In this study we report for the first time that ectodomain shedding by leukocyte ADAM17 participates in regulating neutrophil migration into the inflamed lung. Pulmonary neutrophil recruitment is a critical mechanism of pulmonary inflammatory disorders such as ARDS 
. Excessive neutrophil recruitment into the alveolar compartment in particular has been associated with worsening lung damage and increased mortality 
. We observed in ADAM17-null mice, whose leukocytes lack functional ADAM17, that neutrophil accumulation in the alveolar compartment, although occurring earlier, was overall significantly decreased compared to control mice following their exposure to LPS. This same pattern of rapid but attenuated neutrophil accumulation in the lung air spaces also occurred upon intratracheal injection of ADAM17-null mice with the Gram-positive bacterial constituent lipoteichoic acid (Arndt et al. unpublished results), a functional equivalent to LPS that instead induces neutrophil infiltration into the lung via TLR2. Neutrophil migration into the lung interstitial compartment, however, was not significantly decreased in ADAM17-null mice following LPS challenge, indicating that the targeting of leukocyte ADAM17 does not impair neutrophil infiltration into the interstitium from the pulmonary blood vessels, but does alter their advancement to the alveolar compartment.
ADAM17 expressed by neutrophils is a primary sheddase of L-selectin and TNF-α 
; however, its relevancy in vivo
by neutrophils recruited to the inflamed lung has not previously been examined. We show that the conversion of membrane L-selectin and TNF-α to their soluble forms was greatly impaired in the inflamed lungs of ADAM17-null mice. In contrast to these substrates, the sheddase activity regulating the cleavage of IL-6R is in the early stages of being understood. The cleaved form of the IL-6R binds secreted IL-6 and enhances its pleiotropic activity through the activation of cells that lack expression of IL-6R via trans-signaling through the ubiquitously expressed glycoprotein 130 
. In vitro
studies have implicated ADAM17 in IL-6R shedding 
, yet the biological relevance of ADAM17 in this process has not been directly investigated in vivo
. Our results reveal that in the context of lung inflammation, ADAM17 participates in the shedding of IL-6R, but in contrast to TNF-α and L-selectin, ADAM17 is not the primary sheddase of leukocyte IL-6R. ADAM10 has also been reported to cleave the IL-6R 
, and thus it will be interesting to directly investigate its role in IL-6R shedding in a setting of acute lung inflammation.
It is well recognized that TNF-α induces an extensive array of downstream events that further promote inflammation, and thus the greatly diminished production of soluble TNF-α by ADAM17-deficient leukocytes likely contributed to the reduced levels of alveolar neutrophils as lung inflammation progressed after LPS exposure. For instance, TNF-α signaling has been directly shown to induce the production of neutrophil-tropic chemokines in the alveoli following LPS exposure 
. As CXCL1, CXCL2, and CXCL5 are major chemokines directing neutrophil recruitment into the murine lung 
, we examined their levels in the alveolar compartment of the lung in ADAM17-null and control mice. CXCL2 levels were found to be similar in the two groups of mice. However, CXCL5 and CXCL1 levels in ADAM17-null mice were significantly decreased at 2 hours and 8 hours, respectively, following LPS instillation. CXCL5 is primarily expressed by activated alveolar type II cells 
, and attenuated early production of soluble TNF-α by resident and recruited leukocytes in ADAM17-null mice may have delayed the activation of these cells and the initial production of CXCL5 in the airspace. CXCL1 is secreted by a variety of cells including neutrophils 
, and the time frame of its reduction in alveolar levels in ADAM17-null mice corresponded with the marked reduction in alveolar neutrophil numbers as inflammation progressed following LPS exposure. In contrast to the alveolar spaces, only CXCL5 was decreased in the interstitial compartment of the lung in ADAM17-null mice. The greater reduction in neutrophil-tropic chemokines in the alveolar compartment of ADAM17-null mice implies a specific molecular process accounting for the lack of neutrophil transepithelial migration into the alveolar air spaces at the later time point in our study. Of additional interest is that alveolar neutrophil levels in ADAM17-null mice were initially enhanced upon inducing lung inflammation when compared to control mice. The reasons for this are not clear at this time, but may be the result of still other neutrophil chemoattractants or an enhanced ability of ADAM17-deficient neutrophils to respond to them.
In conclusion, our study demonstrates for the first time that leukocyte ADAM17 regulates acute lung inflammation by modulating intra-alveolar neutrophil levels and the shedding of IL-6R, L-selectin, and TNF-α. It is known that preventing TNF-α activity can increase host susceptibility to infection, and thus it will be important to determine the role of leukocyte ADAM17 in pulmonary defense against bacterial infection. Of interest is that we have recently reported that ADAM17-null mice demonstrate enhanced host resistance, including decreased hematogenous spread of bacterial to the lung, during E. coli
-mediated abdominal sepsis