The immune modulatory effects of omega-3 polyunsaturated fatty acids and iron as applied in an animal pulmonary tuberculosis model
Background: Non-resolving inflammation is characteristic of tuberculosis (TB). This leads to lung tissue damage and anaemia of infection, which is associated with poor clinical outcomes. Iron supplementation may have limited efficacy and may favour bacterial growth. Therefore, anti-inflammatory and pro-resolving host-directed therapy (HDT) have been suggested. Omega-3 long-chain polyunsaturated fatty acids (n-3 LCPUFAs) including eicosapentaenoic and docosahexaenoic acid (EPA/DHA) may provide such a nutritional approach for HDT. Aim: The aim of this thesis was to determine the effects of EPA/DHA and iron supplementation, alone and in combination, on inflammatory and clinical outcomes in Mycobacterium tuberculosis (Mtb)-infected mice and whether these effects were dependent on n-3 polyunsaturated (n-3 PUFA) status prior to infection. Methods: Male C3HeB/FeJ mice were conditioned on n-3 PUFA sufficient or deficient diets for six weeks prior to infection. One week post Mtb infection, n-3 PUFA sufficient mice were randomised to 1) continue on the n-3 PUFA sufficient diet, or were switched to an 2) EPA/DHA-supplemented, 3) iron-supplemented, or 4) EPA/DHA and iron-supplemented diet. Mice conditioned on an n-3 PUFA deficient diet were randomised to 1) continue on the n-3 PUFA-deficient diet, or switched to the 2) n-3 PUFA sufficient, or 3) EPA/DHA-supplemented diets. The mice received these diets for three weeks until euthanasia. Results: The phospholipid fatty acid composition of cell membranes of mice reflected the dietary fatty acid content. Pro-resolving lung lipid mediator profiles were found in the EPA/DHA-supplemented groups in both n-3 PUFA sufficient and low-status arms, and also when combined with iron. Additionally, EPA/DHA supplementation resulted in lower bacterial loads and lung pathology in the sufficient, but not in the low-status group. Iron and EPA/DHA supplementation both individually lowered systemic and lung cytokines, together with improved anaemia of infection markers. However, whilst EPA/DHA supplementation lowered lung T cells, iron resulted in higher lung immune cell counts. Iron also had no effect on lung pathology or bacterial load but lowered body weight gain. There were iron x EPA/DHA interactions to attenuate the lowering effects of iron or EPA/DHA on anaemia of infection, and of iron for higher immune cell counts. Furthermore, compared with the n-3 PUFA sufficient diet, EPA/DHA supplementation provided superior benefits in body weight gain, bacterial load, and lung inflammation in low-status mice. Conclusions: The findings of this thesis showed that EPA/DHA supplementation, after the initial inflammatory response, has antibacterial and inflammation-resolving benefits and improves markers of anaemia of infection in TB, depending on n-3 PUFA status. On the other hand, iron promotes anti-inflammatory effects and improves markers of anaemia, but enhances immune cell recruitment and lowers body weight gain. Providing combination iron and EPA/DHA treatment attenuates their individual beneficial effects. Lastly, in low-status mice, EPA/DHA provides superior effects compared with an n-3 PUFA sufficient diet. Considering this, iron may not be detrimental concerning worsening the bacterial burden in TB, but moreover, n-3 LCPUFA therapy may be a promising approach as HDT in TB.
- Health Sciences