1. Merrimack Pharmaceuticals, Inc., Cambridge, MA 02139, USA.
2. Histo-Scientific Research Laboratories, Mount Jackson, VA 22842, USA.
3. University of California at San Francisco, San Francisco, CA 94110, USA.
4. La Jolla Institute for Immunology, La Jolla, CA 92037, USA.
5. University of California San Diego, La Jolla, CA 92037, USA.