Mouse Hypersensitivity Pneumonitis: Depletion of NK Cells Abrogates the Spontaneous Regression Phase and Leads to Massive Fibrosis

Abstract

The contribution of natural killer cells (NK) in the progression of mouse hypersensitivity pneumonitis induced by repeated intranasal instillations with the thermophilic actinomycete Faeni rectivirgula was examined. These instillations determined a very large increase in the lung index (ca. twofold at 3 weeks), used as a measure of inflammation. In addition, this instillation was associated with a tenfold increase in the number of cells recovered in the bronchoalveolar lavage (BAL) at 3 weeks and thereafter. Most of these cells were macrophages, whereas T lymphocyte numbers increased at 6 weeks and thereafter. The instillations were also associated with a substantial fibrotic response in the lungs, as seen by large increases in hydroxy/proline levels in the lungs. This fibrotic response, however, diminished after 6 weeks of instillations. Similarly, examination of histological preparations of lungs of challenged mice showed that F. rectivirgula induced inflammatory infiltrates of macrophages, lymphocytes, and neutrophils. The severity of the lesions were reduced in mice given more than 6 weeks of the actinomycete challenge. The involvement of NK cells on the development of this pulmonary pathology was determined by infusing F. rectivirguh-challenged mice with anti-NK 1.1 antibody. The depletion protocol was validated by verifying that such treatments effectively blocked lung NK cell activity. Such NK cell-depleted mice responded to the F. rectivirgula challenge with an increased lung index at 9 and 12 weeks, compared to mice challenged with F. rectivirgula and given control antibody. NK cell-depleted mice also responded to the actinomycete with a superior cellular recruitment in the BAL, with this increase mostly mediated by macrophages. Similarly, NK cell-depleted mice developed a fibrotic response that was much higher than that seen in control challenged mice, at 6, 9, and 12 weeks after initiation of the transnasal instillations. This was corroborated by scoring the severity of the histopathological lesions, which showed that NK cell-depleted mice had more severe lesions than challenged control mice. The importance of NK cells was confirmed by demonstrating that mice given anti-NK 1.1, challenged with F. rectivirgula and reconstituted with Percoll gradient-enriched lung NK cells had hydroxyproline levels at 9 and 12 weeks that were comparable to that seen in intact mice, as well as a histopathological score similar to control challenged mice. Overall, this suggests that in the course of a pulmonary inflammatory response, NK cells exert a suppressive effect on cellular recruitment in the BAL, contribute to down-regulating the inflammatory response, and are involved in blocking the appearance of fibrosis.