A Bloody Mess: Dendritic Cells Use Hemophagocytosis to Regulate Viral Inflammation
Authors: Elizabeth Miller, Nina Bhardwaj
Journal: Immunity (2013)

Previous studies have highlighted the immune-dampening effects of apoptotic cell uptake by phagocytes. Ohyagi et al., 2013Ohyagi H. Onai N. Sato T. Yotsumoto S. Liu J. Akiba H. Yagita H. Atarashi K. Honda K. Roers A. Müller W. Kurabayashi K. Hosoi-Amaike M. Takahashi N. Hirokawa M. Matsushima K. Sawada K. Ohteki T. Immunity. 2013; 39 (this issue): 584-598Abstract Full Text Full Text PDF PubMed Scopus (55) Google Scholar expose a unique mechanism of immune regulation during viral infection, which is mediated through phagocytosis of apoptotic red cells by dendritic cells. Previous studies have highlighted the immune-dampening effects of apoptotic cell uptake by phagocytes. Ohyagi et al., 2013Ohyagi H. Onai N. Sato T. Yotsumoto S. Liu J. Akiba H. Yagita H. Atarashi K. Honda K. Roers A. Müller W. Kurabayashi K. Hosoi-Amaike M. Takahashi N. Hirokawa M. Matsushima K. Sawada K. Ohteki T. Immunity. 2013; 39 (this issue): 584-598Abstract Full Text Full Text PDF PubMed Scopus (55) Google Scholar expose a unique mechanism of immune regulation during viral infection, which is mediated through phagocytosis of apoptotic red cells by dendritic cells. Ideally, immunostimulatory and immunoregulatory factors during viral infection should be balanced to allow for sufficient generation of innate and adaptive immunity to facilitate viral clearance, while limiting uncontrolled immune activation that can result in tissue damage and/or death. Apoptotic cells (AC) have long been recognized to participate in immune regulation through their effects on phagocytic cells, including dendritic cells (DCs). The induction of tolerogenic DCs by AC bears important relevance to the fields of autoimmunity and tumor immunology. In contrast, relatively little has been described regarding the modulation of immune responses by ACs throughout the course of viral infections. In this issue of Immunity, Ohyagi et al., 2013Ohyagi H. Onai N. Sato T. Yotsumoto S. Liu J. Akiba H. Yagita H. Atarashi K. Honda K. Roers A. Müller W. Kurabayashi K. Hosoi-Amaike M. Takahashi N. Hirokawa M. Matsushima K. Sawada K. Ohteki T. Immunity. 2013; 39 (this issue): 584-598Abstract Full Text Full Text PDF PubMed Scopus (55) Google Scholar describe a process of imperfect compromise between antiviral immune responses and AC-mediated immune regulation. Secretion of interleukin-10 (IL-10) by inflammatory monocyte-derived dendritic cells (Mo-DCs) as a consequence of hemophagocytosis of apoptotic red blood cells (RBC) protected mice from lethal inflammatory responses following injection with Toll-like receptor (TLR) agonists (primarily CpG and poly I:C) or chronic lymphocytic choriomeningitis virus (LCMV) infection. In the case of LCMV infection, there was elevated and persistent viremia due to compromised immunity. In this study, the authors have described an unusual mechanism of immune regulation during viral infection that provides new perspectives on big picture questions regarding the complex interplay between virus and host. Ohyagi et al. have now demonstrated that hemophagocytosis of apoptotic RBC, following strong Toll-like receptor (TLR) stimulation or LCMV infection, is responsible for secretion of IL-10 by inflammatory moDCs that are rapidly recruited from the bone marrow (Ohyagi et al., 2013Ohyagi H. Onai N. Sato T. Yotsumoto S. Liu J. Akiba H. Yagita H. Atarashi K. Honda K. Roers A. Müller W. Kurabayashi K. Hosoi-Amaike M. Takahashi N. Hirokawa M. Matsushima K. Sawada K. Ohteki T. Immunity. 2013; 39 (this issue): 584-598Abstract Full Text Full Text PDF PubMed Scopus (55) Google Scholar). Transforming growth factor-β (TGF-β) concentrations also rose and were reliant upon IL-10 secretion following hemophagocytosis. They further identified hemophagocytosis by moDCs to be dependent upon type I interferon (IFN) signaling that promoted apoptosis and exposure of phosphatidylserine (PS, "eat me" signal) primarily on RBC, as well as upregulation of AC receptors on moDCs. During LCMV infection, hemophagocytosis and IL-10 secretion were significantly decreased upon blockade of multiple AC receptors simultaneously (Tim 1, Tim4, αVβ3 and αVβ5), with little effect upon blocking Tim1 and Tim4 individually. In vitro experiments further confirmed that these processes were primarily dependent upon PS binding to integrins (αVβ3 and αVβ5) via milk-fat globule epidermal growth factor (EGF) factor 8 (MFG-E8), a protein that bridges PS-expressing AC to integrins on DCs. Though alphaV integrins, in particular αVβ8, have also been implicated in enhancement of TGF-β secretion by DC leading to T regulatory cell development in mouse models of mucosal tolerance (Travis et al., 2007Travis M.A. Reizis B. Melton A.C. Masteller E. Tang Q. Proctor J.M. Wang Y. Bernstein X. Huang X. Reichardt L.F. et al.Nature. 2007; 449: 361-365Crossref PubMed Scopus (405) Google Scholar), here the authors describe immunomodulation of moDC function following binding to αVβ3 or αVβ5 integrins by AC. Furthermore, the findings by Ohyagi et al., 2013Ohyagi H. Onai N. Sato T. Yotsumoto S. Liu J. Akiba H. Yagita H. Atarashi K. Honda K. Roers A. Müller W. Kurabayashi K. Hosoi-Amaike M. Takahashi N. Hirokawa M. Matsushima K. Sawada K. Ohteki T. Immunity. 2013; 39 (this issue): 584-598Abstract Full Text Full Text PDF PubMed Scopus (55) Google Scholar add to the small but growing body of literature implicating AC as a means to modulate immune function during chronic viral infections and identify RBC as a significant, unexpected source of AC in their model. Recently, Frleta et al., 2012Frleta D. Ochoa C.E. Kramer H.B. Khan S.A. Stacey A.R. Borrow P. Kessler B.M. Haynes B.F. Bhardwaj N. J. Clin. Invest. 2012; 122: 4685-4697Crossref PubMed Scopus (40) Google Scholar reported that apoptotic microparticles from dying cells found circulating during acute HIV-1 infection inhibit moDC function by abrogating secretion of proinflammatory cytokines and T cell priming capacity. In contrast to the current study, this process is not found to be mediated by binding to classic AC receptors on moDC or associated with IL-10 secretion. Rather, apoptotic MPs bind and inhibit DCs via the hyaluronate receptor CD44 and induce the production of TGF-β. Whether "good" or "bad," it is clear that a lot hinges on IL-10 in the chronic LCMV model. In seminal studies by Brooks et al., 2006Brooks D.G. Trifilo M.J. Edelmann K.H. Teyton L. McGavern D.B. Oldstone M.B. Nat. Med. 2006; 12: 1301-1309Crossref PubMed Scopus (746) Google Scholar, it was determined that secretion of IL-10 by infected CD8α-CD4+ DCs facilitates the establishment of chronic infection by the clone 13 variant, and its blockade enhances adaptive immunity and viral clearance. IL-10 is known to dysregulate DC function and drive T cells toward "exhaustion." Recent studies have established the dependence of DC IL-10 secretion on type I IFN signaling during chronic LCMV infection (Teijaro et al., 2013Teijaro J.R. Ng C. Lee A.M. Sullivan B.M. Sheehan K.C. Welch M. Schreiber R.D. de la Torre J.C. Oldstone M.B. Science. 2013; 340: 207-211Crossref PubMed Scopus (539) Google Scholar). Ohyagi et al. now provide a mechanistic link between type I IFN and IL-10 and in many ways support these prior studies but reveal an unexpected outcome. Here, despite enhanced cytotoxic T lymphocyte (CTL) function and improved viral control following conditional deletion of Il10 in CD11c+ DCs, viral infection did not resolve as has been previously demonstrated upon disruption of IL-10 signaling (Ohyagi et al., 2013Ohyagi H. Onai N. Sato T. Yotsumoto S. Liu J. Akiba H. Yagita H. Atarashi K. Honda K. Roers A. Müller W. Kurabayashi K. Hosoi-Amaike M. Takahashi N. Hirokawa M. Matsushima K. Sawada K. Ohteki T. Immunity. 2013; 39 (this issue): 584-598Abstract Full Text Full Text PDF PubMed Scopus (55) Google Scholar, Brooks et al., 2006Brooks D.G. Trifilo M.J. Edelmann K.H. Teyton L. McGavern D.B. Oldstone M.B. Nat. Med. 2006; 12: 1301-1309Crossref PubMed Scopus (746) Google Scholar). Instead, these mice displayed heightened damage to tissues and increased mortality attributed to unchecked inflammation (Figure 1) (Ohyagi et al., 2013Ohyagi H. Onai N. Sato T. Yotsumoto S. Liu J. Akiba H. Yagita H. Atarashi K. Honda K. Roers A. Müller W. Kurabayashi K. Hosoi-Amaike M. Takahashi N. Hirokawa M. Matsushima K. Sawada K. Ohteki T. Immunity. 2013; 39 (this issue): 584-598Abstract Full Text Full Text PDF PubMed Scopus (55) Google Scholar). IL-10, therefore, confers a host survival benefit by mitigation of immune-mediated injury. The contribution of direct LCMV infection in inflammatory moDCs to hemophagocytosis and IL-10 production (not directly assessed here), remains unclear, despite the fact that DCs are the major IL-10 producing and virus-infected cells in vivo (Brooks et al., 2006Brooks D.G. Trifilo M.J. Edelmann K.H. Teyton L. McGavern D.B. Oldstone M.B. Nat. Med. 2006; 12: 1301-1309Crossref PubMed Scopus (746) Google Scholar). Given that strong TLR signaling also induces hemophagocytosis and IL-10 (Ohyagi et al., 2013Ohyagi H. Onai N. Sato T. Yotsumoto S. Liu J. Akiba H. Yagita H. Atarashi K. Honda K. Roers A. Müller W. Kurabayashi K. Hosoi-Amaike M. Takahashi N. Hirokawa M. Matsushima K. Sawada K. Ohteki T. Immunity. 2013; 39 (this issue): 584-598Abstract Full Text Full Text PDF PubMed Scopus (55) Google Scholar), antigen seems to be irrelevant. Rather, it is possible that viral RNA elicits hemophagocytosis through myeloid differentiation primary response gene 88 (MyD88), recently identified to be necessary for the development of hemophagocytic lymphohistiocytosis (Krebs et al., 2011Krebs P. Crozat K. Popkin D. Oldstone M.B. Beutler B. Blood. 2011; 117: 6582-6588Crossref PubMed Scopus (48) Google Scholar), and resultant type I IFN production. The effects observed upon inflammatory moDCs therefore might be an indirect consequence of LCMV infection. The divergent outcomes of tissue injury and mortality following disruption of Il10 between the current study and previously established findings in the LCMV model call for some speculation relating to how the magnitude of IL-10 disruption might influence the course of disease. One key difference is that Ohyagi et al. eliminate IL-10 secreting capacity only in CD11c+ DCs versus complete genetic deletion of Il10 as has been previously studied (Brooks et al., 2006Brooks D.G. Trifilo M.J. Edelmann K.H. Teyton L. McGavern D.B. Oldstone M.B. Nat. Med. 2006; 12: 1301-1309Crossref PubMed Scopus (746) Google Scholar), leaving significant residual amounts in the blood (approximately 30% of WT). This decrement in IL-10 was partially effective in boosting virus-specific cytolytic T cell (CTL) number and reducing circulating virus, however, it was not sufficient for viral clearance (Ohyagi et al., 2013Ohyagi H. Onai N. Sato T. Yotsumoto S. Liu J. Akiba H. Yagita H. Atarashi K. Honda K. Roers A. Müller W. Kurabayashi K. Hosoi-Amaike M. Takahashi N. Hirokawa M. Matsushima K. Sawada K. Ohteki T. Immunity. 2013; 39 (this issue): 584-598Abstract Full Text Full Text PDF PubMed Scopus (55) Google Scholar). In the setting of now-lowered concentrations of immunoregulatory factors, this resulted in promoting tissue inflammation, injury, and death. On the other hand, in the complete absence of IL-10, T cell exhaustion is averted, effector T cell responses are established, and virus is quickly cleared with minimal tissue injury (Brooks et al., 2006Brooks D.G. Trifilo M.J. Edelmann K.H. Teyton L. McGavern D.B. Oldstone M.B. Nat. Med. 2006; 12: 1301-1309Crossref PubMed Scopus (746) Google Scholar). Therefore, the concentrations of immunoregulatory factors that are elicited during viral infection, and their maintenance, might ultimately determine the host's outcome. Parallel concepts exist in models of pathogenic versus nonpathogenic SIV in nonhuman primates. Although both develop chronic infection with sustained viremia, nonpathogenic infection is marked by the upregulation of a distinct set of immunoregulatory responses including IL-10 and TGF-β. In contrast, the absence of these dampening factors along with sustained type I IFN signaling leads to chronic immune activation and development of immunodeficiency in pathogenic SIV infection (Jacquelin et al., 2009Jacquelin B. Mayau V. Targat B. Liovat A.S. Kunkel D. Petitjean G. Dillies M.A. Roques P. Butor C. Silvestri G. et al.J. Clin. Invest. 2009; 119: 3544-3555PubMed Google Scholar). Immune regulation in the acute phase of infection might be necessary at times to alleviate mortality from immunopathology while facilitating viral persistence. It remains possible that opportunities for intervention exist during the chronic phases of disease to relieve immune exhaustion and enhance viral control. The critical importance of timing is highlighted in a study of blockade of the inhibitory ligand receptor pair, PD-1/PD-L1, during LCMV infection (Barber et al., 2006Barber D.L. Wherry E.J. Masopust D. Zhu B. Allison J.P. Sharpe A.H. Freeman G.J. Ahmed R. Nature. 2006; 439: 682-687Crossref PubMed Scopus (3012) Google Scholar). Whereas infection of Cd274−/− mice results in early mortality similar to results of Il10−/− moDC seen by Ohyagi et al., 2013Ohyagi H. Onai N. Sato T. Yotsumoto S. Liu J. Akiba H. Yagita H. Atarashi K. Honda K. Roers A. Müller W. Kurabayashi K. Hosoi-Amaike M. Takahashi N. Hirokawa M. Matsushima K. Sawada K. Ohteki T. Immunity. 2013; 39 (this issue): 584-598Abstract Full Text Full Text PDF PubMed Scopus (55) Google Scholar, blockade of PD-L1 later in course of infection relieved CTL exhaustion and lowered viral load. Similarly, positive effects on immune function, viral control, and survival are observed in the chronic SIV model following PD-1 blockade when antibodies are administered even 10 weeks after infection (Velu et al., 2009Velu V. Titanji K. Zhu B. Husain S. Pladevega A. Lai L. Vanderford T.H. Chennareddi L. Silvestri G. Freeman G.J. et al.Nature. 2009; 458: 206-210Crossref PubMed Scopus (601) Google Scholar). Moreover, IL-10R and type I interferon receptor (IFNAR1) blockade in the persistent phase of LCMV infection has been shown to improve viral control and decrease T cell exhaustion in the case of IL-10 neutralization (Teijaro et al., 2013Teijaro J.R. Ng C. Lee A.M. Sullivan B.M. Sheehan K.C. Welch M. Schreiber R.D. de la Torre J.C. Oldstone M.B. Science. 2013; 340: 207-211Crossref PubMed Scopus (539) Google Scholar, Brooks et al., 2006Brooks D.G. Trifilo M.J. Edelmann K.H. Teyton L. McGavern D.B. Oldstone M.B. Nat. Med. 2006; 12: 1301-1309Crossref PubMed Scopus (746) Google Scholar). Future work to fine-tune the kinetics of hemophagocytosis and IL-10 blockade during LCMV infection in this model is warranted to determine whether disruption of signaling at later phases of infection would be of overall benefit. Overt, symptomatic hemophagocytic syndrome (HPS) is a clinical disorder that is associated with infection, genetic defects, malignancy, and autoimmune disease. It is characterized by uncontrolled activation of lymphocytes and monocytes, elevated serum cytokines, multiorgan dysfunction, and often death, some features of which manifest in chronic LCMV infection. HPS's hallmark is phagocytosis of various blood cells by reactive histiocytes in the bone marrow and other lymphoid tissue. Commonly assumed to drive the pathology, the significance of this feature in disease pathogenesis remains unclear. Though HPS is a rare complication of chronic infections, preliminary evidence suggests that lower level, subclinical hemophagocytosis might occur more frequently during chronic infections including HIV. Given the link between type I IFN and IL-10 induction, it is interesting to speculate whether type I IFN elicited during the course of infection programs for HPS and possibly IL-10 production, by promoting apoptotic cell death and phagocytic cell uptake of AC. IL-10 can be elevated in infections such as HIV, though its exact role(s) in immunopathogenesis and regulation during the various phases of infection have yet to be clearly defined. Preliminary studies in HIV again support the notion that timing is critical when it comes to immune regulation. Human IL-10 promoter polymorphisms resulting in higher IL-10 production have been shown to correlate with elevated viral loads during acute infection but might protect against CD4+ T cell loss and disease progression during later stages of infection (Naicker et al., 2009Naicker D.D. Werner L. Kormuth E. Passmore J.A. Mlisana K. Karim S.A. Ndung'u T. CAPRISA Acute Infection Study TeamJ. Infect. Dis. 2009; 200: 448-452Crossref PubMed Scopus (57) Google Scholar). A greater understanding of these kinetics in various viral infections and the underlying mechanisms that govern secretion would be helpful in determining the possible therapeutic utility of cleaning up the "bloody mess" elicited during infection. Monocyte-Derived Dendritic Cells Perform Hemophagocytosis to Fine-Tune Excessive Immune ResponsesOhyagi et al.ImmunitySeptember 12, 2013In BriefBecause immune responses simultaneously defend and injure the host, the immune system must be finely regulated to ensure the host's survival. Here, we have shown that when injected with high Toll-like receptor ligand doses or infected with lymphocytic choriomeningitis virus (LCMV) clone 13, which has a high viral turnover, inflammatory monocyte-derived dendritic cells (Mo-DCs) engulfed apoptotic erythroid cells. In this process, called hemophagocytosis, phosphatidylserine (PS) served as an "eat-me" signal. Full-Text PDF Open Archive