Treatment of Candida albicans mannan-specific downregulatory cell populations with divergent concentrations of monophosphoryl lipid A and intact lipopolysaccharide in vitro abrogates their effect on delayed hypersensitivity.

We have shown previously that splenocytes from mice injected with Candida albicans mannan (MAN) suppress MAN-specific delayed hypersensitivity (DH) when transferred to immunized recipients and that treatment of donor mice with monophosphoryl lipid A (MLA) derived from Salmonella typhimurium or Salmonella minnesota shortly before transfer abrogated the downregulatory activity. We now show that treatment of splenocytes in vitro at 4 degrees C with 5 ng/ml MLA or 0.05 ng/ml S. typhimurium lipopolysaccharide (LPS) for 30 min before transfer also abrogated downregulatory activity. Higher or lower doses of MLA, 5 micrograms or 5 pg, appeared to increase the suppressor activity slightly (5 micrograms) or had no effect (5 pg). LPS induced similar effects but the concentrations of LPS required to show the effects were 100-fold less than those of MLA. The effect of MLA appeared to be on cell(s) in the transfer population involved in MAN-specific DH, in that spleen cells from normal mice treated with MLA prior to transfer had no effect on DH. Finally, the population of MLA-responsive cells mediating downregulation could not be concentrated on MLA-coated plates, suggesting that the MAN-specific downregulatory cell(s) either did not bind to MLA or did not bind to MLA with sufficient avidity to remain attached during the washing procedures. The feasibility of abrogating suppression by treatment of lymphoid cells in vitro will allow a more detailed analysis of the mechanism of abrogation.