Excessive numbers and activity of peripheral blood B cells in infants with Chlamydia trachomatis pneumonia.

The peripheral blood lymphocytes of seven infants who had lower respiratory infections caused by Chlamydia trachomatis (chlamydial pneumonia) were studied for abnormalities that may be related to the hyperimmunoglobulinemia characteristic of this infection. Both proportions and numbers of B cells and plasma cells were strikingly elevated in these infants, as indicted by the percentage of peripheral blood mononuclear cells (PBMC) that reacted with fluorochrome-labeled antibodies to human immunoglobulins. Cells expressing IgM and IgD on their surface, and cells possessing IgM and IgG in their cytoplasm were especially increased above levels found in normal adults, infants, and a group of infants with other infections. Cells from infected infants secreted exceptionally large amounts of IgM, IgG, and IgA when cultured in the absence of added mitogens. These data suggest that chlamydial pneumonia induces substantial B-cell activation during a period of development when antibody responses are normally difficult to stimulate.