Frequency of mast cell precursors in normal tissues determined by an in vitro assay: antigen induces parallel increases in the frequency of P cell precursors and mast cells.

Persisting (P) cells, homogeneous populations of cells that grow in vitro for prolonged periods provided a specific growth factor is present, resemble mast cells in many respects. An in vitro assay based on limit dilution was used to determine the frequency of precursors capable of giving rise to P cells. The incidences of P cell precursors per 10(6) cells in tissues of CBA mice in representative experiments were as follows: bone marrow, 291; spleen, 30; mononuclear blood cells, 11; popliteal lymph node, 0.5; and mesenteric lymph node, 18. P cell precursors appeared to be relatively undifferentiated, non-granulated cells; no cells with metachromatically staining granules were detected in the bone marrow or peripheral blood. Furthermore, mice of the Wf/Wf genotype that were grossly deficient in mast cells had the same frequencies of P cell precursors in bone marrow and spleen as their normal +/+ littermates. In many tissues in which we found P cell precursors, pluripotential hemopoietic stem cells are present. Among nonepithelial cells from the gut mucosa, however, in which there was a 10-fold higher frequency of P cell precursors than in bone marrow cells, pluripotential hemopoietic stem cells were undetectable, indicating the existence of committed P cell precursors distinct from pluripotential hemopoietic stem cells. The frequency of P cell precursors in mesenteric lymph nodes was more than 30-fold higher than in the popliteal lymph nodes, suggesting that antigenic stimulation influences their numbers. This latter notion is supported by the observation that after immunization in the footpad, the number of P cell precursors in ipsilateral popliteal lymph nodes rose about 35-fold. Immunization was also accompanied by a rise in mast cell numbers in draining popliteal nodes. This correlation between P cell precursors and the local production of mast cells was strengthened by the observation that the frequency of P cell precursors in cells from the gut mucosa of mice of Wf/Wf genotype, which are unable to mount an intestinal mastocytosis, was more than 1000-fold lower than in wild type mice. Thus, the precursors of P cells and probably of at least the T cell-dependent subset of mast cells appear to be generated in the bone marrow and seed as non-granulated cells via the blood to peripheral tissues such as spleen, lymph node, and mucosal surfaces. P cells appear to be in vitro counterparts of the mucosal subset of mast cells.