Target cells and key mediators in lead-induced immune modulation
Lead (Pb), is a widely dispersed and persistent environmental contaminant. It is toxic to many organ systems including the immune system. Mechanisms underlying Pb-induced immunotoxicity are not well understood, however its effects appear to be CD4+ T cell-mediated. Using mixed lymphocyte culture (MLC), an in vitro model system for study of cell function, we observed marked enhancement in alloreactive CD4+ T cell proliferation in the presence of Pb, a process termed Pb allo-enhancement. Pb allo-enhancement was characterized by augmented clonal expansion that had both interleukin-2 (IL-2)-dependent and independent components. Pb did not enhance mitogen-stimulated T cell proliferation, suggesting that CD4 + T cells were not the direct target for Pb. Pb allo-enhancement was dependent upon the presence of syngeneic myeloid suppressor cells (MSCs). Pb had some effect on antigen processing/presentation as Pb suppressed the proliferation of transgenic DO11.10 CD4+ T cells in response to whole ovalbumin (requiring antigen processing by antigen presenting cells) but not-to their specific cognate antigenic peptide. However, the primary mechanism by which Pb enhanced alloreactive T cell proliferation was via inhibition of nitric oxide (NO) production by MSCs. This inhibition disrupted normal MSC-dependent regulation of T cell proliferation. While Pb inhibited release of NO, it did not change abundance of enzyme responsible for its synthesis, inducible-nitric-oxide-synthase. Rather, Pb appeared to directly inhibit the function of this enzyme.
Pb had significant effects on cytokine environment in MLC. Pb treatment sharply decreased supernatant IL-2 and IFN-γ, and increased IL-4 and IL-10. These changes indicated a general shift from Thelper-1 (Th1) to Thelper-2 (Th2) in the microenvironment of MLC with Pb treatment. Despite the striking decrease in supernatant IL-2 levels, there was no measurable change in frequency of IL-2-producing CD4+ T cells. Although IL-10 levels dramatically increased with Pb treatment, suggestive of regulatory T cell effector function, CD4+ T cell harvested from Pb-treated MLC showed no regulatory effect on T cell proliferation in fresh MLC. Taken together, data presented in this thesis suggest that Pb enhances alloreactive CD4+ T cell proliferation in MLC by dysregulating NO production in MSCs, and that the cytokine environment is skewed toward Th2.