Cell invasion by Trypanosoma cruzi and its intracellular replication are essential for continuation of the parasite life cycle and for production of Chagas' disease. T. cruzi is able to replicate in nucleated cells and can be killed by activated macrophages. Gamma interferon (IFN-␥) is one of the major stimuli for the activation of macrophages and has been shown to be a key activation factor for the killing of intracellular parasites through a mechanism dependent upon nitric oxide (NO) biosynthesis. We show that although the addition of exogenous tumor necrosis factor alpha (TNF-␣) does not potentiate the trypanocidal activity of IFN-␥ in vitro, treatment of resistant C57BI/6 mice with an anti-TNF-␣ monoclonal antibody increased parasitemia and mortality. In addition, the anti-TNF-␣-treated animals had decreased NO production, both in vivo and in vitro, suggesting an important role for TNF-␣ in controlling infection. In order to better understand the role of TNF-␣ in the macrophage-mediating killing of parasites, cultures of T. cruzi-infected macrophages were treated with an anti-TNF-␣ monoclonal antibody. IFN-␥-activated macrophages failed to kill intracellular parasites following treatment with 100 g of anti-TNF-␣. In these cultures, the number of parasites released at various time points after infection was significantly increased while NO production was significantly reduced. We conclude that IFN-␥-activated macrophages produce TNF-␣ after infection by T. cruzi and suggest that this cytokine plays a role in amplifying NO production and parasite killing.

The increase in Th1 type (CCR1, CCR5, and CXCR3) and Th2 type (CCR2 and CCR3) receptors in both periapical lesions suggests the concomitant occurrence of Th1 and Th2 responses. Furthermore, the prevalent expression of the receptors CCR3, CCR5, CXCR1, and CXCR3 and of the chemokines RANTES, IP-10, and MCP-1 in cysts may point to a role in the progression of granulomas to cysts.