Innate immune responses in peptidoglycan recognition protein L-deficient mice

Peptidoglycan recognition proteins (PGRPs) constitute a family of innate immune recognition molecules. In Drosophila, distinct PGRPs bind to peptidoglycans on gram-positive or gram-negative bacteria and provide essential signals upstream of the Toll and Imd pathways required for immunity against infection. Four PGRPs, PGRP-L, -S, -Ialpha, and -Ibeta, are expressed from three genes in mammals. In this paper, we provide direct evidence that the longest family member, PGRP-L, is a secreted serum protein with the capacity to multimerize. Using gene targeting to create PGRP-L-deficient mice, we demonstrate little contribution by PGRP-L to systemic challenge using gram-negative bacteria (Escherichia coli, slightly less susceptible), Gram-positive bacteria (Staphylococcus aureus), or yeast (Candida albicans). Peritoneal macrophages from PGRP-L-deficient mice produced decreased amounts of the inflammatory cytokines interleukin 6 and tumor necrosis factor alpha when stimulated with E. coli or lipopolysaccharide, but comparable amounts when stimulated with S. aureus, C. albicans, or their cell wall components. Additionally, these cells produced similar amounts of cytokines when challenged with gram-positive or -negative peptidoglycans. In contrast to its critical role in immunity in flies, PGRP-L is largely dispensable for mammalian immunity against bacteria and fungi.