Mechanisms of Alphavirus Midgut dissemination in the Mosquito, Aedes aegypti
The research work of this thesis has focused on the midgut escape barrier and the dissemination mechanism of alphaviruses in Ae. aegypti. Primarily, we identified and characterized Ae. aegypti MMPs that are seemingly involved in midgut BL remodeling during bloodmeal digestion. We identified nine Aemmp genes encoded in the genome of Ae. aegypti. For four of those MMPs, their expression profiles were assessed in the various life stages of the mosquito as well as in midgut and carcass tissues of CHIKV infected and non-infected mosquitoes. Based on antibody-specific detection, AeMMP1 and AeMMP2 were localized to midgut associated tissues. We also showed that AeMMP1 possesses collagenase activity in vitro. This suggests that the BL surrounding the midgut is a potential target of the enzymatic activity of AeMMP1. Using FIB-SEM, we demonstrated for the first time that orally acquired mature CHIK virions exit the mosquito midgut by directly traversing the midgut BL during bloodmeal digestion. CHIK virions localized to the BL as early as 24 h pibm and were present in a high density until 32 h pibm. At 48 h pibm, CHIK virions were present in a lower density and remained so until a subsequent bloodmeal was ingested by the mosquito, at which point CHIK virions were once again associated with the strands of the BL. In SEM, disruptions in the BL adjacent to muscle tissue were also observed early after bloodmeal ingestion. These findings suggest that bloodmeal ingestion imposes changes to the BL structure, which then allows CHIKV to pass through. Furthermore, we showed that alphavirus dissemination from the midgut is not a continuous event during systemic infection of the mosquito; instead there is a relatively narrow time window, less than 48 h pibm, during which de novo synthesized virions are able to exit the midgut. Lastly, we compared the vector competence of two strains of MAYV, which demonstrated the potential of this virus to be transmitted by urban mosquito vectors such as Ae. aegypti and Ae. albopictus. The MAYV strains IQT and TRVL were able, when orally acquired, to infect the midguts of both Ae. aegypti and Ae. albopictus and to disseminate in these mosquitoes to secondary tissues, eventually infecting the salivary glands and being released along with their saliva. CHIKV and MAYV co-infection resulted in similar efficiencies of cotransmission by Ae. aegypti. However, prior systemic infection with MAYV resulted in superinfection exclusion of CHIKV in saliva. Thus, two relatively closely related alphaviruses can antagonize each other in the same mosquito vector depending on the sequence of their acquisition and subsequent infection of their invertebrate hosts.