Antigen-dependent colonies of human peripheral blood lymphocytes: an immunomorphologic study

Human peripheral blood mononuclear cells (PBMC), stimulated by sheep red blood cells (SRBC), focally proliferate in agar and form colonies of anti-SRBC antibody-secreting cells surrounded by hemolytic areas. Two types of colonies develop: type I (diffuse type), which grows deeply into the agar, and type II (compact type), which grows above the former. Immunochemical and ultrastructural studies show that diffuse colonies contain differentiating lymphoid cells, from small lymphocytes to mature plasma cells. About 50% of cells stain positively in their cytoplasm for IgM and only 1-2% for IgG. Most colonies produce light chains of one class, whereas only a few produce both classes. Many cells resemble monocytes or T lymphocytes in their general morphology and lie in close contact with immunoglobulin-positive cells. Compact colonies contain cells not engaged in antibody production. The culture system described here is the first available antigen-dependent colony assay for human PBMC and may be useful for in vitro studies on the mechanism of human B-cell activation.