This is the first report to describe the relationship between immunohistochemical SP-A expression in alveolar epithelial cells in diseased areas (i.e., regenerative/hyperplastic alveolar epithelial cells) and prognosis of IIPs, especially UIP. From the present study, a paucity of immunohistochemical SP-A expression in regenerative/hyperplastic alveolar epithelial cells is considered to predict worse prognosis of IIPs, especially UIP. The presence of significant inverse correlation between the SP-A positive ratio and the FF score, which has already been reported to predict the prognosis of UIP, also supports this result. It is possible that UIP patients whose regenerative/hyperplastic alveolar epithelial cells did not express SP-A had a long-standing illness preceding lung biopsy, and therefore showed poor prognosis. However, there was no correlation between the SP-A positive ratio and the duration of illness or the %VC at the time of lung biopsy. Therefore, we consider this possibility highly unlikely.
We anticipate that more UIP patients might die if they are observed for a longer period. Though we conducted this study retrospectively, UIP patients with stable clinical courses were observed for significantly longer periods compared to those who died from UIP progression. Therefore, it is unlikely that differences in the follow-up period after lung biopsy affected the assessment of prognosis.
Regenerative/hyperplastic alveolar epithelial cells that did not express SP-A expressed other markers such as CK, KL-6, and EMA. Among the cells that did not express SP-A, a few regenerative/hyperplastic alveolar epithelial cells expressing SP-A were intermingled. In addition, type II alveolar epithelial cells seen in the normal alveolar tissue among the diseased lung tissue were stained positively with SP-A. From these findings, we think that the lack of SP-A expression was not an artifact, and did not result from inappropriate fixation or staining techniques.
In a preliminary study, we found that only a few epithelial cells lining the honeycomb expressed SP-A, irrespective of SP-A positive ratio of collapsed and mural fibrosis areas in UIP. Other investigator also reported the paucity of SP-A positive epithelial cells in the honeycomb lesion. Counting results are thus highly influenced by the honeycomb area, and might only reflect the occupied area of honeycomb lung, provided the area is included in the counting. Therefore, we did not include the honeycomb area in the counting. On the other hand, we did not also include normal area in the counting, because the aim of the present study was to investigate the expression of epithelial cell markers such as SP-A in regenerative/hyperplastic alveolar epithelial cells of IIPs.
We examined how corticosteroid or immunosuppressive therapy might affect expression of SP-A in alveolar epithelial cells and found no statistically significant differences in the SP-A positive ratios between treated and untreated UIP or NSIP patients. Therefore, it is unlikely that corticosteroid or immunosuppressive therapy affected the correlation between the SP-A positive ratio and the prognosis for UIP or NSIP patients, although the patient population was small.
The reason a paucity of immunohistochemical SP-A expression related to a worse prognosis in UIP patients is unclear. SP-A is reported to play an important role in the regulation of pulmonary inflammation[20–23]. Therefore, a lack of SP-A expression may induce an exaggerated inflammatory reaction, such as an overproduction of inflammatory cytokines to a given stimulus (a viral infection, for example) and cause deterioration of UIP. We do not know whether the regenerative/hyperplastic alveolar epithelial cells that lacked SP-A expressed other surfactant apoproteins. If expression of surfactant as a whole is poor in lungs of UIP patients, the patients may suffer from a condition similar to respiratory distress syndrome, for which the clinical course is poor. The type II alveolar epithelial cell is the major source of pulmonary surfactant, which functions to reduce surface tension forces and the tendency of the alveolar spaces to collapse. The reduction in surfactant production by the type II cells in IPF[23–26] may promote atelectasis, resulting in the apposition and fibrotic fusion of the denuded alveolar basement membranes. Alveolar epithelial markers, which reflect the capacity of the type II cells to respond appropriately to injury, may predict clinical outcome of IPF.
McCormac et al. reported that IPF patients whose BALF showed a low SP-A to total phospholipid ratio experienced a worse prognosis[8, 9]. Though we did not measure serum or BALF SP-A concentrations in the present study, our current results are consistent with those of McCormac et al. Previous studies report that IPF patients with high SP-A serum concentrations had poor prognoses[10–12]. Takahashi et al. reported a time lag between SP-A or D concentrations in the serum and BALF in radiation-induced lung injury in rats. This may explain the different results reported for the correlation between SP-A or D concentrations in the serum or BALF and IPF prognosis.
The SP-A positive ratio in lung tissue of UIP patients was significantly lower than that for NSIP patients. This difference may reflect the different courses of disease progression in these two types of interstitial pneumonia. Another possible explanation is that characteristics or function of regenerative epithelial cells differ between UIP and NSIP. Though the number of samples studied was small, the different percentages of alveolar epithelial cells staining positive for MIB1 (Ki-67) between UIP and NSIP patients noted in the present study may support this idea. In an investigation of the phenotypes of regenerative epithelial cells in UIP and NSIP patients, Hinata et al. proposed different origins of regenerative epithelial cells in the two interstitial pneumonias.
As the present study was retrospective, a prospective approach is necessary to confirm the relationship between immunohistochemical expression of surfactant protein to the patient prognosis in IIPs, particularly UIP.