8.1.1  N-3 PUFAs regulate the expression of several proinflammatory innate immune components and modulate macrophage response
A ‘cytokine storm’ and activation of the central innate immune pathway linking the NLRP3 inflammasome, IL-1β, TNF-α and IL-6 response is a primary cause of excessive inflammation reported in COVID-19 that negatively impacts cardiovascular system. Therefore, targeting the different components is a promising approach to ameliorate cardiac complications secondary to COVID-19 (Huang et al., 2020). While there is no direct clinical evidence related to the use of n-3 PUFAs in COVID-19 patients, the application of n-3 PUFAs in several inflammatory settings, including cardiovascular disorders, has been demonstrated to ameliorate detrimental immune reactions by several mechanisms (Rogero et al., 2020). The anti-inflammatory effect of n-3 PUFAs seems to be consistent across several previous clinical findings (Calder, Carr, Gombart, & Eggersdorfer, 2020; Fritsche, 2006; Kiecolt-Glaser et al., 2012; Vedin et al., 2008). Intriguingly, Tan et al. recently demonstrated in a randomized controlled study that high-dose n-3 PUFA supplementation (1.5 g/day EPA and 1.0 g/day DHA) markedly reduces plasma levels of IL-6, IL-1β and TNF-α after 4 weeks of therapy in middle or late-aged patients with chronic venous leg ulcers suggesting n-3 PUFAs as an effective low-risk dietary intervention to modulate inflammation (Tan, Sullenbarger, Prakash, & McDaniel, 2018). This study indicates that n-3 PUFAs could have direct modulatory effects on the main components of the cytokine storm IL-6, IL-1β and TNF-α.
N-3 PUFAs can modulate the transcription and expression of inflammatory genes including cytokines, chemokines and adhesion molecules in cardiomyocytes, fibroblasts, endothelial cells, monocytes and macrophages (Collie-Duguid & Wahle, 1996; De Caterina, Cybulsky, Clinton, Gimbrone, & Libby, 1994; Hughes, Southon, & Pinder, 1996; Miles, Wallace, & Calder, 2000; Sanderson & Calder, 1998). This is primarily achieved through the regulation of key transcription factors, such as inhibiting NF-κB (Kumar, Takada, Boriek, & Aggarwal, 2004; Lo, Chiu, Fu, Lo, & Helton, 1999; Novak, Babcock, Jho, Helton, & Espat, 2003; Zhao, Joshi-Barve, Barve, & Chen, 2004) or activating peroxisome proliferator-activated receptors-α/γ (PPARα/γ) (Gani & Sylte, 2008; Zapata-Gonzalez et al., 2008). Activation of PPARα/γ can directly interfere with the activation of NF-κB and prevent its shuttling to the nucleus reducing the inflammatory burst (Matsumoto et al., 2008; Mishra, Chaudhary, & Sethi, 2004; Poynter & Daynes, 1998; Ricote, Huang, Welch, & Glass, 1999; Vanden Berghe et al., 2003). Interestingly, direct activation of PPAR, using PPAR agonists, was proposed as a therapeutic target for blunting and regulating cytokine storm in COVID-19 patients suggesting n-3 PUFAs could have a promising effect (Ciavarella, Motta, Valente, & Pasquinelli, 2020). Another important immunomodulatory mechanism induced by n-3 PUFAs involves activation of G protein-coupled receptor 120 (GPR120), which mediates strong and wide-ranging anti-inflammatory effects. Research from Oh et al. indicates n-3 PUFAs stimulate GPR120 in both monocytic RAW 264.7 cells and primary intraperitoneal macrophages inhibiting TLR4-mediated inflammatory responses. Knockdown of GPR120 attenuates the protective effects attributed to n-3 PUFA consumption (Oh et al., 2010). These studies together provide evidence that n-3 PUFAs mediate anti-inflammatory effects through different mechanistic pathways.
Cardiac macrophages are primarily derived and replenished from inflammatory monocytes in response to an infection with resident macrophages also having a role. Briefly, macrophages will differentiate into classical M1 inflammatory cells to clean cellular and matrix debris (Epelman et al., 2014). Subsequently, M1 macrophages may undergo polarization and transformation to the alternatively activated or reparatory M2 stage which secrete IL-10 to promote resolution and contribute to wound healing and tissue repair (Murray, 2017). Controlling the migration and the polarization of macrophages to the myocardium in the context of COVID-19 is a tentative approach to limit cardiac injury (Frantz & Nahrendorf, 2014; Fujiu, Wang, & Nagai, 2014; Leblond et al., 2015; van Amerongen, Harmsen, van Rooijen, Petersen, & van Luyn, 2007). In COVID-19, an excessive cardiac recruitment and accumulation of pro-inflammatory M1 macrophages potentially aggravates cardiovascular injury. Notably, as M1 macrophages secrete a large variety of chemokines and cytokines such as TNF-α and IL-1β to recruit and activate other immune cells from both the innate and the adaptive immune system. The effect will impede the reparative phase mediated by M2 macrophages and thus aggravates adverse cardiac remodeling (Dewald et al., 2005; Gordon, Pluddemann, & Martinez Estrada, 2014; Murray & Wynn, 2011; ter Horst et al., 2015).
Interestingly, evidence demonstrates n-3 PUFAs and/or their biologically active metabolites have the ability to blunt the expression, production and release of IL-1β, TNF-α, and IL-6 by M1 macrophages (Allam-Ndoul, Guenard, Barbier, & Vohl, 2017; Liu et al., 2014; Mildenberger et al., 2017). Schoeniger et al., showed n-3 PUFAs have the ability to down-regulate inflammatory processes and reduce the production and secretion of pro-inflammatory cytokines from RAW 264.7 macrophages infected with microorganisms, R. equi and P. aeruginosa (Schoeniger, Adolph, Fuhrmann, & Schumann, 2011). Moreover, the inhibitory effects of EPA and DHA on the pro-inflammatory NLRP3 inflammasome pathway has also been well-documented in macrophage cell lines as well as in primary human and mouse macrophages (Iverson et al., 2018; Kumar et al., 2016). Kumar et al., investigated the effects of 15-lipoxygenase (LOX) metabolites of ALA on lipopolysaccharide (LPS) -induced inflammation in RAW 264.7 cells and peritoneal macrophages. The findings revealed the anti-inflammatory effects of these metabolites involve inactivation of the NLRP3 inflammasome complex through the PPAR-γ pathway (Kumar et al., 2016). N-3 PUFAs can increase the phagocytic capacity of macrophages, which has been shown through the engulfment of zymosan particles (Chang, Lee, Kim, & Surh, 2015), Pseudomonas aeruginosa, Rhodococcus equi (Adolph, Fuhrmann, & Schumann, 2012), E.coli (Davidson, Kerr, Guy, & Rotondo, 1998) and apoptotic cells (Chang et al., 2015). It has been suggested the increase in phagocytic capacity of macrophages upon n-3 PUFA treatment could be attributed to changes in the cellular membrane composition and structure caused by the incorporation of the n-3 PUFAs (Hellwing, Tigistu-Sahle, Fuhrmann, Kakela, & Schumann, 2018; Schoeniger, Fuhrmann, & Schumann, 2016). Importantly, n-3 PUFAs have been found to promote M2 polarization in macrophage cell lines and primary mouse macrophages enhancing resolution of inflammation and tissue repair after infection (Chang et al., 2015; Ohue-Kitano et al., 2018). Collectively, the modulatory properties of n-3 PUFAs on the immune system could impart a promising beneficial effect on the cardiovascular system in the context of COVID-19, an effect which needs further exploration and confirmation in larger clinical trials.