The role of the immune system in hexachlorobenzene-induced toxicity

Hexachlorobenzene (HCB) is a persistent environmental pollutant. The toxicity of HCB has been extensively studied after an accidental human poisoning in Turkey and more recently it has been shown that HCB has immunotoxic properties in laboratory animals and probably also in man. Oral exposure of rats to HCB showed stimulatory effects on spleen and lymph node weights and histology, increased serum IgM levels, and an enhancement of several parameters of immune function. Moreover, more recent studies indicate that HCB-induced effects in the rat may be related to autoimmunity. In Wistar rats exposed to HCB, IgM antibodies against several autoantigens were elevated; in the Lewis rat, HCB differently modulated two experimental models of autoimmune disease. Oral exposure of rats to HCB induces skin and lung pathology in the rat. Recently several studies have been conducted to investigate whether these skin and lung lesions can be related to HCB-induced immunomodulation, and these studies will be discussed in this review. HCB-induced skin and lung lesions probably have a different etiology; pronounced strain differences and correlation of skin lesions with immune parameters suggest a specific involvement of the immune system in HCB-induced skin lesions. The induction of lung lesions by HCB was thymus independent. Thymus-dependent T cells were not likely to be required for the induction of skin lesions, although T cells enhanced the rate of induction and the progression of the skin lesions. No deposition of autoantibodies was observed in nonlesional or lesional skin of HCB-treated rats. Therefore, we concluded that it is unlikely that the mechanism by which most allergic or autoimmunogenic chemicals work, i.e., by binding to macromolecules of the body and subsequent T- and B-cell activation, is involved in the HCB-induced immunopathology in the rat. Such a thymus-independent immunopathology is remarkable, as HCB strongly modulates T-cell-mediated immune parameters. This points at a very complex mechanism and possible involvement of multiple factors in the immunopathology of HCB.