Clostridioides difficile meets the adenosine system: the art of manipulating host homeostasis

Abstract

Background: Adenosine is a ubiquitous endogenous molecule capable of influencing several pathophysiological aspects. The adenosine system is extremely complex, starting from the generation of intracellular and extracellular adenosine, the regulation of its levels, and its action on four different receptors that vary in affinity and distribution in the different cell types and tissues. The most relevant effects of adenosine during infections and inflammation are documented on all types of immune cells, including those of adaptive immunity (T lymphocytes, B lymphocytes, regulatory cells) and of natural immunity (macrophages, polymorphonuclear cells, dendritic cells, natural killer). Of interest, the adenosine system is also strongly involved in the pathophysiology of colonic cells. Clostridioides difficile (C. difficile), responsible for 15-20% of all cases of antibiotic-associated diarrhea, is an infection that has been evolving over the past two decades due to the unstoppable spread of C. difficile in the anthropized environment and the progressive human colonization. The pathological activity of C. difficile is due to toxin A (TcdA) and B (TcdB) which profoundly alter the homeostasis of the adenosine system, acting both at the level of its generation and on the expression and regulation of adenosine receptors. The final effect consists in an attenuation of the inflammatory response to favor the persistence of the C. difficile infection.

Conclusion: This review highlights a new ability of C. difficile, through its Tcds, of manipulating the host to its advantage.

Keywords: Clostridioides difficile infection; Clostridioides difficile toxin B; Clostridioides difficiletoxin A; Adenosine; Anti-inflammatory response; Colon pathophysiology; Enteric glial cells; Immune cells; Inflammation.

Fig. 1

Adenosine metabolism and transport in intracellular and extracellular milieu. ADO can be produced from extracellular ATP by the sequential action of the CD39 and CD73 ecto-enzymes expressed on the outside of the cell membrane. Intracellular ADO synthesis can occur by the action of SAH hydrolase acting on SAH and by the action of AK acting on 5'AMP in conjunction with intracellular CD73 ecto-enzyme. Of course, the intracellular and extracellular ADO transport systems, the ENTs, act predominantly on the concentration gradient of ADO. Finally, ADO levels are regulated intracellularly and extracellularly by the action of ADA, which converts ADO into inosine. "Created in BioRender https://BioRender.com/wnmewx7"

Fig. 2

ARs and second messengers signalling pathway activated by extracellular adenosine. Each of the four ARs is characterized by the activation of characteristic signal pathways that define the quality of the cellular response. Some common molecules stimulated by ADO are, activation of MAPKs such as p38, ERK1/2, stimulation of increased intracellular Ca²⁺ levels, cAMP activation, and activation of PKC and activation or inhibition of PKA. Of course, the final outcomes of AR activation depend on the interaction of the signaling pathways and the type of cell involved and its state. "Created in BioRender https://BioRender.com/b01cpt5"

Fig. 3

Interaction of Adenosine with Immune Cells: Main actions mediated by ARs expressed by the immune cells. In innate immunity cells: A1-AR is expressed in PMNs, monocyte-macrophages and DCs; A2A-AR is expressed in PMNs, DCs, monocyte-macrophages, MCs and NK; A2B-AR is expressed in PMNs, macrophages, NK, MCs and MDSCs; A3-AR is expressed in PMNs, monocyte-macrophages, DCs and MCs. In adaptive immunity cells: A1-AR is expressed in B lymphocytes, A2A-AR, and A3-AR are expressed in both B lymphocytes and effector T lymphocytes, A2B-AR is expressed in T lymphocytes, whereas Treg lymphocytes express only A2A-AR and A2B-AR. Breg express A1-AR, A2A-AR and A2B-AR. In the Figure are shown the main effects induced by the interaction of ADO with the predominantly express ARs for each cell type. "Created in BioRender https://BioRender.com/oeb1umq"

Fig. 4

Interactions between C. difficile infection and its TcdA and TcdB with Adenosine system. Both TcdA and TcdB are able to interact with the four ARs as demonstrated in vitro and vivo. This also occurs during CDI in vivo, probably due to the action of the Tcds produced but not exclusively. The main effects of these interactions are reported in the legend (left side). Furthermore, TcdA interacts with P2X7R and PTX3 involved in inflammation and with ADA involved in the regulation of ADO levels, while TcdB is less involved in the inflammatory response and interacts only with P2X7R. In vivo CDI activates Panx1 which, by forming channels for ATP, is able to activate P2X7R. "Created in BioRender https://BioRender.com/ak31a9n"