Clinical signs and gross pathology in SARS-CoV-infected African green monkeys.
Cynomolgus macaques develop pathology upon SARS-CoV infection, and the severity is associated with aging (
46). In this study, we compared SARS-CoV infection in two related nonhuman primate host species, young adult cynomolgus macaques and African green monkeys, focusing on the latter. Four young adult African green monkeys were infected with SARS-CoV HKU39849, and as a control, two were PBS infected. During the 4-day experiment, all animals remained free of clinical symptoms. Their body temperatures, measured by transponders in the peritoneal cavity, demonstrated a rhythmic pattern, with temperatures fluctuating between 35°C at night and 39°C during the day, prior to infection (
Fig. 1A). Between days 1 and 2 after SARS-CoV infection, an increase in body temperature was recorded during the night in all animals. In addition, some African green monkeys showed an increase in body temperature up to 40°C during the first 2 days after SARS-CoV infection (
Fig. 1A). Even at days 3 and 4 postinfection, elevated temperatures were still observed in some African green monkeys. At gross necropsy, 4 days postinfection, the lungs of African green monkeys showed (multi)focal pulmonary consolidation (
Fig. 1B and C). The consolidated lung tissue was gray-red, firm, level, and less buoyant than normal. Strikingly, one animal had severe lesions, with up to 30% of total lung tissue affected (
Fig. 1B). Four young adult cynomolgus macaques infected with SARS-CoV HKU39849 and two PBS-infected cynomolgus macaques were used for comparative analyses (
46). In contrast to infected African green monkeys, no increase in body temperature of up to 40°C during the first 2 days after SARS-CoV infection was observed in macaques (
46). Between days 1 and 2 after SARS-CoV infection, an increase in body temperature was recorded during the night in three out of four animals (
46). Temperatures returned to normal from day 3 postinfection onward (
46). The lungs of young adult macaques showed small, patchy, macroscopic lesions (
Fig. 1C) (
46).
Histopathology in SARS-CoV-infected African green monkeys.
The lesions in the lungs of African green monkeys consisted of acute exudative diffuse alveolar damage characterized by thickened alveolar septa with infiltration of moderate numbers of neutrophils and macrophages, lymphocytes, and plasma cells, and necrosis with karyolysis, karyorrhexis, and loss of cellular detail (
Fig. 2A). Multifocally, moderate hypertrophy and hyperplasia of type II pneumocytes was observed (
Fig. 2A and B). A variable amount of eosinophilic proteinaceous fluid (edema and exudate) with fibrin and eosinophilic hyaline membrane formation (
Fig. 2A and B), few syncytia, and moderate numbers of foamy alveolar macrophages, neutrophils, and erythrocytes (hemorrhage) was seen in alveolar lumina. Mild necrosis, hypertrophy, and hyperplasia of bronchiolar epithelium were encountered (
Fig. 2A). Peribronchiolar, peribronchial, and perivascular infiltrations of moderate numbers of lymphocytes, plasma cells, macrophages, and neutrophils with mild to moderate edema were seen (
Fig. 2A). There was a mild to moderate proliferation of the bronchus-associated lymphoid tissue (BALT), with focal nodular lymphocytic proliferation and formation of follicles with additional exocytosis of neutrophils in the tracheal epithelium (
Fig. 2A). A mild multifocal lymphoplasmacytic tracheobronchoadenitis, characterized by a multifocal mild infiltration of lymphocytes, macrophages, plasma cells, and neutrophils in and surrounding the bronchial seromucous glands with mild necrosis of the epithelium, was observed in all African green monkeys (
Fig. 2A). Two animals showed evidence of multifocal mild lymphoplasmacytic pleuritis consisting of a mild multifocal distention of the pleura with infiltration of a few lymphocytes, plasma cells, macrophages, and neutrophils and mild edema. The lymphoid organs were activated, with proliferation of the tracheobronchial lymph node and spleen, characterized by increased numbers of neutrophils and histiocytes and distinct germinal centers in the lymphoid follicles. No significant lesions were seen in other tissues examined or in the tissues of negative-control animals. The histopathological lesions in the respiratory tracts of SARS-CoV-infected cynomolgus macaques were largely similar in character to those described for the African green monkeys (
46). However, hyaline membrane formation was not observed, and the lesions in macaques were significantly less severe and extensive than those in African green monkeys (
Fig. 3A), which corroborated the gross pathology scores (
Fig. 3B).
Host response to SARS-CoV in African green monkeys and macaques.
To understand why SARS-CoV infection in African green monkeys results in more severe pathology than in young adult macaques, we generated genome-wide mRNA expression profiles from lung tissue with substantial SARS-CoV replication (
Fig. 6A). Principal-component analysis was performed on a single normalized data set of African green monkey samples, together with macaque samples. This unsupervised method clearly separated SARS-CoV-infected lung mRNA expression profiles from PBS control expression profiles for both species. In addition, the infected and uninfected expression profiles of the two species also separated well. Species- and infection-related expression profile differences coincided with principal components 1 and 2, respectively (
Fig. 6B). These results show that species-associated mRNA expression profile differences are maintained upon SARS-CoV infection, suggesting that African green monkeys and macaques respond differently to SARS-CoV infection.
To analyze the host response to infection in African green monkeys and macaques in more detail, mRNA transcripts differentially expressed between infected and uninfected lung samples were identified using the analysis of variance (ANOVA)-like method LIMMA (
47) for both species separately. To obtain a maximum view of infection-induced host response differences between African green monkeys and macaques, the microarray data for the two species were normalized separately. Infected African green monkeys expressed 556 gene transcripts differently from PBS-infected animals. These differently expressed genes were analyzed within the context of molecular pathways, using a functional-analysis approach with biologically related genes (Ingenuity). The most significantly regulated molecular/cellular functions in African green monkeys compared to PBS-infected animals were associated with a proinflammatory response and included cellular growth and proliferation, cell death, cell movement, and cell-to-cell signaling (
Fig. 7A). Upon analysis of the gene transcripts within the context of biological pathways using Ingenuity Knowledge Base, the top gene interaction network in African green monkeys showed a strong antiviral response with differentially expressed type I interferon-stimulated genes and genes associated with the induction of type I interferons (
Fig. 7B), including ISG15, ISG20, DDX58, DHX58, IFI/IFITs, MX1, MX2, IRF7, IRF9, OAS1, and OAS2. In addition, the network contained NF-κB (
Fig. 7B), a transcription factor implicated in proinflammatory host responses and development of ALI/ARDS (
10,
17,
21). Differential expression of genes associated with acute lung injury, inflammation, and/or hypoxia signaling in African green monkeys included C3AR1, CEBPD, CCL2, CCL3, IL1RN, TIMP1, CDKN1A, SPARC, VEGFA, SPP1, CSF1, CSF3R, CD86, KIT, CCL8, CXCL10, PML, SP100, and SOD2 genes. Some of these genes are also upregulated in SARS patient sera or in patients with ARDS (
51,
55). Young adult SARS-CoV-infected macaques expressed 475 gene transcripts differently from PBS-infected animals. These genes were associated with the same molecular/cellular functions observed in African green monkeys, with similar numbers of differentially expressed genes per function (
Fig. 7A). Analysis of the gene transcripts in the context of biological pathways revealed differential expression of genes in both antiviral and proinflammatory responses (
Fig. 7C). These data suggest that African green monkeys and macaques display similar types of responses upon infection, with strong induction of antiviral and proinflammatory pathways, which has been described for SARS-CoV-infected cynomolgus macaques previously but based on regulation of a different array of individual genes within the antiviral and proinflammatory pathways (
7,
46).
As the principal-component analysis suggested that the host response to SARS-CoV infection was different in African green monkeys and macaques, the gene expression profiles of SARS-CoV-infected African green monkeys and macaques (
n = 4) were directly compared using LIMMA (8 samples normalized in a single set). A total of 1,607 gene transcripts were differentially expressed. Upon analysis of these 1,607 gene transcripts within the context of biological processes and pathways using Ingenuity Pathways Knowledge Base, this subset of genes showed indications of an innate host response to viral infection. Among the top significantly differentially regulated (
P < 0.005) functional categories were inflammatory disease, cell movement, and cell-to-cell signaling and interaction, which included genes like those for CCL20, CXCL1, CXCL2, DEFB1, IL1RL1, IL1RN, MMP7, MMP9, IL-8, SERPINE1, SPP1, TFPI2, and VCAM1, which were either up- or downregulated, depending on the gene (
Fig. 8A to C), indicating that they are upregulated in either African green monkeys or macaques. These data suggest that both host species and SARS-CoV infection are factors involved in determining the transcription of cellular genes and that significant differences exist in the proinflammatory host response to SARS-CoV infection, corresponding to the host species.
In order to understand the host responses in the context of host species, we directly compared lung samples from PBS-infected African green monkeys (n = 2) and macaques (n = 2). LIMMA analysis revealed that 2,198 gene transcripts were differentially expressed (change, ≥2-fold; P < 0.05), with categories such as inflammatory disease, cell death, cell movement, and cellular growth and proliferation among the top significantly differentially regulated functions (P < 0.005). Of the 2,198 differentially expressed genes, 917 were also differentially expressed in the direct comparison of SARS-CoV-infected African green monkeys and macaques, but they did not include genes for cytokines/chemokines, such as CCL20, CCL3, and SPP1. Our data indicate that significant differences exist in the basal gene expression levels of African green monkeys and macaques, which may partly explain why differences in pathology were observed after SARS-CoV infection.
To obtain more insight into the differences in the host responses to SARS-CoV infection of African green monkeys and macaques, mRNA expression profiles for specific molecular pathways of infected animals of both species were compared to those of their PBS controls, but also directly against each other. Heat maps were generated for differentially regulated genes involved in ARDS (
Fig. 8A), NF-κB signaling (
Fig. 8B), and cytokine/chemokine signaling (
Fig. 8C), which indicated that the host responses to infection with respect to these pathways in African green monkeys and macaques are different. Surprisingly, inflammatory cytokines that play a major role in mediating and amplifying ALI/ARDS or have neutrophil chemoattractant activity, such as IL-6, IL-8, CXCL1, and CXCL2 (
10,
25,
55), were differentially expressed in macaques, but not in African green monkeys (
Fig. 8C). TaqMan analysis confirmed that IL-6 and IL-8, key players in ALI/ARDS, were differentially expressed at significantly lower levels in African green monkeys than in young adult macaques (
Fig. 9A and B), in contrast to the IFN-β levels, which were not significantly different (
Fig. 9C). Moreover, in the direct comparison of the microarray data on SARS-CoV-infected African green monkeys and macaques, IL-8, CXCL1, and CXCL2 were also differentially expressed (
Fig. 8A to C). These observations suggest that classical pathways known to be involved in development of ALI/ARDS are differently regulated in these two nonhuman primate species. From previous studies, we know that in aged cynomolgus macaques, IL-8 gene expression is further upregulated (
Fig. 9A) (
46) and correlated with significantly more severe pathology. Because African green monkeys showed even more pathology than young adult macaques, other pathogenic pathways besides the induction of genes such as those for IL-6 and IL-8 are most likely involved in the observed pathological changes (
21,
46).
Interestingly, CCL3, SPP1, and CCL20 were induced strongly in African green monkeys, but not in young adult macaques (
Fig. 8A to C and 9D to F). This observation was corroborated by the direct comparison of SARS-CoV-infected African green monkeys with macaques, which showed that CCL20 and SPP1 were differentially expressed (
Fig. 8B and C). It is notable that both these genes were also statistically significantly upregulated in aged macaques compared to young adult macaques (
Fig. 9D to F) (
46). CCL3 is involved in the acute inflammatory state in the recruitment and activation of polymorphonuclear leukocytes, whereas CCL20 is strongly chemotactic for lymphocytes but weakly attracts neutrophils. Osteopontin, encoded by SPP1, has been implicated in a broad range of disease processes, including chronic obstructive pulmonary disease, asthma, multiple sclerosis, rheumatoid arthritis, and cardiovascular disease (
34,
54). Osteopontin has both matrix protein-like and cytokine-like properties and is expressed by different cell types of the immune system (
34,
54). To determine which cell types are responsible for the osteopontin expression, immunohistochemical detection of the protein was performed on lung sections of SARS-CoV-infected African green monkeys (
Fig. 10). SARS-CoV-infected African green monkeys showed marked staining of osteopontin in the alveoli in areas with lesions. Strong expression of osteopontin was observed in infiltrating cells, and staining with an antibody directed against CD68 indicated that the infiltrating cells that expressed osteopontin were mainly macrophages (
Fig. 9). Similar staining was performed on the lungs of young adult and aged macaques, also showing that mainly macrophages were stained for osteopontin expression (data not shown). The immunohistochemistry data, however, do not allow quantification of the amounts of osteopontin expression in the different species.