Dose-dependent cellular and humoral responses in Galleria mellonella larvae following β-glucan inoculation

Galleria mellonella larvae were inoculated with different doses of β-glucan by injection into the haemocoel. Those larvae that had received high doses of β-glucan (15, 30 or 60 μg/larva) demonstrated increased survival following infection with the yeast Candida albicans. High concentrations of glucan induced an increase in haemocyte density and a reduction in yeast proliferation within the haemocoel. Proteomic analysis of glucan-treated larvae revealed increased expression of a variety of peptides some of which may possess antimicrobial properties. Analysis of expression profiles revealed that low doses of β-glucan (3.75 μg/larva) triggered the increased expression of certain peptides (e.g. hemolin) while high dose inoculation was required before the increased expression of others (e.g. archaemetzincin) was evident. These results indicate that low doses of β-glucan induce a limited immune response while high doses induce an immune response that has the potential to curtail the threat within the haemocoel but also withstand a subsequent infection. Immune priming gives insects the ability to withstand a potentially lethal infection if exposed to a low level of the pathogen 24–48 h previously. Immune priming has resource implications and this work indicates that a graded immune response is initiated depending upon the amount of the immune priming agent encountered.