Immune responses in the haemolymph and antimicrobial peptide expression in the abdomen of Apis mellifera challenged with Spiroplasma melliferum CH-1.

Spiroplasma melliferum generally parasitizes honeybees and is one of main pathogens causing 'bee creeping disease' in China. Spiroplasma melliferum can be spread through honeybee pollination, which causes severe economic losses to apiculture. The design of this study was based on previous studies that utilized an in vitro bioassay to investigate the effects of S. melliferum CH-1 infection. We identified invasive S. melliferum CH-1 within Apis mellifera using transmission electron microscopy and investigated the immune response of honeybees infected with S. melliferum CH-1 by assaying the cellular immune response of the haemocytes, the plasma level of phenoloxidase activity and the transcript levels of 5 antimicrobial peptides, including the Abaecin, Apidaecin, Defensin 1, Defensin 2, and Hymenoptaecin gene products. The percentage of granulocytes in the haemolymph of infected honeybees was significantly higher than those of the controls during the early phase of infection, but the percentage of plasmatocytes was significantly higher than those of the controls at the fifth day post-infection. The phenoloxidase activity of the infected honeybees reached a maximum at the second day, and then decreased continuously. Moreover, the transcript levels of the 5 evaluated antimicrobial peptide genes were significantly increased during the early phase of infection and all 5 antimicrobial peptides were significantly decreased during the middle phase of infection. During the late phase of infection, only Defensin 2 and Hymenoptaecin showed significantly increased transcription. These results suggest that the honeybee immune responses could be activated by S. melliferum CH-1 during the early phase of infection and that S. melliferum CH-1 is also capable of circumventing the host defensive mechanisms to complete its life cycle within the honeybee during the middle phase of infection.