| Summary: | Lymphocyte stimulation and proliferation play a pivotal role in the immune response to soluble as well as to cellular, bacterial, and viral antigens. In this study, peripheral blood mononuclear cells (PBMC), mainly composed of lymphocytes, were separated by Ficoll-Hypaque density gradient centrifugation from 50-ml jugular vein blood samples drawn from six captive and five wild-caught brown bears (Ursus arctos) (eight Apennine brown bears from the Italian population; three of undetermined origin). Stimulation of cultured bear PBMC with the two classical T lymphocyte mitogens phytohemagglutinin (PHA) and Concanavalin A (ConA) was followed by a significantly greater proliferative response than that shown by human PBMC (n = 11) (PHA: T = 4.03, d.f. = 20, P = 0.001; ConA: T = 4.25, d.f. = 20, P < 0.0005; Student's t-test, one-way ANOVA). As in humans, the PBMC proliferative response in bears was markedly (>50%) inhibited by addition of transforming growth factor beta (TGFβ) human recombinant cytokine to the culture. Further fractionation provided a cell preparation extremely rich in peripheral blood lymphocytes (PBL) (mean ± SD = 96.1 ± 1.7%). Addition of interleukin 1 (IL1) or interleukin 2 (IL2) human recombinant cytokines to cultured PBL stimulated with a suboptimal concentration of mitogens resulted in a ninefold increase in the lymphocyte proliferative response. Dexamethasone (DEX, a synthetic analog of hydrocortisone) inhibited the bear PBMC proliferative response by 22.2 ± 4.3% (mean ± SD), compared with 46.2 ± 6.9% and 91.8 ± 8.1% (mean ± SD) in humans and mice (n = 11) (Mus domesticus), respectively. Inhibition of the brown bear and human PBMC responses was markedly (>60%) reduced by the addition of IL2. The finding that ILl and IL2 augment and that DEX inhibits bear lymphocyte proliferative response suggests that these cytokines can be used to increase the immune response in vaccinations, and that DEX may hamper several immunologically mediated diseases.
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