The c-di-AMP binding protein NadD from Mesomycoplasma ovipneumoniae functions as a phosphodiesterase that inhibits host inflammatory responses.

Cyclic dimeric adenosine monophosphate (c-di-AMP) is a second messenger commonly found in bacteria, particularly within the Firmicutes and Actinobacteria phyla, as well as in Archaea. Phosphodiesterase (PDE) is an enzyme that plays a crucial role in regulating the homeostasis of bacterial c-di-AMP and is capable of hydrolysing this molecule both intracellularly and extracellularly. This enzymatic activity influences various physiological functions in bacteria, modulates the host immune response, and impacts the pathogenic processes of infectious agents. Mesomycoplasma ovipneumoniae (M. ovipneumoniae) is the primary pathogen responsible for mycoplasma pneumonia in sheep, but its pathogenic mechanisms remain poorly understood. This study aimed to investigate the potential role of c-di-AMP and its binding proteins in M. ovipneumoniae in affecting host immune responses. Our findings confirmed that M. ovipneumoniae produced c-di-AMP in both the intracellular extracts and culture supernatants and that it generated c-di-AMP during the infection of sheep primary alveolar macrophages. Transcriptome analysis revealed that c-di-AMP can activate the immune response in macrophages. Furthermore, we identified and characterized the c-di-AMP binding protein nicotinate-nucleotide adenylyltransferase (NadD), which represents a novel class of PDE containing His-Asp (HD) domains in Mycoplasmas. Notably, our study demonstrated that NadD suppresses inflammatory responses in host cells infected with M. ovipneumoniae. In conclusion, this study provides new insights into the pathogenic mechanisms of M. ovipneumoniae and develops potential targets for the prevention and control of ovine mycoplasma pneumonia, as well as for the development of therapeutic agents.