"Autoimmune rejection" of neonatal heart transplants in experimental Chagas disease is a parasite-specific response to infected host tissue

Abstract

Infection with the protozoan parasite Trypanosoma cruzi often results in chronic heart- and gut-associated disease known as Chagas disease. In this study we show that contrary to previous reports, neonatal hearts transplanted into mice chronically infected with T. cruzi do not exhibit signs of autoimmune-type rejection or any significant inflammatory response. In addition to an absence of inflammation, these syngeneic heart transplants survive for more than 1 year and are absolutely free of parasites as determined by in situ PCR analysis. However, if well-established transplanted hearts in chronically infected mice are directly injected with live parasites, a rapid and dramatic inflammatory response ensues that results in cessation of heart function. Likewise, transplanted hearts established in mice prior to systemic infection with T. cruzi or hearts transplanted into mice during the acute stage of T. cruzi infection become parasitized and develop inflammatory foci. In these cases where the transplanted hearts become parasitized, the ensuing inflammatory response is nearly identical to that observed in the native hearts of T. cruzi-infected mice in terms of cell types present and adhesion molecules and cytokines expressed. Importantly, this response is strikingly different from that observed in the allogeneic heart rejection. These results clearly document that parasitization of heart tissue is both necessary and sufficient for the induction of tissue damage in Chagas disease and strongly argue against a principal autoimmune etiology for this disease.

Figure 1

Neonatal hearts establish and survive for greater than 6 months in mice with chronic (>200 days) T. cruzi infections. The transplanted heart tissue (A and C), although appearing less organized in comparison to the native heart of the same mouse (B and D), nevertheless lacks any evidence of the intense inflammatory response obvious in the native heart (arrows). (A and B, ×63; C and D, ×250.)

Figure 2

Injection of parasites into an established transplant results in a massive infiltration of CD8⁺ T cells. Established heart transplant 14 days after injection of parasites has virtually all of the muscle tissue displaced by inflammatory cells that stain positively (yellow/brown precipitate) for the CD8 molecule (A and B). The native heart of the same animal also exhibits a CD8⁺ cell-dominated inflammation (C). The parasite-injected transplanted heart contains a large number of cells which are producing tumor necrosis factor α (D). (A, ×63; B–D, ×250.)

Figure 3

In situ PCR detection of parasite kDNA in native and transplanted hearts. Detection of a parasite-infected cell (pseudocyst) in the heart of a mouse 14 days postinfection (A) demonstrates the specificity of the in situ PCR for the parasite. Native heart tissue from chronically infected mice (350 days) shows no intact parasite-infected cells but displays a diffuse staining pattern that is more intense in areas of heavy inflammation (arrows in B). The transplanted heart present for 200 days in the same animal as in B shows no evidence of either intact or diffusely distributed parasite kDNA, nor any evidence of inflammation (C). Five days after injection of 10⁶ parasites into a transplanted heart, parasite kDNA is readily detected by in situ PCR (D). The detection of kDNA in the parasite-injected transplanted heart precedes the inflammatory response evident by day 15 postinjection (see Fig. 2). (×250.)