Human endogenous retrovirus onco-exaptation counters cancer cell senescence through calbindin.

Increased levels and diversity of human endogenous retrovirus (HERV) transcription characterize most cancer types and are linked with disease outcomes. However, the underlying processes are incompletely understood. Here, we show that elevated transcription of HERVH proviruses predicted survival of lung squamous cell carcinoma (LUSC) and identified an isoform of CALB1, encoding calbindin, ectopically driven by an upstream HERVH provirus under the control of KLF5, as the mediator of this effect. HERVH-CALB1 expression was initiated in preinvasive lesions and associated with their progression. Calbindin loss in LUSC cell lines impaired in vitro and in vivo growth and triggered senescence, consistent with a protumor effect. However, calbindin also directly controlled the senescence-associated secretory phenotype (SASP), marked by secretion of CXCL8 and other neutrophil chemoattractants. In established carcinomas, CALB1-negative cancer cells became the dominant source of CXCL8, correlating with neutrophil infiltration and worse prognosis. Thus, HERVH-CALB1 expression in LUSC may display antagonistic pleiotropy, whereby the benefits of escaping senescence early during cancer initiation and clonal competition were offset by the prevention of SASP and protumor inflammation at later stages.

E3 ubiquitin ligase HECTD2 mediates melanoma progression and immune evasion.

The ubiquitin-proteasome system maintains protein homoeostasis, underpins the cell cycle, and is dysregulated in cancer. However, the role of individual E3 ubiquitin ligases, which mediate the final step in ubiquitin-mediated proteolysis, remains incompletely understood. Identified through screening for cancer-specific endogenous retroviral transcripts, we show that the little-studied E3 ubiquitin ligase HECTD2 exerts dominant control of tumour progression in melanoma. HECTD2 cell autonomously drives the proliferation of human and murine melanoma cells by accelerating the cell cycle. HECTD2 additionally regulates cancer cell production of immune mediators, initiating multiple immune suppressive pathways, which include the cyclooxygenase 2 (COX2) pathway. Accordingly, higher HECTD2 expression is associated with weaker anti-tumour immunity and unfavourable outcome of PD-1 blockade in human melanoma and counteracts immunity against a model tumour antigen in murine melanoma. This central, multifaceted role of HECTD2 in cancer cell-autonomous proliferation and in immune evasion may provide a single target for a multipronged therapy of melanoma.

Soluble PD-L1 generated by endogenous retroelement exaptation is a receptor antagonist.

Immune regulation is a finely balanced process of positive and negative signals. PD-L1 and its receptor PD-1 are critical regulators of autoimmune, antiviral and antitumoural T cell responses. Although the function of its predominant membrane-bound form is well established, the source and biological activity of soluble PD-L1 (sPD-L1) remain incompletely understood. Here, we show that sPD-L1 in human healthy tissues and tumours is produced by exaptation of an intronic LINE-2A (L2A) endogenous retroelement in the CD274 gene, encoding PD-L1, which causes omission of the transmembrane domain and the regulatory sequence in the canonical 3' untranslated region. The alternatively spliced CD274-L2A transcript forms the major source of sPD-L1 and is highly conserved in hominids, but lost in mice and a few related species. Importantly, CD274-L2A-encoded sPD-L1 lacks measurable T cell inhibitory activity. Instead, it functions as a receptor antagonist, blocking the inhibitory activity of PD-L1 bound on cellular or exosomal membranes.

Restoration of Endogenous Retrovirus Infectivity Impacts Mouse Cancer Models.

Mouse models have been instrumental in establishing fundamental principles of cancer initiation and progression and continue to be invaluable in the discovery and further development of cancer therapies. Nevertheless, important aspects of human disease are imperfectly approximated in mouse models, notably the involvement of endogenous retroviruses (ERVs). Replication-defective ERVs, present in both humans and mice, may affect tumor development and antitumor immunity through mechanisms not involving infection. Here, we revealed an adverse effect of murine ERVs with restored infectivity on the behavior of mouse cancer models. In contrast to human cancer, where infectious ERVs have never been detected, we found that ERV infectivity was frequently restored in transplantable, as well as genetic, mouse cancer models. Such replication-competent, ERV-derived retroviruses were responsible for unusually high expression of retroviral nucleic acids and proteins in mouse cancers. Infectious ERV-derived retroviruses produced by mouse cancer cells could directly infect tumor-infiltrating host immune cells and fundamentally modified the host's immune defenses to cancer, as well as the outcome of immunotherapy. Therefore, infectious retroviruses, variably arising in mouse cancer models, but not in human cancer, have the potential to confound many immunologic studies and should be considered as a variable, if not altogether avoided. Cancer Immunol Res; 6(11); 1292-300. ©2018 AACR.

DNA-binding of the Tet-transactivator curtails antigen-induced lymphocyte activation in mice.

The Tet-On/Off system for conditional transgene expression constitutes state-of-the-art technology to study gene function by facilitating inducible expression in a timed and reversible manner. Several studies documented the suitability and versatility of this system to trace lymphocyte fate and to conditionally express oncogenes or silence tumour suppressor genes in vivo. Here, we show that expression of the tetracycline/doxycycline-controlled Tet-transactivator, while tolerated well during development and in immunologically unchallenged animals, impairs the expansion of antigen-stimulated T and B cells and thereby curtails adaptive immune responses in vivo. Transactivator-mediated cytotoxicity depends on DNA binding, but can be overcome by BCL2 overexpression, suggesting that apoptosis induction upon lymphocyte activation limits cellular and humoral immune responses. Our findings suggest a possible system-intrinsic biological bias of the Tet-On/Off system in vivo that will favour the outgrowth of apoptosis resistant clones, thus possibly confounding data published using such systems.

Prosurvival Bcl-2 family members reveal a distinct apoptotic identity between conventional and plasmacytoid dendritic cells.

Dendritic cells (DCs) are heterogeneous, comprising subsets with functional specializations that play distinct roles in immunity as well as immunopathology. We investigated the molecular control of cell survival of two main DC subsets: plasmacytoid DCs (pDCs) and conventional DCs (cDCs) and their dependence on individual antiapoptotic BCL-2 family members. Compared with cDCs, pDCs had higher expression of BCL-2, lower A1, and similar levels of MCL-1 and BCL-XL. Transgenic overexpression of BCL-2 increased the pDC pool size in vivo with only minor impact on cDCs. With a view to immune intervention, we tested BCL-2 inhibitors and found that ABT-199 (the BCL-2 specific inhibitor) selectively killed pDCs but not cDCs. Conversely, genetic knockdown of A1 profoundly reduced the proportion of cDCs but not pDCs. We also found that conditional ablation of MCL-1 significantly reduced the size of both DC populations in mice and impeded DC-mediated immune responses. Thus, we revealed that the two DC types have different cell survival requirements. The molecular basis of survival of different DC subsets thus advocates the antagonism of selective BCL-2 family members for treating diseases pertaining to distinct DC subsets.

Knockdown of the antiapoptotic Bcl-2 family member A1/Bfl-1 protects mice from anaphylaxis.

Many forms of hypersensitivity reactions and allergic responses depend on deregulated mast cell activity. Several reports suggested that the antiapoptotic Bcl-2 family protein Bcl2a1/Bfl-1/A1 plays a critical role in mast cell survival upon activation. However, its in vivo relevance is poorly understood because of quadruplication of the Bcl2a1 gene locus in mice, hindering conventional knockout studies. In this study, we used a mouse model allowing traceable constitutive knockdown of all A1 isoforms expressed in the hematopoietic system by RNA interference. Knockdown of A1 reduced mast cell numbers in the skin and impaired connective tissue-like mast cell survival upon FcεRI-mediated activation in vitro. In contrast, A1 was dispensable for mucosa-like mast cell differentiation and survival. Moreover, knockdown of A1 prevented IgE-mediated passive systemic and cutaneous anaphylaxis in vivo. Our findings demonstrate that A1 is essential for the homeostasis of connective tissue mast cells, identifying A1 as a possible therapeutic target for therapy of certain types of mast cell-driven allergy symptoms.

Replenishment of the B cell compartment after doxorubicin-induced hematopoietic toxicity is facilitated by STAT1.

STAT1 serves as an important regulator in the response to pathogens, oncogenic transformation, and genotoxic insults. It exerts these effects by shaping the innate and adaptive immune response and by participating in genotoxic stress pathways, leading to apoptosis and inhibition of cell proliferation. We have investigated the role of STAT1 in hematopoietic toxicity induced by doxorubicin in STAT1-proficient and -deficient mice. Whereas the early genotoxic effect of doxorubicin did not depend on STAT1, expression of STAT1 was required for efficient B lymphocyte repopulation in the recovery phase. We found a lower abundance of lymphocyte precursors in the BM of STAT1-deficient animals, which was particularly evident after doxorubicin-induced hematopoietic toxicity. In accordance, colony-forming assays with STAT1-deficient BM cells revealed a decreased number of pre-B colonies. Differentiation from the pro-B to the pre-B stage was not affected, as demonstrated by unaltered differentiation of purified B cell precursors from BM in the presence of IL-7. With the exception of Sca-1, expression of genes implicated in early lymphocyte development in pro-B cells did not depend on STAT1. Our findings indicate a specific requirement for STAT1 in lymphoid development before differentiation to pre-B cells, which becomes particularly apparent in the recovery phase from doxorubicin-induced hematopoietic toxicity.

Targeting antiapoptotic A1/Bfl-1 by in vivo RNAi reveals multiple roles in leukocyte development in mice.

Gene-targeting studies in mice have identified the essential roles of most prosurvival Bcl-2 family members in normal physiology and under conditions of stress. The function of one member, Bcl2a1/Bfl-1/A1, is only poorly understood because of quadruplication of its gene locus in mice, hindering conventional knockout studies. To overcome this problem, we generated mouse models allowing traceable constitutive or reversible ablation of A1 in the hematopoietic system by RNA interference. Knockdown of A1 impaired early stages of T-cell differentiation, B-cell homeostasis, and sensitized transitional as well as follicular B cells to apoptosis induced by ligation of the B-cell receptor. As a consequence, B-cell proliferation in response to mitogens was severely impaired, whereas that of T cells appeared unaffected. Furthermore, depending on the extent of A1 knockdown, granulocytes showed increased spontaneous death in culture or failed to accumulate in significant numbers in vivo. These models highlight the critical role of A1 in leukocyte development and homeostasis, constituting valuable tools for investigating presumed roles of this Bcl-2 family member in immunity, tumorigenesis, and drug resistance.

A1/Bfl-1 in leukocyte development and cell death.

The function of the anti-apoptotic Bcl-2 family member Bcl2a1/Bfl-1/A1 is poorly understood due to the lack of appropriate loss-of-function mouse models and redundant effects with other Bcl-2 pro-survival proteins upon overexpression. Expression analysis of A1 suggests predominant roles in leukocyte development, their survival upon viral or bacterial infection, as well as during allergic reactions. In addition, A1 has been implicated in autoimmunity and the pathology and therapy resistance of hematological as well as solid tumors that may aberrantly express this protein. In this review, we aim to summarize current knowledge on A1 biology, focusing on its role in the immune system and compare it to that of other pro-survival Bcl-2 proteins.

Generation and evaluation of an IPTG-regulated version of Vav-gene promoter for mouse transgenesis.

Different bacteria-derived systems for regulatable gene expression have been developed for the use in mammalian cells and some were also successfully adopted for in vivo use in vertebrate model organisms. However, certain limitations apply to most of these systems, including leakiness of transgene expression, inefficient transgene silencing or activation, as well as limited tissue accessibility of transgene-inducers or their unfavourable pharmacokinetics. In this study, we evaluated the suitability of the lac-operon/lac-repressor (lacO/lacI) system for the regulation of the well-established Vav-gene promoter that allows inducible transgene expression in different haematopoietic lineages in mice. Using the fluorescence marker protein Venus as a reporter, we observed that the lacO/lacI system could be amended to modulate transgene-expression in haematopoietic cells. However, reporter expression was not uniform and the lacO elements introduced into the Vav-gene promoter only conferred limited repression and reversion of lacI-mediated gene silencing after administration of IPTG. Although further optimization of the system is required, the lacO-modified version of the Vav-gene promoter may be adopted as a tool where low basal gene-expression and limited transient induction of protein expression are desired, e.g. for the activation of oncogenes or transgenes that act in a dominant-negative manner.

A simple molecular method for discriminating common filarial nematodes of dogs (Canis familiaris).

Accurate diagnosis of canine filariosis is essential for choosing correct therapeutic approach. Therefore, reliable methods for discriminating among the different filarial infections in dogs are needed. The authors report simple and highly specific molecular methods that identify the three most common filarial nematodes of European dogs: Dirofilaria immitis, D. repens and Acanthocheilonema (syn. Dipetalonema) reconditum, based on (1) PCR amplifications of mitochondrial DNA (12S rDNA and coxI) with general filarial primers followed by digestion with restriction enzymes that generates band polymorphisms clearly discriminating the three species and (2) PCR amplifications with species-specific primers to support the restriction analysis, in particular in the case of multiple infections.