Moving Forward in Sepsis Research
Authors: S. Ananth Karumanchi, Samir M. Parikh
Journal: American Journal of Respiratory and Critical Care Medicine (2013)

To the Editor:

We thank Dr. Perlman and colleagues for their timely and thoughtful perspective on the Proceedings of the National Academy of Sciences of the United States of America study describing large discrepancies between murine and human leukocyte transcriptional profiles obtained in critical illness settings (1, 2). The gulf between human and mouse in sepsis research was vividly illustrated by the recent negative phase 3 trial of Toll-like receptor 4 (TLR4) blockade (3). Yet, the discovery of TLR4 as the body’s sensor for gram-negative endotoxin is rightly considered foundational in immunology and was driven in large part by murine studies recognized by the 2011 Nobel Prize in Physiology or Medicine (4). How should the scientific community resolve this paradox?

The cross-species comparison of expression arrays by Seok and colleagues has injected new, important data into this ongoing conversation (2). Proving the absence of an association between mouse and human is difficult, as highlighted by the methodological issues raised in Perlman and colleagues’ editorial (1). Given the profound implications of Seok and colleagues’ article, clinicians and researchers interested in critical illness would have benefited from an even more intense effort by the authors to compare “apples to apples.” We propose that two additional experiments would have strengthened their conclusions, particularly regarding the dissimilarity in leukocyte responses to endotoxin.

First, Seok and colleagues showed that the mouse transcriptional response to endotoxin was markedly attenuated immediately downstream of TLR4 (2). Because the murine version of TLR4 is considered the true ortholog of the human gene, this surprising result made us wonder whether an adequate exposure to endotoxin was achieved or whether the transcriptional signature of high-TLR4-expressing cells in mice (e.g., monocytes) was swamped out by lower-expressing cells (e.g., lymphocytes). Had Seok and colleagues been able to adjust endotoxin dose or duration to achieve comparable TLR4 signaling responses in their own readout, the subsequent negative findings would have been greatly bolstered.

Second, Seok and colleagues could have compared transcriptional responses to endotoxin of homologous cell types between species—for example, monocytes isolated from each species stimulated with the same endotoxin serotype analyzed at comparable doses and durations of exposure. Had the ex vivo profiles been unexpectedly similar, readers could then have interpreted the in vivo dissimilarity more deeply, for example, by raising the possibility that mouse and human responses to endotoxin do not diverge at the initial molecular events, but rather, in their subsequent effects on the intercellular network found within the body.

Overall, we commend Seok and colleagues on their nuanced approach to the mouse-as-sepsis-model question. We believe that ample evidence exists to support use of the mouse as a discovery tool, but that future mechanistic studies in sepsis should emphasize early validation in the human setting (5). Rather than delaying proof-of-concept human studies, basic and clinical researchers in this field should collaborate early and extensively to ensure that the most “translatable” science is moving forward.