Didemnin B--an immunosuppressive cyclic peptide that stimulates murine hemagglutinating antibody responses and induces leukocytosis in vivo.

Didemnin B (DB) is a cyclic peptide with potent immunosuppressive activity in vitro and in the murine graft-versus-host-reaction (GVHR), the only measure of in vivo immunity tested in our prior studies. Because continued production of mature leukocytes by bone marrow and an intact antibody response are crucial to defense against infection in immunosuppressed patients, we have evaluated the effects of DB on these processes as well. Anti-sheep red blood cell (SRBC) hemagglutinating antibody (hAb) production was induced by i.p. injection of 5 x 10(7) SRBC in CB6F1 mice (5/group) treated with vehicle or DB once/day for six days. Serum was collected on day 7 and hAb titers measured by SRBC agglutination. Control antibody titers were 1/16, while animals receiving DB doses of 0.025, 0.05, 0.10, and 0.20 mg/kg/day yielded titers of 1/37, 1/74, 1/56, and 1/74, respectively. This stimulation of hAb production (4.6 x control) was confirmed by a second experiment. We then studied DB effects (0.1 mg/kg/day x 6 days) on serum hAb titers in separate groups of five mice at 7, 10, 15, and 20 days postimmunization. Control hAb titers were 1/110 on day 7, then dropped to 1/60 on days 10, 15, and 20. DB-treated animals had titers of 1/130 on day 7, and 1/170 on days 10-20. These data show that DB treatment in vivo causes a persisting increase in anti-SRBC hAb titers. Evaluation of DB effects on proliferation and antibody secretion in vitro by three hybridoma cell lines showed a potent inhibition of cell replication but stimulation of antibody production on a per-cell basis in each clone (+26%-+900%, range), suggesting a direct effect on Ig synthesis. During our first in vivo DB studies (0.1 mg/kg/day x 7 days) in mice, we noted that peripheral blood white counts were elevated on day 8 to 21.3 +/- 2.1 x 10(3)/mm3 compared with control (vehicle only) levels of 13.6 +/- 2.0 x 10(3)/mm3 (P less than .01). Kinetic studies showed that by 24 hr after a single i.p. injection of DB (1.0 mg/kg), blood leukocyte, granulocyte, and lymphocyte counts were elevated by 2.5-, 3-, and 2-fold, respectively, but declined rapidly thereafter. 3H-thymidine incorporation (4 hr) by freshly harvested bone marrow leukocytes from DB-treated mice (0.025, 0.05, and 0.10 mg/kg/day x 7 days) was enhanced up to 40% over control (P less than .05), while bone marrow cellularity was increased 200% (P less than .01).(ABSTRACT TRUNCATED AT 400 WORDS)