Critical role of CXC chemokines in endotoxemic liver injury in mice

Dear Editor: We read with great interest the recent paper by Li et al. [1], who reported evidence for a critical role of the CXC che-mokines keratinocyte (KC) and macrophage-inflammatory protein-2 (MIP-2) in the pathophysiology of endotoxin (ET)-induced liver injury. The authors showed increased KC and MIP-2 formation during endotoxemia and found that neutralizing antibodies against these chemokines strongly attenuated parenchymal cell apoptosis, prevented neutrophil extravasation, and reduced liver injury after treatment of mice with galactosamine (Gal)/ET [1]. As neutrophil extrav-asation is a prerequisite of injury in this [2] and other inflammatory liver disease models [3], the data of Li et al. [1] appear to identify the missing link and signal, which trigger neutrophil extravasation and the attack on parenchy-mal cells [4]. Moreover, the authors showed a 68% reduction of hepatocellular apoptosis in animals treated with antibod-ies against KC and MIP-2, suggesting an involvement of CXC chemokines in apoptosis signaling, which would be a novel, unrecognized function of CXC chemokines. However, a closer look at the data raises substantial doubts about the interpretations of these results and the validity of the conclusions drawn from these studies. The first concern relates to the inhibition of apoptosis in this model. The assessment of apoptosis with Hoechst stain is clearly not accurate; e.g., 6% apoptosis in control livers [1] is ten-to 100-fold above baseline values [5]. Conversely, the reduced caspase-3 activity, a critical mediator of Gal/ET-induced apoptosis [6], supports the authors' conclusion that anti-CXC antibodies attenuated apoptosis. However, before postulating a novel discovery, the authors should have checked tumor necrosis factor ␣ (TNF-␣) formation. As apoptosis in this model is entirely TNF-dependent [7], the most likely explanation for reduced apoptosis is reduced TNF formation. In fact, injection of high doses of immunoglobulin G (IgG) has been shown to bind ET, which can reduce TNF formation by Kupffer cells [8]. The observation that all IgG treatment attenuated apoptosis and that the combination of two antibodies (twice the IgG dose) had an additive effect [1] is consistent with this explanation. The second concern is related to the conclusion that CXC chemokines are responsible for neutrophil transmigration. The authors showed no effect of the anti-CXC chemokine antibod-ies on leukocyte adherence in postsinusoidal venules, but they did not investigate sinusoidal adherence. As has been shown in this model, neutrophil transmigration occurs from sinusoids and not from venules [2]. Therefore, assessment of sinusoidal neutrophil sequestration would have been …