The Cellular Immune Response of the Pea Aphid to Foreign Intrusion and Symbiotic Challenge

Abstract

Recent studies suggest that the pea aphid (Acyrthosiphon pisum) has low immune defenses. However, its immune components are largely undescribed, and notably, extensive characterization of circulating cells has been missing. Here, we report characterization of five cell categories in hemolymph of adults of the LL01 pea aphid clone, devoid of secondary symbionts (SS): prohemocytes, plasmatocytes, granulocytes, spherulocytes and wax cells. Circulating lipid-filed wax cells are rare; they otherwise localize at the basis of the cornicles. Spherulocytes, that are likely sub-cuticular sessile cells, are involved in the coagulation process. Prohemocytes have features of precursor cells. Plasmatocytes and granulocytes, the only adherent cells, can form a layer in vivo around inserted foreign objects and phagocytize latex beads or Escherichia coli bacteria injected into aphid hemolymph. Using digital image analysis, we estimated that the hemolymph from one LL01 aphid contains about 600 adherent cells, 35% being granulocytes. Among aphid YR2 lines differing only in their SS content, similar results to LL01 were observed for YR2-Amp (without SS) and YR2-Ss (with Serratia symbiotica), while YR2-Hd (with Hamiltonella defensa) and YR2(Ri) (with Regiella insecticola) had strikingly lower adherent hemocyte numbers and granulocyte proportions. The effect of the presence of SS on A. pisum cellular immunity is thus symbiont-dependent. Interestingly, Buchnera aphidicola (the aphid primary symbiont) and all SS, whether naturally present, released during hemolymph collection, or artificially injected, were internalized by adherent hemocytes. Inside hemocytes, SS were observed in phagocytic vesicles, most often in phagolysosomes. Our results thus raise the question whether aphid symbionts in hemolymph are taken up and destroyed by hemocytes, or actively promote their own internalization, for instance as a way of being transmitted to the next generation. Altogether, we demonstrate here a strong interaction between aphid symbionts and immune cells, depending upon the symbiont, highlighting the link between immunity and symbiosis.

The Combined Effects of Bacterial Symbionts and Aging on Life History Traits in the Pea Aphid, Acyrthosiphon pisum

“…Despite having a reduced or altered immune repertoire compared to previously studied invertebrates (31,50), aphid immune cells have the capacity to phagocytose a range of pathogens (31,32). Secondary symbionts have also been observed inside aphid immune cells, suggesting a role in symbiont control (26,32). The finding of elevated immune cell counts in aphids harboring secondary symbionts differs from the results reported in reference 32, whereby the presence of secondary symbionts resulted in cell counts that were either lower than, or equal to, the symbiont-free controls.…”

“…The finding of elevated immune cell counts in aphids harboring secondary symbionts differs from the results reported in reference 32, whereby the presence of secondary symbionts resulted in cell counts that were either lower than, or equal to, the symbiont-free controls. One possibility is that the relatively recent introduction of symbionts into aphids used in that study (32) may inflict different stresses on host immune responses from those of long-established associations. Another possibility is that aphid genotype or symbiont genotype may impact immune cell proliferation.…”

“…Immune cells were identified as granulocytes, with known phagocytic abilities, as described in reference 31. Recent work has suggested that this group may in fact comprise both granulocytes and plasmatocytes, although a differentiation in function has yet to be shown (32). As both of these cell types have immune function, here we conservatively count all these cells as "immune cells" but do not discriminate between these two cell types.…”

“…Aphid secondary symbionts can inhabit a variety of host tissues but are also found extracellularly in the hemolymph, where they freely come into contact with host immune cells (hemocytes) (14,27). Aphid hemocytes have known phagocytic properties and may play a role in symbiont regulation (14,31,32). Secondary symbionts have been identified within hemocytes, although whether their uptake is passive or active on the part of the host is unknown (32).…”

“…Aphid hemocytes have known phagocytic properties and may play a role in symbiont regulation (14,31,32). Secondary symbionts have been identified within hemocytes, although whether their uptake is passive or active on the part of the host is unknown (32).…”