Within-host mechanisms of immune regulation explain the contrasting dynamics of two helminth species in both single and dual infections

Abstract

Variation in the intensity and duration of infections is often driven by variation in the network and strength of host immune responses. While many of the immune mechanisms and components are known for parasitic helminths, how these relationships change from single to multiple infections and impact helminth dynamics remains largely unclear. Here, we used laboratory data from a rabbit-helminth system and developed a within-host model of infection to investigate different scenarios of immune regulation in rabbits infected with one or two helminth species. Model selection suggests that the immunological pathways activated against Trichostrongylus retortaeformis and Graphidium strigosum are similar. However, differences in the strength of these immune signals lead to the contrasting dynamics of infections, where the first parasite is rapidly cleared and the latter persists with high intensities. In addition to the reactions identified in single infections, rabbits with both helminths also activate new pathways that asymmetrically affect the dynamics of the two species. These new signals alter the intensities but not the general trend of the infections. The type of interactions described can be expected in many other host-helminth systems. Our immune framework is flexible enough to capture different mechanisms and their complexity, and provides essential insights to the understanding of multi-helminth infections.

Fig 1. Experimental data of single and dual infections.

Intensity of infection (A and D), specific IgA antibody response estimated using adult worms (B) or excretory-secretory (ES) products from adult worms as the source of antigen (E), and IL4 cytokine gene expression (C and F) during single (black) and dual (red) infections of T. retortaeformis (A-C) and G. strigosum (D-F). Geometric means and relative dispersions (calculated as product/ratio between the geometric mean and the geometric standard deviation) are presented.

Fig 2. Single infection simulations (blue) and observation data (black).

Mean intensity of infection (A and D), specific IgA response estimated using adult worms as a source of antigen (B and E), and IL4 expression (C and F) over the course of the infection. Observed data (geometric mean multiplied/divided by the S.D. error factor (circle)) and estimated values (star) with the relative 90% confidence interval (shade) are reported for both T. retortaeformis (A-C) and G. strigosum (D-F).

Fig 3. Dual infection simulations (blue) and observation data (red).

Mean intensity of infection (A and D), specific IgA response estimated using excretory-secretory (ES) products from adult parasites as a source of antigen (B and E), and IL4 expression (C and F) over the course of the infection. Observed data (geometric mean multiplied/divided by S.D. error factor) and estimated values (star) with the relative 90% CI (shade) are reported for T. retortaeformis (A-C) and G. strigosum (D-F).