Priming <I>Galleria mellonella</I> (Lepidoptera: Pyralidae) Larvae With Heat-Killed Bacterial Cells Induced an Enhanced Immune Protection Against <I>Photorhabdus luminescens</I> TT01 and the Role of Innate Immunity in the Process

Abstract

The current study investigated the characteristics and mechanism of the invertebrate immune priming using Galleria mellonella (L.) (Lepidoptera: Pyralidae) larvae (host) and Photorhabdus luminescens TT01 (pathogen) as a model. The following parameters of the G. mellonella larvae primed by hemocoel injection of heat-killed cells of TT01 or Bacillus thuringiensis HD-1 were determined at designated times after priming and then compared and analyzed systematically: mortality of the primed larvae against TT01 infection (immune protection level), hemocyte density, phagocytosis and encapsulation abilities ofhemocyte, and antibacterial activity of cell free hemolymph (major innate parameters). The results showed that 1) immune priming increased survival of the larvae against a lethal infection of TT01 and the levels and periods of protection correlated positively to the priming dose; 2) the changes on the levels of protection and the major innate parameters of the larvae primed with either TT01 or HD-1 followed a similar pattern of the convex curve, although the levels and the timing of changes differed significantly among the four innate immune parameters and between two priming bacteria; and 3) the immune protection level at a time after priming was correlated to the overall level of four innate immune parameters of the primed larvae. The current study demonstrated that the immune priming phenomenon of G. mellonella larvae has low level of specificity, and it was achieved mainly by the regulation on the quantity and activity of major innate immune parameters, such as hemocytes, antimicrobial peptides, and enzymes.

Memory and Specificity in the Insect Immune System: Current Perspectives and Future Challenges

“…These findings indicate that the protective effect against an otherwise lethal septic infection with the same pathogen is probably due to “immunological loitering” rather than an enhanced ability to generate a second immune response. These results were further confirmed by another study that examined priming effects on the greater wax moth Galleria mellonella (19). Preinfection experiments revealed that challenging G. mellonella larvae with heat-killed P. luminescens or Bacillus thuringiensis bacteria confers a protective effect by prolonging the survival of insects subsequently infected with either of these pathogens.…”

The specificity of immune priming in silkworm, Bombyx mori, is mediated by the phagocytic ability of granular cells

“…This possibility was supported by our previous study that used Galleria mellonella as the host and found dramatic increases in the counts of granular cells and plasmatocytes (phagocytes) after the larvae were primed with heat-killed bacteria (Wu et al, 2014). The phagocytic index was also a measure of the phagocytic ability of the phagocytes, which was estimated by the average fluorescence intensity of each phagocytic hemocyte and was dependent on the phagocytic receptor (Pearson et al, 2003).…”

“…The in vitro phagocytosis quenching assay was performed according to the method described in our previous study (Wu et al, 2014).…”

Effects of dietary heavy metals on the immune and antioxidant systems of Galleria mellonella larvae

“…Phagocytosis refers to the engulfment of targets, such as bacteria or yeast, by an individual hemocyte and encapsulation results in the binding of hemocytes with larger targets, such as parasitoids and nematodes. In a previous study, we demonstrated that both granular cells and plasmatocytes in the hemolymph of Galleria mellonella phagocytosed FITC-labeled bacteria in vitro (Wu et al, 2014). During an encapsulation response, granular cells contacted the invaders first and then released material that promoted the attachment of plasmatocytes, eventually forming a smooth capsule comprised of overlapping layers of cells (Schmit and Ratcliffe, 1977;Pech and Strand, 1996).…”

“…The fluorescent bacteria were prepared as previously described (Wu et al, 2014). Briefly, TT01 was cultured in Luria-Bertani (LB) broth at 28°C with shaking at 200 rpm to an OD600 of approximately 1.5.…”