Trypanosoma cruzi Infection through the Oral Route Promotes a Severe Infection in Mice: New Disease Form from an Old Infection?

Oral transmission of Chagas disease has been documented in Latin American countries. Nevertheless, significant studies on the pathophysiology of this form of infection are largely lacking. The few studies investigating oral route infection disregard that inoculation in the oral cavity (Oral infection, OI) or by gavage (Gastrointestinal infection, GI) represent different infection routes, yet both show clear-cut parasitemia and heart parasitism during the acute infection. Herein, BALB/c mice were subjected to acute OI or GI infection using 5x104 culture-derived Trypanosoma cruzi trypomastigotes. OI mice displayed higher parasitemia and mortality rates than their GI counterparts. Heart histopathology showed larger areas of infiltration in the GI mice, whereas liver lesions were more severe in the OI animals, accompanied by higher Alanine Transaminase and Aspartate Transaminase serum contents. A differential cytokine pattern was also observed because OI mice presented higher pro-inflammatory cytokine (IFN-γ, TNF) serum levels than GI animals. Real-time PCR confirmed a higher TNF, IFN-γ, as well as IL-10 expression in the cardiac tissue from the OI group compared with GI. Conversely, TGF-β and IL-17 serum levels were greater in the GI animals. Immunolabeling revealed macrophages as the main tissue source of TNF in infected mice. The high mortality rate observed in the OI mice paralleled the TNF serum rise, with its inhibition by an anti-TNF treatment. Moreover, differences in susceptibility between GIversusOI mice were more clearly related to the host response than to the effect of gastric pH on parasites, since infection in magnesium hydroxide-treated mice showed similar results. Overall, the present study provides conclusive evidence that the initial site of parasite entrance critically affects host immune response and disease outcome. In light of the occurrence of oral Chagas disease outbreaks, our results raise important implications in terms of the current view of the natural disease course and host-parasite relationship.

恰加斯病（Chagas disease）的口腔传播已在拉丁美洲国家得到文献证实，但目前针对该感染途径的病理生理学研究仍相对匮乏，系统性研究尤为不足。既往少数针对口腔感染途径的研究，均未意识到：口腔黏膜接种（口腔感染，OI）与灌胃感染（胃肠道感染，GI）属于两种截然不同的感染路径，但二者在急性感染阶段均会出现明确的虫血症与心脏寄生现象。

本研究以5×10^4个体外培养获得的克氏锥虫（Trypanosoma cruzi）锥鞭毛体，分别构建BALB/c小鼠急性口腔感染（OI）与胃肠道感染（GI）模型。结果显示，OI组小鼠的虫血症水平与死亡率均显著高于GI组小鼠。心脏组织病理学检测发现，GI组小鼠的心肌炎症浸润面积更大；而OI组小鼠的肝脏损伤更为严重，同时血清丙氨酸转氨酶（Alanine Transaminase，ALT）与天冬氨酸转氨酶（Aspartate Transaminase，AST）水平显著升高。

细胞因子谱亦存在显著差异：OI组小鼠血清促炎细胞因子（pro-inflammatory cytokine）干扰素-γ（IFN-γ）、肿瘤坏死因子（TNF）水平显著高于GI组。实时荧光定量PCR（Real-time PCR）检测证实，相较于GI组，OI组小鼠心脏组织中TNF、IFN-γ及白细胞介素10（IL-10）的表达水平均显著升高；与之相反，GI组小鼠血清转化生长因子β（TGF-β）与白细胞介素17（IL-17）水平更高。

免疫标记实验显示，巨噬细胞是感染小鼠体内TNF的主要组织来源。OI组小鼠的高死亡率与血清TNF水平升高呈显著正相关，且抗TNF治疗可有效抑制该致死现象。此外，经氢氧化镁处理的小鼠感染实验结果与未处理组一致，提示GI与OI模型的易感性差异主要源于宿主免疫应答的不同，而非胃内pH对寄生虫的直接影响。

综上，本研究确凿证实：寄生虫侵入的初始位点可显著影响宿主免疫应答与疾病转归。鉴于口腔源性恰加斯病暴发事件的频发，本研究结果对当前关于该病自然病程及宿主-寄生虫互作关系的主流认知，具有重要的学术启示价值。