The main goal of our study was to determine the impact of whole-body irradiation and BMT on the repopulation profile and turnover characteristics of lung DC subsets, compared with alveolar and lung macrophages under baseline and inflammatory conditions, using S. pneumoniae as an inflammatory stimulus. To the best of our knowledge, we show for the first time that irradiation/BMT triggers a rapid, greater than 90% depletion of CD103pos DCs from the lung parenchymal tissue of mice by Day 3 after irradiation, whereas CD11bpos DCs are only depleted by approximately 50–60% on Day 5 after treatment. In contrast to these data, alveolar and lung macrophages are depleted much later, by Day 21 after treatment, reaching a maximal depletion of only 50–60%. Moreover, we observed a differentially regulated repopulation of lung tissue within the respective lung DC and macrophage subsets. Whereas lung CD11bpos DCs and lung CD103pos DCs reached their original pool size by Days 10 and 21 after treatment, respectively, the repopulation of alveolar and lung macrophages reached baseline pool sizes on Days 42 and 70 after treatment, respectively. Furthermore, the infection of chimeric mice with low levels of S. pneumoniae triggered a brisk acceleration of the repopulation of all myeloid-derived DC and macrophage subsets in the lung. Altogether, our study provides a comprehensive analysis of the effects of irradiation/BMT on the depletion and repopulation and turnover kinetics of the major myeloid-derived lung DC and macrophage subsets, under both baseline and infection conditions. This information could be particularly important in the development of adjuvant immune enhancement strategies to overcome the immunosuppression of whole-body–irradiated BMT recipients.
Our report contains several novel aspects. First, to the best of our knowledge, we are the first to provide a detailed subset-specific analysis of the depletion and repopulation and turnover kinetics of both CD103pos
DC subsets in a chimeric irradiation/BMT murine model system. Second, we demonstrate that CD103pos
DCs in the lung are more sensitive than CD11bpos
DCs and lung macrophages to irradiation, resulting in an overall transient depletion of more than 90% in chimeric mice. Third, we show a rapid repopulation of the lungs with CD103pos
DC and CD11bpos
DC subsets subsequent to irradiation/BMT within 10 days (CD11bpos
DCs) and 21 days (CD103pos
DCs) after treatment, respectively, whereas alveolar and lung macrophages exhibit a more delayed depletion and repopulation, by Days 21 and 70 after treatment, respectively. Finally, we provide evidence that all myeloid-derived phagocyte subsets examined in the present study, including lung CD103pos
DCs, lung CD11bpos
DCs, and alveolar and lung macrophages, are eliminated from the lung parenchymal and bronchoalveolar compartment subsequent to irradiation by apoptosis-mediated and necrosis-mediated processes. Previous investigators observed a decline in rat airway MHC Class IIpos
cells shortly after irradiation/BMT, but they thought this decline was attributable to the emigration of mature DCs to regional lymph nodes (18
). Our data indicate, however, that elevated levels of apoptosis and necrosis occur in all lung mononuclear phagocyte subsets after irradiation/BMT, indicating that this is the major mechanism underlying the process of depletion. Furthermore, we observed that the draining lymph nodes of the lungs of chimeric mice also contain reduced numbers of CD103pos
DCs and CD11bpos
DCs (data not shown).
One major difficulty when comparing the published turnover kinetics of mononuclear phagocyte subsets in the lungs or in extrapulmonary organ systems is related to the different experimental protocols used in the various reported studies. Here, we used a linear accelerator (also used in the therapeutic whole-body irradiation of BMT recipients) and an irradiation/BMT protocol, resulting in an overall engraftment efficacy of greater than 90%. In a recent study by Ginhoux and colleagues, where parabiosis and separation experiments were performed in BL/6 mice and BL/6 CD45.1pos
mice, relatively short half-lives were also reported for both lung DC subsets (5
). However, comparing these two model systems is difficult, particularly because of the limited clinical relevance of parabiosis model systems. Another study using a different irradiation protocol reported that numbers of macrophages in the alveolar compartment were restored within 30 days after irradiation (17
), whereas in our study, the time needed to restore numbers of macrophages in the alveolar compartment and lung parenchymal tissue comprised approximately 42–70 days. These data clearly indicate that different experimental protocols may affect the comparative analyses of DCs and of macrophage turnover kinetics between different studies.
The origin of various lung DC subsets in the bone marrow and their developmental stages are still subject to ongoing debate. In this regard, a bone marrow precursor of macrophages and DCs, termed the macrophage and DC progenitor, and a common DC precursor were proposed to have the potential to differentiate into monocytes, macrophages, and different DC subsets (19
). Moreover, recent data indicate that two distinct murine blood monocyte populations can serve as DC precursors (21
). Given the nearly complete depletion of CD103pos
DCs in lung parenchymal tissue reported in this study, and furthermore, given that in the present chimeric murine model, bone marrow cells from normal BL/6 mice (characterized by their CD45.2 alloantigen expression) were used as donor bone marrow, the present irradiation and transplantation protocol may also prove useful in identifying candidate molecules that may regulate the developmental pathways of progenitor cells giving rise to CD103pos
lung DCs, as already proven for CCR2 (22
The repopulation and turnover kinetics of lung CD11bpos
DCs and CD103pos
DCs upon infection with S. pneumoniae
constitute another aspect of the present report. We infected mice with a clinical isolate of serotype 19 S. pneumoniae
, using a “low-level” inoculum size of 106
CFUs/mouse. This inoculum size is known to cause mild lobar pneumonia in mice, without causing severe lung tissue destruction or invasive disease progression (15
). Despite this relatively weak bacterial challenge, we found that the rapid depletion of DC subsets was followed by a large recruitment of donor-type CD11bpos
DCs and CD103pos
DCs. These data illustrate that the lung host defense machinery responds to infection not only with a transient increase of lung macrophage pool size, as recently reported (9
), but also with an increased recruitment of lung CD11bpos
DCs and CD103pos
DCs. Interestingly, this transient “overshooting” mobilization of donor-type DCs was particularly evident in the later phase of pneumococcal pneumonia, when both the process of bacterial pathogen elimination and the recruitment of alveolar neutrophils had already returned to baseline (data not shown). However, the relevance of an increased lung DC pool size to the resolution/repair process in pneumococcal pneumonia is not yet clear, as opposed to the relevance of lung macrophages, which we previously showed to play a prominent role in the resolution/repair process subsequent to lung infection with S. pneumoniae
, in terms of regaining lung homeostasis (13
), However, we speculate that lung DC subsets, which are located beneath the alveolar epithelial barrier, may be involved in sensing apoptotic/necrotic neutrophils or responding to inflammatory mediators, thereby possibly contributing to host-derived inflammatory cytokine/chemokine responses subsequent to infection.
In conclusion, this study, to the best of our knowledge, is the first to provide a detailed comparative analysis of the turnover kinetics of lung DC and macrophage subsets under noninfectious conditions, as well as in response to infection with S. pneumoniae. We show that under noninfectious conditions, CD103pos lung DCs were almost completely depleted from lung tissue after irradiation/BMT, indicating their strong radiosensitivity. Moreover, the turnover of lung CD103pos DCs was clearly accelerated compared with lung CD11bpos DCs, and both DC subsets exhibited a strongly accelerated turnover profile relative to alveolar and lung macrophages. Moreover, we demonstrated a brisk repopulation and turnover of all mononuclear phagocyte subsets after low-dose infection with S. pneumoniae. We believe that our newly provided information on the irradiation-induced effects of baseline and inflammation-induced lung DC and macrophage turnover profiles will be of major clinical importance in the development of immune-supportive therapeutic or prophylactic treatment regimens, to lower the risk of both bacterial and viral infections in irradiated BMT recipients.