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IL-5 production and eosinophilia are features of helminth infections, but results concerning the role of IL-5 and eosinophils (EP) in worm control are contradictory. We describe here a novel, IL-5-dependent mechanism of helminth control in vivo, using a fully permissive murine filariasis model, i.e. infection of BALB/c mice with Litomosoides sigmodontis. Worm control was exerted by the formation of inflammatory nodules around adult filariae which initially remained alive but were eventually killed within several weeks. The cell population essential for inflammatory nodule formation was found to be neutrophils (NP) but not EP. Neutralization of IL-5 led to a failure of both EP and NP accumulation at the site of infection (i.e. the thoracic cavity), resulting in cessation of inflammatory nodule formation around worms and in their survival. The role of NP in this process was confirmed by treatment of mice with anti-granulocyte colony stimulating factor (G-CSF) which also resulted in a lack of inflammatory nodule formation and worm killing albeit in the presence of EP. Since IL-5, due to the absence of IL-5 receptors on NP, does not act on these cells directly, it was investigated if anti-IL-5 altered the production of NP-chemotactic cytokines. In anti-IL-5-treated mice, cytokines known to promote NP accumulation like tumor necrosis factor-alpha, G-CSF and KC (IL-8) were found to be strongly reduced, while NP-deactivating cytokines like IL-10 were increased. In conclusion, IL-5 constitutes a cytokine essential for NP-mediated worm control in filarial infection.

The pathways conferring immunity to human filariases are not well known, in part because human-pathogenic filariae do not complete a full life cycle in laboratory mice. We have used the only fully permissive infection of mice with filariae, i.e., infection of BALB/c mice with the rodent filarial nematode Litomosoides sigmodontis. Our previous results showed that worm development is inversely correlated with Th2 cytokine production and eosinophilia. The scope of the present study was to directly elucidate the role of interleukin-5 (IL-5) and eosinophils in controlling the development of L. sigmodonitis after vaccination and in primary infection. BALB/c mice immunized with irradiated third-stage larvae (L3) were confirmed to have elevated IL-5 levels as well as high subcutaneous eosinophilia and to attack and reduce incoming larvae within the first 2 days, resulting in 70% reduction of worm load. Treatment of vaccinated mice with anti-IL-5 antibody (TRFK-5) suppressed both blood and tissue eosinophilia and completely abolished protection. This demonstrates, for the first time in a fully permissive filarial infection, that IL-5 is essential for protection induced by irradiated L3 larvae. In contrast, in primary-infected mice, anti-IL-5 treatment did not modify filarial infection within the 1st month, most likely because during primary infection IL-5-dependent mechanisms such as subcutaneous eosinophilia are induced too late to disturb worm establishment. However, there is a role for IL-5 late in primary infection where neutrophil-dependent worm encapsulation is also under the control of IL-5.

The pathways conferring immunity to filarial infections are not well known, in part because human pathogenic filariae do not develop a full infection cycle in laboratory mice. Using the permissive infection with Litomosoides sigmodontis in BALB/c mice, we have shown previously that worm development is controlled by CD4+ T cells and is inversely correlated with Th2 cytokine production. Here we analyzed the impact of the Xid immunodeficiency on murine filariosis, comparing the course of infection with L. sigmodontis in BALB/c and B1 cell-deficient BALB.Xid mice. In BALB.Xid mice, 2-3 times more adult worms and up to 10 times more microfilariae compared to BALB/c were observed to develop after infection with infective stage 3 larvae (L3). Parasite-specific Th2 cytokine production by cells from the thoracic cavity, the primary location of the parasites, was diminished significantly in BALB.Xid compared to BALB/c mice. In addition, BALB.Xid mice displayed a significantly lower production of antibodies and B cell-derived IL-10 in response to both L. sigmodontis antigen and phosphorylcholine, a molecule we found to be abundant on the surface of L3. Thus, the B cell-defect in BALB.Xid mice may account for susceptibility to murine filarial infection in two ways, i.e. by the lack of antibody to a dominant surface molecule of invading L3 and by less B cell-derived IL-10 resulting in lower parasite-driven Th2 cytokine production.

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