Functional interactions between C-type lectins and clip-domain serine protease homologs in A. gambiae immune responses -

Abstract

Malaria is one of the most shattering tropical diseases. Transmission of malaria to human hosts requires its successful development within its Anopheles mosquito vector. During the last decade, pioneering functional genetic studies in A. gambiae, the major malaria vector in Sub-Saharan Africa, revealed that mosquitoes are not passive to the presence of the malaria parasite, but rather mount potent anti-parasitic responses that play a significant role in eliminating the majority of ookinete stages invading the midgut epithelium. Since then, considerable efforts have been directed at understanding the mosquito immune responses to parasites as well as to other classes of microbes. Melanization is a potent effector mechanism of arthropods that sequesters microbes in a thick melanin coat. A. gambiae mosquitoes rarely melanize Plasmodium ookinetes, and although they do melanize bacteria, melanization does not seem to contribute significantly to anti-bacterial immunity. To further characterize this immune process in mosquitoes, we investigated the role of melanization in anti-fungal defense using the model entomopathogenic fungus Beauveria bassiana. Our results revealed that the melanization response against B. bassiana involves mosquito blood cells, as well as two hemolymph proteins, the complement-like protein TEP1 and the non-catalytic clip-domain serine protease CLIPA8. Importantly, silencing either TEP1 or CLIPA8 abolished hyphal melanization and decreased mosquito tolerance and resistance to natural B. bassiana infections, indicating that the mosquito melanization response contributes significantly to anti-fungal defense. Although melanization of Plasmodium ookinetes is rarely observed in wildtype A. gambiae mosquitoes, certain genotypes, in particular those in which the C-type lectin genes CTL4 and CTLMA2 are silenced, melanize massive numbers of ookinetes. CTL4 and CTLMA2 form a heterodimeric complex whose exact function remains unknown. Here, we show that the CTL complex localizes to microbial surfaces in a