Introduction Acute lung injury (ALI) has a mortality rate of over 30%, with no proven pharmacological treatment. Inhalation of lipopolysaccharide (LPS) in healthy volunteers induces transient inflammation resembling that found in patients with ALI. Inhaled LPS causes neutrophilia that is detectable in bronchoalveolar lavage fluid (BALF) and blood, but its effect on BALF and blood monocyte populations is not well established.
Methods 12 healthy volunteers were recruited and randomly allocated to receive either 60 μg of inhaled LPS or saline (n=6 each arm). Clinical parameters, including temperature, and any reported symptoms were recorded. Full blood counts were taken at baseline and 2, 4, 6, 8 and 24 h post-inhalation. BAL was performed at 8 h. BALF cell populations were analysed morphologically using cytospins and cytometrically by flow cytometry after staining for cell surface markers (alveolar macrophages: CD163, CD206, CCR5; neutrophils/monocytes: HLA-DR, CD14, CD16).
Results 4 LPS volunteers developed pyrexia, two reported cough and one myalgia. The mean maximal increment in temperature was significantly greater in the LPS arm (p=0.047). Compared to saline inhalation, LPS caused a peripheral blood neutrophilia (p=0.006) that was evident from 4 h and greatest at 8 h. There was no significant difference in peripheral blood monocyte counts between treatment arms at any point measured (p=0.87). Although mean total alveolar macrophage numbers were similar between the two groups, their relative proportion in the LPS volunteers was significantly reduced due to the expansion in neutrophil and monocyte populations. Flow cytometry revealed a 24-fold expansion of the neutrophil population following LPS (in parallel with morphological data). These neutrophils were distinguishable by HLA-DR-/CD14-/CD16+ staining. There was a concomitant similar rise in the population of HLA-DR+/CD14+/CD16- ‘classical’ monocytes. Further analysis of these monocytes revealed that macrophage cell surface marker expression was absent.
Conclusion Morphological analysis of BAL fluid in previous LPS inhalation studies has consistently suggested that there is no change in the monocyte population. Using flow cytometry enables a more detailed analysis. This study is the first to clearly demonstrate that an early expansion in the monocyte population accompanies the neutrophil influx seen in BALF 8 h following inhalation of LPS.