Mast Cells Meet Cytomegalovirus: A New Example of Protective Mast Cell Involvement in an Infectious Disease

Abstract

Cytomegaloviruses (CMVs) belong to the β-subfamily of herpesviruses. Their host-to-host transmission involves the airways. As primary infection of an immunocompetent host causes only mild feverish symptoms, human CMV (hCMV) is usually not considered in routine differential diagnostics of common airway infections. Medical relevance results from unrestricted tissue infection in an immunocompromised host. One risk group of concern are patients who receive hematopoietic cell transplantation (HCT) for immune reconstitution following hematoablative therapy of hematopoietic malignancies. In HCT patients, interstitial pneumonia is a frequent cause of death from hCMV strains that have developed resistance against antiviral drugs. Prevention of CMV pneumonia requires efficient reconstitution of antiviral CD8 T cells that infiltrate lung tissue. A role for mast cells (MC) in the immune control of lung infection by a CMV was discovered only recently in a mouse model. MC were shown to be susceptible for productive infection and to secrete the chemokine CCL-5, which recruits antiviral CD8 T cells to the lungs and thereby improves the immune control of pulmonary infection. Here, we review recent data on the mechanism of MC-CMV interaction, a field of science that is new for CMV virologists as well as for immunologists who have specialized in MC.

Keywords: CCL-5 chemokine; CD8 T cells; airway infection; antiviral protection; lung infection; mast cell (MC) degranulation; pneumonia; viral mitochondria-localized inhibitor of apoptosis (vMIA).

Figure 1

MC-dependent wave of serum chemokine CCL-5. (n.s.) Not significant; (*) p < 0.05; (**) p < 0.01. (Yellow bars) Presence of MC. (Open bars) Absence of MC. Bars represent the range of data from individual mice. Median values are indicated. Results are displayed schematically based on original data published in [73].

Figure 2

(Top) Experimental protocol. (BMMC) Bone-marrow-derived MC. MC-deficient mice were reconstituted by intravenous infusion of BMMC derived from MC-sufficient donors. Experiments were performed 4 weeks after engraftment. (A) Two-color immunohistological image showing the confinement of lung infection by CD8 T cells in a nodular inflammatory focus (NIF) on day six after intravenous infection of MC-sufficient B6-WT (wild-type) mice; (a) alveoli lined with alveolar epithelium; (b) bronchioles lined with bronchiolar epithelium; and (s) alveolar septum. The bar marker represents 10 μm. (B) Inverse correlation between CD8 T-cell tissue infiltration and control of lung infection. Bars represent the range of data from individual mice with the median values indicated. (Light yellow bars) Constitutively MC-competent B6-WT mice; (open bars) MC-deficient sash mutants; (dark yellow bars) MC-deficient sash mutants made MC-competent by reconstitution with BMMC. (n.s.) Not significant; (*) p < 0.05; (***) p < 0.001. Results are displayed schematically and in a new arrangement based on original data published in [73].

Figure 3

Evidence for productive mCMV infection of MC in vivo. (A) Gene maps explaining the principle of Cre recombination for generating reporter virus mCMV-rec-egfp. (kbp) Kilobase pairs of the linear, double-stranded DNA genome of mCMV. It circularizes in the infected cell and does not integrate into the cellular genome. The genomic region of interest is shown in detail. Arrows indicate the direction of transcription. (P_hCMV) hCMV promoter–enhancer; (pA) polyadenylation. (B) Experimental protocol. Mcpt5-cre mice were infected intraperitoneally with virus mCMV-flox-egfp. (Flash symbol) Total-body γ-irradiation with a dose of 7 Gy for immunosuppression to facilitate in vivo spread of recombined virus mCMV-rec-egfp. (C) Infection of mouse embryo fibroblast (MEF) monolayer cultures by homogenates of the indicated organs taken on day eight after infection. (Left) eGFP fluorescence and phase-contrast microscopy image showing green-fluorescent MEF infected with MC-derived mCMV-rec-egfp. (Right) Quantitation of green foci of MEF infected with MC-derived mCMV-rec-egfp. Green bars represent range, with median values marked, of data only from individual mice in which recombination has occurred to yield green-fluorescent MEF. Open symbols indicate mice in which recombinations were not detected in the respective organ. (D) Detection of infected cells in liver tissue sections taken on day eight after infection. Cells expressing MC-derived mCMV-rec-egfp were stained in turquoise color by immunohistochemistry (IHC) specific for eGFP. Light hematoxylin counterstain. The bar marker represents 50 μm. Image: (a) iHc, infected hepatocytes; and (b) iEC, infected vascular endothelial cell. Results are displayed schematically and in a new arrangement based on original data published in [80] (C) and in [81] (D).

Figure 4

MC degranulation triggered by the anti-apoptotic mCMV protein vMIA-m38.5. (A) Sketch of the experimental protocol. C57BL/6 mice were infected intraperitoneally with mCMV recombinants containing (grey-shaded capsid) or lacking (empty capsid) gene m38.5. Both virus genomes include gene egfp coding for the fluorescent reporter protein eGFP (green) that identifies infected MC. Peritoneal exudate MC (PEMC) were isolated from the peritoneal cavity 18 hours (h) after infection. (B) Gating strategy for cytofluorometric analysis identifying CD117⁺FcεR1⁺ PEMC that either have remained uninfected (eGFP⁻) or were infected but have not, or not yet, degranulated (eGFP⁺CD107a⁻), or were infected and have already degranulated (eGFP⁺CD107a⁺). (C) Relative quantitation of infected eGFP⁺CD117⁺FcεRI⁺CD107a⁺ PEMC degranulated after infection with either mCMV-egfp expressing vMIA-m38.5 or deletion mutant mCMV-Δm36.5-egfp. Green bars represent range of data from individual mice, with median values marked. (***) highly significant with p < 0.001. Results are displayed graphically modified and in a new arrangement, based on original data published in [77].

Figure 5

Graphical abstract summarizing the current knowledge regarding the role of MC in controlling CMV infection in the mouse model of CMV disease. Reporter protein eGFP (green color) is used to indicate infection. (TCR) T-cell receptor; (pMHC-I) MHC class-I molecule presenting viral peptide to cognate CD8 T cells; (NIF) nodular inflammatory focus; and (vMIA) viral mitochondria-localized inhibitor of apoptosis.