HUMAN ENTERIC DEFENSINS - GENE STRUCTURE AND DEVELOPMENTAL EXPRESSION

Paneth cells, secretory epithelial cells of the small intestinal crypt s, are proposed to contribute to local host defense, Both mouse and hu man Paneth cells express a collection of antimicrobial proteins, inclu ding members of a family of antimicrobial peptides named defensins, In this study, data from an anchored polymerase chain reaction (PCR) str ategy suggest that only two defensin mRNA isoforms are expressed in th e human small intestine, far fewer than the number expressed in the mo use, The two isoforms detected by this PCR approach were human defensi n family members, HD-5 and HD-6, The gene encoding HD-6 was cloned and characterized, HD-6 has a genomic organization similar to HD-5, and t he two genes have a striking pattern of sequence similarity localized chiefly in their proximal 5'-flanking regions, Analysis of human fetal RNA by reverse transcriptase-PCR detected enteric defensin HD-5 mRNA at 13.5 weeks of gestation in the small intestine and the colon, but b y 17 weeks HD-5 was restricted to the small intestine, HD-6 mRNA was d etectable at 13.5-17 weeks of gestation in the small intestine but not in the colon, This pattern of expression coincides with the previousl y described appearance of Paneth cells as determined by ultrastructura l approaches, Northern analysis of total RNA from small intestine reve aled quantifiable enteric defensin mRNA in five samples from 19-24 wee ks of gestation at levels approximately 40-250-fold less than those ob served in the adult, with HD-5 mRNA levels greater than those of HD-6 in all samples, In situ hybridization analysis localized expression of enteric defensin mRNA to Paneth cells at 24 weeks of gestation, as is seen in the newborn term infant and the adult, Consistent with earlie r morphological studies, the ratio of Paneth cell number per crypt was reduced in samples at 24 weeks of gestation compared with the adult, and this lower cell number partially accounts for the lower defensin m RNA levels as determined by Northern analysis, Low levels of enteric d efensin expression in the fetus may be characteristic of an immaturity of local defense, which is thought to predispose infants born prematu rely to infection from intestinal microorganisms.