Investigation of the impact of the mosquito immune system on shaping the transmitted malaria parasite populations

疟疾是一种由蚊子传播的毁灭性疾病。它每年感染约2.2亿人，造成约43万人死亡，其中大多数是撒哈拉以南非洲的儿童。这比15年前的情况要好得多，当时死亡人数几乎是15年前的两倍，这要归功于更有效的药物、更好的保健以及最重要的是加强了蚊虫控制。然而，最新数据表明，这些措施不会产生进一步的影响，因为过去几年的数字保持不变。重要的是，对杀虫剂的广泛抗药性严重阻碍了蚊子的控制。因此，显然必须加强可能导致新干预措施的研究。虽然生产有效疫苗的努力仍在继续，但现在的重点是确定在疟疾寄生虫传播给人类之前阻止它们的新方法。为此目的，对蚊子进行基因改造以防止它们传播疾病和制造对抗蚊子体内寄生虫的疫苗受到了极大的关注和资助。该项目旨在绘制蚊子-寄生虫相互作用发生的分子和进化景观，并提供新的抗疟干预措施的目标清单。我们的研究是基于早期的发现，即只有少数疟疾寄生虫在感染人类的血液中进入蚊子后存活并传播给新的宿主。大多数寄生虫在感染建立之前被蚊子的免疫反应消除。最近，我们发现一种专门的蚊子免疫反应，称为补体样系统，攻击并清除已经受损和不适合的寄生虫。因此，我们假设，这种反应可以作为一个进化的筛子，净化疟疾寄生虫种群从妥协突变为了检验我们的假设，我们将使用超过150啮齿动物疟疾寄生虫线，每一个携带在蚊子感染的早期阶段表达的基因中的有害突变库。这些突变的寄生虫中的一部分会感染小鼠，并传播给正常或部分免疫系统受损的蚊子。这包括补体样途径和另一种在寄生虫进入蚊子后不久就攻击寄生虫的途径。设法存活的寄生虫将通过蚊子叮咬传播回小鼠，小鼠-蚊子-小鼠的传播将再持续四个周期。在每个周期中，将对小鼠中的寄生虫群体进行表征，以确定突变是否影响传播的基因。我们预计，一些携带有害突变的寄生虫只有在蚊子免疫系统被破坏时才会传播。一项关于携带六个此类基因突变的寄生虫的概念验证实验证实了我们的假设。这些实验的结果将是双重的：它将揭示在与蚊子的相互作用中重要的新的寄生虫基因，这些基因可以成为旨在阻止疾病传播的干预措施的目标，它将确定蚊子免疫系统对塑造宿主之间传播的寄生虫种群的影响。后者在描述决定在人群中传播的疟原虫种群组成的力量方面非常重要。在非洲，一些蚊子比其他蚊子具有更强大的免疫系统，在某些情况下，这些蚊子占据不同的地理区域;因此，我们假设它们可以传播不同的寄生虫。第二条研究路线将使用具有不同地理分布并在疟疾流行病学中发挥不同作用的蚊子物种直接调查这一问题。确定哪些基因突变可以由某些蚊子物种传播，而不是其他蚊子物种，可以为疟疾传播格局提供前所未有的见解。

Malaria is a devastating disease transmitted by mosquitoes. It infects about 220 million and kills about 430,000 people every year, mostly children in sub-Saharan Africa. This is a much better situation than 15 years ago, when deaths were almost twice as many, and is due to the combination of more effective medicines, improved health care and, above all, enhanced mosquito control. However, the latest data indicate that these measures can have no further impact as numbers have remain unchanged in the past few years. Importantly, widespread resistance to insecticides has seriously hampered mosquito control. Therefore, it has become evident that research that could lead to new interventions must be intensified. While efforts to manufacture an effective vaccine continues, focus is now placed in identifying new ways to halt malaria parasites before they are transmitted to humans. To this purpose, genetic modification of mosquitoes preventing them from transmitting the disease and manufacturing vaccines that act against parasites inside mosquitoes have received great attention and funding. This project aims to map the molecular and evolutionary landscape in which mosquito-parasite interactions take place and provide an informed list of targets of new antimalarial interventions.Our research is based on earlier findings that only few of the malaria parasites entering a mosquito upon blood feeding on an infected human survive to be transmitted to a new host. Most parasites are eliminated by mosquito immune responses before infection is established. Recently, we discovered that a specialised mosquito immune response, called the complement-like system, attacks and removes already compromised and unfit parasites. Therefore, we hypothesised that this response can act as an evolutionary sieve that purifies malaria parasite populations from compromising mutations.To examine our hypothesis, we will use a library of over 150 rodent malaria parasite lines, each carrying a deleterious mutation in a gene expressed during early stages of mosquito infection. Pools of these mutant parasites will infect a mouse and be transmitted to mosquitoes that are normal or have parts of their immune system disrupted. This includes the complement-like pathway and another pathway that attacks parasites soon after they enter the mosquito. Parasites that manage to survive will be transmitted back to mice through mosquito bites, and the mouse-mosquito-mouse transmission will continue for four additional cycles. In each cycle, the population of parasites in the mice will be characterised to identify genes of which mutations did or did not affect transmission. We expect that some parasites carrying deleterious mutations will be transmitted only when the mosquito immune system is disrupted. A proof-of-concept experiment with parasites carrying mutations in six such genes corroborated our hypothesis. The outcome of these experiments will be twofold: it will reveal novel parasite genes important in interactions with the mosquito, which can be targets of interventions aiming to block disease transmission, and it will identify the impact the mosquito immune system can have on shaping parasite populations transmitted between hosts. The latter can be very important in delineating the forces that determine the composition of the malaria parasite populations circulating among people.In Africa, some mosquitoes have more robust immune systems than others, and in some cases these mosquitoes occupy different geographic regions; hence, we hypothesise that they can transmit different parasites. A second research line will directly investigate this question using species of mosquitoes with different geographic distributions and playing different roles in malaria epidemiology. Determining which gene mutations can be transmitted by some mosquito species but not others can provide unprecedented insights into the malaria transmission landscape.