How does an insect fight off germs? Easy. It builds a wall around them.
"Insects don't have antibodies, like mammals do," explains Jessica Stehr, a graduate student in genetics and entomology. "Their immune systems are non-specific, and they have no memory."
That doesn't mean their defenses aren't effective.
When the insect's body recognizes invasion by fungal pathogens, blood cells called granulocytes rush to the scene, depositing granular matter which surrounds the invader. Then plasmatocytes, another type of blood cell, arrive to establish a barrier, which hardens over a period of several hours. The invading organism, thus isolated and killed, is rendered harmless.
The question is how is all this activity controlled. Up to now, few of the critical enzymes involved in this encapsulating response have been identified. Stehr and her adviser, assistant professor of entomology Diana Cox-Foster, have pinpointed another: FAD-glucose dehydrogenase, or GLD.
Cox-Foster and others had previously found GLD in the molting fluid of fruit flies and other insects, including cockroaches and crop pests; Flies deprived of GLD can't escape from their pupal cases, and so can't molt.
Because the formation of the pupal case in molting uses processes similar to the hardening of encapsulation, Cox-Foster suspected that GLD might also be important in the immune response. In 1992 she recruited Stehr, then a sophomore molecular and cell biology major whom Cox-Foster had known since Stehr's 10th-grade summer, when she did research in Cox-Foster's lab as a student in the Pennsylvania Governor's School for the Agricultural Sciences.
Cox-Foster and Stehr took tobacco young hornworm caterpillars, lovely-looking aqua-blue larvae about the thickness of a pencil, and implanted them with model pathogens: gram-positive and gram-negative bacteria, yeast (a fungus), and latex (as an inert control). Working under a dissecting microscope, Stehr would make a tiny incision in the caterpillar's hind-quarters and insert the pathogen. Then, after an adequate lapse, she removed the now cell-coated implants and analyzed the encapsulating tissue with an enzyme assay using special reagents that change color in response to GLD.
What she found was lots of GLD, particularly in the cells that had gathered around the yeast and latex implants. "This suggests that GLD is more important in the anti-fungal response," Stehr says, "and less important in the anti-bacterial response." In a subsequent experiment using fruit flies, they found corroboration, showing GLD to be important to the fly's defense against yeast.
Cox-Foster and Stehr suspect that GLD works by reacting with other enzymes to produce toxic free radicals, molecules that are particularly chemically active. "These free radicals do one of two things," Stehr says. "Either they actually kill the encapsulated invaders, or they act to help crosslink the blood cells, strengthening the wall around the pathogen."
As a next step, Stehr will look to see whether GLD is also active in fighting parasites like the Cotesia wasp, which lays its eggs in a caterpillar's body. Another important question concerns where GLD comes from: Is it manufactured in the blood cells themselves, or merely transported by them from somewhere else?
Ultimately, she says, "understanding GLD may help in the development of biological controls for agricultural pests."
Jessica E. Stehr received both her B.S. in molecular and cell biology (with honors in entomology) and her M.S. in genetics in May 1994, through the Graduate School's Integrated Undergraduate—Graduate Degree program. Her adviser, Diana L. Cox-Foster, Ph.D., is assistant professor of entomology, 501 Agricultural Science and Industry Building, University Park, PA 16802; 814-865-1022.
Stehr is currently enrolled in the School of Veterinary Medicine of the University of Pennsylvania.