Increased lung neutrophil apoptosis and inflammation resolution

From the authors:

We would like to thank C.G.A. Persson and L. Uller for their interesting observations and suggestions concerning our recently published article [1]. Also, we would like to comment the issues they have raised.

As they point out, we performed our study in patients (and controls), using blood and bronchoalveolar lavage (BAL) neutrophils for in vivo/ex vivo experiments. We did not include tissue samples due to the difficulty of obtaining them, just as C.G.A. Persson and L. Uller acknowledge. We analysed neutrophils from BAL fluid from non-responding community-acquired pneumonia (NCAP) patients to whom bronchoscopy was indicated and had signed informed consent. Bronchoscopy has the advantage of being a technique that is able to sample around 106 alveoli and, in addition, it is indicated for patients with NCAP [2]. All this could give relevant information about NCAP and, in fact, it did, as reflected in results that shed light on the relationship of some cytokine levels (interleukin (IL)-6, -8 and -10) and apoptosis rates in NCAP patients with clinical outcome.

We agree that more studies are needed to address several issues that emerge from our findings. Specifically, the engulfment of apoptotic neutrophils in NCAP patients is one of them. As they indicate, the lack of engulfment of apoptotic neutrophils can imply secondary necrosis which, in turn, can also be an important pathogenic event to analyse. We also agree that apoptosis alone is not sufficient to resolve inflammation; therefore, in our study we also investigated inflammatory local and systemic cytokine profiles and clinical resolution of infectious parameters. In fact, our results clearly show that neutrophil apoptosis is associated with lower proinflammatory cytokine levels and faster clinical stability. This is a finding that would probably not have been changed if we had obtained lung biopsies.

Another comment refers to transepithelial elimination of neutrophils during resolution of NCAP. The interesting proposed hypothesis of inflammatory resolution through egression of leukocytes is very attractive [3]. C.G.A. Persson and L. Uller argue that elimination of leukocytes from the airways with bronchial inflammation (transepithelial egression) is a mode of resolving inflammation. This challenging interpretation is nicely presented in human and animal models in asthma and chronic obstructive pulmonary disease. However in pneumonia, the acute inflammatory response of host against microorganisms is a completely different model of airway inflammation. In pneumonia, innate responses and alveolar macrophages orchestrate the inflammatory response in lung parenchyma, and neutrophils exit the pulmonary circulation at the capillary level, that is, mainly in alveolar air spaces. Obviously, our design was not directed to evaluate the neutrophil migration across the alveolar or epithelial lining, although this topic merits attention.

Referring to the comments that evidence of neutrophil apoptosis in inflammation resolution is not very strong, it is worth mentioning that our results and those from other researchers seem to indicate the contrary [4–6]. We have found a significant correlation between neutrophil apoptosis and pneumonia outcome. Our results agree with the comment made by Matute-Bello and Martin [7], indicating that studies in humans suggest that neutrophil apoptosis is inhibited early in acute respiratory distress syndrome but returns to normal as inflammation resolves. These authors also indicate other steps that must be explored to understand all the process, but they were not the objective of our present work. Droemann et al. [8] have results in concordance with ours, and the differences in the rate of neutrophil apoptosis are probably due to the initial phase of pneumonia (24 h).

So, we consider that the study of cytokine patterns and neutrophil apoptosis in BAL fluid from NACP is currently an excellent available option to evaluate the ongoing inflammatory process ex vivo. With regard to the role of the transepithelial exit of leukocytes from the airway proposed by C.G.A. Persson and L. Uller for asthma and chronic obstructive pulmonary disease, it should not be directly translated to pneumonia, an acute infection with a completely different restitutio in integrum. Finally, we totally agree that regarding the host response against micro-organisms and resolution of inflammation, there are still many missing important pieces and new approaches are welcome.

• ©ERS 2012