The Inhibition of Lymphocyte Blastogenesis By L-Asparaginase

The effect of the enzyme L-asparaginase on in vitro lymphocyte blastogenesis was studied. E.coli L-asparaginase has a reversible inhibitory effect on phytohemagglutinin (PHA) or specific antigen-induced lymphocyte blastogenesis. Five and one international units (I.U.) L-asparaginase per milliliter of culture media, suppressed lymphocyte blastogenesis almost completely. 0.5 I.U./ml. L-asparaginase showed a significant suppression and 0.1 I.U./ml. showed mild suppression. The effect was not due to direct toxicity of the enzyme, asparagine breakdown products, nor impure products of the enzyme preparation. The lymphocytes freed of L-asparaginase by washing after culture with the enzyme for up to 72 hours had normal responsiveness to PHA. The inhibitory effects of 0.1 I.U./ml. and 0.5 I.U./ml. of L-asparaginase were completely or partially counteracted by simultaneous administration of 2µmole/ml. L-asparagine, respectively. L-glutamine also counteracted the effect of L-asparaginase partially, but to a lesser degree than L-asparagine. Cell-density studies showed that 1 I.U./ml. of L-asparaginase, sufficient to inhibit the lymphocyte blastogenesis almost completely at the cell density of 0.5 x 10⁶ and 1 x 10⁶ lymphocytes, had less effect at higher cell population of 2.5 x 10⁶ and 5 x 10⁶. Lymphocytes once stimulated by PHA were partially resistant to L-asparaginase added during a later phase of the incubation period. The possible mechanism of this inhibition of L-asparaginase on lymphocyte blastogenesis was discussed. It is suggested that depletion of L-asparagine (both exogenous and endogenous) from the culture media would be the probable cause of this inhibitory effect.