Murine filariasis: interleukin 4 and interleukin 5 lead to containment of different worm developmental stages

We compared the impact of IL-4 and IL-5 deficiency during the fully permissive infection of BALB/c mice with the rodent filaria Litomosoides sigmodontis. IL-5, in contrast to IL-4, is crucial for the containment of adult worms during short- and long-term infections. IL-5 KO mice allowed development of more larvae into adult worms and showed up to 200 times more adult worms persisting during chronic infection (day 60 until 200 post-infection). This increased persistence was accompanied by a reduction in inflammatory nodules around adult filariae. In contrast, adult worm survival and nodule formation did not differ between BALB/c wild-type mice and BALB/c IL-4 KO or BALB/c IL-4 receptor (IL-4R) alpha-chain KO mice. In both IL-4 and IL-5 KO mice microfilaraemia was greatly enhanced (160-fold) and prolonged compared to wild-type mice. This extent of susceptibility to microfilariae required the presence of adult worms in a full infection cycle since upon intraperitoneal injection of microfilariae alone they were removed from BALB/c, BALB/c IL-4 KO and BALB/c IL-4R alpha-chain KO mice with equivalent kinetics, and since microfilarial survival was only slightly increased in IL-5 KO mice (factor of 5 vs. factor of 160 in full infection). In conclusion, IL-4 and IL-5 dependent effector pathways operate against different stages of filarial worms, and IL-5 has a greater impact on parasite containment than IL-4.