Chickens (Gallus domesticus) are immune to infection by the deadly anthrax pathogen (Bacillus anthracis). The fowl's innate defense is its high body temperature, which the anthrax bug cannot abide.

This built-in anti-anthrax barricade is one of the four innate immunity barriers that are the first-line defenses of most living organisms against the pathogens - bacterial, viral, fungal and parasitical - that constantly assail all forms of life. These urgent defense mechanisms come in four echelons: physical, anatomic, scavenging and inflammatory.

Hot-blooded chickens are emblematic of the physiological protection afforded by innate immunity. Besides temperature, interferon and complement secretions also stave off early infective attacks.

Skin and mucus membranes are shields, which also pack a chemical punch. Thus, the skin's dermal layer secretes a fatty-acid substance, sebum, which slows down the growth of most microorganisms.

Scavenging molecules, notably macrophages and neutrophils, lie in wait for invaders, which they engulf by endocytosis and phagocytosis.

Then comes the innate response, typically weaponized by histamine secretion.

These front-line innate holding operations snap into action within hours, to stun the microbes into submission while the slower rear-guard adaptive immune response stockpiles antibodies for the long haul, repelling the antigenic enemy.

"The Caenorhabditis elegans nematode worm," molecular geneticist Frederick Ausubel told BioWorld Today, "is more primitive than an insect such as the fruit fly, and it's an excellent model organism that has provided scientists with insights into basic biological processes. We hope that our current study provides the basis for now using the worm to study immune function."

A Harvard Medical School faculty member and a principal investigator at Massachusetts General Hospital in Boston, Ausubel is senior author of a paper in today's Science, dated July 27, 2002. It bears the title: "A conserved p38 MAP kinase pathway in Caenorhabditis elegans innate immunity." Ausubel had previously found that a strain of Pseudomonas aeruginosa from an infected human patient could also kill C. elegans. So, as reported in today's Science, he and his team set out to determine whether the immune responses of human and worm are similar. (See BioWorld Today, March 3, 1999, p. 1.)

Kinase Pathway Links Worms, Humans

They chemically mutagenized worms, leaving them with weakened immune defenses and exposed them to bacterial pathogens. They identified two genetic mutations linked with this susceptibility in genes that are part of a p38 MAP kinase signaling pathway known to be involved in the innate immune response of humans.

"Because it's the only known component of the immune system common to both C. elegans and Homo sapiens," Ausubel pointed out, "this pathway probably represents the most ancient evolutionary aspect of innate immunity.

"Our overall finding," he told BioWorld Today, "was that we identified C. elegans mutants that had an innate immune deficiency that were more susceptible to pathogens. And the mutations underlying that phenotype were in this highly conserved MAP kinase pathway, including the p38 component. This signaling element of innate immunity has been conserved all the way between C. elegans and mammals. That's thus far the most ancient aspect of innate immunity that has been identified from an evolutionary perspective.

"The esp genes in the worms corresponded to the isolated mutants that were more susceptible. We called the genes esp for enhanced susceptibility to pathogens.' The key component that's relevant to our paper is this p38 MAP kinase molecule, which is clearly involved in the activation and production of immune system cytokines, which turn on the human innate immune response. The killing assay that we used is simply to transfer C. elegans from a Petri plate, where they are feeding on an innocuous bacterium, usually E. coli, to a new Petri dish that contains the virulent pathogen Pseudomonas aeruginosa.

"C. elegans eat Escherichia coli bacteria; that's what they do," Ausubel went on. "They have a normal life span of two or three weeks, when they are feeding on E. coli. When they're feeding on Pseudomonas, on the other hand, they die in about 48 hours. We screened for susceptible mutants that died even more quickly - 24 hours. Those were our potential candidates for immune-deficient worms. And there's a little trick here. From a geneticist's viewpoint, mutants that we marked are dead. That's how we identified them. But we needed to recover the progeny in order to work with them. A dead organism didn't help us any, but fortunately nematodes are hermaphrodite, and they had fertilized eggs in them. So even though the parents died very quickly when exposed to the Pseudomonas, they were filled with viable eggs. We were able to recover those mutants that way even though their parents had been killed.

"There were eight mutant candidate immune-deficient worms coming from that screen, hyper-susceptible to killing. And two of those eight mutants, esp-2 and esp-8, had the best, most similar, phenotypes. So, we focused on those, and when we cloned the gene corresponding to the two mutations, fortuitously those two genes encoded the triple-MAP kinase and double-mapped kinase that had already been shown to be part of a MAP-kinase cascade."

Treating Hospital Patients - Conceptually

"Overall," Ausubel summed up, "the major point of these experiments is that worms have innate immunity. It wasn't known until this point, really, that they have anything at all resembling innate immunity that's present in other animals. But it's clear that they have a conserved innate immune response that shares this p38 signaling pathway with other organisms, including mammals.

"Now we're trying to find out," he said, looking ahead, "what are the other components of the signaling pathway. What's upstream of this p38 kinase cascade, and what's downstream. We're using both genetic and biochemical techniques. Conceptually," Ausubel suggested, "there are potential pharmaceutical compounds that target this pathway for anti-inflammatory or anti-sepsis drugs. It's been suggested you could even use C. elegans to screen for such compounds. The septic response is activated through this same signaling cascade. And if we can block it effectively, then we have a potential way for treating patients in hospitals who come down with septic reactions to bacterial infections. That's really a future goal," he concluded.