Homeostatic pulmonary microenvironment is responsible for alveolar macrophages resistance to endotoxin tolerance

Introduction

Endotoxin tolerance (ET) is a modification of immune response to a second challenge with lipopolysaccharide (LPS), which results in a decreased production of proinflammatory cytokines, and is considered partly responsible for the susceptibility to infectious processes in hospitalized patients [1]. We previously observed an absence of ET of alveolar macrophages (AM) to LPS in an ex vivo murine model of endotoxin tolerance [2]. We hypothesized that this singularity could be mediated by granulocyte-macrophage colony-stimulating factor (GM-CSF) (known to be predominantly produced by type II pneumocytes) and interferon-gamma (INFγ), two cytokines known to prevent the occurrence of ET [3]. The objectives were to confirm the absence of tolerance of AM to LPS and to assess the respective roles of GM-CSF and INFγ in this phenomenon and the cellular origin of INFγ.

Methods

We used different wild-type mice strains (BALB/c, C57BL/6,129SV), and KO mice lacking different leukocytes subset rag2^-/-, rag2gc^-/-, cd3e^-/-, μ^-/-, il-15^-/- and Ja18^-/-. We used an ex vivo model consisting of intravenous injection of LPS 20 hours prior to an in vitro stimulation of AM, peritoneal macrophages and monocytes with LPS. We pretreated the wild-type mice with anti-cytokines antibodies, and KO mice with B cells and NK cells adoptive transfer.

Results

We confirmed the absence of AM tolerance to endotoxin in all the strain of wild-type mice. Inhibiting either GM-CSF or INFγ in vivo at homeostasis led to a decrease in TNF production by AM during the in vitro stimulation by LPS, suggesting the involvement of these cytokines in the prevention of tolerance within the lungs. The fact that AM from rag2^-/-, rag2gc^-/-, μ^-/- could be tolerated, the fact that adoptive transfer of B lymphocytes in these deficient mice restores the wild-type response, and the presence of INFγ mRNA in the lungs at homeostasis in wild-type mice and before and after adoptive B-lymphocyte transfer in KO mice demonstrated the involvement of these cells in the wild-type phenotype.

Conclusion

We confirm the resistance of AM to endotoxin tolerance. Both GM-CSF and INFγ within the lung microenvironment at homeostasis are involved in this phenomenon. B lymphocytes play a key role in the local expression of INFγ.