Therefore, our first attempt was to characterize mast cells in these specimens according to their mucosal (tryptase) and connective (chymase, tryptase, and carboxypeptidases) tissue classification.
32,33 Our finding confirms the presence of round/oval metachromatic cells in corneoscleral tissues and provides evidence on the presence of tryptase (AA1) immunoreactive mast cells nearby the palisades of Vogt. The morphology of these mast cells appeared either intact (resting) or degranulated (active), suggesting a dynamic contribution of mast cells close to niches. A comparison between intact and degranulated mast cells was not carried out as the degranulation route also might be influenced by tissue sampling and/or handling.
34 In fact, low temperatures, freezing/thawing, and some fixatives (Carnoy's fixative, glutaraldehyde, or formaldehyde buffered solutions) might trigger membrane perturbation as well as induce an antidromic nervous stimulation or any other microenvironment insult, resulting in alteration of overall membrane integrity.
34,35 The observation of only 30% of AA1-positive cells out of total CD45-positive cells suggests the presence of other immune cells populating the limbal junction.
31,36 The specific expression of tryptase (AA1) and the almost undetectable immunoreactivity for chymase (CC1) and chymase/tryptase coexpression, confirmed by epifluorescent and molecular analysis, would suggest a “potential” tryptase contribution in the local mast cell–driven tissue homeostasis and parainflammation.
31,36 As shown elsewhere, tryptase takes part in some physiologic activities (tissue airway homeostasis, vascular relaxation and contraction, gastrointestinal smooth muscle activity, intestinal transport, and coagulation pathway) by degrading some components of the cellular matrix (merely fibronectin), regulating cell trafficking and taking part in local homeostasis.
37–40,41 Galli and coworkers
42 highlighted the concept that mast cells can function as immunoregulatory cells, prospecting the concept of OFF/resting or ON/degranulating functional configurations. Although increased levels of systemic/local tryptase are frequently associated with Th2-driven chronic inflammatory and fibrotic ocular conditions, the presence of tryptase by itself does not represent an exclusive indication of anaphylaxis and/or allergy.
30,41,43,44 Mast cell survival inside tissues is guaranteed by several cytokines and growth factors and particularly by cKit and Stem Cell Factor (SCF). The results on cKit and SCF demonstrated no significant changes in limbal expression with respect to the nearby areas. This result is in line with previous studies suggesting that cKit is not restricted to mast cells.
16,20,29–31 Both IgE-dependent and no IgE-dependent activations drive mast cell activity and particularly degranulation.
16,20,29–31 The high-affinity IgE receptor (FcεRI) was investigated at both biochemical and molecular levels. On the contrary, FcεRI surface receptor was not detected by immunofluorescence and only weakly expressed by molecular analysis, opening the question of whether no IgE-dependent mast cell activity is close to the niches. According to literature, FcεRI is weakly expressed upon a nonallergic background and works as an inducible short-term life receptor in cultures exposed to IgE.
45