Production and biological role of hepatocyte and myeloid cell-derived interleukin-1 receptor antagonist in a model of systemic inflammation
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* C Lamacchia
* G Palmer
* E Rodriguez
* D Talabot-Ayer
* C Gabay

## Background

Interleukin-1 receptor antagonist (IL1Ra) is a specific IL1 inhibitor that possesses anti-inflammatory activities in experimental models and in patients. Interestingly, administration of IL1Ra, particularly in animal models, can ameliorate the physiological consequences of septic shock. The contribution of the IL1/IL1Ra balance during endotoxaemia was confirmed by the high susceptibility of IL1Ra deficient mice to a lethal dose of lipopolysaccharide (LPS). Previous in vitro data demonstrated that both hepatocytes and myeloid cells produce IL1Ra in large amounts in response to LPS.

## Objectives

The aim of this study was to determine the role of hepatocyte- and myeloid cell-derived IL1Ra in the production of circulating IL1Ra and in the control of survival in response to endotoxin in vivo.

## Methods

Conditional knockout mice in which IL1Ra production has been specifically targeted in hepatocytes (IL1RadeltaH), myeloid cells (IL1RadeltaM) or in both cellular sources (IL1RadeltaH+M) were generated in a pure C57BL/6 genetic background by using the LoxP/Cre-recombinase system. Systemic inflammation was induced by intraperitoneal (ip) injection of 10 mg/kg LPS in IL1RadeltaH, IL1RadeltaM, IL1RadeltaH+M and control mice. IL1Ra was quantified by ELISA in different organs and sera 4 or 18 h after injection. LPS (10 mg/kg) was also injected ip in the different conditional IL1Ra knockout mouse lines as well as in constitutively IL1Ra knockout (IL1Ra–/–) and control mice for a survival test.

## Results

The levels of circulating IL1Ra were decreased by 15% and 57% in IL1RadeltaH and IL1RadeltaM mice, respectively, 4 h after LPS administration compared with control mice. IL1Ra levels were very low in the circulation of IL1RadeltaH+M mice (decreased by 92% and 75% 4 h and 18 h after LPS injection, respectively). In addition, IL1Ra levels were decreased by 56%, 20% and 76% in the livers of IL1RadeltaH, IL1RadeltaM and IL1RadeltaH+M mice, respectively, in response to LPS. We observed a strong decrease of IL1Ra levels in the spleen (70%) and lung extracts (50%) in both IL1RadeltaM and IL1RadeltaH+M mice versus control mice. Finally, IL1Ra–/–, IL1RadeltaM and IL1RadeltaH+M mice were more susceptible than control mice to the lethal effects of endotoxin, whereas IL1RadeltaH mice exhibited a similar lethality profile as control mice.

## Conclusions

We showed that (1) hepatocytes and myeloid cells are the two major cellular sources of hepatic and circulating IL1Ra in response to LPS; (2) myeloid cells represent an important source of IL1Ra in the spleen and lung after LPS challenge; (3) IL1Ra production by myeloid cells, but not hepatocytes, is critical for survival during endotoxaemia.