Compartment-specific immune response in Mytilus galloprovincialis to Vibrio splendidus revealed by haemolymph proteomics

The Mediterranean mussel Mytilus galloprovincialis relies on its innate immune system to defend against pathogens, with haemocytes serving as the primary immune cells in its haemolymph. However, the role of haemolymph serum in immune defence has been underexplored. To address this gap, the present study investigated the immune response in both haemocyte and serum compartments of the haemolymph following Vibrio splendidus infection using high-throughput quantitative proteomic analysis. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) with isobaric tandem mass tags (TMT) enabled the identification and quantification of 2310 proteins in haemocytes and 629 in serum, making this the most comprehensive proteomic analysis of M. galloprovincialis haemolymph to date. Our results support the hypothesis that serum contains a higher proportion of immune-related proteins, highlighting its specialised role in host defence. The distinct protein profiles observed between haemocytes and serum provide additional evidence of a functional compartmentalisation of the immune response. Only a few immune proteins were shared between both compartments, helping to distinguish those that act within haemocytes from those released into the serum to play a key role in systemic defence. This study reveals that V. splendidus infection triggers a coordinated, yet compartmentalised, immune response across haemolymph components. These findings provide new insights into the complexity of the mussel immune system and underscore the importance of analysing both haemocytes and serum to better understand immune defence mechanisms, not only in M. galloprovincialis but also in other bivalve species.