Scientific Abstract

Proposal No.  IBD-0148R
Principal Investigator:  Michael Zasloff, M.D., Ph.D.
Applicant Organization:  Georgetown University (Washington, District of Columbia, U.S.A.)
Project Title:  Antimicrobial peptide expression and its relationship to commensal bacteria in the transplanted small bowel
Period of Award:  July 25, 2005 – July 31, 2008

Antimicrobial peptides (AMPs) are components of the innate immune system, the system of defense that is “hardwired” to respond to specific classes of microbes.  Sites such as the bowel wall, the conjunctivae, the oral epithelium, and skin elaborate a cocktail of peptides that provide these surfaces with a protective shield against potential pathogens.  This system permits the body to actively defend the epithelium without resorting to the mobilization of destructive inflammatory mechanisms.  In addition, antimicrobial peptides also help maintain control over the species of commensals with which animals co-exist, by suppressing the growth of certain sensitive microbial species.

It is generally accepted that inflammatory bowel disease (IBD) represents a condition in which host intestinal-microbial relationships are disturbed, resulting in chronic intestinal inflammation.  Some recent reports have suggested that antimicrobial peptide expression in Crohn’s disease is deficient, from both Paneth cells and lumenal enterocytes.

We will explore the relationship between antimicrobial peptide expression and microbial populations in the gut.  We will study the human small bowel in the setting of small intestinal transplantation.  Patients undergoing this procedure are routinely monitored by frequent and regularly scheduled endoscopic biopsies through an ileal diversionary stoma over a three month course.  We propose to identify the constellation of genes of the innate immune system expressed by the crypt epithelium during the course of small intestinal transplantation, from the time of implantation, through initiation of feeding, and into the period of stable, normal functioning prior to surgical closure.  We anticipate that both known and novel antimicrobial products will be identified.  We will also follow the progression of microbial species that come to populate the initially sterile small intestine and correlate their presence with the induction of antimicrobial defenses.  Intestinal bacterial species diversity will be determined using high-throughput gene sequencing techniques not requiring cultivation.  We anticipate that as antimicrobial peptide expression is induced by the presence of microbes, certain species will rise or fall in abundance dependent on their sensitivity to the AMPs to which they are exposed.  About 25% of our patient population will likely experience acute graft rejection, a process in which certain crypt cells (including Paneth cells) become depleted.  We anticipate that in these individuals, bacterial populations will respond with respect to species diversity, organisms sensitive to the activity of Paneth cell defensins increasing in abundance.  Such patients will be treated with a short course of additional immunosuppressive agents.  We anticipate that following recovery, a shift in gut bacterial populations will return to a composition that might resemble the pre-rejection condition, at least with respect to the antimicrobial peptide sensitivity.  The knowledge gained from these human studies could provide insights to understand the nature of the disturbed host-microbial interactions that occur in IBD.