Eosinophil lysis and plasma extravasation.

I read with interest two recent Editorials, one by ALBERA and GHIO [1] on eosinophil activation (in eosinophilic pneumonia) and the other by KUBES [2], which discusses plasma extravasation (in animal tissues). While these editorials no doubt deal extremely well with topical notions in these fields, there are a few additional aspects that might be of general interest. With regard to eosinophil activation there is a potentially very important in vivo phenomenon that has been totally neglected so far [3]. I am referring to the eosinophil lysis process that quickly releases clusters of free eosinophil granules (Cfegs) [3]. Eosinophil lysis and generation of Cfegs are a feature of most eosinophilic diseases and can be induced acutely by inflammatory challenges in vivo [3]. The Cfegs, containing potent proteins, appear in disease tissues close to important target cells and organs [3]. Cfegs may also abound in the airway lumen [3]. Indeed, the common measurements of eosinophilic cationic protein (ECP) in cell-free samples of airway surface liquids may, to a high degree, reflect ECP contained in Cfegs [3]. The presence of Cfegs, that is the result of nonapoptotic death and disappearance of eosinophil cells, would also ruin simple correlations between analysed levels of ECP and eosinophil numbers in lumenal samples [3]. During the last 50 yrs, the occurrence of eosinophil lysis and Cfegs in eosinophilic pneumonia has been illustrated repeatedly [3]; one may even regard them as a typical feature of this disease [3]. Although ALBERA and GHIO [1] are in the very best company, I wonder about the reasons for their failure to notice this phenomenon. Is it possible that Cfegs generation has been completely ignored by scientists around the world because it is an in vivo event that is outside the "all important" in vitro research paradigms? I admire the ability of KUBES [2] to deal succinctly with diverse data on tissue plasma changes reported to occur in response to nitric oxide active agents. However, with regard to airways plasma exudation responses there are several methodological and conceptual aspects that should, perhaps, be considered, particularly when findings from different laboratories are compared. For example, it may not be trivial whether workers have given drugs, mediators, etc., intravenously or topically, on the airway mucosa. ERJEFÄLT et al. [4], thus, found that topical L-NAME produced a potent exudative response that could not be mimicked by intravenous L-NAME. In the work that prompted the editorial by KUBES [2], only intravenous injections were employed [5]. Furthermore, it may be important to consider how socalled extravasation was actually analysed in this latter work [5]: 1) Only total tissue Evans blue dye was determined [5]. This is a good measure of total albumin, but provides a less reliable distinction between the albumin that is extravascular and the albumin that inevitably remains intravascular. The claim that the intravascular pool in airway mucosa can be washed away [5], is not well founded, (see [6] and references cited therein); 2) Most importantly, the authors [2, 5] fail to determine and discuss lumenal entry of plasma. This flaw is remarkable. Not only because lumenal entry of plasma is a major aspect of the original demonstration of airways plasma extravasation in response to L-NAME [4]. Neglecting the lumenal fate of extravasated plasma is unfortunate also in view of the fact that this is a quick and almost quantitative mode of elimination of the tissue plasma (briefly described below and most recently reviewed in [6]). In human airways, as well as in guinea-pig trachea, lumenal entry of plasma proteins, ranging from albumin to α2-macroglobulin, starts within a minute after topical challenge of the mucosa [6]. The epithelial passage, through ubiquitous paracellular pathways, appears to be regulated by tiny increases in the hydrostatic pressure load (caused by the extravasated plasma itself) on the basolateral aspects of columnar epithelial cells [6]. There is an exceedingly elegant mechanism by which unsieved plasma can enter the airway lumen, without prior development of any oedema, harmful effects on the epithelial lining or compromising the function of the animal or human airway epithelial lining as an absorption barrier [6]. These findings identify the mucosal exudation response as, basically, a powerful first-line defence mechanism of the intact airway surface [6]. There is a potential link between the eosinophil aspect discussed above and the plasma exudation mechanisms. Eosinophil lysis and Cfegs generation evidently occurs in vivo and particularly at sites where the tissue is flooded with extravasated plasma [3]. I would not be surprised if, in the dynamic brew of multipotent plasma-derived proteins and peptides (a milieu not readily created in vitro), we will find molecular clues to the "ultimate activation" of eosinophils that characterize eosinophilic pneumonia, bronchial asthma and other eosinophilic diseases [3].