Surfactant Protein D Is Altered in Experimental Malaria-Associated Acute Lung Injury/Acute Respiratory Distress Syndrome

Abstract

Surfactant protein D (SP-D) is in the collectin family of C-type lectins and plays an important role in the regulation of inflammation and the innate immune defense against pathogens. This protein has been proposed as a biomarker for acute lung injury. However, the expression of SP-D in the lung and the circulating levels of SP-D during malaria infection have received limited attention. Therefore, the aim of this study was to determine the location and expression of the SP-D protein in lung tissue and to measure the plasma level of SP-D in experimental malaria-associated acute lung injury/acute respiratory distress syndrome (ALI/ARDS). Malaria-infected mice induced by Plasmodium berghei ANKA were classified into two groups, namely, the ALI/ARDS and non-ALI/ARDS groups, according to lung histopathology. The lungs of uninfected mice were used as a control group. The location and expression of SP-D in the lung tissues were investigated by immunohistochemical staining and Western blot analysis. In addition, the level of SP-D in plasma and lung homogenate was measured by an enzyme-linked immunosorbent assay. Immunohistochemical staining of SP-D was significantly increased in the lung tissues of the malaria-infected mice in the ALI/ARDS group compared with that in the malaria-infected mice in the non-ALI/ARDS group and the mice in the control group (p < 0.05). The levels of SP-D in the plasma and lung homogenate were significantly increased in the malaria-infected mice in the ALI/ARDS group compared with those in the malaria-infected mice in the non-ALI/ARDS group and the mice in the control group (p < 0.05). There was a significant positive correlation between SP-D in the plasma and SP-D in the lung homogenate (r _s = 0.900, p = 0.037). In conclusion, this study demonstrated increased expression levels of SP-D in the lung tissue and high levels of plasma SP-D in the malaria-infected mice with ALI/ARDS compared with those in the mice in the other groups. The current study supports that the elevation of the plasma SP-D level may provide useful biological confirmation of the diagnosis of ALI/ARDS during malaria infection.

Figure 1

Representative photomicrographs of SP-D expression in the lung tissues. (a) Lung tissue of the mice in the control group, (b) lung tissue of the malaria-infected mice in the non-ALI/ARDS group, and (c) lung tissue of the malaria-infected mice with ALI/ARDS. All the images are magnified 400×. Scale bar = 20 μm. Notice alveolar type I cells are thin and flat whereas alveolar type II cells are cuboidal. Alveolar macrophages are pleomorphic in shape, adhered to the internal surface of alveolar septum.

Figure 2

Western blot analysis of SP-D expression in the lung tissues from the malaria-infected mice with ALI/ARDS, malaria-infected mice in the non-ALI/ARDS group, and mice in the control group (n = 5 mice per group). (a) Representative immunoblot images for SP-D and (b) the quantification of SP-D expression by Western blot analysis. ∗Significance of p < 0.05 compared among groups using Kruskal–Wallis test.

Figure 3

SP-D levels in the plasma and lung homogenate from the malaria-infected mice with ALI/ARDS, malaria-infected mice in the non-ALI/ARDS group, and mice in the control group (n = 5 mice per group). (a) SP-D in the plasma and (b) SP-D in the lung homogenates. ∗Significance of p < 0.05 compared among groups using Kruskal–Wallis test. The data are presented as the mean ± SEM.

Figure 4

(a) Correlations between SP-D in the plasma and SP-D in the lung homogenate. (b) Correlations between SP-D in the plasma and the staining intensity scores for SP-D in the malaria-infected mice with ALI/ARDS (n = 5). The data were analyzed using Spearman's rank correlation test.