Malarial infection is initiated when the sporozoite form of the Plasmodium parasite is inoculated into the skin by a mosquito. Sporozoites invade hepatocytes in the liver and develop into the erythrocyte-infecting form of the parasite, the cause of clinical blood infection. Protection against parasite development in the liver can be induced by injection of live attenuated parasites that do not develop in the liver and thus do not cause blood infection. Radiation-attenuated sporozoites (RAS) and genetically attenuated parasites are now considered as lead candidates for vaccination of humans against malaria. Although the skin appears as the preferable administration route, most studies in rodents, which have served as model systems, have been performed after i.v. injection of attenuated sporozoites. In this study, we analyzed the early response to Plasmodium berghei RAS or wild-type sporozoites (WTS) injected intradermally into C57BL/6 mice. We show that RAS have a similar in vivo distribution to WTS and that both induce a similar inflammatory response consisting of a biphasic recruitment of polymorphonuclear neutrophils and inflammatory monocytes in the skin injection site and proximal draining lymph node (dLN). Both WTS and RAS associate with neutrophils and resident myeloid cells in the skin and the dLN, transform inside CD11b+ cells, and induce a Th1 cytokine profile in the dLN. WTS and RAS are also similarly capable of priming parasite-specific CD8+ T cells. These studies delineate the early and local response to sporozoite injection into the skin, and suggest that WTS and RAS prime the host immune system in a similar fashion.

Background To be transmitted to vertebrate hosts via the saliva of their vectors, arthropod-borne viruses have to cross several barriers in the mosquito body, including the midgut infection and escape barriers. Yellow fever virus (YFV) belongs to the genus Flavivirus , which includes human viruses transmitted by Aedes mosquitoes, such as dengue and Zika viruses. The live-attenuated YFV-17D vaccine has been used safely and efficiently on a large scale since the end of World War II. Early studies have shown, using viral titration from salivary glands of infected mosquitoes, that YFV-17D can infect Aedes aegypti midgut, but does not disseminate to other tissues. Methodology/Principal findings Here, we re-visited this issue using a panel of techniques, such as RT-qPCR, Western blot, immunofluorescence and titration assays. We showed that YFV-17D replication was not efficient in Aedes aegypti midgut, as compared to the clinical isolate YFV-Dakar. Viruses that replicated in the midgut failed to disseminate to secondary organs. When injected into the thorax of mosquitoes, viruses succeeded in replicating into midgut-associated tissues, suggesting that, during natural infection, the block for YFV-17D replication occurs at the basal membrane of the midgut. Conclusions/Significance The two barriers associated with Ae . aegypti midgut prevent YFV-17D replication. Our study contributes to our basic understanding of vector–pathogen interactions and may also aid in the development of non-transmissible live virus vaccines.

Disease caused by the Zika virus (ZIKV) is a public health emergency of international concern. Recent epidemics have emerged in different regions of the world and attest to the ability of the virus to spread wherever its vector, Aedes species mosquitoes, can be found. We have compared the transmission of ZIKV by Ae. aegypti (PAEA strain originating from Tahiti) and by a French population of Ae. albopictus to better assess their competence and the potential risk of the emergence of ZIKV in Europe. We assessed the transmission of ZIKV by Ae. albopictus in temperatures similar to those in Southern France during the summer. Our study shows that the extrinsic incubation period of Ae. aegypti for transmission was shorter than that of Ae. albopictus. Both vectors were able to transmit ZIKV from 10 to 14 days post-infection. Ae. aegypti, however, had a longer transmission period than the French population of Ae. albopictus. Although the salivary glands of both vectors are highly infected, transmission rates of ZIKV to saliva remain relatively low. These observations may suggest that the risk of emergence of ZIKV in Europe could be low.

20 Background 21 To be transmitted to vertebrate hosts via the saliva of their vectors, arthropod-borne viruses have 22 to cross several barriers in the mosquito body, including the midgut infection and escape barriers.23 Yellow fever virus (YFV) belongs to the genus Flavivirus, which includes human viruses transmitted 24 by Aedes mosquitoes, such as Dengue and Zika viruses. The live-attenuated YFV-17D vaccine has 25 been used safely and efficiently on a large scale since the end of World War II. Early studies have 26 shown, using viral titration from salivary glands of infected mosquitoes, that YFV-17D can infect 27 Aedes aegypti midgut, but does not disseminate to other tissues.28 Methodology/Principal Findings 29Here, we re-visited this issue using a panel of techniques, such as RT-qPCR, Western blot, 30 immunofluorescence and titration assays. We showed that YFV-17D replication was not efficient in 31Aedes aegypti midgut, as compared to the clinical isolate YFV-Dakar. Viruses that replicated in the 32 midgut failed to disseminate to secondary organs. When injected into the thorax of mosquitoes, 33 viruses succeeded in replicating into midgut-associated tissues, suggesting that, during natural 34 infection, the block for YFV-17D replication occurs at the basal membrane of the midgut. Our NGS 35 analysis revealed that YFV-Dakar genome exhibited a greater diversity than the vaccine strain; a trait 36 that may contribute to its ability to infect and disseminate efficiently in Ae. aegypti. 37 Conclusions/Significance 38The two barriers associated with Ae. aegypti midgut prevent YFV-17D replication. Our study 39 contributes to our basic understanding of vector-pathogen interactions and may also aid in the 40 development of non-transmissible live virus vaccines.41 42 3 43 Author summary 44 Most flaviviruses, including yellow fever virus (YFV), are transmitted between hosts by mosquito 45 bites. The yellow fever vaccine (YFV-17D) is one of the safest and most effective live virus vaccine 46 ever developed. It is also used as a platform for engineering vaccines against other health-threatening 47 flaviviruses, such as Japanese encephalitis, West Nile, Dengue and Zika viruses. We studied here the 48 replication and dissemination of YFV-17D in mosquitoes. Our data showed that YFV-17D replicates 49 poorly in mosquito midgut and is unable to disseminate to secondary organs, as compare to a YFV 50 clinical isolate. Our study contributes to our basic understanding of the interactions between viruses 51 and their vectors, which is key for conceiving new approaches in inhibiting virus transmission and 52 designing non-transmissible live virus vaccines. 53 54 Introduction 55 Arborviruses, which are transmitted among vertebrate hosts by blood-feeding arthropod vectors, 56 put billions of people at risk worldwide. Viral infection in arthropod is usually persistent. Following 57 uptake of an infectious blood meal by a female mosquito, arbovirus must initiate a productive 58 infection of the midgut epithelium, which consists of a ...