The immunological activities of human prealbumin preparation were investigated in vitro. Prealbumin was effective at inducing precursors in nylon wool-nonadherent spleen cells of nu/ nu mice to express Thy-1 antigens. At 100-250 μg/ml a four- to sixfold enhancement of cytotoxic index was observed. In the presence of prealbumin, mitogenic responses of spleen cells from normal C57B1/6J mice to phytohemagglutinin (PHA) or concanavalin A (Con A) but not lipopolysaccharide (LPS) were increased. The most significant effect of prealbumin at the optimal concentration (250 μg/ml) on PHA-induced proliferation of spleen cells was found in mice 12 months of age. An in vitro plaque-forming cell (PFC) assay showed prealbumin to exert a stimulating effect on PFC formation in spleen cells from normal C57B1/6J mice at different ages. No such activity was exhibited by human serum albumin preparation. In comparison with prealbumin, synthetic facteur thymique serique (FTS) was tested in mitogenic responses and the PFC assay. FTS showed no influence on responses of mouse spleen cells to PHA, Con A, or LPS. At concentrations of 100-200 μg/ml, however, FTS enhanced PFC formation from normal spleen cells. The findings indicate that it is unlikely that FTS is responsible for the activities of the prealbumin preparation.
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