6.3. Re-epithelialization of newly formed intraluminal connective tissue with laminin-5 γ 2 chain in BOOP and UIP

The present study shows that the laminin-5 γ 2 chain is synthesized and widely expressed in regenerating pneumocytes in both BOOP and UIP. The proportional number of laminin-5 γ 2 chain-positive type II pneumocytes as well as the number of cells synthesizing laminin-5 γ 2 chain mRNA was equal in both diseases. This finding suggests that in both diseases there is an attempt to re-epithelialize the injured alveolar septa with laminin-5 γ 2 chain positive epithelial cells and thus stabilize the adhesions between the re-epithelializing cells and underlying stroma. The importance of laminin-5 for epithelial cell adhesion has been shown in several previous studies (Rousselle et al. 1991, Pulkkinen et al. 1994a,b, Kivirikko et al. 1995, Gagnoux-Palacios et al. 2001). Moreover, it has been shown that the short arm of the laminin-5 γ 2 chain plays a pivotal role in the incorporation of laminin-5 into the ECM and in cell adhesion (Gagnoux-Palacios et al. 2001).

The extent of the re-epithelialization of the newly formed connective tissue was statistically wider in BOOP when compared to that in UIP. In UIP the histological finding was heterogenous, poorly and well re-epithelialized intraluminal lesions occurring simultaneously. This gives rise to the question whether the low extent of re-epithelialization only reflects temporal heterogeneity of UIP. However, the clear morphological and structural abnormality of epithelial cells strongly favors the theory of disturbed or delayed re-epithelialization in UIP. In BOOP, the regenerating epithelium was morphologically uniform and usually showed appropriate layering above intraluminal connective tissue lesions. This regular histology contrasted with the histology of UIP, where there was disordered layering of regenerating epithelial cells, morphological variety of the cells, and exfoliation of laminin-5 γ 2 chain-positive epithelial cells into alveolar spaces. The morphological difference between the diseases suggests that despite the synthesis of laminin-5 γ 2 chain, the attachment of the cells to the underlying stroma is disturbed in UIP. This may be due to a plethora of different factors, like altered extracellular processing of laminin-5 γ 2 chain or dysfunction of other laminin-5 chains, laminin receptors or other BM components. Loss of processed laminin-5 in the ECM may also expose epithelial cells to increased apoptosis (Esco et al. 2001) which may be of great importance in the pathogenesis of UIP (Uhal et al. 1998).

Laminin-5 γ 2 chain mRNA in situ hybridization correlated with the immunohistochemical results, showing that the laminin-5 γ 2 chain is synthesized in epithelial cells, with no detectable synthesis by stromal cells. This is well in line with the epithelial nature of laminin-5 γ 2 chain observed in earlier studies (Pyke et al. 1994, Virtanen et al. 1995, Pyke et al. 1995). The small amount of positive signals in in situ hybridization corresponds to the normal wound healing model, in which laminin-5 γ 2 mRNA has been observed only at the leading edge of the migrating epithelium (Pyke et al. 1994). At the same time, the small amount of laminin-5 γ 2 chain mRNA contrasts with the wide immunohistochemical expression of the protein. Obviously, in non-neoplastic cells the synthesis of laminin-5 γ 2 chain is strictly regulated and momentary.

In immunoelectron microscopy, positive labeling for laminin-5 γ 2 chain was seen at BMs of type I pneumocytes and regenerating type II pneumocytes in UIP, as well as in basal cells of a normal bronchus. BM stability is mediated by connections between laminin-5, and laminins-6 and -7 via integrin α3β 1 (Champliaud et al. 1996, DiPersio et al. 1997, Rousselle et al. 1997). In skin, laminin-5 also bridges hemidesmosomes with collagen type VII, which provides significant force to the connection between epithelial cells and the underlying stroma (Rousselle et al. 1997). In this study, no typical hemidesmosomes were seen in regenerating pneumocytes. In a previous ultrastructural study on BOOP, no hemidesmosomes were seen either (Myers & Katzenstein 1988). However, in a study on diverse human fibrotic lung disorders, hemidesmosomes were found in a subpopulation of cuboidal cells probably derived from bronchiolar cells and associating with advanced fibrosis (Kawanami et al. 1982). The presence of laminin-5 does not implicate that there are hemidesmosomes in epithelial cells (Leivo et al. 1996), but it is possible that in addition to the re-epithelialization after alveolar injury, the adhesive properties of laminin-5 also contribute to the interstitial remodeling by forming connections between subpopulation of epithelial and stromal cells via hemidesmosomes.