Necator americanus and helminth co-infections: further down-modulation of hookworm-specific type 1 immune responses

Abstract

Background: Helminth co-infection in humans is common in tropical regions of the world where transmission of soil-transmitted helminths such as Ascaris lumbricoides, Trichuris trichiura, and the hookworms Necator americanus and Ancylostoma duodenale as well as other helminths such as Schistosoma mansoni often occur simultaneously.

Methodology: We investigated whether co-infection with another helminth(s) altered the human immune response to crude antigen extracts from either different stages of N. americanus infection (infective third stage or adult) or different crude antigen extract preparations (adult somatic and adult excretory/secretory). Using these antigens, we compared the cellular and humoral immune responses of individuals mono-infected with hookworm (N. americanus) and individuals co-infected with hookworm and other helminth infections, namely co-infection with either A. lumbricoides, Schistosoma mansoni, or both. Immunological variables were compared between hookworm infection group (mono- versus co-infected) by bootstrap, and principal component analysis (PCA) was used as a data reduction method.

Conclusions: Contrary to several animal studies of helminth co-infection, we found that co-infected individuals had a further downmodulated Th1 cytokine response (e.g., reduced INF-γ), accompanied by a significant increase in the hookworm-specific humoral immune response (e.g. higher levels of IgE or IgG4 to crude antigen extracts) compared with mono- infected individuals. Neither of these changes was associated with a reduction of hookworm infection intensity in helminth co-infected individuals. From the standpoint of hookworm vaccine development, these results are relevant; i.e., the specific immune response to hookworm vaccine antigens might be altered by infection with another helminth.

Figure 1. Fecal egg counts in hookworm mono- and co-infected individuals.

Footnotes: Circles represent individual values for eggs per gram of feces (epg) and are shown on a logarithmic scale. Boxes indicate the median and the quartiles for each group and the whiskers indicate the 95% ranges. Groups are split in hookworm mono-infected (HW), co-infected with A. lumbricoides (HW+ASC), co-infected with S. mansoni (HW+SM), and triple-infected individuals (HW+ASC+SM). Kruskal-Wallis test on differences in hookworm egg counts between groups was not statistically significant (p = 0.523).

Figure 2. Robust principal component analysis (PCA) of log-transformed serum antibody values in response to hookworm antigens.

Footnotes: The principal component scores for individuals mono- (•) and co-infected (▴) with hookworm are shown. The respective mean values are shown as open symbols, with 95% confidence ellipses (p value for bivariate T2 test is 0.006). The arrows show the strongest loadings, i.e. contributions of the original variables to the principal components.