Infection stage-dependent modulation of macrophage activation in Trypanosoma congolense-resistant and -susceptible mice

Abstract

The contribution of cytokines and chemokines to resistance and susceptibility to African trypanosomiasis remains controversial. In the present study, the levels of type I and type II cytokines and of the MCP-1 chemokine were compared during the early and late stages of Trypanosoma congolense infection in susceptible BALB/c and resistant C57BL/6 mice. Moreover, the status of macrophage activation was compared in these animals by analyzing the inducible nitric oxide synthase-arginase balance, tumor necrosis factor secretion, and expression of the FIZZ1 and YM genes. Data show that changing from a predominant type I cytokine environment in the early stage of infection to a predominant type II cytokine environment and an enhanced MCP-1 secretion in the late stage of infection correlates with resistance to T. congolense. Concomitantly, macrophage activation evolves from a classical to a predominant alternative phenotype. We further confirmed that the simultaneous occurrence of type I/type II cytokines in the early stage of infection in susceptible BALB/c mice, reflected by the presence of macrophages exhibiting a mixed classical/alternative activation phenotype, is associated with uncontrolled parasite growth and early death. Interleukin-4 (IL-4) and IL-13 signaling did not influence the susceptibility of BALB/c mice to T. congolense infection and interestingly were not the main trigger to alternative macrophage activation. In T. congolense-resistant C57BL/6 mice, our results corroborated the induction of FIZZ1 and YM gene expressions with the alternative pathway of macrophage activation. In susceptible BALB/c mice, however, YM but not FIZZ1 induction reflected the emergence of alternatively activated macrophages. Hence, the FIZZ1 and YM genes may be useful markers to discriminate between distinct populations of alternatively activated macrophages.

FIG. 1.

Cytokine levels in serum from BALB/c and C57BL/6 mice infected with T. congolense. During the early (6 days) and late (8 weeks) stages of infection, IFN-γ (solid bars), IL-10 (dotted bars), and IL-4 (open bars) cytokines were quantified in the serum of BALB/c and C57BL/6 mice (mean ± standard error, n = 3). Cytokine levels in serum from noninfected animals were below the detection limit (<5 pg/ml). a, significantly higher compared to noninfected animals; b, significantly lower compared to the early stage of infection; c, significantly higher compared to the early stage of infection.

FIG. 2.

Status of macrophage activation during T. congolense infection in BALB/c and C57BL/6 mice. Spontaneous and concanavalin A-induced NO₂ (a) and tumor necrosis factor (b) secretion was quantified in the supernatants of peritoneal exudate cells from BALB/c (open bars) and C57BL/6 (solid bars) mice (mean ± standard error, n = 3) in the early (6 days) and late (8 weeks) stages of infection and compared to that in noninfected (N) animals. In parallel, arginase activity was quantified in peritoneal exudate cell lysates (c), and expression levels of the β-actin, FIZZ1, and YM genes was analyzed in noninfected (N) and infected (I) adherent peritoneal exudate cells (d). The number of PCR cycles performed is indicated in parentheses. a, significantly higher compared to noninfected animals; b, significantly lower compared to the early stage of infection; c, significantly higher compared to the early stage of infection.

FIG. 3.

Role of IL-4 and IL-13 in macrophage activation status during T. congolense infection in BALB/c mice. During the early stage (6 days) of infection, spontaneous and concanavalin A (ConA)-induced NO₂ (a) and tumor necrosis factor (b) secretion was quantified in the supernatants of peritoneal exudate cells from IL-4^−/−, IL-4Rα^−/−, and wild-type BALB/c mice (mean ± standard error, n = 3). In parallel, arginase activity was quantified in lysates from peritoneal exudate cells (c). Dotted lines represent levels in peritoneal exudate cells from naïve animals. Finally, expression levels of the β-actin, FIZZ1, and YM genes were analyzed in adherent peritoneal exudate cells from the noninfected (N) and infected (I) animals (d). The number of PCR cycles performed is indicated in parentheses. a, significantly higher than in noninfected animals; b, significantly higher than in BALB/c mice.