Influence of cytokines on macrophage tolerance to lipopolysaccharide

Macrophages have great significance in immune response, being important participants in innate immunity. They are capable of both directly and indirectly fighting pathogens by regulating the surrounding cells via biologically active substances. In response to various stimuli, macrophages may switch from basal step to a pro- or anti-inflammatory state, polarizing into the M1 or M2 phenotype. M1 macrophages have a pro-inflammatory phenotype, being activated under the influence of cell wall lipopolysaccharide from Gram-negative bacteria, or some pro-inflammatory cytokines. M2 macrophages acquire an anti-inflammatory phenotype upon activation of some immune response receptors (Fcγ and TLR), cytokines IL-4, IL-13, IL-10 and other stimuli. Macrophages are also capable of alleviating their immune response depending on the duration and/or frequency of inflammatory signal, thus developing immune tolerance effect. Of particular importance is tolerance to bacterial lipopolysaccharide, when the macrophages acquire refractoriness to repeated stimulation compared to the primary one, producing less cytokines and chemokines. The aim of our study was to assess the role of cytokines and chemokines CCL2, CXCL1, CXCL9, CXCL12, IL-1b, IL-4, IL-6, IL-7, IL-8, IL- 15, IL-22, TNFα on immune response of macrophages to lipopolysaccharide and emergence of immune tolerance. Primary monocytes were obtained from venous blood of healthy donors. E. coli lipopolysaccharide (LPS) was used to stimulate monocytes and differentiated macrophages. We have shown that pre-treatment of primary human macrophages with recombinant cytokines IL-4 and TNFα enhances the inflammatory response to repeated stimulation with lipopolysaccharide, i.e. weakens the development of tolerance. This effect was expressed as increased production of cytokines (TNFα, IL-6, IL-10) and IL-8 chemokine in presence of recombinant IL-4, and TNFα production with recombinant TNFα. At the same time, recombinant IL-4 was also able to enhance the inflammatory signal of macrophages upon a single LPS stimulation, thus increasing the TNFα and IL-8 secretion. We have shown that some cytokines may affect the tolerance of macrophages to LPS. This phenomenon may involve transition of macrophages from one polarization state to another, or to an intermediate step. Modulation of macrophage phenotype and its immune response opens the way to the development of new therapeutic approaches to inflammatory diseases, where tolerance to lipopolysaccharide plays a significant role in pathogenesis, such as sepsis and atherosclerosis.