Human immune responses to infective stage larval-specific chitinase of filarial parasite, Onchocerca volvulus, Ov-CHI-1

Abstract

BACKGROUND: Ov-CHI-1 is a chitinase specifically expressed in the infective stage larvae of the human filarial parasite Onchocerca volvulus. Evidence has show that it could be a vaccine candidate, however, there is no data available regarding the immunological status of people naturally exposed to infective stage larvae and thus provoked by this antigen. METHOD: We analysed the Ov-CHI-1-specific immune response present in four endemic foci of human onchocerciasis (Ecuador, Nigeria, Togo and Cameroon) by enzyme-linked immunosorbent assays and T-cell proliferation assays. RESULTS: In these foci of infection, antibodies to Ov-CHI-1 were found to be present in only 22% of individuals from Ecuador, but were detected in 42-62% of infected individuals in the three foci from West Africa (Nigeria, Togo and Cameroon). There was found to be no relationship between antibody level and age, gender, or infection intensity as indicated by microfilarial density and numbers of skin nodules. The isotype response to Ov-CHI-1 was dominated by the presence of IgG3, IgG1 was present to a lesser extent. Our results show a positive correlation between N- and C-termini of Ov-CHI-1 in their ability to provoke humoral and cellular immune responses in the human. Peripheral blood mononuclear cell (PBMC) proliferative responses to Ov-CHI-1 when assayed, were found to be significantly higher in the individuals from endemic areas and there was a statistically elevated response to Ov-CHI-1 in the infected individuals when compared to putative immune individuals. CONCLUSION: Ov-CHI-1 is an antigen that we have found strongly induces both humoral and cellular immune responses in humans.

Figure 1

ELISA analysis of the IgG response to recombinant Ov-CHI-1 in human onchocerciasis. Figure 1A, Comparison of the response in sera of endemic individuals from 4 endemic foci with uninfected controls from the United Kingdom. Mean values are indicated by a horizontal line. Figure 1B, Comparison of the response in the sera of infected (INF) and putatively immune (PI) individuals from 4 endemic foci and from uninfected UK controls. Box and whisker plots illustrate the group mean values (horizontal bar), data points falling within the 25th to 75th percentile (boxed) and the range (vertical lines). * P = < 0.05 vs. UK controls † P = < 0.05 vs. Individuals from Eucador

Figure 2

IgG subclass analysis by ELISA on Ov-CHI-1 full-length antigen using 77 selected sera (see text). The whole profile of IgG1-IgG4 responses is shown in Figure 2A. Figure 2B shows the relationship between microfilarial skin density and anti-Ov-CHI-1 IgG3 level in these sera. The relationship of IgG1 with IgG3 is shown in Figure 2C.

Figure 3

Schematic representation of filarial chitinase gene and the position of recombinant Ov-CHI-1 used in our studies

Figure 4

ELISA analysis of the IgG response to recombinant Ov-CHI-1 5' or 3' antigen in human onchocerciasis. Figure 4A: Comparison of the response in the sera of infected (INF) and putatively immune (PI) individuals. Figure 4B: The relationship of response to 5' antigen and 3' antigen for individual sera. The line of liner regression is shown for data sets in which there is a statistically significant correlation between responses to the two antigens (determined by Spearman's Rank Correlation Coefficient).

Figure 5

Human cellular immune responses to Ov-CHI-1. PBMC were isolated from individuals were stimulated with recombinant Ov-CHI-1 proteins. The results are expressed in stimulation indices. Figure 5A: the response to the full-length antigen. Figure 5B: the response to 3' antigen of Ov-CHI-1. * P = < 0.05 vs. controls ** P = < 0.01 vs. controls

Figure 6

Analysis of IgG responses to Ov-CHI-1 categorized by age for individual individuals from four endemic regions. Figure 6A: Nigeria, Figure 6B: Ecuador, Figure 6C: Togo, Figure 6D: Cameroon.

Figure 7

Analysis of IgG responses of human onchocerciasis to Ov-CHI-1 categorized by gender and comparison of responses in the sera of infected (INF) and putatively immune (PI) individuals from four endemic foci.

Figure 8

Analysis of IgG responses to Ov-CHI-1 plotted against microfilarial skin densities for individual individuals from four endemic regions. Figure 8A: Nigeria, Figure 8B Ecuador, Figure 8C Togo, Figure 8D. Cameroon. The line of linear regression is shown on each graph.

Figure 9

Analysis of IgG responses to Ov-CHI-1 with respect to skin nodule number for individual subject from four endemic regions. Figure 9A: Cameroon, Figure 9B: Ecuador, Figure 9C: Nigeria.