Pattern recognition receptors: A comparative study between flies and mice

Pattern recognition receptors are a class of host immune proteins that bind directly to microbial molecules, participate in immune signaling, and are necessary for immune responses. We investigated both the Drosophila melanogaster Toll and Mus musculus Peptidoglycan Recognition Protein-Long (Pgrp-L) genes as candidates for pattern recognition receptors.; Toll is expressed in Drosophila plasmatocytes. Using an assay we developed to visualize phagocytosis, we found that Toll plasmatocytes fail to engulf bacteria to the same extent as do wild type plasmatocytes. Tagged Toll transgenic flies were made to determine the intracellular localization of Toll and to assess rescue of the mutant phagocytosis phenotype, however, these data are not conclusive. Thus, there is insufficient evidence to classify Toll as a pattern recognition receptor.; We determined that mouse Pgrp-L is expressed in dendritic cells and the spleen, consistent with its hypothesized role as a pattern recognition receptor. We found mouse Pgrp-L expressed in blastocysts, but its biological role at this stage of development is uncertain. Mouse Pgrp-L cDNA was cloned and when expressed in the RAW 246.7 macrophage cell line, an NF-kappaB-luciferase reporter was activated. This activity suggests that Pgrp-L may signal through NF-kappaB. The entire transcription unit of Pgrp-L was deleted in embryonic stem cells (ES cells) by recombination targeting. The ES cells were transfected with Cre recombinase and subjected to ganciclovir selection, resulting in the deletion of a selectable marker cassette. Mice derived from these ES cells contained only wild type alleles of Pgrp-L, despite initial genotyping by PCR. At this time, Pgrp-L should not be considered a pattern recognition receptor due to a lack of a discernable immune phenotype in homozygous mice containing another Pgrp-L knockout allele.; We conclude that there is insufficient evidence at this time to conclude that Drosophila Toll and Mus musculus Pgrp-L are pattern recognition receptors. Future investigation into the physical association of Toll with a microbial molecule, and determination of an immune phenotype of homozygous Pgrp-L knockout mice, would suffice to classify these proteins as pattern recognition receptors.