Immunomodulatory role of omega 3 polyunsaturated fatty acid-derived pro-resolving lipid mediators on cell-mediated adaptive immunity

Inflammation represents a crucial physiological process mounted by immune system to react to tissue damage and pathogens. Acute inflammation, which is usually the quickest immune reaction to occur upon the insurgence of an infection, is a widely conserved mechanism that is triggered and regulated mostly by cells of the innate immunity (i.e. dendritic cells, monocyte/macrophages, neutrophils and NK cells) and is meant to be the first line of defense against microbes. Since innate immune cells only recognize a relatively limited number of pathogen-associated molecular patterns, higher efficiency requires the action of the specialized cells of the adaptive immune system (i.e. T and B lymphocytes), which potentiate innate functions. Acute inflammation can have two main outcomes: on one hand, prolonged immune reactions can lead to chronic inflammation, which involves the concerted action of macrophages and lymphocytes; on the other hand, inflammation can undergo resolution with complete return to tissue homeostasis. Resolution of inflammation is a pivotal process that is meant to actively quench phlogistic signals. Even though this mechanism has been thought for a long time to act in a passive way, in recent years it has been unequivocally established that it is instead a finely regulated and active process, governed by a new class of lipid mediators, which are synthesized by essential polyunsaturated fatty acids such as arachidonic acid (AA), eicosapentaenoic acid (EPA) and docosahexanenoic acid (DHA). These molecules, which have been termed pro-resolving lipid mediators (SPMs), orchestrate self-limitation of inflammation mostly by acting on innate immune cells; even though a growing number of works are starting to suggest that SPMs might also act on adaptive immunity, our current knowledge of the immunoregulatory actions exerted by SPMs on chronic inflammation and its main effectors, the lymphocytes, is still very scarce. Thus, we investigated the immunomodulatory effect of crucial DHA-derived bioactive lipids such as RvD1, RvD2 and Maresin 1 on pivotal populations of cell-mediated adaptive immunity, i.e. CD4+ and CD8+ cells, as well as the most important T lymphocyte subsets (TH1, TH2, TH17 and iTreg). In order to achieve this goal we analyzed immune functions by means of polychromatic flow cytometry alongside with well-established molecular biology and biochemistry techniques such as qRT-PCR, western blot and ELISA assays. We found that DHA-derived SPMs strongly inhibit the production of pro-inflammatory cytokines (i.e. IFN-?, TNF-a and IL-17) in circulating CD4+ and CD8+ lymphocytes, and reduced the production of IL-2, a pivotal lymphocyte mitogen, without inducing apoptosis or necrosis. In order to further characterize the complete spectrum of immunoresolving actions elicited by DHA-derived SPMs, we assayed their ability to modulate de novo generation of T helper cells (TH1, TH2 and TH17) and induced regulatory T cells (iTreg) from naïve T cell precursors. RvD1, RvD2 and MaR1 were able to reduce intracellular production and release of TH1 and TH17 signature cytokines (IFN-? and IL-17 respectively), as well as inhibiting the expression of their pivotal transcription factors (T-bet and RORc), without affecting in any way TH2 function. On the other hand they enhanced the immunoregulatory properties of iTreg cells by enhancing the expression of their key transcription factor Foxp3, while boosting the release of the anti-inflammatory cytokine IL-10 and the expression of their immunosuppressive receptor CTLA-4. These effects were mediated by the two well-known SPMs receptors, GPR32 and ALX/FPR2. The direct evidence of a role for SPMs in modulating cell-mediated immunity holds huge importance in that it unveils a new level of regulation that the immune system may use to balance inflammatory responses, and considerably improve our knowledge of the vast network of bioactive lipids that regulate inflammatory processes. This could be crucial in theorizing possible future therapeutic approaches for many T cell-dependent chronic inflammatory diseases.