Genome-wide transcriptomic profiling of Anopheles gambiae hemocytes reveals pathogen-specific signatures upon bacterial challenge and Plasmodium berghei infection

Abstract

Background: The mosquito Anopheles gambiae is a major vector of human malaria. Increasing evidence indicates that blood cells (hemocytes) comprise an essential arm of the mosquito innate immune response against both bacteria and malaria parasites. To further characterize the role of hemocytes in mosquito immunity, we undertook the first genome-wide transcriptomic analyses of adult female An. gambiae hemocytes following infection by two species of bacteria and a malaria parasite.

Results: We identified 4047 genes expressed in hemocytes, using An. gambiae genome-wide microarrays. While 279 transcripts were significantly enriched in hemocytes relative to whole adult female mosquitoes, 959 transcripts exhibited immune challenge-related regulation. The global transcriptomic responses of hemocytes to challenge with different species of bacteria and/or different stages of malaria parasite infection revealed discrete, minimally overlapping, pathogen-specific signatures of infection-responsive gene expression; 105 of these represented putative immunity-related genes including anti-Plasmodium factors. Of particular interest was the specific co-regulation of various members of the Imd and JNK immune signaling pathways during malaria parasite invasion of the mosquito midgut epithelium.

Conclusion: Our genome-wide transcriptomic analysis of adult mosquito hemocytes reveals pathogen-specific signatures of gene regulation and identifies several novel candidate genes for future functional studies.

Figure 1

The global transcriptomic profiles of adult female An. gambiae hemocytes. (a)Bar chart show the gene functional group distribution of all genes for which any transcription was detected in An. gambiae hemocytes, in at least 1 of the 5 experimental comparisons. (b) Bar chart showing the number and functional class of all the genes either enriched or under-represented in hemocytes relative to whole adult female mosquitoes (whole), or significantly differentially regulated in hemocytes either following challenge with heat-killed bacteria (E. coli and M. luteus) or infected with different stages of the rodent malaria parasite P. berghei (P. b. 24 hours and P. b. 19 days). Red and green arrows indicate, respectively, genes up- and down-regulated in hemocytes relative to either whole adult females (whole) or hemocytes from naïve control mosquitoes (E. coli, M. luteus, P. b. 24 hours and P. b. 19 days). (c) and (d) Proportional-area Venn diagrams illustrating the distribution of genes significantly differentially transcribed in hemocytes following either bacterial challenge (c) or different stages of infection with the malaria parasite P. berghei (d). Numbers in brackets outside circles indicate the total number of genes differentially regulated by each species, or stage, of pathogen. Numbers in brackets inside circles indicate the pooled total number of genes differentially regulated by either both stages of malaria parasite infection (c) or both species of bacteria (d). Numbers by red and green triangles indicate the number of genes up- and down-regulated, respectively, within each segment of the three segments formed by the two upper-most circles of each figure. IMM = immunity-and apoptosis-related; RED/STE = redox and oxidoreductive stress; PROT = proteolysis; CYT/STR = cytoskeletal and structural; TRP = transport; MET = metabolism; R/T/T = replication, transcription and translation; DIV = diverse; and UNK = unknown.

Figure 2

Total number of hemocytes and hemocyte types (granulocytes, oenocytoids and prohemocytes) in An. gambiae. (a) Total number of hemocytes and abundance of hemocyte types (granulocytes, oenocytoids, or prohemocytes) per mosquito 24 h post-injection with E. coli, M. luteus, or PBS (mock). (b) Total number of hemocytes and abundance of hemocyte types in day 4 non-fed mosquitoes, and mosquitoes (24 h or 19 days) after feeding on a blood meal containing P berghei or non-infected blood meal. (c) Influence of bacterial species (E. coli or M. luteus) on phagocytosis by An. gambiae granulocytes. (d) Influence of bacterial species on the proportion of granulocytes with internalized bacteria that are melanized. (e) Influence of bacterial species on the proportion of melanized oenocytoids. A minimum of 10 mosquitoes were bled per treatment. Results for (a) and (b) are presented as means ± SE. Results for (c-e) are given as means ± SE for phagocytic or melanized cells relative to the total number of cells (100) counted per sample.

Figure 3

Comparison of differential transcription according to functional class – specific responses. The number of differentially regulated transcripts within each functional class is expressed as a percentage of the total number transcripts detected in hemocytes for each experimental comparison. (a) The percentage of genes, for each functional class, expressed in hemocytes which were differentially regulated following either bacterial challenge and/or malaria parasite infection. Dashed line indicates the overall percentage of each functional class in the differentially expressed genes in hemocytes; this percentage of transcripts is expected to be differentially expressed within each functional class if there is no association between functional class and differential transcription. Functional classes exhibiting significant over- or under-representation among expressed transcripts are indicated by asterisks (χ²-test; see main text for details). (b) The same as (a) but for each challenge. IMM = immunity-and apoptosis-related; RED/STE = redox/stress/mitochondrial; PROT = proteolysis; CYT/STR = cytoskeletal/structural; TRP = transporters; MET = metabolism; R/T/T = replication/transcription/translation; DIV = diverse; and UNK = unknown.