IgG1 and IgG4 antibody responses to the Anopheles gambiae salivary protein gSG6 in the sympatric ethnic groups Mossi and Fulani in a malaria hyperhendemic area of Burkina Faso

Abstract

Human antibody response to the Anopheles gambiae salivary protein gSG6 has recently emerged as a potentially useful tool for malaria epidemiological studies and for the evaluation of vector control interventions. However, the current understanding of the host immune response to mosquito salivary proteins and of the possible crosstalk with early response to Plasmodium parasites is still very limited. We report here the analysis of IgG1 and IgG4 subclasses among anti-gSG6 IgG responders belonging to Mossi and Fulani from Burkina Faso, two ethnic groups which are known for their differential humoral response to parasite antigens and for their different susceptibility to malaria. The IgG1 antibody response against the gSG6 protein was comparable in the two groups. On the contrary, IgG4 titers were significantly higher in the Fulani where, in addition, anti-gSG6 IgG4 antibodies appeared in younger children and the ratio IgG4/IgG1 stayed relatively stable throughout adulthood. Both gSG6-specific IgG1 and IgG4 antibodies showed a tendency to decrease with age whereas, as expected, the IgG response to the Plasmodium circumsporozoite protein (CSP) exhibited an opposite trend in the same individuals. These observations are in line with the idea that the An. gambiae gSG6 salivary protein induces immune tolerance, especially after intense and prolonged exposure as is the case for the area under study, suggesting that gSG6 may trigger in exposed individuals a Th2-oriented immune response.

Figure 1. Seasonal variation of the gSG6-specific IgG1 and IgG4 antibody titers in the sympatric ethnic groups Mossi and Fulani.

Scatter plots reporting IgG1 (left panel) and IgG4 (right panel) antibody titers among gSG6 IgG responders of the Mossi and Fulani ethnic groups in the three different surveys. C, unexposed controls. Bars indicate median values. Number of individuals analyzed (n) and average age in years ±95% CI were as follows. August: Mossi n = 60 (22.0±4.5), Fulani n = 63 (16.1±3.9); October: Mossi n = 56 (13.0±3.4), Fulani n = 30 (21.2±7.0); March: Mossi n = 32 (15.3±5.1), Fulani n = 29 (11.6±4.2); Controls n = 44 (33.1±7.1). P values determined according to the Kruskal-Wallis test. Pairwise comparisons refer to the Mann-Whitney U test (*, 0.01

Figure 2. Seasonal variation of the IgG response to the P. falciparum CSP in Mossi and Fulani.

Scatter plot of OD values representing the IgG response to the P. falciparum CSP among gSG6 IgG responders of the Mossi and Fulani ethnic groups in the three different surveys. C, unexposed controls (n = 28). Bars, number of Mossi and Fulani analyzed, p values and pairwise comparisons as in Figure 1. Note that three data points are outside the axis limits.

Figure 3. Distribution of anti-gSG6 IgG1 and IgG4 and of anti-CSP IgG in Mossi and Fulani according to different age groups.

Box plots showing IgG1 and IgG4 response to gSG6 (A-D) and IgG response to CSP (E-F) among gSG6 IgG responders from the three different surveys. Boxes display median values, 25^th and 75^th percentiles. Whiskers represent 5–95 percentiles and dots the outliers. Anti-gSG6 IgG1 and IgG4 antibody titers are expressed in ng/ml, anti-CSP IgG levels as OD₄₀₅. The five different age groups (years) are indicated at the bottom. Data from the three different surveys were pooled and the number of individuals for each age group is given in parenthesis. Left panels (A, C, E) refer to Mossi and right panels (B, D, F) to Fulani as indicated. P values were determined by the Kruskal-Wallis test and pairwise comparisons were according to Mann-Whitney U test (*, 0.01<p<0.05; **, 0.001<p<0.01; ***, p<0.001).

Figure 4. IgG4/IgG1 ratio by age group in Mossi and Fulani.

Scatter plot reporting the IgG4/IgG1 ratio by age group in Mossi (n = 120) and Fulani (n = 95). Data from the three different surveys were pooled together and individuals with no detectable IgG1 or IgG4 (or both) were excluded from the analysis. The five different age groups (years) are indicated at the bottom. The number of individuals for each age group is given in parenthesis. Bars indicate median values. P values determined according to the Kruskal-Wallis test. Pairwise comparisons refer to the Mann-Whitney U test (*, 0.01

Figure 5. Humoral response to the gSG6 and CSP proteins according to age.

Scatter plot reporting the antibody responses to gSG6 (IgG1, black; IgG4, red) and to CSP (IgG, green) as function of age among gSG6 IgG responders of the Mossi (n = 148) and Fulani (n = 122) ethnic groups from the three different surveys. Anti-gSG6 IgG1 and IgG4 are expressed as titers (ng/ml, left Y axis); IgG response to CSP is expressed as OD₄₀₅ (right Y axis). The best-fit lines are shown (Mossi, solid lines; Fulani dashed lines). Spearman correlation coefficients: (i) anti-CSP IgG (Mossi, r = 0.53, p<0.0001; Fulani, r = 0.41, p<0.0001); (ii) anti-gSG6 IgG1 (Mossi, r = −0.25, p = 0.0019; Fulani, r = −0.27, p = 0.0024); (iii) anti-gSG6 IgG4 (Mossi, r = −0.12, p = 0.1359; Fulani, r = −0.29, p = 0.0013). Note that twenty-six data points are outside the axis limits.