Protein kinase A affects Galleria mellonella (Insecta: Lepidoptera) larval haemocyte non-self responses

We used the protein kinase A (PKA) specific activator Sp-8-Br-cAMPS and type I inhibitor Rp-8-Br-cAMPS alone and in combination to define the role of PKA in the non-self responses of larval Galleria mellonella haemocytes in vitro and in vivo. Active PKA depressed haemocyte responses whereas PKA inhibition enhanced activities, including bacterial phagocytosis, the number of haemocytes with adherent bacteria, bacterial-induced haemocytic protein release and haemocyte adhesion to slides in vitro, as well as in vivo bacterial removal from the haemolymph. Non-attached haemocytes had more PKA activity than attached haemocytes; therefore, active PKA limited haemocyte response to foreign materials. We found that (i) PKA inhibitor alone induced non-self responses, including haemocyte protein discharge and lowered haemocyte counts in vivo, and induced nodulation; (ii) the enzyme activator produced effects opposite to those of the inhibitor; and (iii) together, the modulators offset each others' effects and influenced haemocyte lysate PKA activity. These findings establish PKA as a mediator of haemocytic non-self responses.