Host-pathogen interactions upon the first and subsequent infection of Galleria mellonella with Candida albicans

Insects are able to develop enhanced resistance in response to repeated infection. This phenomenon is called immune priming. In this work, so-called "primed" Galleria mellonella larvae were re-infected with a lethal dose of Candida albicans 48 h after injection of a non-lethal dose, while "non-primed" larvae were infected only with a lethal dose. The increased resistance of the primed larvae correlated with a slower rate of body colonisation by the fungus. Changes in the protein profiles were detected in the whole hemolymph of the primed insects. The analysis of low-molecular weight proteins and peptides obtained with the use of three different organic solvents and comparative quantitative HPLC analysis thereof showed that the primed larvae did not have higher amounts of any infection-inducible polypeptides than the non-primed larvae. Moreover, electrophoresis of low-molecular weight polypeptides revealed an even lower level of immune-induced peptides in the primed larvae than in the non-primed ones. Furthermore, the defence activity of larval hemolymph, i.e. the antifungal, antibacterial, and lysozyme-type activity, was up-regulated in the primed larvae at the time of re-infection and, consequently, at the early time points after the infection with the lethal dose. Twenty four hours after the infection, these parameters were equally high in the non-primed and primed larvae. Accordingly, at the time of the injection of the lethal dose, certain immune-inducible genes were up-regulated. However, 24 h after the infection with the lethal dose, their expression in both groups was incomparably higher than at the time of the infection and, in most cases, it was as high in the primed larvae as in the non-primed ones. We found that only anti yeast-like activity was enhanced 24 h after the re-infection. This correlated with results obtained by testing the priming effect in heterologous systems: the primed animals did not exhibit higher resistance to the other pathogens tested.