Suppression of allograft immunity by 3,4,5,3',4',5'-hexachlorobiphenyl. II. Effects of exposure on mixed lymphocyte reactivity and induction of suppressor cell activity in vitro.

Abstract

Previous studies in our laboratory have established the sensitivity of the in vivo allogeneic cytotoxic T lymphocyte (CTL) response to suppression by 3,4,5,3',4',5'-hexachlorobiphenyl[(345)2-HxCB], a toxic, Ah receptor-binding polychlorinated biphenyl isomer. The present studies have examined possible cellular mechanisms for this suppression. A modest dose-dependent suppression of the proliferative response to alloantigen in mixed lymphocyte culture (MLC) was observed with lymphocytes from B6 mice exposed to 10 or 100 mg/kg (345)2-HxCB while the CTL response generated in MLC was significantly suppressed only following exposure to 100 mg/kg (345)2-HxCB. The amount of time between treatment with (345)2-HxCB and sacrifice, which ranged from 2 to 23 days, did not appear to influence the degree of immunosuppression produced by (345)2-HxCB exposure. Mitomycin C-treated lymphocytes from B6 mice treated with (345)2-HxCB were not suppressive when added as third party cells to an independent MLC. However, if the mice were alloimmune, lymphocyte-mediated suppression of the MLC response was observed and directly correlated with the magnitude of the CTL response present in the same population. Thus, (345)2-HxCB-treated mice which had less CTL activity as compared to vehicle-treated mice also had less suppressor activity. Further analysis indicated that stimulator cell lysis by the CTL was likely to be responsible for the inhibitory activity of the alloimmune lymphocytes rather than suppressor cells per se. Avoidance of stimulator cell lysis by using H-2-incompatible MLC stimulator cells revealed the existence of antigen-nonspecific suppressor activity that was greater with lymphocytes from vehicle-treated than from (345)2-HxCB-treated mice, suggesting that both CTL and suppressor cell activities were suppressed by (345)2-HxCB exposure. Direct addition of (345)2-HxCB to lymphocyte cultures in vitro indicated a lack of direct toxicity of (345)2-HxCB on lymphoproliferative responses to mitogen or alloantigen at concentrations equal to or less than 1 x 10(-6) M. Thus, the in-vitro functional integrity of lymphocytes obtained from (345)2-HxCB-treated mice coupled with the lack of a direct lymphotoxic effect of (345)2-HxCB in vitro suggest an indirect mechanism of action for (345)2-HxCB-mediated suppression of CTL activity in vivo. Previous reports implicating suppressor cell induction and/or activation by Ah-receptor-binding halogenated aromatic hydrocarbons that mediate the inhibition of CTL generation were not confirmed in these studies.