Localization of chlamydial group Antigen in McCoy cell monolayers infected with Chlamydia trachomatis or Chlamydia psittaci.

Chlamydial inclusions were demonstrated by indirect immunofluorescence (IF) with antiserum to the chlamydial group antigen when McCoy cell monolayers infected with either Chlamydia trachomatis or Chlamydia psittaci were fixed in formaldehyde or paraformaldehyde, provided the monolayer was not allowed to dry. If these monolayers were then air dried and restained by IF with the same antiserum but with a different fluorescence conjugate, group antigen associated with inclusion-containing McCoy cells but independent of the inclusions was revealed. This antigen was not restricted to infected cells but appeared to radiate out from them, suggesting that group antigen was released from infected cells. Similar host cell-associated antigen could be shown by IF of glutaraldehyde-fixed, air-dried monolayers, but inclusions could not be stained by IF before these preparations were dried, presumably because antibody could not penetrate glutaraldehyde-fixed cells. Electron microscopic immunoperoxidase studies of paraformaldehyde-fixed, wet monolayers located group antigen within inclusions on the outer membrane of chlamydial organisms and on single-membrane vesicles. However, when dried monolayers were labeled with the same immunoperoxidase technique, no intracellular labeling occurred, but dense staining was seen at the surface of infected cells and on adjacent membranous material. These observations are compatible with the postulate that replicating chlamydiae produce outer membrane blebs containing group antigen, which are excreted by the host cells during the chlamydial developmental cycle.