Interleukin-12 modulates T-cell responses to microfilariae but fails to abrogate interleukin-5-dependent immunity in a mouse model of onchocerciasis

Abstract

Infection of mice with microfilariae of Onchocerca lienalis induces high levels of protective immunity to reinfection, which is dependent on interleukin (IL)-5 but not IL-4. Here, we have investigated the effect of exogenous IL-12 administration during either the priming or effector phases of the immune response. When administered during priming, IL-12 induced down-regulation of parasite-specific serum immunoglobulin (Ig)E and up-regulation of IgG2a. Antigen-specific IL-4 responses were strongly suppressed, whilst blood eosinophil levels were partially reduced. When administered during a challenge infection, IL-12 did not significantly influence the balance of antibody isotypes, but partially reduced eosinophil production. Antigen-specific IL-4 responses were again completely ablated. Unusually, interferon-gamma (IFN-gamma) responses were not significantly affected following IL-12 administration, either during priming or after challenge infections. Moreover, despite a fall in antigen-specific IL-5 production, the expression of IL-5-dependent immunity, as determined by reduction in worm recoveries, was fully maintained. These data demonstrate that parasite-induced IL-4 can be abrogated without affecting protective immunity to Onchocerca microfilariae in mice. In view of the established role of IL-4 in pathogenesis, this may have important implications for the development of immunoprophylaxis aimed at microfilariae and the alleviation of pathology in onchocerciasis.

Figure 1

Effect of interleukin (IL)‐12 treatment on systemic immune responses during a primary infection of mice with living microfilariae (Mf) of Onchocerca lienalis. Parasite‐specific serum immunoglobulin (Ig)E and IgG2a levels were measured 15 days after primary (b, c) and challenge (e, f) infections. Peripheral eosinophil levels were measured at various time‐points, as indicated. Groups of animals (n = 8) received either recombinant (r)IL‐12 or phosphate‐buffered saline (PBS) over the period prior to challenge. Bars represent mean percent eosinophil, or mean OD 405 readings (± SE). NMS, normal mouse serum. ¹*P < 0·05 compared with PBS‐treated mice.

Figure 2

Effect of interleukin (IL)‐12 treatment on antigen‐specific spleen cell cytokine production during the priming of mice with living microfilariae (Mf) of Onchocerca lienalis. Groups of animals received either recombinant (r)IL‐12 or phosphate‐buffered saline (PBS) over the period prior to challenge. Spleen cells collected 15 days after challenge were cultured with 10 µg/ml parasite antigen. Splenocytes from naive mice were included as controls. Levels of interferon‐γ (IFN‐γ), IL‐4 and IL‐5 were measured using enzyme‐linked immunosorbent assay (ELISA). Bars represent mean (± SE) cytokine concentration in culture supernantants (n = 4). *P < 0·05 compared with PBS‐treated mice.

Figure 3

Effect of interleukin (IL)‐12 treatment on systemic immune responses during a challenge infection with living microfilariae (Mf) of Onchocerca lienalis. Groups of mice (n = 8) were sensitized with a primary infection of microfilariae to induce protective immunity and received either recombinant (r)IL‐12 or phosphate‐buffered saline (PBS) during the period following challenge. Parasite‐specific immunoglobulin (Ig)E and IgG2a were measured 15 days after challenge. Peripheral eosinophilia was monitored at various time‐points, as indicated. Bars represent mean (± SE) OD readings at 405 nm or mean (± SE) per cent eosinophilia. NMS, normal mouse serum. *P < 0·05 compared with PBS‐treated mice.

Figure 4

Effect of interleukin (IL)‐12 treatment during a challenge infection with living microfilariae (Mf) of Onchocerca lienalis on antigen‐specific spleen cell cytokine production. Groups of mice were sensitized with a primary infection of microfilariae to induce protective immunity and received either recombinant (r)IL‐12 or phosphate‐buffered saline (PBS) during the period following challenge. Spleen cells collected 15 days after challenge were cultured with 10 µg/ml parasite antigen. Splenocytes from naive mice were included as controls. Levels of interferon‐γ (IFN‐γ), IL‐4 and IL‐5 were measured using enzyme‐linked immunosorbent assay (ELISA). Bars represent mean (± SE) cytokine concentration in culture supernantants (n = 4). *P < 0·05 compared with PBS‐treated mice.