Immune response of Galleria mellonella after injection with non-lethal and lethal dosages of Candida albicans

The immune response of Galleria mellonella to injection with non-lethal and lethal dosages of Candida albicans was compared. Larvae infected with the non-lethal dosage (2 x 10(4) cells/larva) did not show significant morphological changes, while those infected with the lethal dosage (2 x 10(5) cells/larva) showed inhibition of motility and cocoon formation and became darker around the area of injection after 24 h. While the administration of the lower dosage caused approx. 5- and 20-fold induction of genes for gallerimycin and galiomycin, respectively, the injection with the higher dosage induced approx. 25 and 120-fold expression of the respective genes. Similar differences were obtained for the insect metalloproteinase inhibitor (IMPI) and hemolin gene transcripts. The relatively low level of immune gene expression was confirmed by an assay of hemolymph antifungal activity, which was detected only in larvae infected with lethal dosage of C. albicans. Furthermore, greater amounts of immune-inducible peptides were detected in the hemolymph extracts in the same group of larvae. The stronger humoral immune response was not correlated with survival. Phenol oxidase (PO) activity was induced only in the hemolymph of larvae infected with the non-lethal dose; injection of the lethal dose resulted in strong inhibition of this enzyme after 24 h. We showed that PO is susceptible to regulation by immune priming with the non-lethal dose of C. albicans. The activity of this enzyme was enhanced in primed larvae at the time of re-injection. When both primed and non-primed larvae received 2 x 10(5) cells, the inhibition of PO was stronger in the primed group. G. mellonella infected with the lethal dose of C. albicans died despite the strong induction of humoral defence mechanisms. The priming-enhanced activity of PO was correlated with increased resistance to subsequent infection.