussion focused on defining the major challenges and opportunities concerning the impact of metabolism on anti-tumor immunity and cancer immunotherapy as well as what tools, technologies, resources or community efforts are required to accelerate research in this area. Overall, future studies need to consider how cancer cell metabolic pathways differ from activated lymphocytes in order to define a therapeutic window for cancer therapy. Further, studies aimed at reprogramming the metabolic qualities of T cells with the goal of improving immunotherapy were considered a promising avenue.

Vesicle transfer and cell fusion: Emerging concepts of cell-cell communication in the tumor microenvironment

Cell-cell fusion and vesicle-mediated transfer are fundamental biological processes that are emerging as novel mechanisms for re-programming cells in the tumor microenvironment. Both cell-cell fusion and intercellular transfer of vesicles (including microvesicles and exosomes) allow for the transfer of information among tumor cells, between tumor cells and tumor stroma, and between tumor cells and the host immune system, which could have profound implications for our understanding of tumor initiation and progression. The National Cancer Institute's Division of Cancer Biology sponsored a recent workshop (December 4-6, 2010) entitled, Vesicle Transfer and Cell Fusion: Emerging Concepts of Cell-Cell Communication in the Tumor Microenvironment to assess the current state of the science in these two scientific areas. Co-chaired by Drs. Huang-Ge Zhang (University of Louisville) and Madhav Dhodapkar (Yale University) this workshop brought together, for the first time at the NIH, leaders in the field to assess the effects of vesicle transfer and cell-cell fusion on cancer initiation, progression and metastasis. This meeting report includes brief summaries of the presentations and identifies the major questions, roadblocks, and opportunities. The meeting report is presented here to highlight research priorities and to stimulate basic and translational research efforts to better understand the contributions of cell-cell fusion and vesicle transfer to cancer.

Three novel downstream promoter elements regulate MHC class I promoter activity in mammalian cells

BACKGROUND

MHC CLASS I TRANSCRIPTION IS REGULATED BY TWO DISTINCT TYPES OF REGULATORY PATHWAYS: 1) tissue-specific pathways that establish constitutive levels of expression within a given tissue and 2) dynamically modulated pathways that increase or decrease expression within that tissue in response to hormonal or cytokine mediated stimuli. These sets of pathways target distinct upstream regulatory elements, have distinct basal transcription factor requirements, and utilize discrete sets of transcription start sites within an extended core promoter.

METHODOLOGY/PRINCIPAL FINDINGS

We studied regulatory elements within the MHC class I promoter by cellular transfection and in vitro transcription assays in HeLa, HeLa/CIITA, and tsBN462 of various promoter constructs. We have identified three novel MHC class I regulatory elements (GLE, DPE-L1 and DPE-L2), located downstream of the major transcription start sites, that contribute to the regulation of both constitutive and activated MHC class I expression. These elements located at the 3' end of the core promoter preferentially regulate the multiple transcription start sites clustered at the 5' end of the core promoter.

CONCLUSIONS/SIGNIFICANCE

Three novel downstream elements (GLE, DPE-L1, DPE-L2), located between +1 and +32 bp, regulate both constitutive and activated MHC class I gene expression by selectively increasing usage of transcription start sites clustered at the 5' end of the core promoter upstream of +1 bp. Results indicate that the downstream elements preferentially regulate TAF1-dependent, relative to TAF1-independent, transcription.

The NIH Human Microbiome Project

The Human Microbiome Project (HMP), funded as an initiative of the NIH Roadmap for Biomedical Research (http://nihroadmap.nih.gov), is a multi-component community resource. The goals of the HMP are: (1) to take advantage of new, high-throughput technologies to characterize the human microbiome more fully by studying samples from multiple body sites from each of at least 250 "normal" volunteers; (2) to determine whether there are associations between changes in the microbiome and health/disease by studying several different medical conditions; and (3) to provide both a standardized data resource and new technological approaches to enable such studies to be undertaken broadly in the scientific community. The ethical, legal, and social implications of such research are being systematically studied as well. The ultimate objective of the HMP is to demonstrate that there are opportunities to improve human health through monitoring or manipulation of the human microbiome. The history and implementation of this new program are described here.

Upstream stimulatory factor regulates constitutive expression and hormonal suppression of the 90K (Mac-2BP) protein

We previously reported that hormones important for the normal growth and function of FRTL-5 rat thyroid cells, TSH, or its cAMP signal plus insulin or IGF-I, could transcriptionally suppress constitutive and gamma-interferon (IFN)-increased synthesis of the 90K protein (also known as Mac-2BP). Here we cloned the 5'-flanking region of the rat 90K gene and identified a minimal promoter containing an interferon response element and a consensus E-box or upstream stimulator factor (USF) binding site, which are highly conserved in both the human and murine genes. We show that suppression of constitutive and gamma-IFN-increased 90K gene expression by TSH/cAMP plus insulin/IGF-I depends on the ability of the hormones to decrease the binding of USF to the E-box, located upstream of the interferon response element. This site is required for the constitutive expression of the 90K gene. Transfection with USF1 and USF2 cDNAs increases constitutive promoter activity, attenuates the ability of TSH/cAMP plus insulin/IGF-I to decrease constitutive or gamma-IFN-increased 90K gene expression but does not abrogate the ability of gamma-IFN itself to increase 90K gene expression.

A T lymphocyte-specific transcription complex containing RUNX1 activates MHC class I expression

MHC class I expression is subject to both tissue-specific and hormonal regulatory mechanisms. Consequently, levels of expression vary widely among tissues, with the highest levels of class I occurring in the lymphoid compartment, in T cells and B cells. Although the high class I expression in B cells is known to involve the B cell enhanceosome, the molecular basis for high constitutive class I expression in T cells has not been explored. T cell-specific genes, such as TCR genes, are regulated by a T cell enhanceosome consisting of RUNX1, CBFbeta, LEF1, and Aly. In this report, we demonstrate that MHC class I gene expression is enhanced by the T cell enhanceosome and results from a direct interaction of the RUNX1-containing complex with the class I gene in vivo. T cell enhanceosome activation of class I transcription is synergistic with CIITA-mediated activation and targets response elements distinct from those targeted by CIITA. These findings provide a molecular basis for the high levels of MHC class I in T cells.

Distinct transcriptional pathways regulate basal and activated major histocompatibility complex class I expression

Transcription of major histocompatibility complex (MHC) class I genes is regulated by both tissue-specific (basal) and hormone/cytokine (activated) mechanisms. Although promoter-proximal regulatory elements have been characterized extensively, the role of the core promoter in mediating regulation has been largely undefined. We report here that the class I core promoter consists of distinct elements that are differentially utilized in basal and activated transcription pathways. These pathways recruit distinct transcription factor complexes to the core promoter elements and target distinct transcription initiation sites. Class I transcription initiates at four major sites within the core promoter and is clustered in two distinct regions: "upstream" (-14 and -18) and "downstream" (+12 and +1). Basal transcription initiates predominantly from the upstream start site region and is completely dependent upon the general transcription factor TAF1 (TAF(II)250). Activated transcription initiates predominantly from the downstream region and is TAF1 (TAF(II)250) independent. USF1 augments transcription initiating through the upstream start sites and is dependent on TAF1 (TAF(II)250), a finding consistent with its role in regulating basal class I transcription. In contrast, transcription activated by the interferon mediator CIITA is independent of TAF1 (TAF(II)250) and focuses initiation on the downstream start sites. Thus, basal and activated transcriptions of an MHC class I gene target distinct core promoter domains, nucleate distinct transcription initiation complexes and initiate at distinct sites within the promoter. We propose that transcription initiation at the core promoter is a dynamic process in which the mechanisms of core promoter function differ depending on the cellular environment.

The GTP-binding domain of class II transactivator regulates its nuclear export

The transcriptional coactivator class II transactivator (CIITA), although predominantly localized in the nucleus, is also present in the cytoplasm. The subcellular distribution of CIITA is actively regulated by the opposing actions of nuclear export and import. In this study, we show that nuclear export is negatively regulated by the GTP-binding domain (GBD; aa 421-561) of CIITA: mutation or deletion of the GBD markedly increased export of CIITA from the nucleus. Remarkably, a CIITA GBD mutant binds CRM1/exportin significantly better than does wild-type CIITA, leading to the conclusion that GTP is a negative regulator of CIITA nuclear export. We also report that, in addition to the previously characterized N- and C-terminal nuclear localization signal elements, there is an additional N-terminal nuclear localization activity, present between aa 209 and 222, which overlaps the proline/serine/threonine-rich domain of CIITA. Thus, fine-tuning of the nucleocytoplasmic distribution of coactivator proteins involved in transcription is an active and dynamic process that defines a novel mechanism for controlling gene regulation.

Transcriptional coactivator, CIITA, is an acetyltransferase that bypasses a promoter requirement for TAF(II)250

The CIITA coactivator is essential for transcriptional activation of MHC class II genes and mediates enhanced MHC class I transcription. We now report that CIITA contains an intrinsic acetyltransferase (AT) activity that maps to a region within the N-terminal segment of CIITA, between amino acids 94 and 132. The AT activity is regulated by the C-terminal GTP-binding domain and is stimulated by GTP. CIITA-mediated transactivation depends on the AT activity. Further, we report that, although constitutive MHC class I transcription depends on TAF(II)250, CIITA activates the promoter in the absence of functional TAF(II)250.

TAF(II)250-independent transcription can be conferred on a TAF(II)250-dependent basal promoter by upstream activators

TAF(II)250, a component of the general transcription factor, TFIID, is required for the transcription of a subset of genes, including those involved in regulating cell cycle progression. The tsBN462 cell line, with a temperature-sensitive mutation of TAF(II)250, grows normally at 32 degrees C, but when grown at 39.5 degrees C, it differentially arrests transcription of many, but not all, genes. The present studies examine the basis for the requirement for TAF(II)250. We show that the basal promoter of a major histocompatibility complex class I gene requires TAF(II)250. This dependence can be overcome by select upstream regulatory elements but not by basal promoter elements. Thus, the coactivator CIITA rescues the basal promoter from the requirement for TAF(II)250, whereas introduction of a canonical TATAA box does not. Similarly, the SV40 basal promoter is shown to require TAF(II)250, and the presence of the 72-base pair enhancer overcomes this requirement. Furthermore, the SV40 72-base pair enhancer when placed upstream of the basal class I promoter renders it independent of TAF(II)250. These data suggest that the assembly of transcription initiation complexes is dynamic and can be modulated by specific transcription factors.

Upstream stimulatory factor regulates major histocompatibility complex class I gene expression: the U2DeltaE4 splice variant abrogates E-box activity

The tissue-specific expression of major histocompatibility complex class I genes is determined by a series of upstream regulatory elements, many of which remain ill defined. We now report that a distal E-box element, located between bp -309 and -314 upstream of transcription initiation, acts as a cell type-specific enhancer of class I promoter activity. The class I E box is very active in a neuroblastoma cell line, CHP-126, but is relatively inactive in the HeLa epithelial cell line. The basic helix-loop-helix leucine zipper proteins upstream stimulatory factor 1 (USF1) and USF2 were shown to specifically recognize the class I E box, resulting in the activation of the downstream promoter. Fine mapping of USF1 and USF2 amino-terminal functional domains revealed differences in their abilities to activate the class I E box. Whereas USF1 contained only an extended activation domain, USF2 contained both an activation domain and a negative regulatory region. Surprisingly, the naturally occurring splice variant of USF2 lacking the exon 4 domain, U2DeltaE4, acted as a dominant-negative regulator of USF-mediated activation of the class I promoter. This latter activity is in sharp contrast to the known ability of U2DeltaE4 to activate the adenovirus major late promoter. Class I E-box function is correlated with the relative amount of U2DeltaE4 in a cell, leading to the proposal that U2DeltaE4 modulates class I E-box activity and may represent one mechanism to fine-tune class I expression in various tissues.

HIV Tat represses transcription of the beta 2-microglobulin promoter

The MHC class I complex, which binds and presents peptide antigen, is composed of a class I heavy chain and the beta2-microglobulin light chain. HIV-1, which induces a profound immunodeficiency in infected individuals, encodes proteins that cause decreased expression of class I heavy chain. We now report that the HIV Tat protein, which is a potent transactivator of viral transcription, is also a potent repressor of the beta2-microglobulin gene. Repression is mediated through the basal promoter of the beta2-microglobulin gene, which is shown to be predominantly regulated by an initiator element. Tat repression is further augmented by the short viral transcript, TAR, which interacts with Tat. Tat-mediated repression of beta2-microglobulin expression, together with its known repression of class I gene transcription, provides an effective mechanism by which HIV could prevent cell surface expression of the MHC class I complex and avoid immune surveillance.

HIV-1 tat binds TAFII250 and represses TAFII250-dependent transcription of major histocompatibility class I genes

HIV Tat, a transactivator of viral transcription, represses transcription of major histocompatibility (MHC) class I genes. Repression depends exclusively on the C-terminal domain of Tat, although the mechanism of this repression has not been known. We now show that repression results from the interaction of Tat with the TAFII250 component of the general transcription factor, TFIID. The C-terminal domain of Tat binds to a site on TAFII250 that overlaps the histone acetyl transferase domain, inhibiting TAFII250 histone acetyl transferase activity. Furthermore, promoters repressed by Tat, including the MHC class I promoter, are dependent on TAFII250 whereas those that are not repressed by Tat, such as SV40 and MuLV promoters, are independent of functional TAFII250. Thus, Tat repression of MHC class I transcription would be one mechanism by which HIV avoids immune surveillance.

Elf-1 regulates basal expression from the T cell antigen receptor zeta-chain gene promoter

In mature T cells, limited synthesis of the TCR-zeta subunit is primarily responsible for regulating surface expression of TCRs. Transcription of zeta is directed by a complex promoter that includes two potential binding sites for the Ets family of transcription factors at -52 (zEBS1) and -135 (zEBS2). Mutation of these two sites results in a marked reduction of transcription from this promoter. Using electrophoretic mobility shift analysis, Elf-1 was demonstrated to be the Ets family member that binds to these sites. One site, zEBS1, matches the optimal Elf-1 consensus sequence in eight of nine bases, making it the best match of any known mammalian Elf-1 binding site. A role for Elf-1 in TCR-zeta trans-activation was confirmed by ectopic expression of Elf-1 in COS-7 cells. This resulted in an increase in TCR-zeta promoter activity that mapped to zEBS1 and zEBS2. Additional support for the involvement of Elf-1 in TCR-zeta trans-activation derives from the finding that a GAL4-Elf-1 fusion protein trans-activated TCR-zeta promoter constructs that had been modified to contain GAL4 DNA binding sites. These results demonstrate that Elf-1 plays an essential role in the trans-activation of a constitutively expressed T cell-specific gene, and that trans-activation occurs in the context of the native promoter in both lymphoid and nonlymphoid cells. Taken together with the existing literature, these data also suggest that the requirement for inducible factors in Elf-1-mediated trans-activation may decrease as the affinity and number of Elf-1 sites increase.

Characterization of two class I genes from the H2-M region: evidence for a new subfamily

We cloned, sequenced, and mapped two divergent major histocompatibility class Ib genes from BALB/c mice. M9d and M10d both have the potential to encode full-length class I molecules, but transcripts were not readily detectable. M9 is 86% similar to M1 in its nucleotide sequence and maps next to it on YAC clones. M9 is only 64% similar to M10 and 60% to H2-K k. Probes from M10 define a new subfamily of eight class I genes in C3H mice; five cluster directly distal to H2-T1, and three are located between M9-1-7-8 and M6-4-5 in the H2-M region.

HIV Tat protein requirements for transactivation and repression of transcription are separable

The HIV Tat protein, primarily characterized as a transcriptional activator of the viral long terminal repeat (LTR), is also a potent repressor of major histocompatibility complex (MHC) class I transcription. In the present study, we demonstrate that these two functional activities are distinct and mediated by discrete, but overlapping, structural domains of Tat. Tat repressor activity depends on C-terminal sequences, whereas transactivation depends on N-terminal sequences; both functions require core sequences. The repressor activity requires a domain encompassing the region encoded by the second exon of the Tat gene, beginning at amino acid 73, with a C-terminal limit between amino acids 80 and 83. Tat repressor function also depends on the presence of a lysine at position 41, located within the core of the protein. Tat repressor activity is independent of two N-terminal domains essential for transactivation: the acidic segment and the cysteine-rich region. Conversely, Tat transactivation is independent of the second exon-encoded region of Tat. As further support for this novel model of separable Tat functions, we show that in murine fibroblasts, Tat represses class I promoter activity, but does not transactivate the HIV LTR. We propose that distinct structural domains mediate the two functionally distinct activities associated with the Tat protein.

A spontaneously arising mutation in the DLAARN motif of murine ZAP-70 abrogates kinase activity and arrests thymocyte development

Development of immature CD4+ CD8+ thymocytes into functionally mature CD4+ and CD8+ T cells is driven by selection events that require signals transduced through the T cell antigen receptor (TCR). Transduction of TCR signals in the thymus involves tyrosine phosphorylation of the protein tyrosine kinase ZAP-70 by p56(lck) and results in induction of ZAP-70 enzymatic activity. We have identified a novel, spontaneously arising point mutation within a highly conserved motif (DLAARN) in the kinase domain of murine ZAP-70 that uncouples tyrosine phosphorylation of ZAP-70 from induction of ZAP-70 kinase activity. Mice homozygous for this mutation are devoid of mature T cells because thymocyte development is arrested at the CD4+ CD8+ stage of differentiation. The developmental arrest is due to the inability of CD4+ CD8+ thymocytes to propagate TCR signals in the absence of ZAP-70 kinase activity despite tyrosine phosphorylation of TCR-associated ZAP-70 molecules.

Measure of transient transfection efficiency using beta-galactosidase protein

Although the control elements which regulate the transcriptional activity of promoter sequences are largely determined by the use of reporter plasmids in transient transfection analyses, controlling variability in these experiments can often be a vexing problem. Problems arise when the promoter of the internal control plasmid, used to correct for transfection efficiency, either affects test promoter strength or is itself regulated by trans-acting factors or inducing agents used to study the test promoter. Here we report the use of beta-galactosidase protein as an unbiased standard of transfection efficiency. beta-Galactosidase protein is readily internalized by adherent cell lines when incorporated into a calcium phosphate precipitate; significant enzyme activity can be recovered up to 72 h after transfection. Use of beta-galactosidase protein as a control obviates the concerns associated with promoter-dependent reporter plasmids as controls.

Repression of the nonclassical MHC class I gene H2-M1 by cis-acting silencer DNA elements

H2-M1 is a non-classical major histocompatibility complex (MHC) class I gene that is highly divergent from classical class I genes; M1 was the first gene in the recently classified M region of the mouse MHC to be cloned. Although the M1 DNA sequence contains normal splice sites, open reading frames within its exons, and a recognizable promoter, no M1 transcripts were detected in various healthy mouse tissues. However, M1 transcripts were detected in transfected L cells and in vivo in brains of M1 transgenic mice, albeit at very low levels, and the level of expression is correlated with transgene copy number. Analysis of the M1 promoter region identified a competent promoter capable of directing transcription, but whose expression is repressed by two strong upstream silencer elements, one mapping between -184 base pairs (bp) and -266 bp and the other between -1149 bp and -1702 bp. These studies suggest that M1 expression is highly regulated and restricted either temporally or to a very limited number of cell types.

HIV Tat represses transcription through Sp1-like elements in the basal promoter

MHC class I genes are potently repressed by HIV Tat, which transactivates the HIV LTR. Tat represses class I transcription by binding to complexes associated with a novel promoter element, consisting of Sp1-like DNA binding sites. Transcription by other Sp1-dependent promoters, such as MDR1 and the minimal SV40 promoters, is also repressed by Tat, whereas the human beta-actin promoter is neither activated by Sp1 nor repressed by Tat. Tat repression can be overcome by a strong enhancer element. Thus, the SV40 72 bp enhancer element confers protection from Tat-mediated repression on both the minimal SV40 promoter and the class I promoter. Surprisingly, Tat can activate the class I promoter in the presence of both the HIV TAR element and a strong upstream enhancer. These data demonstrate that Tat differentially affects Sp1-responsive promoters, depending on promoter architecture.

MHC class I gene expression is negatively regulated by the proto-oncogene, c-jun

The trans-acting factor AP-1 is a heterodimeric complex composed of c-Jun and c-Fos family proteins which bind and regulate genes containing a TPA responsive enhancer element. Although AP-1 binding sites have been identified within the regulatory region of major histocompatibility complex (MHC) class I genes in vitro, the role of AP-1 in regulating MHC class I transcription in vivo has not been investigated previously. The present study demonstrates that expression of c-Jun results in decreased MHC class I promoter activity as determined in cotransfection assays of an MHC class I reporter construct with a c-Jun expression construct. The c-Jun responsive element is located between bp -440 and -431 upstream of initiation of transcription as determined both functionally and by direct binding of purified c-Jun. Furthermore, over-expression of c-Jun reduced the steady state levels of endogenous MHC class I RNA in murine L cells by approximately 10-fold. These data indicate that c-Jun/AP-1 acts as a negative trans-acting factor that down-regulates MHC class I gene expression.

Repression of MHC class I gene promoter activity by two-exon Tat of HIV

Major histocompatibility complex (MHC) class I molecules are the major receptors for viral peptides and serve as targets for specific cytotoxic T lymphocytes. Human immunodeficiency virus-type 1 (HIV-1) specifically decreased activity of an MHC class I gene promoter up to 12-fold. Repression was effected by the HIV-1 Tat protein derived from a spliced viral transcript (two-exon Tat). These studies define an activity for two-exon Tat distinct from that of one-exon Tat and suggest a mechanism whereby HIV-1-infected cells might be able to avoid immune surveillance, allowing the virus to persist in the infected host.

Membranolytic and nucleolytic activities of cytolytic T lymphocyte clones

In an attempt to see if the nucleolytic and membranolytic activities of cytolytic T lymphocytes (CTL) were totally independent and could be expressed independently, we cloned CTL and determined their membranolytic and nucleolytic activities. If the two lytic mechanisms were completely independent and could be independently expressed by individual CTL, we anticipated that we would find CTL clones exhibiting only one or the other activity. Initial examination of membranolytic and nucleolytic activities in 99 newly established CTL clones revealed a poor correlation (r = 0.4) between the two activities. In addition, some clones expressed membenolytic activity without nucleolytic activity, and others, nucleolytic activity without membenolytic activity. The results suggest that CTL have 2 or more separate and independent mechanisms that lead either to the membranolytic or to the nucleolytic lesions in target cells.

The protein kinase C inhibitor 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride (H-7) inhibits PMA-induced promiscuous cytolytic activity but not specific cytolytic activity by a cloned cytolytic T lymphocyte

Phorbol 12-myristate 13-acetate (PMA) induces the cytolytic T lymphocyte (CTL) clone 4D (H-2b anti-H-2d) to promiscuously kill the inappropriate target EL-4 (H-2b). The protein kinase C (PKC) inhibitor 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride (H-7) inhibited the PMA-induced promiscuous lympholysis. The concentration of H-7 that inhibited PMA-induced lympholysis by 50% (IC50) was calculated to be 4 microM, which closely approximates the reported IC50 of H-7 of 6 microM for PKC activity in vitro. In striking contrast, specific cytolysis of appropriate P815 (H-2d) target cell by CTL clone 4D was not inhibited by concentrations of H-7 which inhibited PMA-induced promiscuous lympholysis. These results indicate that PMA-induced promiscuous lympholysis of inappropriate target cell is triggered via activation of PKC, whereas PKC activation is not obligatory in triggering CTL clone 4D to specifically kill appropriate target cells. Thus, these data suggest that cloned CTL have two or more triggering mechanisms than may initiate one or more cytolytic pathways.

Cytotoxic T lymphocyte (CTL)-mediated cytolysis proceeds in the absence of Na+/H+ antiport activity: regulation of cytosolic pH by the Na+/H+ antiport in a cloned CTL

Cytotoxic T lymphocyte (CTL)-mediated cytolysis of specifically bound target cells (TC) is thought to be triggered by cross-linking the T-cell antigen receptor (TcR). Biochemical events associated with TcR cross-linking include increased intracellular calcium levels [Ca2+]i, hydrolysis of phosphatidylinositol (PI), and an increase in intracellular pH [pH]i. Whereas CTL-mediated cytolysis of some TC is calcium-dependent, and PI hydrolysis is speculated to trigger the CTL lethal hit via activation of PKC, little is known about changes in [pH]i relating to activation of the lethal hit stage. We report regulation of [pH]i in a cloned CTL by the electroneutral Na+/H+ antiport during activation with PMA and specific antigen-bearing TC. Furthermore, using 5-(N-methyl-N-isobutyl) amiloride (MIBA), a potent antiport inhibitor, we demonstrate that Na+/H+ exchange is not required for activation of CTL cytolytic activity.

Protein kinase C (PKC) inhibitors inhibit primary (1-0) but not secondary (1-1) or cloned cytotoxic T lymphocytes

The isoquinolinesulfonamide PKC inhibitors H-7 and H-8 inhibit primary, in vivo generated cytotoxic T lymphocyte (CTL) activity by 50% at concentrations approximating their reported Ki values for PKC, 6 uM and 15 uM respectively. However, a greater than ten-fold higher concentration of H-7 (100 uM) is required to reduce secondary or clone 8F CTL-mediated cytotoxicity by 50%. At this concentration H-7 is also reported to inhibit calmodulin (CaM)-dependent enzymes. To distinguish between the effect of 100 uM H-7 on PKC versus CaM the napthalenesulfonamide CaM antagonist W-7 was investigated. W-7 inhibited primary, secondary and clone 8F CTL-mediated cytolysis by 50% near its reported Ki value for CaM-dependent kinase activity, 12 uM. We conclude that W-7 and 100 uM H-7 reduce cytolysis by inhibiting CaM-dependent reactions and not PKC. Thus, these findings indicate that primary killers require both PKC- and CaM-dependent activation pathways for lethal hit delivery, whereas highly lytic cultured CTL use only one pathway dependent upon CaM.

Demonstration of cell-mediated cytotoxicity to allogeneic and xenogeneic tissue in the American cockroach, Periplaneta americana, using a combination in vivo/in vitro assay

Previous reports have claimed that insects lack the ability to reject integumentary allografts. However, when we followed the fate of the epidermal layer underneath the inert cuticle, we observed that the American cockroach not only rejected xenografts, but also demonstrated impressive reactivity against allografts. We have now developed a combination in vivo/in vitro assay that can quantitate the extent of allograft reactivity in the roach. Pieces of filter paper (3 X 4-mm) were implanted under the second and fourth tergites of donor animals. After 7 days, implants seeded with host hemocytes were removed, washed, and incubated in culture medium containing 3H-thymidine for 24 hr. Labeled grafts were reimplanted into paired animals to detect cytotoxicity as follows: grafts removed from under the second tergite were placed back into their original positions to serve as autograft controls; grafts removed from under the fourth tergite were reciprocally transferred between paired animals; and grafts were recovered after various time intervals and processed for scintillation counting. There was no significant difference in counts between allografts and autografts at day 1. Autografts sampled on days 3, 5, 7, and 10 had significantly higher counts than allografts, with peak reactivity occurring between days 3 and 5. This indicates that allogeneic cells were selectively destroyed, and confirms our recent data from conventional grafting studies that insects have the ability to react to allografts.

Primary integumentary allograft reactivity in the American cockroach, Periplaneta americana

Previous reports have failed to demonstrate integumentary allograft rejection in insects. We realized however, that these studies may not have fully appreciated the structure of the insect exoskeleton. Since the subcuticlar epidermal layer constitutes the only living tissue associated with insect integument, its destruction would indicate that the animal recognized and responded to the foreign tissue. Thus, we investigated allograft reactivity in the American cockroach, Periplaneta americana, by observing the fate of the epidermal portion of the integument. Each animal in a pair received a 3 X 4-mm integumentary allograft from its partner, as well as a 3 X 4-mm control autograft. The transplants were then examined histologically for signs of epidermal destruction at 0, 1, 3, 5, 7, and 10-70 days (in 10-day increments) posttransplantation. The results indicated that significant rejection of the allografts began by day 3, with peak reactivity occurring by day 7 when 92% of the grafts were scored as rejected. At later periods (greater than 20 days), the graft sites showed signs of repopulation by host epidermal cells. The allograft reaction was found to lag behind the xenograft reaction, which showed peak activity after only 1 day posttransplantation. Even so, allograft rejection in this insect occurred quite rapidly (as compared with some other invertebrates), and would appear to be due to a cytotoxic reaction against the epidermal layer.