Eosinophil deficiency compromises parasite survival in chronic nematode infection

Abstract

Immune responses elicited by parasitic worms share many features with those of chronic allergy. Eosinophils contribute to the inflammation that occurs in both types of disease, and helminths can be damaged or killed by toxic products released by eosinophils in vitro. Such observations inform the widely held view that eosinophils protect the host against parasitic worms. The mouse is a natural host for Trichinella spiralis, a worm that establishes chronic infection in skeletal muscle. We tested the influence of eosinophils on T. spiralis infection in two mouse strains in which the eosinophil lineage is ablated. Eosinophils were prominent in infiltrates surrounding infected muscle cells of wild-type mice; however, in the absence of eosinophils T. spiralis muscle larvae died in large numbers. Parasite death correlated with enhanced IFN-gamma and decreased IL-4 production. Larval survival improved when mice were treated with inhibitors of inducible NO synthase, implicating the NO pathway in parasite clearance. Thus, the long-standing paradigm of eosinophil toxicity in nematode infection requires reevaluation, as our results suggest that eosinophils may influence the immune response in a manner that would sustain chronic infection and insure worm survival in the host population. Such a mechanism may be deployed by other parasitic worms that depend upon chronic infection for survival.

FIGURE 1

Intestinal infection promotes myositis, neutrophilia, eosinophilia and a peripheral Th2 response. A, H&E stained sections of tongue collected from C57BL/10 mice at 20 days post-intravenous infection (I.V.) or 24 days post-oral infection. Arrows indicate infiltrating cells and arrowheads indicate larva within the nurse cell. Scale bar = 100 μm. B, The cellular response around nurse cells was quantified by measuring the distance from the capsule to the margin of the infiltrate. (Note: Cellular infiltrates do not attain sufficient size for measurement prior to 17 dpoi/dpi.) C, Numbers of white blood cells (WBC) recovered from diaphragms of mice after intravenous or oral infection. D, Differential counts of leukocytes recovered from diaphragms of mice after intravenous or oral infection. For C and D, values at 12 and 20 dpi should be compared with values at 16 and 24 dpoi. E, Cytokines produced in cultures of antigen-stimulated cervical lymph node (CLN) cells collected at 12 dpi or 16 dpoi. N.D. = not detected. Values represent means ± SD, n = 4 - 8 mice. *p <0.05, ** p < 0.001, ***p < 0.0001.

FIGURE 2

Eosinophils promote survival of T. spiralis larvae in muscle. A, Larval burdens in muscles of C57BL/6 or PHIL mice, 28 dpoi. B, Infectivity of L₁ recovered from C57BL/6 or PHIL muscles 24 dpoi. The viability of L₁ is reflected in their ability to transmit infection to C57BL/6 mice. C, Muscle burdens in BALB/c or ΔdblGATA^−/− mice, 28 dpoi. D, Kinetics of adult worm expulsion from small intestines of C57BL/6 or PHIL mice. E, Number of developing nurse cells per low power field (LPF) at 12 dpoi. F, Detection of tyvelose (red color) in muscle larvae, 15 dpoi. G, H&E stained sections of tongue collected from mice at 12 and 28 dpoi. Arrowheads indicate T. spiralis larva and arrows indicate infiltrating cells. Scale bar = 100 μm. Values are means ± SD, n = 3 - 4 mice. *p <0.05, ** p < 0.001.

FIGURE 3

Parasite killing is associated with a Th1 immune response. A, Cytokines and B, nitric oxide (NO) end-products measured in antigen-stimulated cultures of CLN cells collected from PHIL and C57BL/6 mice at 17 dpoi. C, Detection of iNOS in skeletal muscle (brown color), 17 dpoi. Arrowhead indicates larva and arrows indicate cellular infiltrates. Scale bar = 100 μm. D, Design of L-NIL treatment study. E, Effect of L-NIL treatment on NO-end products in cultures of CLN, 24 dpoi. F, Effect of L-NIL treatment on larval burdens 24 dpoi. Values represent means ± SD, n = 3 - 4 mice. *p <0.05, ** p < 0.001.

FIGURE 4

Enhanced Th1 response in eosinophil deficient mice enhances parasite killing. A, Cytokines and B, NO end-products measured in cultures of antigen-stimulated CLN cells collected 24 dpoi. C, Detection of iNOS in muscle (arrows indicate positive areas) 24 dpoi. Scale bar = 100 μm. D, Larval burdens in muscles of IL-10^−/− and IL-10^−/−/PHIL mice, 28 dpoi. E, Effect of aminoguanidine (AMG) treatment on NO end-products in CLN cultures and F, larval burdens in IL-10^−/− and IL-10^−/−/PHIL mice, 24 dpoi. Treatment design replicated that in Fig. 3. Values represent means ± SD, n = 3 - 4 mice. *p <0.05, ** p < 0.001.