No evidence for priming response in Galleria mellonella larvae exposed to toxin protein PirA2B2 from Photorhabdus luminescens TT01: An association with the inhibition of the host cellular immunity.

There is accumulating evidence that many invertebrates including insects can acquire enhanced immune protection against subsequently pathogens infection through immune priming. However, whether the toxin protein from pathogenic bacteria can induce such priming response remains unclear. Here we cloned, expressed and purified the toxin Photorhabdus insect-related proteins A2B2 (PirA2B2) from Photorhabdus luminescens TT01. We primed Galleria mellonella with sublethal dose of PirA2B2 and then challenged the larvae with viable P. luminescens TT01 at 48 h after priming. We found no evidence for immune priming in G. mellonella larvae exposed to PirA2B2. Priming the larvae with PirA2B2 did not improve their resistance in a subsequent challenge with P. luminescens TT01. Whereas a robust priming response was observed when the larvae exposed to lipopolysaccharide (LPS) extracted from P. luminescens TT01. Because the larvae primed with LPS showed significant higher resistance against P. luminescens TT01 infection than those of the PBS and BSA controls. Furthermore, we investigated the changes of the cellular immune parameters, such as hemocyte counts, phagocytic activity and encapsulation ability of the hemocytes, after priming. We found that the toxin PirA2B2 significantly decreased the cellular immunity of the larvae, whereas the LPS significantly increased them. These results indicated that the degree of priming response in G. mellonella correlated positively to the levels of cellular immune parameters, and the underlying mechanism in regulating the immune priming of invertebrates was not homologous to that of the immunological memory of vertebrates.