Enhancement of weak mixed lymphocyte-type reactions by hydrophilic polymers

Two hydrophilic polymers, dextran and polyethylene glycol, were found to enhance in vitro MLR-type reactions which are weak or nonexistent in normal culture media. BALB/c and C57BL/6 thymocytes were cultured for 4 days in Eagle's MEM + 10% FCS, with or without the x-irradiated lymphomas LSTRA AND EL-4, respectively. By thymidine incorporation no stimulation was observed with EL-4 in plain culture whereas occasionally a very small stimulation was observed with LSTRA. Four percent dextran (w/v, m.w. greater than or equal to 40,000) improves stimulation (E/C) by about 2-fold. The effect of polyethyleneglycol m.w. 6,000 (PEG-6) is much more pronounced. At the optimal concentration of 4 to 5%, PEG-6 causes an increase of 5- to 20-fold in the stimulation index. The PEG-6 effect can also be observed microscopically by the appearance of numerous blasts clustered around the tumour cells. PEG-6 by itself has little effect on thymocytes in the absence of tumor cells. In the case of one-way allogeneic MLR under suboptimal ratios, dextrans (m.w. greater than or equal to 40,000) enhance the reaction dramatically. We speculate that these effects are occurring via a change in the solvent (such as solvent exclusion) in a way which enhances immunologic interactions under conditions which are tolerable to cells in vitro.