Thymus-dependent in vivo suppression of IgE synthesis in a murine IgE-secreting hybridoma

In previous studies we demonstrated that BALB/c mice bearing ascitic tumors of the IgE-secreting hybridoma B53 (ε, κ, anti-dinitrophenyl) developed large numbers of Lyt-1^-2+ FcεR(+) T lymphocytes (T cells with membrane Fc receptors) in response to the elevated serum IgE concentration. The development of FcγR(+) T lymphocytes was followed by a progressive decrease in the levels of serum IgE in spite of continued proliferation of the hybridoma cells. This sequence of events suggested that the IgE-secreting hybridoma triggered a suppressive immunoregulatory circuit of the host that inhibited IgE expression by the hybridoma cells. The present studies were undertaken to investigate the basis for the subsequent decline in serum IgE levels in mice with B53 tumors and to identify host factors that might be involved in this process. We observed that ascitic B53 cells recovered at increasing time points from BALB/c mice exhibited a selective decline in steady state levels and rates of synthesis of ε-heavy chain protein and mRNA. The expression of κ-light chain protein and mRNA appeared relatively unchanged. The decrease in ε-heavy chain gene expresion did not occur when B53 tumors were passaged in nu⁺/nu⁺ mice or in BALB/c mice depleted of Lyt-2⁺ cells (suppressor/cytotoxic cell lineage), but did occur in nu⁺/nu⁺ mice reconstituted with neonatal BALB/c thymus and in BALB/c mice depleted of L3T4⁺ cells (helper/inducer cell lineage). That FcεR(+) T lymphocytes were directly involved in the inhibition of IgE expression was supported by the earlier and more pronounced inhibition of B53 IgE in mice infused with FcεR(+) T lymphocytes. We conclude from these findings that: 1) the decline in serum IgE levels that occurs toward the end of each generation of in vivo passage of the B53 hybridoma is due to decreased production of IgE by the hybridoma cells, 2) the decreased production of IgE is due to a selective loss of ε mRNA expression, 3) the decrease production of IgE by B53 cells is dependent on the presence of Lyt-2⁺ cells, and 4) FcεR(+) T lymphocytes participate in the mechanism by which IgE production is suppressed.