Alternative Activation of Macrophages in Mice Peritoneal Cavities and Diaphragms by Newborn Larvae of Trichinella spiralis

Background: Trichinellosis is a serious zoonosis with a worldwide distribution. Fecund adult worms in the intestine release newborn larvae (NBL) that enter the general circulation from 4 days post infection (dpi). Alternatively activated macrophages in the peritoneal cavities and the diaphragms in Trichinella spiralis infected mice have been reported. However, a role of newborn larvae is poorly understood. Methods: The total numbers of peritoneal macrophages in mice infected with 500 muscle-stage larvae were counted during early infection and then total RNA was extracted. Peritoneal macrophages from uninfected C57BL/6 mice were incubated with IL-4 or LPS as a control, or co-cultured with live NBL, and peritoneal macrophages were obtained from mice injected with live or frozen dead NBL into peritoneal cavity. Total RNA was extracted from these macrophages. Two types of gene expression, classical and alternative activation, were examined in the macrophages and diaphragms of the infected mice using semi-quantitative reverse transcription-PCR. Results: The number of peritoneal macrophages in T. spiralis infected mice increased significantly. mRNA peak expression of alternative activation markers, Ym1 and arginase-1 (Arg1), was confirmed in the peritoneal macrophages and in diaphragm of mice around 15 dpi, while mRNA expression of classical activation markers, TNFα, IP-10, and iNOS was not detected. Injection of live NBL into the peritoneal cavities induced mRNA expression of Ym1 and Arg1 in the peritoneal macrophages of mice 9 dpi. However, dead NBL did not induce such gene expression. Alternative activation was not detected in the peritoneal macrophages co-cultured with NBL in vitro. Conclusion: Gene expression of alternative activation makers, Ym1 and Arg1, was confirmed in the peritoneal macrophages and diaphragms of mice infected with T. spiralis. However, gene expression of classical activation markers was not detected. Live NBL induced an alternative activation of peritoneal macrophages in vivo, but not in vitro.