Priming Galleria mellonella (Lepidoptera: Pyralidae) larvae with heat-killed bacterial cells induced an enhanced immune protection against Photorhabdus luminescens TT01 and the role of innate immunity in the process.

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.