NK-mediated elimination of mutant lymphocytes that have lost expression of MHC class I molecules

Rev1 overexpression accelerates N-methyl-N-nitrosourea (MNU)-induced thymic lymphoma by increasing mutagenesis

Rev1 has two important functions in the translesion synthesis pathway, including dCMP transferase activity, and acts as a scaffolding protein for other polymerases involved in translesion synthesis. However, the role of Rev1 in mutagenesis and tumorigenesis in vivo remains unclear. We previously generated Rev1-overexpressing (Rev1-Tg) mice and reported that they exhibited a significantly increased incidence of intestinal adenoma and thymic lymphoma (TL) after N-methyl-N-nitrosourea (MNU) treatment. In this study, we investigated mutagenesis of MNU-induced TL tumorigenesis in wild-type (WT) and Rev1-Tg mice using diverse approaches, including whole-exome sequencing (WES). In Rev1-Tg TLs, the mutation frequency was higher than that in WT TL in most cases. However, no difference in the number of nonsynonymous mutations in the Catalogue of Somatic Mutations in Cancer (COSMIC) genes was observed, and mutations involved in Notch1 and MAPK signaling were similarly detected in both TLs. Mutational signature analysis of WT and Rev1-Tg TLs revealed cosine similarity with COSMIC mutational SBS5 (aging-related) and SBS11 (alkylation-related). Interestingly, the total number of mutations, but not the genotypes of WT and Rev1-Tg, was positively correlated with the relative contribution of SBS5 in individual TLs, suggesting that genetic instability could be accelerated in Rev1-Tg TLs. Finally, we demonstrated that preleukemic cells could be detected earlier in Rev1-Tg mice than in WT mice, following MNU treatment. In conclusion, Rev1 overexpression accelerates mutagenesis and increases the incidence of MNU-induced TL by shortening the latency period, which may be associated with more frequent DNA damage-induced genetic instability.

Depletion of NK Cells Resistant to Ionizing Radiation Increases Mutations in Mice After Whole-body Irradiation

BACKGROUND

Ionizing radiation is a very powerful genetic mutagenic agent. Although immune cells are very sensitive to radiation, their sensitivity varies between different types of immune cell. We hypothesized that radiation-resistant immune cells survive after irradiation and then play a role in removing mutant cells.

MATERIALS AND METHODS

Splenic lymphocytes and mice were irradiated with γ-rays. Cell populations were analyzed using flow cytometry after dyeing with antibodies and expression of B-cell lymphoma 2 (BCL2) was measured by western blot analysis. To deplete natural killer (NK) cells, anti-asialo GM1 antiserum was used. Micronuclei in polychromatic erythrocytes were measured by May-Grunwald/Giemsa staining. H-2Kb loss variant in T-cells induced by irradiation of B6C3F1 mice were detected by flow cytometry.

RESULTS

When splenic lymphocytes were irradiated in vitro, B cells notably died, while NK cells did not. In vivo, on the third day after whole-body irradiation, the total number of lymphocytes in the spleen decreased rapidly, but the proportion of NK cells was approximately three times higher than that of the normal control group. In addition, it was confirmed that high expression of BCL2 in NK cells was maintained after irradiation, whereas that of B-cells was not. Removal of NK cells by injection with anti-asialo GM1 antiserum immediately after irradiation increased the micronuclei of polychromatic erythrocytes in the bone marrow and the variant fraction with H-2kb loss in the spleen.

CONCLUSION

These results provide important evidence that radioresistant NK cells apparently survive by escaping apoptosis in the early stages after irradiation, and work to eliminate mutant cells resulting from γ-ray irradiation. Future studies are needed to reveal why NK cells are resistant to radiation and the in-depth mechanisms involved in the elimination of radiation-induced mutant cells.

HIV-1 Nef: a master manipulator of the membrane trafficking machinery mediating immune evasion

BACKGROUND

Many viral genomes encode a limited number of proteins, illustrating their innate efficiency in bypassing host immune surveillance. This concept of genomic efficiency is exemplified by the 9 kb RNA genome of human immunodeficiency virus 1 (HIV-1), encoding 15 proteins sub-divided according to function. The enzymatic group includes proteins such as the drug targets reverse transcriptase and protease. In contrast, the accessory proteins lack any known enzymatic or structural function, yet are essential for viral fitness and HIV-1 pathogenesis. Of these, the HIV-1 accessory protein Nef is a master manipulator of host cellular processes, ensuring efficient counterattack against the host immune response, as well as long-term evasion of immune surveillance. In particular, the ability of Nef to downmodulate major histocompatibility complex class I (MHC-I) is a key cellular event that enables HIV-1 to bypass the host's defenses by evading the adaptive immune response.

SCOPE OF REVIEW

In this article, we briefly review how various pathogenic viruses control cell-surface MHC-I, and then focus on the mechanisms and implications of HIV-1 Nef-mediated MHC-I downregulation via modulation of the host membrane trafficking machinery.

CONCLUSION

The extensive interaction network formed between Nef and numerous membrane trafficking regulators suggests that Nef's role in evading the immune surveillance system intersects multiple host membrane trafficking pathways.

SIGNIFICANCE

Nef's ability to evade the immune surveillance system is linked to AIDS pathogenesis. Thus, a complete understanding of the molecular pathways that are subverted by Nef in order to downregulate MHC-I will enhance our understanding of HIV-1's progression to AIDS.

NK cell self tolerance, responsiveness and missing self recognition

Natural killer (NK) cells represent a first line of defense against pathogens and tumor cells. The activation of NK cells is regulated by the integration of signals deriving from activating and inhibitory receptors expressed on their surface. However, different NK cells respond differently to the same stimulus, be it target cells or agents that crosslink activating receptors. The processes that determine the level of NK cell responsiveness have been referred to collectively as NK cell education. NK cell education plays an important role in steady state conditions, where potentially auto-reactive NK cells are rendered tolerant to the surrounding environment. According to the "tuning" concept, the responsiveness of each NK cell is quantitatively adjusted to ensure self tolerance while at the same time ensuring useful reactivity against potential threats. MHC-specific inhibitory receptors displayed by NK cells play a major role in tuning NK cell responsiveness, but recent studies indicate that signaling from activating receptors is also important, suggesting that the critical determinant is an integrated signal from both types of receptors. An important and still unresolved question is whether NK cell education involves interactions with a specific cell population in the environment. Whether hematopoietic and/or non-hematopoietic cells play a role is still under debate. Recent results demonstrated that NK cell tuning exhibits plasticity in steady state conditions, meaning that it can be re-set if the MHC environment changes. Other evidence suggests, however, that inflammatory conditions accompanying infections may favor high responsiveness, indicating that inflammatory agents can over-ride the natural tendency of NK cells to adjust to the steady state environment. These findings raise many questions such as whether viruses and tumor cells manipulate NK cell responsiveness to evade immune-recognition. As knowledge of the underlying processes grows, the possibility of modulating NK cell responsiveness for therapeutic purposes is becoming increasingly attractive, and is now under serious investigation in clinical studies.

Generation of loss of heterozygosity and its dependency on p53 status in human lymphoblastoid cells

Loss of heterozygosity (LOH) is a critical event in the development of human cancers. LOH is thought to result from either a large deletion or recombination between homologous alleles during repair of DNA double-strand breaks (DSBs). These types of genetic alterations produce mutations in the TK gene mutation assay, which detects a wide mutational spectrum, ranging from point mutations to LOH-type mutations. TK6, a human lymphoblastoid cell line, is heterozygous for the thymidine kinase (TK) gene and has a wild-type p53 gene. The related cell lines, TK6-E6 and WTK-1, which are p53-deficient and p53-mutant (Ile237), respectively, are also heterozygous for the TK gene and LOH-type mutation can be detected in these cells. Therefore, comparative studies of TK mutation frequency and spectrum with these cell lines are useful for elucidating the role of p53 in generating LOH and maintaining genomic stability in human cells. We demonstrate here that LOH and its associated genomic instability strongly depend on the p53 status in these cells. TK6-E6 and WTK-1 are defective in the G1/S checkpoint and in apoptosis. Unrepaired DSBs that escape from the checkpoint can potentially initiate genomic instability after DNA replication, resulting in LOH and a variety of chromosome changes. Moreover, genomic instability is enhanced in WTK-1 cells. It is likely that the mutant p53 protein in WTK-1 cells increases LOH in a dominant-negative manner due to its abnormal recombination capacity. We discuss the mutator phenotype and genomic instability associated with p53 inactivation with the goal of elucidating the mechanisms of mutation and DNA repair in untargeted mutagenesis.

The innate immune response to tumors and its role in the induction of T-cell immunity

Recent genetic studies have resurrected the concept that the adaptive and innate immune systems play roles in tumor surveillance. Natural killer (NK) cells recognize many tumor cells but not normal self cells, and they are thought to aid in the elimination of nascent tumors. Two main strategies are employed by NK cells to recognize tumor targets. Many tumor cells down-regulate class I major histocompatibility complex (MHC) molecules, thus releasing the NK cell from the inhibition provided by class I MHC-specific inhibitory receptors ('missing self recognition'). More recently, it has become clear that a stimulatory receptor expressed by NK cells, T cells and macrophages (NKG2D) recognizes ligands (MHC class I chain related [MIC], H6O, retinoic acid early inducible [Rae1] and UL16 binding proteins [ULBP]) that are up-regulated on tumor cells and virally infected cells but are not expressed well by normal cells. Ectopic expression of these ligands on tumor cells leads to the potent rejection of the tumors in vivo. Importantly, mice that previously rejected the ligand+ tumor cells develop T-cell immunity to the parental (ligand-) tumor cells. The recognition of induced-self ligands as a strategy to recognize abnormal self sets a precedent for a new immune recognition strategy of the innate immune system.

Elevated in vivo frequencies of mutant T cells with altered functional expression of the T-cell receptor or hypoxanthine phosphoribosyltransferase genes in p53-deficient mice

We have studied the effects of a defect in the p53 gene on spontaneous and radiation-induced somatic mutation frequencies in vivo by measuring T-cell receptor (TCR) and hypoxanthine phosphoribosyltransferase (HPRT) mutant frequencies (MFs) in p53 deficient mice both before and after exposure to X-irradiation. In the absence of irradiation, the TCR and HPRT mutant frequencies were roughly two-fold higher in p53 null (-/-) mice than in wild-type (+/+) mice. Unexpectedly, the TCR and HPRT MFs were slightly lower in heterozygote p53 (+/-) than in wild-type (+/+) mice, however. After 2 weeks 2Gy whole body irradiation the TCR and HPRT MFs were about two-fold higher in the p53 null (-/-) and p53 (+/-) mice than in the wild-type. Taken together, these findings suggest that a defect in the p53 gene may lead to TCR and HPRT mutants being recovered at higher frequencies in both irradiated and unirradiated mice, but it should be emphasized that the effects we have observed are not particularly strong, albeit that they are statistically significant. Interestingly, several of the highest TCR MF values that we observed in the course of our experiments were recorded in p53 (-/-) animals that had developed thymomas and hence appeared to be cancer prone.

Possible role of natural killer cells in negative selection of mutant lymphocytes that fail to express the human leukocyte antigen-A2 allele

Increased frequencies of cells carrying mutations at several loci have been found in the blood cells of atomic-bomb (A-bomb) survivors upon testing four or five decades after the bombing. Interestingly, though, we have been unable to demonstrate any radiation-associated increases in the frequencies of mutant blood cells in which human leukocyte antigen (HLA)-A expression has been disrupted; this is true both of preliminary tests on the T cells of a small subset of A-bomb survivors and of the much more extensive study reported here in which we screened a much larger group of survivors for HLA-A2 loss mutations in B cells and granulocytes as well as in T cells. In attempting to explain our inability to detect any increases in HLA-A2-negative cell numbers in HLA-A2 heterozygous individuals exposed to A-bomb irradiation, we decided to test the hypothesis that HLA-A mutant lymphocytes might well have been induced by radiation exposure in much the same way as every other type of mutant we encountered, but may subsequently have been eliminated by the strong negative selection associated with their almost inevitable exposure to autologous natural killer (NK) cells in the bloodstream of each of the individuals concerned. We now report that mutant B lymphocyte cell lines that have lost the ability to express the HLA-A2 antigen do indeed appear to be much more readily eliminated than their parental heterozygous counterparts during co-culture in vitro with autologous NK cells. We make this claim first because we have observed that adding autologous NK cells to in vitro cultures of HLA-A2 heterozygous B or T cell lines appeared to cause a dose-dependent decrease in the numbers of HLA-A2-negative mutants that could be detected over a period of 3 days, and second because when we used peripheral blood HLA-A2 heterozygous lymphocyte cultures from which most of the autologous NK cells had been removed we found that we were able to detect newly-arising HLA-A2 mutant T cells in substantial numbers. Taken together, these results strongly support the hypothesis that autologous NK cells are responsible for eliminating mutant lymphocytes that have lost the ability to express self-HLA class I molecules in vivo, and may well therefore explain why we have been unable to detect increased frequencies of HLA-A2 mutants in samples from any of the 164 A-bomb survivors whose HLA-A2 heterozygote status made their lymphocytes suitable for our tests.

Recognition of tumor cells by the innate immune system

There has been a rapid increase in our understanding of the cellular components of the innate immune system, the receptors used to distinguish changes in homeostasis, and how these components integrate into an anti-tumor effector response. Recently, significant progress has been made in the identification of ligands for receptors that activate NK cells, and the results have implications for the recognition of tumor cells.