Rapid changes in shape and number of MHC class II expressing cells in rat airways after Mycoplasma pulmonis infection

Abstract

Mycoplasma pulmonis infection in rodents causes a chronic inflammatory airway disease with a strong immunological component, leading to mucosal remodeling and angiogenesis. We sought to determine the effect of this infection on the shape and number of dendritic cells and other major histocompatibility complex (MHC) class II expressing cells in the airway mucosa of Wistar rats. Changes in the shape of subepithelial OX6 (anti-MHC class II)-immunoreactive cells were evident in the tracheal mucosa 2 days after intranasal inoculation with M. pulmonis. By 1 week, the shape of the cells had changed from stellate to rounded (mean shape index increased from 0.42 to 0.77). The number of OX6-positive cells was increased 6-fold at 1 week and 16-fold at 4 weeks. Coincident with these changes, many columnar epithelial cells developed OX6 immunoreactivity, which was still present at 4 weeks. We conclude that M. pulmonis infection creates a potent immunologic stimulus that augments and transforms the OX6-immunoreactive cell population in the airways by changing the functional state of airway dendritic cells, initiating an influx of MHC class II expressing cells, and activating expression of MHC class II molecules by airway epithelial cells.

Fig. 1

OX6-immunoreactive cells in rat tracheal mucosa (A–C). Vibratome cross-sections 150 μm in thickness comparing mucosal thickness (arrowheads) and amount of OX6 immunoreactivity in (A) pathogen-free rat, (B) M. pulmonis-infected rat at 1 week, and (C) M. pulmonis-infected rat at 4 weeks. Cartilage is located beneath the mucosa in all specimens. (D) Tracheal whole mount from pathogen-free rat: OX6-positive cells just beneath epithelium have dendritic shape, with multiple, branched cytoplasmic processes. (E) Tracheal whole mount from rat 1 week after M. pulmonis infection: OX6-positive cells near epithelium are rounder and more numerous than corresponding cells in pathogen-free rat. Scale bar in (E) applies to all figures. Bar=60 μm (A–C), 10 μm (D, E).

Fig. 2

Shape index of OX6-positive cells. Bar graph showing changes in shape index (4πA/P², where A is the projected cell area and P is the projected cell perimeter) of OX6-positive cells in tracheal whole mounts of pathogen-free rats (control) and rats infected with M. pulmonis. Rounder cells have values closer to 1. Values are means±SE, n=5 rats per group. *Significantly different from control, P<0.05.

Fig. 3

Number of OX6-positive cells. Bar graph showing number of OX6-positive cells in subepithelial tracheal mucosa of pathogen-free rats (control) and rats infected with M. pulmonis (n=5 rats per group). OX6-immunoreactive epithelial cells were not counted. Values are means±SE of counts made on tracheal whole mounts. *Values significantly different from control, P<0.05.

Fig. 4

Tracheal epithelial cells viewed from luminal surface of whole mounts. Comparison of OX6 immunoreactivity in tracheal epithelium of (A) pathogen-free rat, (B) M. pulmonis-infected rat at 2 days, and (C) M. pulmonis-infected rat at 1 week. Epithelial cells in pathogen-free rat (arrowheads) have little or no OX6 immunoreactivity, but some OX6-positive cells located beneath the epithelium have cytoplasmic processes (arrows) that extend into the epithelium. OX6-positive epithelial cells (arrows) are scattered in the epithelium of infected rat at 2 days. Many OX6-positive columnar epithelial cells (arrows) are present in infected rat at 1 week. Most of the immunoreactivity in these cells appears to be in intracellular granules. Scale bar in (C) applies to all figures. Bar=8 μm (A–C).