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Classical (M1) and alternative (M2) polarization of mononuclear cells (MNCs) such as monocyte and macrophages is known to occur in response to challenges within a microenvironment, like the encounter of a pathogen. LPS, also known as endotoxin, is a potent inducer of inflammation and M1 polarization. LPS can also generate an effect in MNCs known as endotoxin tolerance, defined as the reduced capacity of a cell to respond to LPS activation after an initial exposure to this stimulus. Using systems biology approaches in PBMCs, monocytes, and monocyte-derived macrophages involving microarrays and advanced bioinformatic analysis, we determined that gene responses during endotoxin tolerance were similar to those found during M2 polarization, featuring gene and protein expression critical for the development of key M2 MNC functions, including reduced production of proinflammatory mediators, expression of genes involved in phagocytosis, as well as tissue remodeling. Moreover, expression of different metallothionein gene isoforms, known for their role in the control of oxidative stress and in immunomodulation, were also found to be consistently upregulated during endotoxin tolerance. These results demonstrate that after an initial inflammatory stimulus, human MNCs undergo an M2 polarization probably to control hyperinflammation and heal the affected tissue.

The innate immune response is the first line of defense against microorganisms. However, its over-stimulation can result in excessive inflammation that can lead to deadly syndromes like sepsis. Initially, septic patients exhibit a high immune response, which later shifts towards an immunosuppressive state. This state may be related with a phenomenon called endotoxin tolerance, which is a reduced capacity to respond to antigens. Recent studies have demonstrated that cationic host defense peptides, such as LL-37, have broad immunomodulatory activities like diminishing inflammatory responses. Therefore, the purpose of these two phenomena may be to reduce the immune response and control excessive inflammation. Although much studied, mechanisms that lead to their development are not clear. In this work, time course analysis by RT-qPCR, western blot, and microarrays on human peripheral blood mononuclear cells and monocytic cell line, have shown similar patterns in the up- and down-regulation of important immune players like pro/anti-inflammatory mediators, and negative regulators during the development of endotoxin tolerance and anti-endotoxin activity of LL-37. Results from this work may not only help clarify how each of these phenomena occur, but also may find a link between them. This work has been supported by Genome British Columbia and Genome Prairie for the Pathogenomics of Innate Immunity Research Program, and by the Foundation for the National Institutes of Health and Canadian Institutes for Health Research through the Grand Challenges in Global Health Initiative.