Fish Oil-Fed Mice Have Impaired Resistance To Influenza Infection

md_a

Member
Joined
Aug 31, 2015
Messages
468
Fish Oil-Fed Mice Have Impaired Resistance to Influenza Infection
Published:
23 June 2009

Abstract
Dietary fish oils, rich in (n-3) PUFA, including eicosapentaenoic acid and docosahexaenoic acid, have been shown to have antiinflammatory properties. Although the antiinflammatory properties of fish oil may be beneficial during a chronic inflammatory illness, the same antiinflammatory properties can suppress the inflammatory responses necessary to combat acute viral infection. Given that (n-3) fatty acid-rich fish oil supplementation is on the rise and with the increasing threat of an influenza pandemic, we tested the effect of fish oil feeding for 2 wk on the immune response to influenza virus infection. Male C57BL/6 mice fed either a menhaden fish oil/corn oil diet (4 g fish oil:1 g corn oil, wt:wt at 5 g/100 g diet) or a control corn oil diet were infected with influenza A/PuertoRico/8/34 and analyzed for lung pathology and immune function. Although fish oil-fed mice had lower lung inflammation compared with controls, fish oil feeding also resulted in a 40% higher mortality rate, a 70% higher lung viral load at d 7 post infection, and a prolonged recovery period following infection. Although splenic natural killer (NK) cell activity was suppressed in fish oil-fed mice, lung NK activity was not affected. Additionally, lungs of infected fish oil-fed mice had significantly fewer CD8+ T cells and decreased mRNA expression of macrophage inflammatory protein-1-α, tumor necrosis factor-α, and interleukin-6. These results suggest that the antiinflammatory properties of fish oil feeding can alter the immune response to influenza infection, resulting in increased morbidity and mortality.

Discussion
The favorable effects of dietary fish oils stem from the potential of (n-3) fatty acids to reduce excessive inflammation (3,38). Studies have shown beneficial antiinflammatory properties of dietary fish oils on chronic diseases such as rheumatoid and osteoarthritis, inflammatory bowel disease, cardiovascular disease, type 2 diabetes, and Alzheimer's disease (39). However, suppression of the immune system may be deleterious when inflammation is required to eliminate the invading pathogen, such as during influenza virus infection (28). Following infection with influenza, a controlled and coordinated immune response is essential for resolving the infection. The availability of (n-3) fatty acid-rich fish oil supplements and the increasing inclusion of (n-3) fatty acids in prepared foods (40) add to the importance of understanding how the immunosuppressive properties of fish oils may impact the host's ability to respond to influenza virus infection.

Mice fed a fish oil-rich diet had decreased lung pathology following influenza infection compared with controls; however, the fish oil-fed mice also had a prolonged recovery time, significantly increased viral titer and importantly, a significantly higher mortality rate. Thus, in the case of influenza virus infection, reduced lung inflammation was associated with a poor outcome p.i. To understand the direct effect of fish oil feeding on the immune response to influenza virus infection, numbers and phenotypes of cells infiltrating the lung tissue were measured. At d 3 p.i., fish oil-fed mice had fewer numbers of NK cells infiltrating their lungs compared with control mice. NK cells provide an early defense against influenza virus infection and are important in reducing viral load prior to activation of the adaptive immune response (36). NK cell trafficking to the site of infection is dependent on a variety of inflammatory mediators, including the expression of a cytokine/chemokine gradient, upregulation of adhesion molecules, and activation of G-protein–coupled receptors (41–43). Fatty acids of the (n-3) group have been shown to decrease surface expression of vascular adhesion molecules and to alter G-protein–coupled membrane receptors (38,44,45), suggesting that fish oil feeding may have interfered with the signaling required for NK cell trafficking into the lungs. However, other possibilities for reduced NK in the lung include a reduction in general numbers, resulting in fewer NK cells trafficking to the lung.

Although the NK cell number was reduced in both the spleen and lungs of fish oil-fed mice, impairment in NK activity was found only in splenic NK cells. The differences between spleen and lung NK activity may be due to exposure to cytokines. There are a number of cytokines that enhance NK cytotoxicity, including IL-12, IL-18, and IFNα and β (36,42); therefore, NK activity in the lungs of fish oil-fed mice may have increased due to exposure to the cytokine milieu at the site of infection. During influenza virus infection, production of inflammatory cytokines occurs at the site of infection and therefore the spleen is not exposed to this localized inflammatory response. Indeed, Yaqoob et al. (8) demonstrated enhanced NK activity in PUFA-treated NK cells following exposure to IFNγ.

Fish oil feeding also affected numbers of neutrophils in the lung following infection. During early virus-induced inflammatory responses, neutrophils rapidly traffic into infected airways, where they play a critical role in limiting virus replication and activating innate immunity (37,46). Neutrophil migration is controlled in part by the release of chemokines, cytokines, and leukotrienes. MIP-1α has been shown to play a critical role in each aspect of neutrophil trafficking, including rolling, stationary adhesion, and tissue recruitment in vivo (47,48). In addition, studies by Ramos et al. (48) demonstrated that ovalbumin-induced neutrophil migration in immunized mice was mediated by MIP-1α via the release of TNFα and leukotriene B4. Moreover, influenza-infected MIP-1α knockout mice exhibited reduced lung inflammation and delayed viral clearance compared with infected wild type mice (49). In our study, we showed that fish oil-fed mice lacked MIP-1α and TNFα mRNA induction. Together, these data suggest that the failure to upregulate MIP-1α and TNFα mRNA in fish oil-fed mice may have resulted in reduced neutrophil trafficking to the lungs following influenza infection.

On d 7 and 10 p.i., CD8+ T cells were lower than in controls in the lung of influenza-infected fish oil-fed mice. Influenza-specific CD8+ T cells kill infected target cells by direct lysis and also play an essential role in influenza virus clearance and controlling morbidity (50–52). In mice that lack CD8+ T cells, infection with influenza A/PR/8/34 led to increased viral replication and mortality (53). CD4+ T cells, on the other hand, help to resolve inflammation during influenza infection; however, they are not essential for viral clearance (54,55). Our data suggest that reduction in CD8+ T cell numbers p.i. coupled with reduced numbers of neutrophils likely contributed to the increase in lung virus titer in the fish oil-fed mice.

The recruitment of T cells is dependent on cytokine-induced expression of adhesion molecules. For example, TNFα and IL-1β stimulate endothelial cells to increase expression of adhesion molecules, selectins, and intergrins (56,57). As expected with an influenza virus infection, cytokine mRNA expression of TNFα peaked at d 7 in control mice; however, this induction did not occur in fish oil-fed mice, suggesting that a lack TNFα may have contributed to the decrease in CD8+ T cell trafficking to the lung. Alternatives to trafficking include increased apoptosis and/or failure to proliferate in response to antigen stimuli (58,59). However, we examined lung tissue for increased apoptosis by TUNEL staining and found no differences in apoptosis between diet groups (data not shown).

The number of CD4+ T cells in the lung, however, was not affected in fish oil-fed influenza infected mice. Fish oil feeding may have altered pathways required for CD8+ T cell and not CD4+ T cell trafficking. For example, although both CD4+ and CD8+ T cells express CXCR3, administration of anti-CXCR3 antibody reduced CD4+ T cell infiltrate in the brain, whereas CD8+ trafficking was not affected (60). Similarly, CCR5 is also expressed on both CD4+ and CD8+ T cells, although only CD4+ T cell trafficking was affected by lack of CCR5 expression in a mouse model of hepatitis virus (61). Interestingly, mRNA for MIP-1α, the ligand for CCR5, was underexpressed in the fish oil-fed mice. Our results suggest that chemokines and perhaps their receptor expression may play a key role in the immunomodulatory effects of fish oil during influenza infection.

TNFα is produced by infected lung epithelial cells, activated macrophages, dendritic cells, neutrophils, T cells (CD8+ and CD4+), and NK cells (26,62). During viral infection, TNFα exerts antiviral activity (63), enhances the recruitment of leukocytes to the site of infection, and activates innate immune responses (64,65). IL-6 induction has pleiotropic effects, including the activation of NK cells and macrophages and stimulation of T cell differentiation during influenza infection (64,66). The potential for (n-3) fatty acids to reduce proinflammatory cytokines has been shown previously (67). For example, studies of fish oil-fed mice have demonstrated that injection with lipopolysaccharide decreased the ex vivo production of TNFα, IL-1β, and IL-6 by peritoneal macrophages and decreased TNFα, IL-1β, and IL-6 concentrations in circulation (14,17,68).

Together, these data suggest that the antiinflammatory properties of fish oil that resulted in reduced neutrophils, NK cells, and CD8+ T cells in the lung and decreased expression of mRNA for proinflammatory cytokines likely led to the increased viral titer and subsequent higher mortality rate in the fish oil-fed mice. However, the viral titers began to decrease in both fish oil-fed and control groups at a time when the former were beginning to die. This may be a case of survivor bias and those mice that ultimately died may have had a higher viral titer. We also investigated other possibilities for the high mortality rate of fish oil-fed mice, including impaired liver and/or kidney function, spread of virus to the brain, and increased lung cell apoptosis. All of these possibilities were negative (data not shown). In the future, other possibilities to explain the increased death rate of fish oil-fed mice will include an investigation to determine whether lung tissue repair mechanisms are impaired (69) or if fish oil-fed influenza-infected mice have increased and/or longer fevers potentially leading to brain damage and ultimately death (70,71).

Further mechanistic studies are needed to determine how PUFA can influence the immune response to influenza infection. For example, studies have shown that lipid raft disruptions by (n-3) fatty acids can affect signaling pathways in T lymphocytes and disrupt immunological synapse formation needed to activate T cells (6,72). However, potential adverse effects of these alterations in vivo during an influenza virus infection have not been investigated.

In this study, we utilized both a physiologically relevant concentration of dietary fish oil supplementation and a natural route of administration of a viral pathogen. Results from our study suggest that fish oil consumption has the potential to increase the severity of an influenza virus infection and perhaps other viral illnesses as well.

Fish Oil-Fed Mice Have Impaired Resistance to Influenza Infection
 

Similar threads

Back
Top Bottom