Galleria mellonella larvae are capable of sensing the extent of priming agent and mounting proportionatal cellular and humoral immune responses.

Larvae of Galleria mellonella are useful models for studying the innate immunity of invertebrates or for evaluating the virulence of microbial pathogens. In this work, we demonstrated that prior exposure of G. mellonella larvae to high doses (1×10(4), 1×10(5) or 1×10(6) cells/larva) of heat-killed Photorhabdus luminescens TT01 increases the resistance of larvae to a lethal dose (50 cells/larva) of viable P. luminescens TT01 infection administered 48h later. We also found that the changes in immune protection level were highly correlated to the changes in levels of cellular and humoral immune parameters when priming the larvae with different doses of heat-killed P. luminescens TT01. Priming the larvae with high doses of heat-killed P. luminescens TT01 resulted in significant increases in the hemocytes activities of phagocytosis and encapsulation. High doses of heat-killed P. luminescens TT01 also induced an increase in total hemocyte count and a reduction in bacterial density within the larval hemocoel. Quantitative real-time PCR analysis showed that genes coding for cecropin and gallerimycin and galiomycin increased in expression after priming G. mellonella with heat-killed P. luminescens TT01. All the immune parameters changed in a dose-dependent manner. These results indicate that the insect immune system is capable of sensing the extent of priming agent and mounting a proportionate immune response.