Background
Cytomegalovirus (CMV) is a beta-herpes virus with high infection rate in humans, and the infection can be lifelong, without acute disease in healthy hosts. Unfortunately, reactivation from latency is a major cause of morbidity and mortality in immunocompromised hosts. Murine (M)CMV recapitulates many of the physiopathological characteristics of human CMV infection, and as such can serve as a model for studying the immunobiology of CMV infection of humans. In general, the immune response against CMV involves various types of cells, such as natural killer (NK) cells and T cells, which participate in restricting the primary infection and dampening reactivation [
1]. The activation of both CD8
+ T cells and NK cells heavily relies on their cross-talk with dendritic cells (DCs) [
2], which serve as a link between innate and adaptive immunity and also play a significant role in mediating the immune response to CMV infection. DCs can be divided into two major subsets: conventional DCs (cDCs) and plasmacytoid DCs (pDCs), which are both present in mouse lungs [
3‐
5].
During infection, DCs recognize CMV double-stranded DNA mainly through the Toll-like receptor 9, which triggers a distinct signaling pathway and results in the production of inflammatory cytokines and type I interferon [
6,
7]. In addition, upon activation, DCs up-regulate the production of co-stimulatory molecules and change the expression of surface chemokine receptors in order to migrate to the lymphoid tissues. In the spleen and lymph nodes, DCs up-regulate MHC molecules. After encountering antigen-presenting mature DCs, naïve T cells are activated and become antigen-specific T effector cells, a process that is crucial for the initiation of adaptive immunity. Mechanistic studies have also shown that CMV can evade the immune system by paralyzing DCs, specifically by impairing the function and cytokine secretion capacity of monocyte-derived DCs (moDCs), rendering them incapable of inducing proliferation of T-cells [
8‐
11].
CD11c, also known as integrin α
x, is a commonly used marker of DCs; however, it can also be expressed on T cells, NK cells, monocytes, macrophages, neutrophils and some B cells [
12]. An increasing number of studies have aimed to characterize the relation of CD11c with functions of the various immune cells, recognizing this factor’s importance beyond a cell phenotype marker. Indeed, it has been demonstrated that mouse DCs strongly downregulate CD11c expression upon activation and that this process is triggered by Toll-like receptor signaling [
13]. In addition, CD11c
+CD8
+ T cells have been reported as remarkably efficient producers of IFN-γ and to play an important role in mediating its related cytotoxic effects, ultimately aiding in viral clearance and tumor regression [
14,
15]. In a comparative analysis of CD11c
+ cells with CD11c
− liver NK cells, the former displayed an activated phenotype and enhanced effector functions, facilitating contributions to early hepatic IFN-γ production during adenovirus infection [
16]. The collective findings reported in the literature have suggested that the expression pattern of CD11c on immune cells might be related with their activation and function.
In the study reported herein, in vivo experiments were used to address the expression pattern of CD11c on various immune cells in the lung following dissemination of MCMV infection, with the expression level of the specific cell surface markers monitored during the infection.
Methods
Animals and MCMV infection
Female 4-week-old BALB/c specific pathogen-free mice, weighing 8–10 g, were purchased from a local experimental animal research center (Wuhan, China). All mice were fed normal diet for 3 days and then randomly divided into the following three groups (n = 12 each): MCMV infection, lipopolysaccharide (LPS) stimulation and untreated control. The respective treatments included intraperitoneal inoculation with 200 μL salivary gland homogenate containing 5 × 103 plaque forming units (PFUs) of MCMV Smith strain, with Escherichia coli LPS (0.25 μg/g; Sigma-Aldrich, USA) or DMEM (Gibco, USA). At 1, 3 and 7 days after injection, lungs were harvested aseptically under ether anesthesia.
Preparation of pulmonary single-cell suspension
After carefully discarding the thoracic lymph nodes and thymus, the lungs were dissected and submerged in ice-cold tissue culture medium (RPMI-1640 supplemented with 5% fetal calf serum, 2-mercaptoethanol and penicillin/streptomycin; procured from Gibco, Hyclone and Sigma-Aldrich, USA, respectively). Following thorough mincing, the tissues were treated with 1 mg/mL collagenase type II (Gibco) and 0.02 mg/mL DNase I (Roche Diagnostics Corporation, Switzerland). The samples were then incubated in a humidified 5% CO2 incubator at 37 °C for 30–45 min, with mechanical shaking every 15 min to help digestion. Next, the samples were vigorously agitated using glass pipettes, treated with more freshly prepared 1 mg/mL collagenase type II and 0.02 mg/mL DNase I, and incubated for an additional 15 min. The digested tissues were then centrifuged, resuspended in PBS containing 10 mM EDTA, and incubated for 5 min on a shaker at room temperature. Following a 7-min lysis of red blood cells, the samples were washed in PBS and RPMI-1640, and passed through a 75 μm cell-strainer. The final samples were resuspended in RPMI-1640 with a drop of fetal calf serum, and incubated on ice until processing for immunofluorescent labeling.
Immunolabeling of single-cell suspension for flow cytometry
100 μL of sample, containing of 1 × 106 cells, was first incubated with Fc receptor- blocking antibody (anti-CD16/CD32; BD Pharmingen, USA) for 5 min to reduce non-specific binding. Next, the sample was labeled for 20 min in the dark at 4 °C, with the following anti-CD primary antibodies: PE hamster anti-mouse CD11c (BD Pharmingen, USA), FITC rat anti-mouse CD86 (BD Pharmingen), APC anti-mouse MHC Class II (eBiosciences, USA). Labeled cells were washed three times with PBS supplemented with 2% bovine serum albumin (Sigma-Aldrich) and 0.1% NaN3, and fixed. Flow cytometric analysis was performed on a Becton-Dickinson LSRII (USA).
Validation of disseminated MCMV infection
Spleen and small lung-portion specimens obtained from each mouse were stored at −80 °C until analysis. MCMV infections were detected to verify the MCMV infection group by using qPCR to amplify the MCMV gB gene DNA (at 1 day post infection, dpi) and plaque assay to detect MCMV infection viral titers (at 3 and 7 dpi). For plaque assay, the organs were first homogenized in 1 mL of DMEM (supplemented with 4% fetal calf serum) and diluted in 1:10 steps. Diluted homogenates were then layered on murine embryonic fibroblasts (MEFs) and incubated at 37 °C for 60 min, after which the supernatants were discarded and cells were overlaid with 1% carboxymethylcellulose (Sigma-Aldrich)-DMEM containing 4% fetal calf serum to prevent secondary viral spread. Finally, the cells were incubated at 37 °C for 5–7 days, when viral titers were determined.
Assessment of cell types among the increased CD11cint cells
At 7 dpi, pulmonary single-cell suspension was obtained and labeled using the method described above but with the following labeling antibodies: APC anti-mouse CD11c, FITC anti-mouse MHC Class II, PE anti-mouse NKp46, PE/Cy7 anti-mouse CD19, PerCP/Cy5.5 anti-mouse CD3ε, PE anti-mouse F4/80, PE/Cy7 anti-mouse Ly-6G, PerCP/Cy5.5 anti-mouse Siglec H (all from BioLegend, USA), FITC anti-mouse CD4 and PE anti-mouse CD8a (both from eBioscience).
Analysis of MCMV-specific CD8+ T cells
Tetramer complexes (produced by HelixGen Company, China) of APC-labeled mouse H-2Dd incorporating the AGPPRYSRI nonapeptide (encoded by the MCMV gene m164) were added to the 7-dpi pulmonary single-cell suspensions, along with FITC anti-mouse CD8a, PerCP/Cy5.5 anti-mouse CD3ε and PE anti-mouse CD11c (all from BioLegend or eBioscience).
Analysis of CD11cint CD8+ T cells in spleen and blood
At 7 dpi, peripheral blood was collected in 5 mM EDTA-containing tubes, to prevent clotting. After that, the spleen was obtained and passed through a 70 μM cell strainer (by mechanical means, utilizing the thumb-piece of a plunger removed from a 1 mL syringe). Following two rounds of red blood cell lysis, each spleen sample was then washed in PBS and resuspended in RPMI-1640 supplemented with a drop of fetal calf serum. Aliquots (100 μL each) of spleen single-cell suspension (1 × 106 cells) and peripheral blood were labeled using the method described above but with the following labeling antibodies: APC anti-mouse CD11c, FITC anti-mouse CD8a, PE anti-mouse CD4 and PerCP/Cy5.5 anti-mouse CD3ε (all from BioLegend or eBioscience). After labeling, the spleen samples were washed and fixed, while the peripheral blood samples were subjected to two rounds of red blood cell lysis and then washed and fixed.
Expression pattern of CD11c and B220 on NK cells
At 1, 3, 5 and 7 dpi, pulmonary and spleen single-cell suspensions were obtained and labeled using the method described above with the following labeling antibodies: PE anti-mouse CD11c, FITC anti-mouse B220, PerCP/Cy5.5 anti-mouse CD3ε and PE/Cy7 anti-mouse NKp46 (all antibodies from BioLegend).
Post-acquisition data analysis
Analysis of flow cytometry data was performed on the WinMDI (version 2.09) software and FlowJo V10 software (TreeStar, USA). Values are presented as mean and standard deviation (SD). Statistical analyses were carried out using the Mann-Whitney and Kruskal-Wallis tests. P values <0.05 were considered statistically significant.
Discussion
An interesting finding of the present study is the generation of a large population of CD11c
int cells in the lung tissue in response to MCMV infection. Through screening with labeling antibodies, the CD11c
int cells were first identified as CD3ε
+ T cells and then specified as CD8a
+ CTLs. CD11c was first reported to be expressed by some CTLs in humans, suggesting their potential contribution to conjugate formation between CTL and target cells [
22]. It was subsequently reported that CD11c
+CD8
+ T cells display signs of recent activation and are more efficient producers of IFN-γ, ultimately aiding in targeted cell lysis (as shown in vitro) and induction of viral clearance (as shown in vivo) [
14].
Though the exact function of CD11c remains unknown, in different studies, CD11c
+ CTL has been demonstrated to be able to exert both immunoregulation and effector functions [
23]. In the present study, MCMV induced a large expansion of CD11c
intCD8
+ T cells (also known as CD11c
+CD8
+ T cells) at 7 dpi, which is consistent with the findings from the Beyer et al. [
14] study of respiratory syncytial virus. However, in contrast to that previous study, the present study observed a higher proportion of lung CD11c
int CTLs (75 ~ 85% vs 40 ~ 50%). Meanwhile, no expansion of CD11c
− CTLs or of CD4
+ T cells was observed, but MCMV-specific CTLs (mostly CD11c
int cells) were detected, indicating the possibility of CD11c
int CTLs playing a key role in anti-MCMV adaptive immune response. Interestingly, a recent study confirmed the proliferation of CD11c
+CD8
+ T cells induced by in vitro CMV antigen in human peripheral blood mononuclear cells [
15]. The authors also reported that CD11c
+CD8
+ T cells represent an active effector phenotype and that the degree of CD11c expression in intra-tumor CD8
+ T cells corresponds with their level of activation. Collectively, these studies indicate that CD11c expression is closely related with the function of CTLs involved in both viral clearance and tumor regression, and suggest its potential as a marker for the evaluation of host immune response and prognosis.
NK cells are essential for the control of a broad range of virus infections, including the widely studied CMV. The activation and function of NK cells depend on the balance of inhibitory and activating signals that are induced by the receptors expressed on their surfaces [
24]. In the C57BL/6 mice, infection with m157-bearing MCMV led to rapid proliferation of Ly49H
+ NK cells and better control of the viral infection. In contrast, the BALB/c mice, which lack Ly49H expression, are highly susceptible to MCMV infection [
25]. Our result showed a decrease of NK cells in BALB/c at 7dpi, when C57BL/6-infected counterparts still displayed a high percentage of NK cells in lung, which was consistent with the failure of the former mouse strain to control MCMV. In addition, the distribution of CD11c expression was also different in NK cells of the two mouse lines (with and without Ly49H expression). B220
−CD11c
− and B220
−CD11c
int cells were found to be the major constituents of NK cells in the lungs in the control groups of the BALB/c and C57BL/6 mice, respectively. After the MCMV infection, the B220
−CD11c
int cells proliferated and emerged as the major constituent in the BALB/c mice. Both BALB/c and C57BL/6 mice upregulated their B220 expression levels on B220
−CD11c
int NK cells at 3–5 dpi, and a significantly higher proportion of induced B220
+CD11c
int NK cells was observed in the C57BL/6 mice. These results indicate that B220
+CD11c
int NK cells might represent a more effective type of NK cells, accounting for the observed resistance of C57BL/6 mice against MCMV infection.
Previous studies of the B220
+CD11c
int NK cells have revealed that, as rapidly cycling cells, they can exert a highly effective cytotoxic activity and are highly effective secretors of IFN-γ upon stimulation [
17,
19]. Yet, apart from Ly49H, it remains unknown whether or not the apparently discrepant expression patterns of CD11c and B220 (especially, the different performance of B220
+CD11c
int NK cells) is also related with the anti-MCMV ability of NK cells in mice or has some other function.
As a commonly used DCs marker, CD11c has also been exploited as a target for in vivo depletion of DC populations [
26,
27], namely by means of transgenic mice (i.e. Itgax-DTR-Tg mice expressing the diphtheria toxin (DT) receptor under the CD11c promoter) pretreated with DT before experimentation. In such studies, the results of depletion of CD11c
+ subsets have been attributed largely to the DC-specific effect. However, results of our study showed that a portion of the NK cells and CD8
+ CTLs can express CD11c. And the percentage of CD11c
int cells in both cell types can be greatly increased during MCMV infection. Thus, it is possible that targeting of CD11c-positive cells for depletion might result in depletions of CD8
+ CTL or NK cell subsets as well. Further investigation is needed to determine the validity of such a hypothesis.
The difference of the lung anti-MCMV immune response that was found in the present study to exist between the two mice types examined involves not only the NK cells but also the CD8
+ T cells response. A prominently higher level of CD11c
intCD8
+ T cells was detected in the C57BL/6 mice after MCMV infection, as compared to the BALB/c mice. It seems that C57BL/6 can induce, first, higher levels of NK cells and, then, higher levels of CD8
+ T cells upon exposure to m157-bearing MCMV; these features may aid in bringing the infection under control as quickly as possible. In a previous study of Ly49H
+ mice (compared to Ly49H
− mice) it was found that, after 6 dpi, the NK cells negatively regulated the anti-viral activity of CTLs, in order to suppress excessive immune response [
1]. However, further study is still needed to confirm whether CD11c
int NK cells (the major NK cells represented), in particular, contribute to this process.
NKp46 is a type I transmembrane glycoprotein, which is involved in the control of various bacterial and viral infections [
28,
29]. While NKp46 has been evidenced as an important mediator of the host response to influenza virus infection [
30], little is known about its role in CMV infection and the findings in the literature are contradictory [
28,
31,
32]. A recent in vitro study revealed that human NKp46 does not play a role in the anti-HCMV responses of decidual NK (dNK) cells [
31]. Another study demonstrated a lack of difference in the early control of MCMV infection between NCR1
gfp/gfp mice and control mice [
28]; yet, when a different group investigated
Noé mice, which carry a point-mutation within the
NCR1 gene, they demonstrated a greater responsiveness of NK cells in vivo and a greater resistance phenotype to MCMV infection [
32]. It seems that hyper-responsiveness of NK cells is associated with low NKp46 expression, as well as high
Helios transcription [
32,
33]. In the present study, lower expression of NKp46 on NK cells was found in C57BL/6 mice, compared to BALB/c mice with or without MCMV infection. After MCMV infection, decreased expression of NKp46 was detected in both mice lines. This is consistent with the results reported by Siewiera et al. [
31], in which co-culture of dNK cells and HCMV-infected fibroblasts led to downregulation of NKp46. However, whether decline of NKp46 represents a manipulative strategy by CMV to evade the host immune system or a sign of increased reactivity remains unknown.