**3. Focus on Mast Cells, SARS-CoV-2 and Microbiota**

In the context of the wide variety of cells involved in SARS-CoV-2, two types of cells that are at the forefront of the pathogenesis are mast cells (MCs) and platelets, and a control over them could represent biomarkers and targets at the same time for an interesting therapeutic strategy.

MCs are innate immunity cells present in mucous membranes and connective tissue, strategically located at the interface with the external environment such as the skin, lungs and intestines, where they act as gatekeepers for attack of pathogens [70,88], and they play, themselves, pathogenic roles in many inflammatory responses. MCs organize the inflammatory response and are crucial early participants in responses to viral infection. Once activated, in a very short time MCs release mediators classified as dependent or independent of degranulation. These molecules contribute to inflammation and changes at the site of infection. Mast cells also can be activated by a variety of both bacterial and viral products, and, consequently, to release a very wide spectrum of proinflammatory and immuno-regulatory molecules. In addition, many studies have analysed the ability of MCs to contract common viruses and release molecules such as histamine and leukotrienes [89,90]. MCs are resistant to productive infection with respiratory syncytial virus but have a protective response that includes the production of cytokines and chemokines that promote the recruitment of antiviral effector cells [89]. MCs can be activated directly by active viral infection or by contact with viral particles. Activation of MCs leads to the production of a variety of mediators, including large amounts of interferons (IFNs) by human virus-infected cells. In addition to initiating an antiviral state in neighbouring cells, a storm of chemokines and cytokines promote the local recruitment of effector cells. IFN also acts in an autocrine manner to further promote the production of MCs. The molecules released by MCs also act

by improving lymph nodes hypertrophy. Furthermore, the involvement of local dendritic cells promotes the development of a subsequent acquired immune response [91]. Mast cells can influence T cell proliferation and cytokine production [92]. In addition, MCs produce proteases that are increased in COVID-19 sera and lung districts [93]. Mast cells contain the serine protease ACE2 [94].

MCs play a leading role in many pathophysiological conditions, in which there is a condition of chronic silent inflammation. IL-1β, IL-6 and IL-8 are typical of silent chronic inflammation and MCs are both producers and effectors of these cytokines. Furthermore, MCs are involved in inflammatory responses and psychological stress [70]. MCs are about 2–3% of the immune cellular pool of the lamina propria, and in the muscular and serous layers (3000–25,000 MCs/mm3) [10]. Variations in the number of MCs are observed in the elderly, as with aging there are changes in connective and mucous tissues as well as other changes closely related to CNS disorders, including depression and anxiety [95]. An increase in the number of MCs during aging has also been observed in human organs and organs of other mammals and vertebrate animals.

MCs play a crucial role in host–microbiota communication, as they can help influence microbiota status and host conditions by modifying their activation [10,96]. MCs can contribute to the maintenance of intestinal homeostasis and their activation is linked to a variety of factors, motor abnormalities and dysfunctions of the intestinal epithelial barrier [10,26]. MCs establish functional signalling pathways with the nervous system and nerves in the gut. Their activation induces sensitization of the nerves, and these, in turn, can condition the release of mediators from MCs. This crosstalk is critical in the generation of symptoms or in the pathogenesis of inflammatory disorders [97,98].

MC responses to virus and other pathogens provide excellent tools for modifying local immune responses and could represent an attractive target for COVID-19 treatment, vaccination, and other immunotherapeutic uses. As is known, MCs can be activated by PAMPS through TLRs. Interestingly, MCs have been shown to express the renin–angiotensin system, the angiotensin 2 converting enzyme ectoprotease required for binding of SARS-CoV-2 and serine proteases [70,99]. This could lead to the secretion of proinflammatory mediators in a targeted and selective manner, without release of histamine or tryptase, as has already been described for the release of IL-6 in response to IL-1β from human MC cultures [70].

MCs could be a potential target to control SARS–CoV-2, for example employing known MC stabilizing agents [96]. Interestingly, some specific probiotic strains are able to stabilize MCs, especially *L. rhamnosus* GG [100]. Oral administration with *L. rhamnosus* JB-1 induces inhibition of peritoneal MC degranulation [101].

Relationships established between gut microbiota composition, cytokine storm, and MC activation in SARS-CoV-2 patients suggest that the gut microbiome is extremely involved in the severity of the pathology. Finally, an intestinal dysbiosis condition could then contribute to the condition of feeding those persistent symptoms that characterize the outcomes of SARS-CoV-2.
