**5.2 Inflammatory mediators**

In response to infection and inflammation, a variety of immune cells, both nonspecific and specific, generate crucial cell signaling mediators called cytokines. They contain a wide scale of mediators, including chemokines, Interferons (IFNs), ILs, lymphokines, and TNFs, that regulate both humoral and cellular immune responses, and modulate the maturation, growth, and responsiveness of specific cell populations [14]. Cytokines can induce pro-inflammatory or anti-inflammatory responses, and ascorbic acid seems to regulate systemic and leukocyte-derived cytokines in a complex system [14, 15, 23].

Ascorbic acid reduced Lipopolysaccharide (LPS)-induced production of the pro-inflammatory cytokines TNF-α and IFN-γ, and elevated anti-inflammatory IL-10 generation, while having no impact on IL-1β levels. Additionally, in vitro administration of ascorbic acid to peripheral blood monocytes isolated from subjects with pulmonary infection reduced the production of the pro-inflammatory cytokines TNF-α and IL-6 [14]. However, another research initiated that in vitro supplementation of peripheral blood monocytes with ascorbic acid and/or vitamin E increased LPS-stimulated TNF-α production, but had no effect on IL-1β production. In addition, incubation of ascorbic acid with virus-infected human and murine fibroblasts increased production of antiviral IFN. Administration of control subjects with 1 g/day ascorbic acid (with and without vitamin E) was exhibited to increase peripheral blood mononuclear cell-derived IL-10, IL-1, and TNF-α after induction with LPS. Thus, the role of ascorbic acid on cytokine production seems to depend on the cell type and/or the inflammatory inducer. Recent study has showed that ascorbic acid supplementation of microglia and resident myeloid-derived macrophages in the central nervous system changes activation of the cells and generate of the pro-inflammatory cytokines TNF, IL-6, and IL-1β. This is indicative of an anti-inflammatory phenotype [14, 23].

Ascorbic acid's roles in modulating cytokines have been identified in preclinical studies utilizing Gluo knockout mice. In ascorbic acid-deficient Gulo knockout mice infected with influenza virus, pro-inflammatory cytokines TNF-α and IL-1α/β in lungs were elevated, while antimicrobial cytokine IFN-α/β was reduced. In addition, when ascorbic acid was given to Gulo mice that had polymicrobial peritonitis, isolated neutrophils' production of the pro-inflammatory cytokines TNF-α and IL-1β was reduced [14]. Another research in septic Gulo mice supplemented with 0.2 g/kg parenteral ascorbic acid has shown reduced production of the inhibitory cytokines TGF-β and IL-10 by Tregs. In this research, the change in IL-4 production and augmented IFN-γ production was also assessed, indicating immune-regulating roles of ascorbic acid in severe infection. In general, ascorbic acid seems to normalize cytokine production, likely via its gene-modulating roles [14, 15, 23].

Pathogens and stress will induce basophils, eosinophils, and mast cells to produce immune mediator, histamine. Histamine induces vasodilation and elevated capillary permeability, resulting in the classic allergic symptoms of runny nose and eyes [14]. The deficiency of ascorbic acid was related with increased plasma histamine concentrations and ascorbic acid would reduce histamine concentrations in researches using guinea pigs, an ascorbic acid-requiring animal model. Increased histamine production was shown to elevate the utilization of ascorbic acid in this research. Consistent with the animal researches, human experimental researches with oral ascorbic acid (125 mg/day to 2 g/day) and intravenous ascorbic acid (7.5 g infusion) have reported reduced histamine concentrations, particularly in subjects with allergic diseases compared with infectious diseases. The precise mechanisms responsible for the in vivo reduce in histamine concentrations after ascorbic acid supplementation are currently inconclusive although ascorbic acid has been suggested to "detoxify" histamine. Furthermore, the effect of ascorbic acid administration on histamine concentrations are not evaluated in all researches [14, 23].
