The radiation-attenuated schistosome vaccine induces high levels of protective immunity in the absence of B cells

Abstract

Radiation-attenuated cercariae of Schistosoma mansoni elicit consistently high levels of protective immunity in mice. The cell-mediated pulmonary effector mechanisms have been well characterized but the role of B cells and antibodies remains ill defined. We have compared the immune responses of B-cell-deficient (muMT) mice and their wild-type (WT) counterparts following exposure to the attenuated vaccine. Both groups mounted a T helper type 1 (Th1)-biased response in the skin-draining lymph nodes after vaccination. Interferon-gamma was the dominant cytokine secreted by airway leucocytes after challenge in both muMT and WT mice, but there was a somewhat greater Th2 component in the former animals. The cellular infiltrates observed in the airways, and the pulmonary effector foci, were of similar composition in the two groups although some large foci were present in the muMT mice. There was a marked dichotomy in the protection induced in muMT animals by a single vaccination, with two-thirds showing levels similar to their WT counterparts, demonstrating that cell-mediated mechanisms alone can provide adequate protection. The remaining muMT mice had a mean worm burden identical to that of their challenge controls. A possible explanation is that a proportion of the muMT animals have a genetic defect closely associated with the mu-heavy-chain locus on chromosome 12, which affects their ability to mount a protective cell-mediated response. Three vaccinations enhanced the immunity of WT animals, most likely by augmenting antibody-mediated mechanisms. In contrast, no enhancement was seen in muMT mice, suggesting that the cell-mediated response is not boosted by multiple exposures to attenuated larvae.

Figure 1

SLAP-specific cytokine production by LN cells on days 5 and 15 postvaccination. Lymphocytes from WT and μMT mice (2×10⁵/well) were cultured in the presence of SLAP for 72 hr before determination of (a) IFN-γ and (b) IL-4 production. Bars are means±SEM (n = 3 to n = 5 mice).

Figure 2

BAL cell populations at day 14 postchallenge. The proportions of lymphocytes, macrophages and granulocytes were determined on the basis of size and granularity by flow cytometry and then multiplied by haemocytometer cell counts. Bars are means±SEM (n = 5 mice).

Figure 3

Cytokine production by BAL cells on day 14 postchallenge. BAL leucocytes from WT and μMT mice (4×10⁵/well) were cultured with or without SLAP for 72 hr before determination of (a) IFN-γ, (b) IL-4, (c) IL-5 and (d) IL-13 production. Cells were pooled within groups of mice (n = 5) and error bars are for replicate ELISA wells.

Figure 4

Worm burdens of individual WT (▴, ▵) and μMT (• ○) mice 5 weeks after challenge; C animals (▵,○) and V animals (▴, •). Horizontal lines are means (with percentage protection in V relative to C groups shown alongside). Values in brackets are percent-age protection with V μMT outliers removed. Two representative experiments of five are shown.

Figure 5

Frequency distribution of protection levels in individual WT and μMT mice (data from five experiments, n = 27 and n = 30, respectively). Protection was calculated for each V animal by comparing its worm burden to the mean worm burden of the relevant C group.

Figure 6

Mean worm burdens±SEM of C, 1×V and 3×V groups of WT and μMT mice, 5 weeks after challenge. Values above the bars represent percentage protection calculated as described in the Materials and Methods.