Regional variation in the correlation of antibody and T-cell responses to Trypanosoma cruzi

Abstract

Chagas disease, caused by the protozoan parasite Trypanosoma cruzi, is a major cause of morbidity and mortality in Central and South America. Geographic variations in the sensitivity of serologic diagnostic assays to T. cruzi may reflect differences in T. cruzi exposure. We measured parasite-specific T-cell responses among seropositive individuals in two populations from South America with widely varying antibody titers against T. cruzi. Antibody titers among seropositive individuals were significantly lower in Arequipa, Peru compared with Santa Cruz, Bolivia. Similarly, the proportion of seropositive individuals with positive T-cell responses was lower in Peru than Bolivia, resulting in overall lower frequencies of interferon-γ (IFNγ)-secreting cells from Peruvian samples. However, the magnitude of the IFNγ response was similar among the IFNγ responders in both locations. These data indicate that immunological discrepancies based on geographic region are reflected in T-cell responses as well as antibody responses.

Figure 1.

Differences in absorbance values of seropositive individuals in two different geographic regions. Sera from individuals in Bolivia (Santa Cruz) and Peru (Arequipa region) were tested using a Chagatek assay for detection of T. cruzi-specific antibodies. Seropositives were determined as described in Materials and Methods. SN = seronegative; SP = seropositive.

Figure 2.

Comparison of T. cruzi-specific T-cell responses by country stratified by Chagatek serologic status. PBMCs were stimulated with media alone or T. cruzi antigen (Tc21 strain) in an ELISPOT assay as described. Data are expressed as IFNγ SFUs induced by T. cruzi antigen. Data are displayed as a box and whiskers graph, with minimum and maximum values shown. (A) IFNγ responses from SN or SP individuals in Peru and Bolivia. (B) SFUs of only IFNγ responders.

Figure 3.

IFNγ recall responses to various T. cruzi strains in Peruvians. PBMCs from Peruvian individuals were stimulated with whole-parasite lysate from each strain as shown. Data are displayed as a box and whiskers graph, with minimum and maximum values shown. P > 0.05 for all strains.

Figure 4.

Scatterplot of T-cell responses and Chagatek absorbance values by country. Scatterplot analysis of absorbance values versus ELISPOT responses revealed two distinct patterns and simple linear regression to estimate the relationship between the two measures of T. cruzi exposure (controlling for age and sex). Among Bolivian specimens, a positive association was observed, where a 1 log unit change in ELISPOT responses resulted in an X-unit change in absorbance. Conversely, among Peruvian specimens, the change in absorbance as a function of Tc21 response was lower and marginally significant. Note that the y axes scales are different in the two graphs.

Figure 5.

Median ELISPOT IFNγ spot count by number of positive serology tests for Peruvian samples. Upper panel shows IFNγ-producing cells stratified by number of positive serology tests for the same individual grouped by the number of positive tests. Data are displayed as a box and whiskers graph, with minimum and maximum values shown on a log-scale y axis. Lower panel shows median spot count and P values comparing the median spot count for one, two, three, or four serological tests with no positive serological tests (REF). IQR = interquartile range.