Date Published: August 28, 2018
Publisher: Public Library of Science
Author(s): Yvonne Lueder, Katrin Heller, Christiane Ritter, Kirsten A. Keyser, Karen Wagner, Xiaokun Liu, Martin Messerle, Felix R. Stahl, Stephan Halle, Reinhold Förster, Annette Oxenius.
Human cytomegalovirus (CMV) and mouse cytomegalovirus (MCMV) infection share many characteristics. Therefore infection of mice with MCMV is an important tool to understand immune responses and to design vaccines and therapies for patients at the risk of severe CMV disease. In this study, we investigated the immune response in the lungs following acute infection with MCMV. We used multi-color fluorescence microscopy to visualize single infected and immune cells in nodular inflammatory foci (NIFs) that formed around infected cells in the lungs. These NIFs consisted mainly of myeloid cells, T cells, and some NK cells. We found that the formation of NIFs was essential to reduce the number of infected cells in the lung tissue, showing that NIFs were sites of infection as well as sites of immune response. Comparing mice deficient for several leukocyte subsets, we identified T cells to be of prime importance for restricting MCMV infection in the lung. Moreover, T cells had to be present in NIFs in high numbers, and CD4 as well as CD8 T cells supported each other to efficiently control virus spread. Additionally, we investigated the effects of perforin and interferon-gamma (IFNγ) on the virus infection and found important roles for both mechanisms. NK cells and T cells were the major source for IFNγ in the lung and in in vitro assays we found that IFNγ had the potential to reduce plaque growth on primary lung stromal cells. Notably, the T cell-mediated control was shown to be perforin-independent but IFNγ-dependent. In total, this study systematically identifies crucial antiviral factors present in lung NIFs for early containment of a local MCMV infection at the single cell level.
The immune response against CMV infection in humans and murine cytomegalovirus (MCMV) in mice has been studied for decades . Understanding immune-cell mediated control of CMV infection is of essential interest since immunocompromised patients are particularly susceptible to CMV-disease . From earlier human and mouse studies, it is known that virus-specific T cell responses contribute to virus control in both species [3,4]. It is thought that both CD8 T cell-mediated killing of virus-infected cells and secretion of cytokines by CD8 and CD4 T cells contribute to protective immunity [4,5]. Since different lines of evidence indicate that T cells can control CMV infection, the adoptive transfer of HCMV-specific T cells has been used in clinical trials . In patients that suffer from CMV reactivation or primary infection during periods of severe immunosuppression, the infusion of donor HCMV-specific T cells was found to improve the control of infection [7,8]. However, it remains unclear how CD8 T cells are able to control CMV infection, given that both HCMV and MCMV encode for proteins that efficiently down-modulate surface major histocompatibility complex class I (MHCI) on infected cells . This immune evasion mechanism can prevent virus-specific T cells from recognizing their target cells and thus interferes with CD8 T cell-mediated killing . Other immunoevasins interfere with the presentation of antigen via major histocompatibility complex class II (MHCII) molecules to avoid recognition by CD4 T cells [11,12] but they also downregulate ligands of natural killer (NK) cell receptors to avoid cytotoxic killing .
The immune response initiated by CMV infection can protect the host from severe disease. However, several immunosuppressive conditions can lead to recurrence of an unnoticed primary infection and during pregnancy primary infection or reactivation can lead to vertical transmission of the virus to the neonate . Therefore, it is important to understand why the immune system can control, but not eradicate CMV infection in different organs. There are still many uncertainties regarding the antiviral factors needed to control CMV infection. In the mouse model, a classical read-out to assess the viral load of an organ is to determine viral titers via plaque assays or quantification of reporter virus-expressed luciferase activity within an organ of interest. Although these approaches can give a general overview of the viral load in individual organs, the localization of infected cells, the infected cell type, and the micro-anatomical context remain enigmatic.