Anopheles gambiae larvae mount stronger immune responses against bacterial infection than adults: evidence of adaptive decoupling in mosquitoes

Abstract

BackgroundThe immune system of adult mosquitoes has received significant attention because of the ability of females to vector disease-causing pathogens while ingesting blood meals. However, few studies have focused on the immune system of larvae, which, we hypothesize, is highly robust due to the high density and diversity of microorganisms that larvae encounter in their aquatic environments and the strong selection pressures at work in the larval stage to ensure survival to reproductive maturity. Here, we surveyed a broad range of cellular and humoral immune parameters in larvae of the malaria mosquito, Anopheles gambiae, and compared their potency to that of newly-emerged adults and older adults.ResultsWe found that larvae kill bacteria in their hemocoel with equal or greater efficiency compared to newly-emerged adults, and that antibacterial ability declines further with adult age, indicative of senescence. This phenotype correlates with more circulating hemocytes and a differing spatial arrangement of sessile hemocytes in larvae relative to adults, as well as with the individual hemocytes of adults carrying a greater phagocytic burden. The hemolymph of larvae also possesses markedly stronger antibacterial lytic and melanization activity than the hemolymph of adults. Finally, infection induces a stronger transcriptional upregulation of immunity genes in larvae than in adults, including differences in the immunity genes that are regulated.ConclusionsThese results demonstrate that immunity is strongest in larvae and declines after metamorphosis and with adult age, and suggest that adaptive decoupling, or the independent evolution of larval and adult traits made possible by metamorphosis, has occurred in the mosquito lineage.Electronic supplementary materialThe online version of this article (doi:10.1186/s13071-017-2302-6) contains supplementary material, which is available to authorized users.

Chemical depletion of phagocytic immune cells reveals dual roles of mosquito hemocytes in Anopheles gambiae anti-Plasmodium immunity

“…Phenoloxidse (PO) activity was measured in pools of perfused hemolymph from pre-treated mosquitoes (n=15, 10 μl per mosquito) under naïve conditions, 24 h post-blood feeding, and 24 h post- P. berghei infection. Following perfusion, 10 μl of the total perfused hemolymph was mixed with 90 μl 3, 4-Dihydroxy-L-phenylalanine (L-DOPA, 4 mg/ml) dissolved in nuclease free water as previously described (42). PO activity was measured at 490 nm every 5 min for 30 min, then the final activity was measured at 60 min using a microplate reader (Molecular Devices).…”

Transstadial immune activation in a mosquito: Adults that emerge from infected larvae have stronger antibacterial activity in their hemocoel yet increased susceptibility to malaria infection

“…Circulating hemocytes were collected by perfusion from 6‐day‐old adults at 24‐hr post‐treatment with E. coli as previously described (League et al, ). An additional group that did not receive an E. coli injection as adults was also analyzed.…”

“…Metamorphosis involves considerable shifts in the ecology and physiology of many invertebrates, including alterations to the immune system (League, Estevez‐Lao, Yan, Garcia‐Lopez, & Hillyer, ; League & Hillyer, ; Russell & Dunn, ). In mosquitoes, metamorphosis has decoupled the larval and adult immune systems such that immunity is strongest in larvae and declines after eclosion and with adult age (League et al, ).…”

“…Metamorphosis involves considerable shifts in the ecology and physiology of many invertebrates, including alterations to the immune system (League, Estevez‐Lao, Yan, Garcia‐Lopez, & Hillyer, ; League & Hillyer, ; Russell & Dunn, ). In mosquitoes, metamorphosis has decoupled the larval and adult immune systems such that immunity is strongest in larvae and declines after eclosion and with adult age (League et al, ). However, multiple reports have demonstrated that the conditions experienced by mosquito larvae can influence adult immune traits (Araujo, Gil, & e‐Silva, ; Moller‐Jacobs, Murdock, & Thomas, ; Murdock, Paaijmans, Cox‐Foster, Read, & Thomas, ; Muturi, Kim, Alto, Berenbaum, & Schuler, ; Suwanchaichinda & Paskewitz, ; Telang, Qayum, Parker, Sacchetta, & Byrnes, ; Vantaux, Ouattarra, Lefevre, & Dabire, ).…”

“…The aquatic habitats of mosquito larvae are rife with microorganisms that can invade the hemocoel (body cavity) (Bartholomay & Michel, ; Granados, ; Kalucy & Daniel, ; Petersen, Chapman, & Woodard, ; Sweeney, Inman, Bland, & Wright, ; Washburn, Egerter, Anderson, & Saunders, ; Yassine, Kamareddine, & Osta, ). Consequently, mosquito larvae mount powerful cellular and humoral immune responses against pathogens in their hemocoel (Biron et al, ; Dimopoulos, Richman, Muller, & Kafatos, ; Duncan et al, ; Kalucy & Daniel, ; League et al, ; League & Hillyer, ; Meredith, Hurd, Lehane, & Eggleston, ; Richman et al, ; Shin et al, ). Recently, we demonstrated that a larval‐acquired infection in the hemocoel is transstadially transmitted to the hemocoel of an adult, which may have important implications for adult immunity (Brown, Thompson, & Hillyer, ).…”