Acknowledgments
This work was partly supported by NIH contract HHSN268200500019C (Thomas B. Kepler), grant GM-41514 (Michael D. Cahalan), the George E. Hewitt Foundation for Medical Research (Melanie P. Matheu), and by the National Science Foundation under grant DMS-1106516 (Mike West). Any opinions, findings, and conclusions or recommendations expressed in this work are those of the authors and do not necessarily reflect the views of the Hewitt Foundation, NIH, and/or NSF.