Kinetics of changes in lymphocyte sub-populations in mouse lungs after intrapulmonary infection with M. bovis (Bacillus Calmette-Guerin) and identity of cells responsible for IFNγ responses

Abstract

Gamma interferon (IFNγ) plays a key role in host defense against pulmonary mycobacterial infections. A variety of lymphocyte subsets may participate in producing pulmonary IFNγ responses, but their relative contributions after mycobacterial infection have not been clearly elucidated. To address this question, C57Bl/6 female mice were infected by intrapulmonary instillation of 2.5×10⁴ BCG (Mycobacterium bovis Bacillus Calmette-Guerin). Lymphocyte populations in lung interstitium were examined at different time points after the infection. BCG load in lungs peaked between 4 and 6 weeks post-infection and declined to very low levels by the 12th week of infection. Recovery of lung interstitial lymphocytes doubled by 4-6 weeks after infection and declined thereafter. Flow cytometric analysis of the lung-derived lymphocytes revealed that about 5% of the these cells made IFNγ in control mice, and this baseline IFNγ production involved T (CD3⁺NK1.1⁻), NK (CD3⁻NK1.1⁺) and NKT (CD3⁺NK1.1⁺) cells. As the BCG lung infection peaked, the total number of CD3⁺ T cells in the lungs increased threefold at 5 - 6 weeks post-infection. There was a marked increase (sixfold) in the number of T cells secreting IFN 5-6 weeks post-infection. Some increase was also noted in the NKT cells making IFNγ, but the numbers of NK cells making IFNγ in BCG-infected lungs remained unaltered. Our results suggest that whereas NK and NKT cells contribute to baseline IFNγ secretion in control lungs, expansion in the IFNγ-producing T-cell population was essentially responsible for the augmented response seen in lungs of BCG-infected mice.