SummaryIt has frequently been suggested that loss of Bz-microglobulin (B2m) in tumor cells may lead to malignant progression due to escape from immunological recognition. Here, we directly tested the role of fl2m expression in tumorigenicity. A ~2m loss mutant (C4.4-25-), selected from the murine lymphoma Eb4, showed a marked reduction in tumorigenicity as compared with Eb4 in normal C57B1/6 (B6) mice. The reduced tumorigenicity was directly related to ~2m expression. Transfection of an intact murine ~2m gene markedly increased the tumorigenic potential. The reduced tumorigenicity of C4.4-25-compared with fl2m transfected cells was observed also in athymic B6 nu/nu mice, but was abolished in B6 mice depleted of natural killer (NK) 1.1-positive cells. These results show that restoration of B2m expression can promote tumorigenicity and demonstrate for the first time that induction of major histocompatibility complex class I expression by transfection can lead to escape from NK cells in vivo.

A panel of dendritic epidermal T cell (DETC) lines, and hybridomas derived from them, has been shown to spontaneously secrete lymphokines in the absence of added stimuli, which suggests that these cells are autoreactive. These cell lines are characterized by the expression of a V gamma 1.1C gamma 4/V delta 6 type T cell receptor (TcR), but several of the DETC lines also express a second TcR. Sequence analyses of these gamma/delta TcR revealed that the gamma chains were identical and that the delta chains, while not identical, were quite restricted in diversity, indicating that these receptors may recognize a common or closely related group of antigens. Analysis of hybridomas derived from newborn thymocytes identified six hybridomas that spontaneously secrete lymphokines. Five hybrids expressed a V gamma 1.1C gamma 4/V delta 6 receptor and one hybrid a V gamma 1.1C gamma 4/V delta 4 receptor that had a close structural relationship to the DETC gamma/delta TcR associated with spontaneous lymphokine secretion. gamma/delta TcR of the C gamma 4 type expressed by splenic hybridomas that did not spontaneously secrete lymphokines revealed no such relationship. Curiously, like the DETC, several of the thymocyte hybridomas that spontaneously secreted lymphokines expressed a second TcR, V gamma 2C gamma 1 or V gamma 3C gamma 1, apparently in association with the same delta chain that paired with the C gamma 4 chain. The presence of spontaneous lymphokine-secreting gamma/delta T cells with such highly homologous TcR in both the thymus and skin suggests a thymic origin for the autoreactive DETC and that these cells recognize a common or closely related group of self-antigens.

Several reports have suggested that an inverse correlation exists between major histocompatibility complex class I expression and the susceptibility to natural killer (NK)-mediated lysis. For example, the increased class I expression induced by interferon-gamma was always accompanied by an increased resistance to NK lysis. Likewise, class I loss variants were often more NK susceptible than their normal counterparts. To investigate whether the inverse correlation between class I expression and NK susceptibility was fortuitous or whether the class I molecules were directly responsible for this effect we resorted to gene transfection studies. From the murine thymoma line EL4 and H-2Db- and Kb-negative variant S3 was selected. This variant was highly susceptible to NK lysis. S3 was found to have a defect in beta 2-microglobulin gene expression. Therefore, restoration of Db and Kb expression could be achieved by transfection with the beta 2-microglobulin gene. This resulted in a strong decrease in susceptibility to NK lysis to the level of the H-2+ parental EL4. Transfection with class II genes had no effect. Blocking of the class I molecules on the H-2+ cells with anti-H-2b F(ab')2 fragments increased the susceptibility to NK cells to the level of the H-2- variant S3. These data demonstrate that the class I molecules on the targets are directly responsible for regulation of NK susceptibility but the mechanism is not clear. Possibly the class I molecules interfere with the unknown NK target structures.