A cloned cell line, Mk1, possessing Ia antigens and accessory cell activity.

A new cell line, termed Mk1, was established by subcloning from M1+ cells derived from murine myeloid leukemia cell line cells, M1−, by coculture with conditioned medium containing a differentiation factor harvested from secondary cultures of embryonic fibroblasts. Mk1 cells, consisting of five clones (A to E), are dishadherent cells with a few long processes and are nonspecific esterase-positive. These cell line cells have been maintained in the absence of conditioned medium. The capacity to ingest latex particles is low in Mk1 cells, but is significantly higher in both M1+ and Mm1 cells, the latter of which is a clonal subline of M1− cells. Fc receptors for IgG are always observed on 60 to 70% of each Mk1 cell clone, as well as on M1+ and Mm1 cells. Nevertheless, the phagocytic activity for EA (IgG antibody-sensitized sheep erythrocytes) is remarkably low in Mk1 and M1+ cells (less than 1 EA/cell), in comparison to Mm1 cells (more than 10 EA/cell). Another marked difference between Mk1 and Mm, cells is the expression of la antigen; la antigens detected by anti-laq ((A × AQR)F1 anti-B10.T(6R)) and anti-lak (A.TH anti-A.TL) are found in the former, but not in the latter. The expression of la antigen in Mk1 cells is dependent on the cell cycle (the highest in the G1/S, and the lowest in the M phase), and is accompanied by the appearance of accessory cell activities for lymphocytes in primary and secondary anti-SRBC PFC responses as well as with the production of monokines that substitute for A cells in secondary anti-SRBC PFC responses and augment Con A responses of syngeneic thymocytes. Despite the expression of la antigens, Mk, cells lack the ability to stimulate allogeneic lymphocytes in MLR, suggesting that either la on Mk, cells is different than that on other effective stimulator cells or that Mk, cells lack additional features that contribute to MLR stimulation. Thus, Mk1 and Mm1 cells seem to be counterparts in the macrophage lineage; Mk1 cells as a model of A cells that are poorly phagocytic and la antigen-positive, and Mm, cells as a model of scavengers that are highly phagocytic and lack A cell activity.