The availability of cloned lines of bone marrow stromal cells could facilitate the analysis of their role in hemopoietic cell development. The 266AD cell line was isolated from a colony of lipid-accumulating bone marrow cells growing in a collagen gel. 266AD cells have subsequently been maintained by passage in tissue culture plastic flasks about every 10 days for greater than 10 mo. Subconfluent cultures of cells are fibroblast-appearing, but in confluent cell sheets, prominent foci of lipid-containing cells develop in both uncloned and four separate cloned cell lines. Supernatants from confluent cultures containing lipid-laden cells contain granulocyte- macrophage colony-stimulating activity (GM-CSA) for normal bone marrow cells and can induce differentiation of Abelson virus transformed murine promonocytic leukemia cells. 266AD cells were originally isolated in the presence of hydrocortisone, but hydrocortisone is not necessary for lipogenesis to occur. Growth of bone marrow cells in a collagen gel matrix provided a way to isolate stromal cells, and the 266AD cell line provides a means to examine the relationships between stromal cell lipogenesis and regulation of granulopoiesis.
Virus-producing, tumorigenic, promonocytic leukemia cell lines were derived from Abelson murine leukemia virus-infected mice. This study shows that, of these 25 cloned lines, 22 were capable of extensive differentiation. It also shows that granulocyte-macrophage colony-stimulating activity increased the proportion of differentiating cells in 17/22 lines. Cells from agar colonies with a diffuse colony morphology had an increased expression of mature macrophage phenotypic characteristics, and a reduced proliferative capacity in vitro, compared to cells from agar colonies with a compact colony morphology. Cells from diffuse colonies also produced less Abelson virus, and were less tumorigenic in vivo than cells from compact colonies. Together, these results suggest some Abelson virus-producing leukemic cells are not blocked in their capacity to differentiate and are capable of reversing the transformed phenotype.
Adoptive immunotherapy, the transfer of spleen cells from immunized mice to mice with a small tumor, was usually curative for mice with the P815 mastocytoma provided that steps were taken to prevent the generation of tumor-induced suppressor cells in the recipient animal. However, failure of adoptive immunotherapy of the P815 tumor, resulting in regrowth of either the primary intradermal or a metastatic tumor, was observed in 10 out of 112 animals receiving graded doses of 7.5 x 10(7) to 3.0 x 10(8) immune spleen cells. Examination of the ten tumors in mice that failed to respond to therapy revealed that seven of them were significantly less susceptible than the original P815 tumor to rejection in vivo by transferred anti-P815-specific effector cells. In addition, nine of the ten therapy-failure tumors were also less susceptible than the original P815 tumor to lysis in vitro by P815-specific, but not DBA/2-specific, cytotoxic T lymphocytes. Sensitivity to lysis by tumor-specific cytotoxic T cells was not, however, strongly correlated with sensitivity to rejection in vivo by P815-specific effector spleen cells. Neither in vivo sensitivity to rejection, nor sensitivity to cytotoxic T cells, was correlated with alterations in class I major histocompatibility complex antigen expression. These results suggest that the survival and outgrowth of variant tumor cells was frequently the cause of failure of specific adoptive immunotherapy of the P815 tumor, and that selection for cells with a reduced sensitivity to killing by cytotoxic T cells was only one mechanism that might lead to an immunotherapeutic failure.
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