Kinetics of phagocytosis of Chlamydia psittaci by mouse fibroblasts (L cells): separation of the attachment and ingestion stages

Abstract

The kinetics of phagocytosis of C. psittaci (6BC) by monolayers of mouse fibroblasts (L cells) was studied with an assay that distinguished between the attachment and ingestion phases of phagocytosis. At multiplicities of 10 and 100, 50% infectious doses (ID50)/L cell, virtually all of the inoculated C. psittaci were attached and ingested after 60 min at 37.degree. C. At multiplicities of 500-5000 ID50/L cell, the initial rates of attachment and ingestion of C. psittaci to L cells increased with the multiplicity of infection, but phagocytosis stopped even though many chlamydial cells remained free in suspension and readily available for attachment to the host-cell monolayers. Phagocytosis probably ceased because the L cells were injured when they took up large numbers of chlamydial cells. The injury prevented direct determination of the number of potential binding sites for C. psittaci on each L cell. This number is large enough to make the rates of chlamydial attachment and ingestion predominantly dependent on the multiplicity of infection.