Differential susceptibility to acute Trypanosoma cruzi infection in BALB/c and C57BL/6 mice is not associated with a distinct parasite load but cytokine abnormalities

Abstract

Inoculation of Trypanosoma cruzi, Tulahuén strain, into C57BL/6 and BALB/c mice led to an acute infection characterized by marked parasitaemia, myocardial inflammation and thymocyte depletion. While C57BL/6 mice showed a progressive and lethal disease, BALB/c mice partly recovered. To characterize these murine models more effectively, we studied the parasite burden, serum levels of major infection outcome-related cytokines, the in vitro features of T. cruzi infection in peritoneal macrophages and the immunophenotype of thymic cells. The greater disease severity of T. cruzi-infected C57BL/6 mice was not linked to an increased parasite load, as parasitaemia, myocardial parasite nests and amastigote counts in peritoneal macrophages were not different from those in BALB/c mice. Cortical thymocyte loss was accompanied by the presence of apoptotic bodies and fragmented nuclear DNA, whereas fluorocytometric analysis at 17 days postinfection (p.i.) revealed a more pronounced loss of CD4+ CD8+ cells in C57BL/6 mice. This group displayed higher levels of TNF-alpha on days 14 and 21 p.i., in the presence of lower IL-1beta and IL-10 concentrations by days 14 and 21, and days 7 and 14 p.i., respectively. Day-21 evaluation showed higher concentrations of nitrate and TNF-alpha soluble receptors in C57BL/6 mice with no differences in IFN-gamma levels, with respect to the BALB/c group. Increased morbidity of C57BL/6 T. cruzi-infected mice does not seem to result from an aggravated infection but from an unbalanced relationship between pro- and anti-inflammatory mediators.

Fig. 1

Thymic alterations in T. cruzi-infected BALB/c and C57BL/6 mice. Apoptosis visualization in thymocytes. (a) Sections from thymus medulla (1) and cortex (2) of control and infected mice, stained by TUNEL. (b) Agarose gel electrophoresis of DNA from mouse thymuses. Lanes 1 and 3 correspond to non infected C57BL/6 and BALB/c, respectively. Lanes 2 and 4 correspond to 21-day infected C57BL/6 and BALB/c, respectively. Each lane contains DNA from two different animals loaded independently. (c) Cell cytometric analysis of DP thymocytes from acutely T. cruzi-infected mice; data correspond to mean ± s.e.m. values of six mice/group (one representative of three independent sets of experiments). *P < 0·002. □, Non-infected; ▪, infected.

Fig. 2

Circulating levels of TNF-α and its soluble receptors in LPS-challenged mice. (a) The relative increase in either TNF-α or its soluble receptors was calculated according the following formula: values in LPS-challenged mice/baseline values at day 21 p.i. (mean ± s.e.m.); #P < 0·01 when compared with BALB/c mice. □, BALB/c; ▪, C57BL/6. (b) Results indicate the mean ± s.e.m. (pg/ml) of 4–5 mice/group (one of two experiments with similar results). Different from C57BL/6 mice, *P < 0·05 and **P < 0·01.