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，导致约43万人死亡，其中大部分是撒哈拉以南非洲的儿童。这比15年前要好得多，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、改善的卫生保健以及最重要的是加强了蚊子控制。然而，最新的数据表明，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，广泛的杀虫剂耐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，对蚊子进行基因改造以防止它们传播疾病，并制造针对蚊子体内寄生虫的疫苗得到了极大的关注和资助。该项目旨在绘制蚊子和寄生虫相互作用的分子和进化图景，并提供新的抗疟疾干预措施的靶标清单。我们的研究基于早期的发现，即通过吸食受感染的人的血液进入蚊子的疟疾寄生虫中，只有少数存活下来并传播给新的宿主。在确定感染之前，大多数寄生虫都通过蚊子的免疫反应来消除。最近，我们发现一种特殊的蚊子免疫反应，称为补体样系统，攻击并清除已经受到损害的和不适合的寄生虫。因此，我们假设这种反应可以作为一个进化筛子，净化疟疾寄生虫种群，使其免受危害的突变。为了检验我们的假设，我们将使用一个超过150个啮齿动物疟疾寄生虫品系的文库，每个品系在蚊子感染的早期阶段表达的一个基因中携带有害突变。这些突变寄生虫将感染老鼠，并将其传播给正常或部分免疫系统被破坏的蚊子。这包括补体样途径和另一种在寄生虫进入蚊子后不久攻击它们的途径。成功存活的寄生虫将通过蚊子叮咬传播回老鼠，老鼠-蚊子-老鼠的传播将持续四个额外的周期。在每个周期中，将对老鼠体内的寄生虫种群进行特征描述，以确定哪些基因突变影响了传播，哪些基因突变没有影响传播。我们预计，只有当蚊子的免疫系统被破坏时，一些携带有害突变的寄生虫才会传播。一项对携带6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：它将揭示在与蚊子相互作用中重要的新寄生虫基因，这些基因可能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形成在宿主之间传播的寄生虫种群的影响。后者在描述决定人与人之间传播的疟疾寄生虫种群组成的力量方面可能非常重要。在非洲，一些蚊子的免疫系统比其他蚊子更强大，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。第二个研究线将利用不同地理分布和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一些蚊子物种传播，而不是其他蚊子物种可以传播，这可以为疟疾传播格局提供前所未有的洞察。

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.