Our results reveal that PGI2 synthase OE+ mice are protected against RSV-induced illness and have decreased viral replication, while mice that lack IP had exacerbated weight loss and have delayed viral clearance in comparison. These results reveal that PGI2 signaling through IP protects against RSV-induced disease.
We found that production of the stable urinary metabolite of PGI2
is increased by RSV infection. This result is not surprising because a cyclooxygenase enzyme, COX-2, that produces PGI2
is induced as a result of inflammatory stimuli such as tumor necrosis factor alpha that can be generated in the lung as a result of RSV infection (2
). However, the expression of the PGI2
metabolite closely corresponds with the period of weight loss that occurs with RSV infection, suggesting that PGI2
may be an important factor in protection from RSV-induced illness. Measures of acute respiratory illness, such as tachypnea, have been noted as early as 1 day after RSV infection (25
) and do not correspond to the urinary PGI2
metabolite that we assayed.
Studies of the murine model of RSV infection reveal that a vigorous adaptive immune response is a key factor in disease severity (11
). For instance, when CD4+
lymphocyte subsets are depleted by administration of neutralizing antibody, there is no illness as defined by weight loss or decreased activity, yet RSV replication is prolonged (11
). This suggests that the host immune response instead of the viral cytocidal effect is an important determinant of RSV-induced illness. However, we found in our experiments with PGI2
mice and their respective controls that signaling through IP not only protected against weight loss but also decreased viral replication while decreasing IFN-γ production. Given the small yet statistically significant changes in viral titers that occurred in the PGI2
groups in comparison with the respective wild-type mice, it is uncertain whether these changes in viral titers influenced the more dramatic changes seen in weight changes in the groups in response to RSV infection; however, this possibility cannot be ruled out.
Of further note, we found that RSV-specific antibody levels were significantly lower in the PGI2 synthase OE+ mice and tended to be higher in the IP−/− mice. The most likely explanation for the decreased IFN-γ levels and anti-RSV antibody titers in the PGI2 synthase OE+ mice is an inhibition in peak viral replication in these animals, as a lower antigen load resulted in a decreased need for a robust adaptive immune response to the virus. This suggests that the protective effect of PGI2 is more likely through cells of the innate immune response, such as NK cells or macrophages.
is known to have a direct effect on NK cells. Iloprost, a PGI2
analog, increases the NK lytic activity of spleen cells in an ex vivo model of experimental neoplastic metastasis (6
). Natural killer cells are activated by both IFN-α and IFN-β (13
affects NK cell function as well as monocyte and macrophage activity. SM-10906, a stable PGI2
analog, suppressed the production of tumor necrosis factor and interleukin-1 in lipopolysaccharide-stimulated rat pleural resident monocytic cells (19
). In addition, PGI2
regulates the production of tumor necrosis factor alpha in an in vitro assay of murine peritoneal macrophages stimulated with zymosan (21
). Carbacyclin, a PGI2
analog, reduces tumor necrosis factor alpha production from peritoneal macrophages by 50% while increasing the production of interleukin-10 (21
has only recently been recognized to have immunomodulatory effects in vivo. In murine pulmonary allergen challenge models, mice lacking IP have augmented allergic inflammation as characterized by increases in plasma extravasation, leukocyte accumulation, and type 2 cytokine production in the airway (24
). In addition, allergically sensitized IP−/−
mice have increased serum antigen-specific IgE and total IgE levels (24
). In wild-type mice, PGI2
production is induced by exposure to an allergen, and IP is expressed by CD4+
type 2 lymphocytes but not by type 1 lymphocytes (24
). In vitro, PGI2
and its stable analog carbaprostacyclin augment interleukin-10 production by CD4+
type 2 cells (15
also has antineoplastic effects. In carcinogenesis models, PGI2
mice have significantly reduced numbers of lung tumors that are proportional to PGI2
synthase transgene expression (16
There are only a few reports on PGI2
in viral infection, but some suggest that PGI2
may regulate virus-induced illness. For instance, in human immunodeficiency virus-infected patients, the plasma half-life of PGI2
is diminished to15 to 161 s, compared to 9 to 12 min in noninfected patients; human immunodeficiency virus-infected patients with central nervous system manifestations of their disease have even greater decreases (mean, 34 s) compared to those without such symptoms (mean, 84 s) (20
). In lung microvascular endothelial cells from sheep with bluetongue virus infection, PGI2
is decreased compared to infected endothelial cells from cattle (8
). This decrease in PGI2
production in blue tongue-infected ovine endothelial cells is interesting in that marked pulmonary edema and microvascular thrombosis occur in sheep infected with bluetongue virus but rarely if ever occurs in bluetongue virus-infected cattle (8
Our histopathology results also suggest that signaling through IP has protective effects against lung edema caused by RSV infection. We found that PGI2 synthase overexpression decreased edema in the lungs of RSV-infected mice of the FVB background, while the inability to signal through IP led to edema in the alveoli in C57BL/6 background mice. These results suggest that the protective effect of signaling through IP might be a result not only of immunomodulation of RSV infection, but also of preventing vascular leaks leading to lung edema.
In the only prior report investigating the effect of PGI2
on viral infection in mice, Zavagno and colleagues used a BALB/c mouse model of vaccinia virus infection to investigate the role of the different prostaglandins on illness and viral clearance (26
). They found that mice treated with either aspirin or indomethacin, which are cyclooxygenase inhibitors and therefore inhibitors of prostaglandin synthesis, had a marked increase in mortality for vaccinia virus infection over nontreated mice (26
). The mice treated with the nonsteroidal anti-inflammatory agents had delayed viral clearance with inhibition of the antibody response, whereas control mice had a higher survival rate with lower virus yield and normal antibody responses. These studies showed that PGD2
conferred little or no protection to mice against the lethal effects of vaccinia virus, while mice treated with PGE1
had a dramatic increase in mortality after infection. In contrast, the mice treated with PGI2
had greatly enhanced survival.
In conclusion, we found that mice overexpressing PGI2 synthase were protected against RSV-induced weight loss, while mice lacking the PGI2 receptor IP had exacerbated weight loss after RSV infection. In addition, the mice that overexpressed PGI2 synthase had decreased peak viral replication, lower lung IFN-γ levels, and lower serum RSV-specific antibody levels, suggesting that PGI2 overexpression decreased RSV antigen load and downregulated the adaptive immune response. In contrast, IP−/− mice had delayed viral clearance, higher IFN-γ levels in the lung, and slightly increased serum RSV-specific antibody levels, suggesting that the inability to signal through IP increased RSV antigenemia and led to an augmented adaptive immune response.
These results reveal that PGI2 signaling through its receptor IP protected against RSV-induced illness and decreased viral replication. Therefore, PGI2 may be a useful therapy in the prophylaxis of infants at risk for the severe consequences of RSV infection.