Lipocalin-2 Test in Distinguishing Acute Lung Injury Cases from Septic Mice Without Acute Lung Injury

Objective To explore whether the amount of lipocalin-2 in the biofluid could reflect the onset of sepsis-induced acute lung injury（ALI） in mice. Methods Lipopolysaccharide（LPS, 10 mg/kg） injection or cecal ligation and puncture（CLP） was performed to induce severe sepsis and ALI in C57 BL/6 male mice randomly divided into 5 groups（n=10 in each group）： group A（intraperitoneal LPS injection）, group B（intravenous LPS injection via tail vein）, group C（CLP with 25% of the cecum ligated）, group D（CLP with 75% of the cecum ligated）, and the control group（6 sham-operation controls plus 4 saline controls）. All the mice received volume resuscitation. Measurements of pulmonary morphological and functional alterations were used to identify the presence of experimental ALI. The expressions of lipocalin-2 and interleukin（IL）-6 in serum, bronchoalveolar lavage fluid（BALF）, and lung tissue were quantified at both protein and mRNA levels. The overall abilities of lipocalin-2 and IL-6 tests to diagnose sepsis-induced ALI were evaluated by generating receiver operator characteristic curves（ROC） and computing area under curve（AUC）. Results In both group B and group D, most of the “main features” of experimental ALI were reproduced in mice, while group A and group C showed septic syndrome without definite evidence for the presence of ALI. Compared with septic mice without ALI（group A＋group C）, lipocalin-2 protein expression in septic mice with ALI（group B＋group D） was significantly up-regulated in BALF（P〈0.01） and in serum（P〈0.01）, and mRNA expression boosted in lung tissues（all P〈0.05）. Lipocalin-2 tests performed better than IL-6 tests in recognizing sepsis-induced ALI cases, evidenced by the larger AUC of the former（BALF tests, 0.8800 versus 0.6625; serum tests, 0.8500 versus 0.7000）. Using a dual cutoff system to diagnose sepsis-induced ALI, BALF lipocalin-2 test exhibited the highest positive likelihood ratio（13.000） and the lowest negative likelihood ratio（0.077） among the tests of lipocalin-2 and IL-6 in blood and BALF. A statistically significant correlation was found between lipocalin-2 concentration in BALF and that in serum（Spearman r=0.8803,P〈0.0001）. Conclusions Lipocalin-2 expression is significantly up-regulated in septic ALI mice compared with those without ALI. Lipocalin-2 tests with a dual cutoff system could be an effective tool in distinguishing experimental ALI cases.