Proteins recognized by anti-5-HTR1B and anti-5-HTR2B antibodies in <i>Galleria mellonella</i> haemocytes are affected by <i>Conidiobolus coronatus</i> infection and its fungal metabolites

Galleria mellonella is now a robust invertebrate model for host–pathogen interaction, neuroimmune responses, and immunomodulatory mechanisms. In this study, we investigated the expression of two proteins recognized by 5-HTR1B or 5-HTR2B antibodies, in the immune response of larvae of G. mellonella to infection with the entomopathogenic fungus Conidiobolus coronatus and topical application of its β-carboline metabolites, harman and norharman. Using immunocytochemical methods and Enzyme-Linked Immunosorbent Assay (ELISA)-based determination, we demonstrated time-dependent upregulation of both receptors in haemocytes of larvae following fungal infection, suggesting the activation of serotonergic signalling pathways upon immune challenge. Similarly, topical treatment with harman and norharman resulted in dose-dependent upregulation of proteins recognized by anti-5-HTR1B and anti-5-HTR2B antibody expression, with maximal receptor expression observed upon application of 1250 ppm of the compounds. Notably, proteins immunopositive to 5-HTR1B antibody were particularly elevated following high-concentration norharman treatment, whereas proteins immunopositive to 5-HTR1B antibody reached peak levels 48 h following fungal infection. These findings agree with the hypothesis that serotonergic mechanisms control haemocyte function following both infection with pathogens and neuroactive fungal metabolites. The receptor-specific patterns further suggest that 5-HTR1B and 5-HTR2B play distinct roles in controlling immune processes. Our results illustrate an insect neuroimmune interface that shares features with aspects of vertebrate immunoregulation, such as potential overlap with cytokine signal transduction. With the evolutionarily conserved presence of serotonin receptors and their immunological importance, this study provides novel data on receptor-mediated immune regulation in invertebrates and reinforces G. mellonella as a powerful model for neuroimmunology.