Innate immune gene expression and function studied in insects and mammals

Sammanfattning: Insects and mammals possess innate immune responses to effectively recognize and eliminate invading microorganisms. Insects, who share with mammals conserved molecules for immune defense, are good model systems for innate immunity studies. To identify genes related to immunity in the cabbage looper Trichoplusia ni, a differential display PCR technique was employed to isolate genes that become up-regulated upon bacterial challenge of T. ni larvae. Using the PCR bands as probes to screen a library from T. ni immunized larvae, we obtained cDNA clones for T. ni lysozyme, cecropin, lebocin, gloverin and a peptidoglycan recognition protein (PGRP). T. ni cecropin shows highest homology to Hyalophora cecropia cecropin A. Lebocin shows 35% identity to Bombyx mori lebocin. A downstream insertion element was found at the C-terminus of the non-coding region of T. ni lebocin cDNA. Gene expression studies revealed that these five cloned genes are normally silent and activated quickly upon bacterial infection. Transcripts were found mainly in immune organs such as fat body and hemocytes of bacteria-infected larvae. Gloverin is synthesized as a precursor including a signal peptide and a prosegment, and is processed to a 14 kDa mature protein upon export into immunized larvae hemolymph. Recombinant expression of gloverin with a six-histidine tag in the baculovirus system enabled us to obtain a proprotein. This progloverin inhibited the growth of E. coli and the activity is comparable to that of H. gloveri mature gloverin. Peptidoglycan is a bacterial cell wall component that can trigger immune responses in insects and mammals. PGRP has sequence homology to an amidase, bacteriophage T3 lysozyme. We have shown that PGRP is a 19 kDa humoral protein that binds to peptidoglycan with high affinity but lacks amidase activity. We have also shown that PGRP is a ubiquitous immune protein by cloning the corresponding genes from human and mouse. The mammalian PGRP is strongly expressed in hematopoietic tissues. A family of twelve rather diverse PGRP proteins was identified in the fruit fly Drosophila melanogaster. Based on the length of gene transcripts, the twelve genes are grouped into short and long PGRPs. The short group contains seven genes that are predicted to encode exported proteins. The long group members code for intracellular, membrane-spanning or extracellular proteins. Their gene expression patterns are versatile and can be tissue specific or developmentally regulated. Two members produced as recombinant proteins bind to peptidoglycan.