Myeloid-specific Acat1 ablation attenuates inflammatory responses in macrophages, improves insulin sensitivity, and suppresses diet-induced obesity

Macrophages are phagocytes that play important roles in health and diseases. Acyl-CoA:cholesterol acyltransferase 1 (ACAT1) converts cellular cholesterol to cholesteryl esters and is expressed in many cell types. Unlike global Acat1 knockout (KO), myeloid-specific Acat1 KO ( Acat1^-) does not cause overt abnormalities in mice. Here, we performed analyses in age- and sex-matched Acat1^-M/-M and wild-type mice on chow or Western diet and discovered that Acat1^-M/-M mice exhibit resistance to Western diet-induced obesity. On both chow and Western diets, Acat1^-M/-M mice display decreased adipocyte size and increased insulin sensitivity. When fed with Western diet, Acat1^-M/-M mice contain fewer infiltrating macrophages in white adipose tissue (WAT), with significantly diminished inflammatory phenotype. Without Acat1, the Ly6C^hi monocytes express reduced levels of integrin-β₁, which plays a key role in the interaction between monocytes and the inflamed endothelium. Adoptive transfer experiment showed that the appearance of leukocytes from Acat1^-M/-M mice to the inflamed WAT of wild-type mice is significantly diminished. Under Western diet, Acat1^-M/-M causes suppression of multiple proinflammatory genes in WAT. Cell culture experiments show that in RAW 264.7 macrophages, inhibiting ACAT1 with a small-molecule ACAT1-specific inhibitor reduces inflammatory responses to lipopolysaccharide. We conclude that under Western diet, blocking ACAT1 in macrophages attenuates inflammation in WAT. Other results show that Acat1^-M/-M does not compromise antiviral immune response. Our work reveals that blocking ACAT1 suppresses diet-induced obesity in part by slowing down monocyte infiltration to WAT as well as by reducing the inflammatory responses of adipose tissue macrophages.

MicroRNA-155 plays a critical role in the establishment of latency in murine γ-herpesvirus infection (VIR1P.1148)

Gammaherpesvirus infection in immunosuppressed populations is associated with severe disease. MicroRNA-155 (miR155) has been shown to play significant roles in the immune response, including in the formation of germinal centers (GC) and the development and maturation of T follicular helper cells (Tfh). We sought to determine the relative contribution of miR-155 in the T and B cell compartments to latent murine gammaherpesvirus infection. Mice deficient in miR-155 had a 100-fold decrease in latent viral load, measured by qPCR, and exhibited decreases in both follicular helper cells and germinal center B cells. However, infection of mixed bone marrow chimeric mice showed a similar latent viral burden in both WT and miR-155 deficient B cells. Therefore the role of miR-155 may not be B cell intrinsic, but is likely mediated in part due to the regulation that miR155 exerts on the T follicular helper cell population. Studies using a YFP marker virus also address roles for miR-155 in virus reactivation and maintenance of latency. Further elucidating the relative role of miR155 on these cell populations will provide key insights into the development of latency in this model system.

Differentiation and function of murine gammaherpesvirus specific CD4+ T cells (VIR7P.1052)

Gammaherpesvirus infection in AIDS patients is associated with severe disease. CD4+ T cells are essential for immune control of viral infection and prevention of malignancy. Progressive loss of CD4+ T cells results in an erosion of immune surveillance, which may lead to gammaherpesvirus associated tumors. A recently generated murine gammaherpesvirus-68 gp150-specific TCR transgenic mouse provides a model for studying CD4+ T cells more precisely. We aimed to study differentiation of MHV-68-specific CD4+ T cells and the contribution of the CD4+ T cells in immune responses independent of CD8+ T cells and B cells. The gp150-specific CD4+ T cells differentiated into two interconvertible populations based on Ly6C expression. The majority of these cells differentiated into Th1 cells, producing IFN-γ and expressing T-bet but not eomes, GATA3, RORγ or Bcl6. Ly6C expression was positively correlated with T-bet expression, IFN-γ, TNF-α and granzyme B production. The gp150-specific CD4+ T cells prolonged survival of MHV-68-infected mice and controlled growth of MHV-68-immortalized SL1 cells in gp150-Tg mice without CD8+ T cells and B cells. Their antiviral functions were dependent on IFN-γ, and the CD4+ T cells also had cytolytic activity. In summary, CD4+ T cells that arise during MHV-68 infection are mostly Th1 and display antiviral ability in the absence of CD8+ T cells and B cells through IFN-γ.

Dissecting the role of retinoic acid receptor isoforms in the CD8 response to infection

Vitamin A deficiency leads to increased susceptibility to a spectrum of infectious diseases. The studies presented dissect the intrinsic role of each of the retinoic acid receptor (RAR) isoforms in the clonal expansion, differentiation, and survival of pathogen-specific CD8 T cells in vivo. The data show that RARα is required for the expression of gut-homing receptors on CD8(+) T cells and survival of CD8(+) T cells in vitro. Furthermore, RARα is essential for survival of CD8(+) T cells in vivo following Listeria monocytogenes infection. In contrast, RARβ deletion leads to modest deficiency in Ag-specific CD8(+) T cell expansion during infection. The defective survival of RARα-deficient CD8(+) T cells leads to a deficiency in control of L. monocytogenes expansion in the spleen. To our knowledge, these are the first comparative studies of the role of RAR isoforms in CD8(+) T cell immunity.

Characterization of IL-10 producing CD8 T cells that limit immune surveillance during chronic virus infection. (105.34)

Gammaherpesvirus infection in AIDS patients is associated with severe disease. Progressive loss of CD4 T cells results in an erosion of immune surveillance, which may lead to gammaherpesvirus associated tumors. Studies by our group and others have used murine γHV-68 infection in combination with CD4 depletion to model this breakdown in immune surveillance. Mice initially control the infection, but virus reactivates after 6 weeks, indicating a lack of long-term immune surveillance. We previously showed mice had elevated IL-10 and therapeutic IL-10R blockade reduced reactivation of the virus, implicating IL-10 as a key factor suppressing immune surveillance. We identified a population of IL-10 producing CD8 T cells only in the infected mice lacking CD4 T cells but not in intact mice. Here we characterize IL-10+ CD8 T cells and elucidate how they arise in both the acute and chronic phase of the infection. In the acute phase of the infection less IL-10 producing CD8s arose in the absence of CD4 help or CD40-CD40L signaling, while in the chronic phase IL-10+ CD8s only arose in the absence of CD4 help. IL-10 producing CD8s in the chronic phase showed an anergic phenotype in terms of proliferation in vitro. In addition, they suppressed the proliferation of effector CD8 T cell in vitro. Dissection of the mechanisms involved in the generation and the function of these suppressive IL-10 producing CD8 T cells may expose new targets for therapeutic intervention for AIDS patients.

Retinoic acid modulation of CD8 effector and memory differentiation. (110.4)

A plethora of work implicates important effects of the Vitamin A derivative, retinoic acid (RA), in myeloid differentiation, while fewer studies explore the role of RA on lymphoid cells. Most work has looked at the role of RA on CD4 T cells, focusing on regulatory CD4 T cell differentiation and maintenance of this phenotype along with some Th1 functions. There is little information about the role of RA in CD8 T cell differentiation, and even less on cell-intrinsic effects in the CD8 T cell. In this study we explored the role of RA on effector and memory differentiation in a direct and cell intrinsic manner in the context of vaccinia virus infection. We used dominant negative RA alpha-receptor (RARα DN) expressing mice and bone marrow chimeric mice containing both WT and RARα DN compartments. We observed the loss of the short-lived effector cell phenotype and an enhanced memory precursor phenotype at the effector and contraction phase of the CD8 response in the absence of RA signaling. Further, the lack of RA increased the proportion of central memory CD8s. Vitamin A is used in conjunction with various vaccines (e.g. Measles vaccine), these results will elucidate the effect of RA in shaping the effector and memory response helping to tailor the supplement to better enhance the outcome of vaccination.

MiR-155 modulates effector and memory CD8 T cell differentiation in response to murine γ herpes virus. (110.13)

MicroRNAs (miRNAs) have emerged as a major regulatory network with profound effects on many physiological processes. They are particularly important for determining cell fate decisions. Several processes mediate CD8 T cell differentiation. While many studies have revealed the importance of transcriptional and epigenetic regulation, it is likely that miRNAs also contribute to this programming. miRNAs are critical in determining B and CD4 T cell lineage, yet little is known about their effect on CD8 T cell differentiation. Studies with in vitro skewed central memory CD8 T cells show a down regulation of miR-155 relative to skewed effector memory cells. Using murine gammaherpesvirus 68 as a model, we examined the influence of miR-155 on CD8 differentiation in vivo. Our results showed lowered CD8 responses in the absence of miR-155 accompanied by a phenotypic skewing towards a memory precursor phenotype at the acute phase, which resulted in more central memory cells. The resulting memory cells in the absence of miR-155 were as functionally competent as their wild type counterparts, as revealed by their ability to recall to the same antigen. Our data indicate the importance of mIR-155 in effector differentiation, which may be a critical piece of the puzzle in CD8 differentiation giving useful information for vaccine research.

CD4+ T cell regulation of CD25 expression controls the development of effector CD8+ T cells in acute and chronic virus infections. (137.14)

Both CD4+ T cell help and IL-2 have been postulated to ‘program’ activated CD8+ T cells for memory cell development. However, the link between these two signals has not been well elucidated. We have studied effector and memory CD8+ T cell differentiation following infection with vaccinia virus and murine gammaherpes virus in the absence of both CD4+ T cells and CD25. Expression of CD25 on antigen-specific CD8+ T cells peaked 3-4 days after infection and was dependent on CD4+ T cell help. CD4+ T cell or CD25-deficiency led to normal early effector CD8+ T cell differentiation, but a subsequent lack of accumulation of CD8+ T cells resulting in overall decreased memory cell generation. Interestingly, KLRG1high CD127low short-lived effector cells were diminished in the absence of IL-2 signaling, while memory precursors remained intact. Interestingly, the mononucleosis-like expansion of Vβ4+ T cells seen in mice infected with MHV-68 was impaired in a similar fashion to antigen specific responses. Upon secondary antigen encounter, CD25-deficient CD8+ T cells were capable of undergoing robust expansion, but short-lived effector development was again impaired. These results demonstrate that CD4+ help and IL-2 signaling are linked via CD25 upregulation, which controls the expansion and differentiation of antigen-specific effector CD8+ T cells.

Strain-dependent requirement for IFN-γ for respiratory control and immunotherapy in murine gammaherpesvirus infection (42.1)

IFN-γ and perforin are important effector mechanisms used by CD8 T cells to clear virus-infected cells. Previous research has shown that either IFN-γ or pfp was non-essential for viral control after intranasal infection of mice with MHV-68. In this study, we use IFN/pfp double knockout mice to address if these two effector molecules play redundant roles in the control of acute infection with MHV-68 in Balb/C mice. Pfp KO mice and WT mice cleared infectious virus from the lungs, even following high dose infection. However IFN-γ KO and IFN/pfp DKO groups had higher virus titers in the lungs at day 10 post-infection and both groups had higher mortality rates. In IFN/pfp DKO mice, the virus titer and mortality rate were significant higher than IFN-γ KO mice, indicating a role for pfp in protection from disease. WT mice given IFN-γ blocking Ab also showed significant higher viral titers. In contrast, IFN-γ KO mice on a B6 background controlled respiratory infection comparably to WT mice. These data showed that pfp plays a redundant role in the control of virus replication but IFN-γ plays an essential role in Balb/C mice infected with MHV-68. We conclude that there is a marked strain-dependent difference in the effector mechanisms needed to control acute MHV-68 infection between B6 and Balb/C mice. In addition we show that immune therapy that re-establishes viral control after spontaneous reactivation in CD4-deficient mice depends upon pfp in B6 mice but IFN-γ in Balb/C mice.

Distinction of the memory B cell response to cognate antigen versus bystander inflammatory signals

The hypothesis that bystander inflammatory signals promote memory B cell (B(MEM)) self-renewal and differentiation in an antigen-independent manner is critically evaluated herein. To comprehensively address this hypothesis, a detailed analysis is presented examining the response profiles of B-2 lineage B220(+)IgG(+) B(MEM) toward cognate protein antigen in comparison to bystander inflammatory signals. After in vivo antigen encounter, quiescent B(MEM) clonally expand. Surprisingly, proliferating B(MEM) do not acquire germinal center (GC) B cell markers before generating daughter B(MEM) and differentiating into plasma cells or form structurally identifiable GCs. In striking contrast to cognate antigen, inflammatory stimuli, including Toll-like receptor agonists or bystander T cell activation, fail to induce even low levels of B(MEM) proliferation or differentiation in vivo. Under the extreme conditions of adjuvanted protein vaccination or acute viral infection, no detectable bystander proliferation or differentiation of B(MEM) occurred. The absence of a B(MEM) response to nonspecific inflammatory signals clearly shows that B(MEM) proliferation and differentiation is a process tightly controlled by the availability of cognate antigen.

A novel inhibitory mechanism of mitochondrion-dependent apoptosis by a herpesviral protein

Upon viral infection, cells undergo apoptosis as a defense against viral replication. Viruses, in turn, have evolved elaborate mechanisms to subvert apoptotic processes. Here, we report that a novel viral mitochondrial anti-apoptotic protein (vMAP) of murine γ-herpesvirus 68 (γHV-68) interacts with Bcl-2 and voltage-dependent anion channel 1 (VDAC1) in a genetically separable manner. The N-terminal region of vMAP interacted with Bcl-2, and this interaction markedly increased not only Bcl-2 recruitment to mitochondria but also its avidity for BH3-only pro-apoptotic proteins, thereby suppressing Bax mitochondrial translocation and activation. In addition, the central and C-terminal hydrophobic regions of vMAP interacted with VDAC1. Consequently, these interactions resulted in the effective inhibition of cytochrome c release, leading to the comprehensive inhibition of mitochondrion-mediated apoptosis. Finally, vMAP gene was required for efficient γHV-68 lytic replication in normal cells, but not in mitochondrial apoptosis-deficient cells. These results demonstrate that γHV-68 vMAP independently targets two important regulators of mitochondrial apoptosis-mediated intracellular innate immunity, allowing efficient viral lytic replication.

Apoptosis is a conserved cell death program that contributes to restriction of viral replication and elimination of infected cells. Whether triggered via internal inducers such as DNA damage or via external stimuli such as engagement of the death receptor, apoptosis takes place through a cascade of regulated internal proteolytic digestion, resulting in a collapse of cellular infrastructure, mitochondrial potential, genomic fidelity, and cell membrane integrity. Indeed, apoptosis represents a predominant form of virally infected cell demise. In response, viruses have evolved numerous ways of circumventing this host-cell apoptosis. Most of the DNA viruses including murine γ-herpesvirus 68 (γHV-68) are genetically equipped with anti-apoptotic ability to ensure viral replication and propagation. The authors have identified a new viral mitochondrial protein (vMAP) of γHV-68 that interacts with Bcl-2 and voltage-dependent anion channel 1 (VDAC1) in a genetically separable manner. These interactions markedly suppress Bax mitochondrial translocation and activation and inhibit cytochrome c release, leading to the comprehensive inhibition of mitochondrion-mediated apoptosis. The authors also demonstrate that vMAP gene is required for efficient γHV-68 lytic replication in normal cells, but not in mitochondrial apoptosis-deficient cells. These findings are entirely novel and significantly advance our understanding of how virus escapes host intracellular apoptosis-mediated innate immunity.

Simultaneous analysis of in vivo CD8+ T cell cytotoxicity against multiple epitopes using multicolor flow cytometry

CD8+ T cells play a critical role in host defense against infections and tumors. Analysis of cytotoxic function of antigen-specific CD8+ T cells in animal models would be important in optimizing vaccine design against infections and tumors. In vivo cytotoxicity assays using fluorescent cellular dyes have been used as a popular alternative to traditionally used in vitro (51)Cr-release assays. With the identification of multiple epitopes in various pathogen models, methods to simultaneously analyze cytotoxicity of CD8+ T cells to multiple epitopes in vivo would assist studies which aim to generate protective CD8+ T cell immunity to multiple epitopes. In this study, we evaluate the use of multiple fluorescent cellular dyes for the in vivo cytotoxicity assay. The use of 3 dyes allowed us to analyze the cytotoxicity of antigen-specific CD8+ T cell populations to multiple epitopes generated by virus infections, as well as their functional avidity, in vivo. Our studies extend the use of in vivo cytotoxicity assays to allow direct comparisons of cytotoxicity to various epitopes in the same animal and may also be applicable to assessment of in vitro cytotoxicity of human CD8+ T cells specific for multiple viral or tumor antigens in clinical settings.

Co-infection of malaria and gamma-herpesvirus: exacerbated lung inflammation or cross-protection depends on the stage of viral infection

In order to study the interaction between a gamma-herpesvirus and malaria we established a co-infection model that involves infection of mice with murine gamma-herpesvirus (MHV-68) and Plasmodium yoelii non-lethal strain (PYNL). To investigate the interaction between acute malaria and the lytic stage of MHV-68, the timing of infections was chosen such that the peak virus and parasite burdens would be present at the same time. Under this condition, we observed significant mortality in co-infected mice and aggressive lung inflammation with a marked influx of neutrophils and megakaryocytes. If mice were latently infected with MHV-68 and then co-infected with malaria we noticed significantly less viral load and parasitaemia. Using MHC/peptide tetramer staining we found that acute malaria reduces the anti-MHV-68 CD8+ T cell response in the animals that develop severe disease. Our study provides important fundamental information, which will be of use when devising strategies to combat infections with more than one agent, a situation that often occurs naturally.