Complement system contributes to modulate the infectivity of susceptible TcI strains of Trypanosoma cruzi

BACKGROUND Trypanosoma cruzi is a protozoan parasite and an etiological agent of Chagas disease. There is a wide variability in the clinical outcome of its infection, ranging from asymptomatic individuals to those with chronic fatal mega syndromes. Both parasite and host factors, as well as their interplay, are thought to be involved in the process. OBJECTIVES To evaluate the resistance to complement-mediated killing in two T. cruzi TcI strains with differential virulence and the subsequent effect on their infectivity in mammalian cells. METHODS Tissue-culture derived trypomastigotes of both strains were incubated in guinea pig serum and subjected to flow cytometry in order to determine their viability and complement activations. Trypomastigotes were also incubated on host cells monolayers in the presence of serum, and infectivity was evaluated under different conditions of complement pathway inhibition. Relative expression of the main parasite-specific complement receptors between the two strains was assessed by quantitative real-time polymerase chain reaction. FINDINGS In this work, we showed that two TcI strains, one with lower virulence (Ninoa) compared to the other (Qro), differ in their resistance to the lytic activity of complement system, hence causing a compromised ability of Ninoa strain to invade mammalian cells. These results correlate with the three-fold lower messenger RNA (mRNA) levels of complement regulatory protein (CRP), trypomastigote-decay acceleration factor (T-DAF), and complement C2 receptor inhibitor trispanning (CRIT) in Ninoa compared to those in Qro. On the other hand, calreticulin (CRT) mRNA and surface protein levels were higher in Ninoa strain and promoted its infectivity when the lectin pathway of the complement system was inhibited. MAIN CONCLUSIONS This work suggests the complex interplay of CRP, T-DAF, CRIT, and CRT, and the diagnostic value of mRNA levels in the assessment of virulence potential of T. cruzi strains, particularly when dealing with isolates with similar genetic background.

背景 克氏锥虫（Trypanosoma cruzi）是一种原生动物寄生虫，同时也是恰加斯病（Chagas disease）的病原体。该寄生虫感染的临床结局存在显著异质性，涵盖无症状感染者至出现慢性致命性巨综合征的患者。目前认为，寄生虫自身因素、宿主因素以及二者的相互作用均参与了感染的发生发展进程。
研究目的 评估两株毒力存在差异的克氏锥虫TcI株对补体介导杀伤作用的抗性，及其对哺乳动物细胞感染性的后续影响。
研究方法 将两株菌株的组织培养来源锥鞭毛体（trypomastigote）在豚鼠血清中孵育，随后通过流式细胞术（flow cytometry）检测其存活率与补体激活情况。此外，将锥鞭毛体与宿主细胞单层共孵育并添加血清，在补体通路抑制的不同条件下评估其感染性。通过实时定量聚合酶链反应（quantitative real-time polymerase chain reaction），检测两株菌株主要寄生虫特异性补体受体的相对表达水平。
研究结果 本研究显示，两株克氏锥虫TcI株——毒力较低的Ninoa株与毒力较高的Qro株——在补体系统裂解活性的抗性上存在差异，导致Ninoa株侵袭哺乳动物细胞的能力受损。上述结果与Ninoa株中补体调节蛋白（complement regulatory protein, CRP）、锥鞭毛体衰变加速因子（trypomastigote-decay acceleration factor, T-DAF）以及补体C2受体抑制性跨膜蛋白（complement C2 receptor inhibitor trispanning, CRIT）的信使RNA（messenger RNA, mRNA）水平较Qro株低3倍的结果相符。另一方面，Ninoa株的钙网蛋白（calreticulin, CRT）mRNA及其表面蛋白水平更高，且在补体凝集素通路被抑制时可促进其感染性。
主要结论 本研究表明，CRP、T-DAF、CRIT与CRT之间存在复杂的相互作用，且mRNA水平可用于评估克氏锥虫菌株的毒力潜力，尤其适用于遗传背景相似的分离株的毒力评估。