Fasciola hepatica suppresses a protective Th1 response against Bordetella pertussis

Abstract

Fasciolosis, like other helminth infections, is associated with the induction of T-cell responses polarized to the Th2 subtype. Respiratory infection with Bordetella pertussis or immunization with a pertussis whole-cell vaccine (Pw) induces a potent Th1 response, which confers a high level of protection against bacterial challenge. We have used these two pathogens to examine bystander cross-regulation of Th1 and Th2 cells in vivo and provide evidence of immunomodulation of host T-cell responses to B. pertussis by a concomitant infection with Fasciola hepatica. Mice with a coinfection of F. hepatica and B. pertussis exhibited a Th2 cytokine profile in response to F. hepatica antigens, similar to those infected with F. hepatica alone. By contrast, the Th1 response to B. pertussis antigens was markedly suppressed and the bacterial infection was exacerbated following infection with F. hepatica. Furthermore, an established Th1 response induced in mice by infection with B. pertussis or by parenteral immunization with Pw was also suppressed following infection with F. hepatica. This immunomodulatory effect of B. pertussis-induced responses by F. hepatica infection is significantly reduced, but not completely abrogated, in IL-4 knockout mice. Our findings demonstrate that Th2-inducing parasites can exert bystander suppression of protective Th1 responses to infection or vaccination with a bacterial pathogen and that the modulation is mediated in part by IL-4 and, significantly, is effective at both the induction and effector stages of the Th1 response.

FIG. 1

F. hepatica suppresses B. pertussis-specific IFN-γ production in coinfected mice. BALB/c mice were infected with either B. pertussis (BP) or F. hepatica (FH) or were concurrently infected with B. pertussis and F. hepatica. Three weeks after infection, spleen cells were stimulated in vitro with B. pertussis sonicate (BPS), FHA, LFH, PMA and anti-CD3, or medium only, and cytokine levels were assessed in supernatants 3 days later. Cytokine concentrations represent means ± SE after subtraction of background control values with medium only (IFN-γ, 1.5 to 2.7 ng/ml; IL-4, 11 to 36 pg/ml) for four mice per experimental group and are representative of four experiments. ∗∗, P < 0.01 versus mice infected with B. pertussis alone.

FIG. 2

F. hepatica infection delays bacterial clearance in mice infected with B. pertussis. BALB/c mice were infected either with B. pertussis alone (▴) or concurrently with F. hepatica (■). Subsequently, mice were sacrificed at various times to assess the numbers of viable bacteria in the lungs. Results are reported as the mean numbers of B. pertussis CFU for individual lungs from four mice from each group at each time point and are representative of two experiments. ∗, P < 0.05 versus mice infected with B. pertussis alone; ∗∗, P < 0.01 versus mice infected with B. pertussis alone.

FIG. 3

F. hepatica infection suppresses an established Th1 response to B. pertussis. BALB/c mice were infected with B. pertussis and allowed to recover for 6 weeks. The convalescent mice were then infected with 10 metacercariae of F. hepatica. Mice infected with B. pertussis (BP) or F. hepatica (FH) alone served as controls. Antigen-specific cytokine production by spleen cells was assessed 3 weeks after infection with F. hepatica. Cytokine concentrations represent means ± SE after subtraction of background controls (IFN-γ, 1.5 to 2.7 ng/ml; IL-4, 11 to 36 pg/ml) for four mice per experimental group and are representative of three experiments. ∗∗, P < 0.01 versus mice infected with B. pertussis alone; ∗∗∗, P < 0.001 versus mice infected with B. pertussis alone.

FIG. 4

F. hepatica infection suppresses a Th1 response induced with Pw. Mice were immunized with Pw and boosted 4 weeks later. Two weeks after the second immunization, mice were infected with F. hepatica (FH). Controls consisted of mice that received either F. hepatica infection or pertussis immunization only. Cytokine production by spleen cells was assessed 2 weeks after F. hepatica infection, following stimulation in vitro with B. pertussis sonicate (BPS) and LFH. Cytokine concentrations represent means ± SE after subtraction of background control values (IFN-γ, 2.2 to 2.4 ng/ml; IL-2, 0.02 to 0.2 U/ml; IL-4, 11 to 36 pg/ml; IL-5, 12 to 25 pg/ml) for four mice per experimental group and are representative of three experiments. ∗, P < 0.05 versus mice immunized with Pw alone; ∗∗∗, P < 0.001 versus mice immunized with Pw alone.

FIG. 5

F. hepatica infection reduces the protective efficacy of Pw in mice. BALB/c mice were immunized with Pw (0 and 4 weeks), and 1 week later a proportion of these mice were infected with 10 metacercariae of F. hepatica (FH). Respiratory infection of mice with B. pertussis was performed by aerosol challenge 1 week after infection with F. hepatica. Naive mice and mice that were immunized with Pw and subsequently infected with B. pertussis without a preceding F. hepatica infection served as controls. Mice were killed from all groups at various times after aerosol challenge to assess the numbers of viable bacteria in the lungs. Results are reported as the mean numbers of B. pertussis CFU for individual lungs from four mice at each time point and are representative of two experiments. ∗, P < 0.05 versus mice immunized with Pw alone.

FIG. 6

Effect of F. hepatica infection on antigen-specific cytokine production in IL-4^−/− mice immunized with Pw. IL-4^−/− and wild-type C57BL/6 mice were immunized with Pw and boosted after 4 weeks. Two weeks after the second immunization, mice were infected with F. hepatica (FH). Mice infected with F. hepatica or immunized with Pw only served as controls. Cytokine production were assessed 2 weeks after F. hepatica challenge by stimulating spleen cells in vitro with B. pertussis sonicate (BPS), LFH, or PMA and anti-CD3. Cytokine concentrations represent means ± SE after subtraction of background control values (IFN-γ, 2.9 to 3.8 ng/ml; IL-4, <10 pg/ml) and are representative of two experiments. ∗∗, P < 0.05 versus mice immunized with Pw alone; ∗∗, P < 0.01 versus mice immunized with Pw alone; ∗∗∗, P < 0.001 versus mice immunized with Pw alone.