In in vitro production and cloning of the P cell, a bone marrow-derived null cell that expresses H-2 and Ia-antigens, has mast cell-like granules, and is regulated by a factor released by activated T cells.

A technique, based on selective culture conditions, is described for the preparation from mouse spleen of a pure population of Ig-negative, Thy.1-negative and Lyt.1- and Lyt.2-negative nonadherent cells, termed persisting (P) cells. Antigens coded for by the H2-K, H2-D, and I regions were readily detectable on the surface of P cells. P cells grew for long periods in vitro provided that a factor produced by activated T cells, but distinct from T cell growth factor, was present. It was demonstrated that P cells were bone marrow derived. When P cells were examined in suspension, a variable number of fine cytoplasmic projections could be seen waving from the cell, often at 1 pole. The nuclei varied in shape, being round, in general, but with bilobed and irregular forms occurring, and the cytoplasm contained large numbers of characteristic metachromatic granules. P cells had readily detectable Fc receptors but did not phagocytose latex particles or antibody-coated erythrocytes. P cells formed colonies in soft agar in the presence of growth factors, and cloned lines could be propagated from these colonies. The relationship of P cells to mast cells and Ia-positive accessory cells and their possible role in T cell activation is discussed.