Delayed presence of alternatively activated macrophages during a Francisella tularensis infection

Enhancement of Macrophage Immunity against Chlamydial Infection by Natural Killer T Cells

Lung macrophage (LM) is vital in host defence against bacterial infections. However, the influence of other innate immune cells on its function, including the polarisation of different subpopulations, remains poorly understood. This study examined the polarisation of LM subpopulations (monocytes/undifferentiated macrophages (Mo/Mφ), interstitial macrophages (IM), and alveolar macrophages (AM)). We further assessed the effect of invariant natural killer T cells (iNKT) on LM polarisation in a protective function against Chlamydia muridarum, an obligate intracellular bacterium, and respiratory tract infection. We found a preferentially increased local Mo/Mφ and IMs with a significant shift to a type-1 macrophage (M1) phenotype and higher expression of iNOS and TNF-α. Interestingly, during the same infection, the alteration of macrophage subpopulations and the shift towards M1 was much less in iNKT KO mice. More importantly, functional testing by adoptively transferring LMs isolated from iNKT KO mice (iNKT KO-Mφ) conferred less protection than those isolated from wild-type mice (WT-Mφ). Further analyses showed significantly reduced gene expression of the JAK/STAT signalling pathway molecules in iNKT KO-Mφ. The data show an important role of iNKT in promoting LM polarisation to the M1 direction, which is functionally relevant to host defence against a human intracellular bacterial infection. The alteration of JAK/STAT signalling molecule gene expression in iNKT KO-Mφ suggests the modulating effect of iNKT is likely through the JAK/STAT pathway.

The effect of BCG vaccination on macrophage phenotype in a mouse model of intranasal Mycobacterium bovis challenge

In order to develop improved vaccinations against tuberculosis, it is essential to understand the effect of vaccination on the immune response, and to overcome the mechanisms by which mycobacteria regulate this immune response. In this study, we examine the effect of intradermal vaccination with Mycobacterium bovis bacille Calmette-Guèrin on macrophage phenotype following intranasal challenge with virulent Mycobacterium bovis. Preserved lung tissues used in the present study were obtained from a previous vaccination trial in BALB/c mice. Vaccinated mice showed less extensive pulmonary lesions along with a significant decrease in bacterial lung burden when compared to control mice. Immunohistochemical markers of classically activated macrophages (iNOS) and alternatively activated macrophages (Arg1, FIZZ1) were applied to lung sections. Vaccination led to a statistically significant decrease in the number of Arg1+ macrophages. The presence of macrophages that expressed Arginase 1 in pulmonary lesions was much smaller than the presence of macrophages expressing iNOS. The low presence of Arg1+ macrophages induced by vaccination may be caused by Th1 polarization and may reduce alternative activation of macrophages, with an overall more effective intracellular killing of bacteria.

Modeling early events in Francisella tularensis pathogenesis

Computational models can provide valuable insights into the mechanisms of infection and be used as investigative tools to support development of medical treatments. We develop a stochastic, within-host, computational model of the infection process in the BALB/c mouse, following inhalational exposure to Francisella tularensis SCHU S4. The model is mechanistic and governed by a small number of experimentally verifiable parameters. Given an initial dose, the model generates bacterial load profiles corresponding to those produced experimentally, with a doubling time of approximately 5 h during the first 48 h of infection. Analytical approximations for the mean number of bacteria in phagosomes and cytosols for the first 24 h post-infection are derived and used to verify the stochastic model. In our description of the dynamics of macrophage infection, the number of bacteria released per rupturing macrophage is a geometrically-distributed random variable. When combined with doubling time, this provides a distribution for the time taken for infected macrophages to rupture and release their intracellular bacteria. The mean and variance of these distributions are determined by model parameters with a precise biological interpretation, providing new mechanistic insights into the determinants of immune and bacterial kinetics. Insights into the dynamics of macrophage suppression and activation gained by the model can be used to explore the potential benefits of interventions that stimulate macrophage activation.